Vet Clin Equine 20 (2004) 41–61

Sepsis in adults and foals

Marie-France Roy, DMV
Center for the Study of Host Resistance, Montreal General Hospital Research Institute,
McGill University Health Center, 1650 Cedar Avenue, Room L11-513,
Montreal, Que´bec H3G 1A4, Canada

Severe sepsis is estimated to affect more than 750,000 persons annually in

the United States alone, with an overall mortality rate of approximately
30%, increasing to 40% in the elderly and to more than 60% in severe cases
with multiple failing organs [1]. In equine medicine, no epidemiologic data
pertaining to the incidence and mortality associated with severe sepsis per se
exist. Several conditions routinely seen in equine practice are infectious in
origin or associated with endotoxemia and thus have the potential to induce
severe systemic inflammatory responses and eventually to progress to severe
sepsis and septic shock. Moreover, clinical experience tells us that horses
seem to be prone to developing severe sepsis compared with other farm
animal species, and experimental data suggest that they are far more
susceptible to the effects of endotoxins [2]. For these reasons, it is important
for equine practitioners to be aware of the pathogenesis and potential
treatment of sepsis in horses. Unfortunately, critical care of equine patients
is still in its infancy, and its development has been slowed by different
constrains, such as the large size of the horse and economic factors. It is
hoped, however, that equine practitioners will soon be armed with more
guidelines and therapeutic options in the treatment of sepsis in horses as we
refine our understanding of its pathophysiology, become more aware of its
manifestations and associated complications, and perform more random-
ized and controlled clinical trials specifically aimed at equine patients. In the
following review, some of the current concepts regarding the pathophysi-
ology of sepsis in horses and humans are discussed. Additionally, a few of
the existing treatments of equine sepsis are briefly reviewed, and recent
studies in human beings that have shown for the first time that the mortality
from sepsis could be reduced by therapeutic interventions are presented.

E-mail address: marie-france.roy@mail.mcgill.ca

0749-0739/04/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.cveq.2003.12.005
42 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61

Definitions
To clarify the remainder of this review, it is worth defining the terms
presently used in the scientific literature to describe patients with sepsis.
These definitions were adopted in 1991 at a Consensus Conference of the
American College of Chest Physicians and the Society of Critical Care
Medicine to make the use of sepsis-related terms in the scientific literature
uniform [3]. Since then, additional terms have been added as understanding
of the pathophysiology of sepsis has grown [4]. Although these definitions are
conceptually useful and allow better communication among scientists, they
nevertheless have some limitations and should not serve as the sole entry
criteria for clinical trials [5]. The unfamiliar reader may refer to Table 1 and
to the following paragraph for an overview of the current sepsis terminology
and abbreviations.
The body’s normal response to trauma or infection is to build an
inflammatory response at the site of injury with the goals of containing the
infectious agent, repairing tissue damage, and ultimately restoring homeo-
stasis. In severe cases, the inflammatory response may spill into the systemic

Table 1
Definitions and inclusion criteria for sepsis-related terminology
Terms and abbreviations Definitions
Systemic inflammatory Systemic inflammatory response to a variety of
response syndrome (SIRS) severe clinical insults manifested by two or
more of the following conditions: (1) fever
or hypothermia; (2) tachycardia;
(3) tachypnea or hypocapnia;
and (4) leukopenia, leukocytosis, or increased
circulating immature neutrophils
Sepsis SIRS induced by infection
Severe sepsis Sepsis associated with organ dysfunction,
hypoperfusion, or hypotension
Septic shock Sepsis-induced hypotension despite adequate
fluid resuscitation along with the presence of
perfusion abnormalities (eg, lactic acidosis,
oliguria, altered mental status)
Multiple organ dysfunction Presence of organ dysfunction in an
syndrome (MODS) acutely ill patient such that homeostasis
cannot be maintained without interventions
Compensatory anti-inflammatory Describes a patient with increased circulating
response syndrome (CARS) levels of anti-inflammatory mediators, leukocyte
anergy, or increased susceptibility to infection
Mixed anti-inflammatory response Describes a patient presenting with
syndrome (MARS) features of CARS and SIRS
Data from: Bone RC, Balk RA, Cerra FB, Dellinger RP, Fein AM, Knaus WA, et al.
Definitions for sepsis and organ failure and guidelines for use of innovative therapies in sepsis.
The ACCP/SCCM Consensus Conference Committee. American College of Chest Physicians/
Society of Critical Care Medicine. Chest 1992;101(6):1644–55; and Bone RC. Sir Isaac Newton,
sepsis, SIRS, and CARS. Crit Care Med 1996;24(7):1125–8.
 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61 43

circulation with pathologic consequences at sites remote from the focus of

infection, a state known as the systemic inflammatory response syndrome
(SIRS). Although there may be several reasons for the development of SIRS
(eg, severe trauma, extensive burns), it is called sepsis if initiated by an
infectious process. Clinically, human beings are said to have sepsis if they
have a documented infection plus at least two of the following criteria: fever
or hypothermia, tachycardia, tachypnea or hypocapnia, leukocytosis,
leukopenia, or an increased number of circulating immature neutrophils.
Although no consensus criteria for sepsis have been established for equine
patients, one might think that similar inclusion criteria could apply to them.
Acutely ill patients with evidence of organ dysfunction such that homeostasis
cannot be maintained without intervention are said to have multiple organ
dysfunction syndrome (MODS), and sepsis is said to be severe if it is
accompanied by MODS. In our equine patients, organ dysfunction may
include laminitis, evidence of coagulopathy (eg, thrombosis, increased
prothrombin time [PT] and partial thromboplastin time [PTT], decreased
platelets), pulmonary dysfunction (eg, arterial hypoxemia), gastrointestinal
dysfunction (eg, ileus), renal dysfunction (eg, oliguria), or cardiovascular
dysfunction (eg, hypotension, need for vasopressors, lactic acidosis). Septic
patients may rapidly progress to a state where they remain hypotensive
despite adequate fluid resuscitation: this is called septic shock. Conversely, at
the same time that a proinflammatory response is developing, the normal
body’s response to stress also consists of building a counterbalancing anti-
inflammatory response, which may, if it becomes too pronounced, lead to
a syndrome of immune suppression called the compensatory anti-inflamma-
tory response syndrome (CARS). Finally, patients may experience alternat-
ing or overlapping episodes of SIRS and CARS, a situation referred to as the
mixed anti-inflammatory response syndrome (MARS).

Overview of clinical sepsis in horses

Although any type of infection (bacterial, viral, or fungal) may lead to the
development of sepsis, bacteria and their products are the most frequent
initiators of severe sepsis in horses. In young foals, neonatal bacterial sepsis is
the most common presentation, often with no primary site of infection
detectable. Other frequent causes of sepsis in foals are infectious enteritis (eg,
Clostridium or Salmonella spp, rotavirus) and bacterial pneumonia. In adults,
sepsis is commonly found during gastrointestinal disturbances (colic,
proximal enteritis, or colitis) and is probably most often caused by excess
circulating bacterial products, such as endotoxins, rather than true bacterial
infection. Other conditions associated with sepsis in adult horses include
severe bacterial pneumonia or pleuropneumonia, clostridial myositis,
retained placenta, and metritis.
Horses presenting with severe SIRS show clinical signs that are obvious
and familiar to most practitioners. The most common findings include
44 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61

tachycardia, tachypnea, dark injected mucous membranes with or without

a toxic ring, altered capillary refill time, injected sclera, and fever. In addition,
sepsis patients may be depressed or lethargic or may exhibit signs of
abdominal discomfort. If the cardiovascular system is compromised from
hypovolemia or as a result of severe sepsis, peripheral pulses may be weak
to absent, the extremities cold, and the jugular filling poor. It is important
to realize that the early clinical signs may not always be obvious and may
be easily overlooked by a nonsuspecting observer. This situation is most
common and has more serious consequences in young foals, in whom early
sepsis may manifest as a slightly depressed mentation; decreased activity
level; decreased borborygmi; increased abdominal fullness; lack of daily
weight gain; slightly injected mucous membranes or sclera; and mild in-
creased heart rate, respiratory rate, or body temperature. It is thus impor-
tant that critically ill patients be followed regularly with at least a daily
complete physical examination by an experienced veterinarian and frequent
reassessment of critical clinical parameters throughout the day.
Various alterations of laboratory data may be found in patients with
sepsis. Monitoring these changes aids in determining the severity of the
patient’s condition and is useful in assessing response to treatment. Adult
horses and foals with acute SIRS often present with severe leukopenia
because of neutropenia, along with circulating immature and toxic
neutrophils. Hypovolemia and pulmonary dysfunction may be reflected by
increased packed cell volume (PCV), lactic acidosis, and arterial hypoxemia.
Blood glucose alterations are common in foals but may also be seen in adult
horses. Changes in creatinine concentration and liver and muscle enzyme
activities may reflect end-organ dysfunction, damage, or hypoperfusion.
Alteration of the blood coagulation profile is also relatively frequent,
including prolonged PT and PTT, decreased antithrombin III (ATIII) levels,
thrombocytopenia, and increased fibrinogen degradation products.

Overview of the pathophysiology of sepsis

The balancing forces: systemic inflammatory response syndrome and
compensatory anti-inflammatory response syndrome
In an attempt to illustrate the events underlying the development of sepsis
and its consequences, Bone [4] has proposed a model based on the opposing
effects of the host pro- and anti-inflammatory responses. Although this model
is certainly oversimplified, it nevertheless gives a useful starting point to
understanding sepsis. According to this point of view (Fig. 1), after an insult,
which is of infectious origin in the case of sepsis, a localized proinflammatory
response ensues, shortly followed by a balancing anti-inflammatory response.
If these responses are appropriate and the insult not overwhelming, the
invaders are neutralized, tissue damage is minimized and repaired, and
homeostasis is restored. In the case where homeostasis is not rapidly restored,
 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61 45

Fig. 1. The balancing forces. A proper balance between the body’s pro- and anti-inflammatory
responses is necessary to restore homeostasis. Adverse consequences occur when one or the
other response is disproportionate in relation to the inciting event or in relation to each other.

the localized pro- and anti-inflammatory responses may soon become

systemic, leading to a whole body response: SIRS and CARS. Again, if the
pro- and anti-inflammatory responses balance each other, homeostasis is
restored. If SIRS predominates, severe sepsis with end-organ damage or
shock may develop. In contrast, if CARS predominates, the patient presents
a shift from a proinflammatory to an anti-inflammatory response, anergy,
increased lymphocyte apoptosis, and increased susceptibility to nosocomial
infections [6]. Alternatively, the patient may go through overlapping episodes
of SIRS and CARS or show evidence of both responses at once, the situation
described as MARS.
Although this model was destined to describe the state of sepsis patients in
human medicine, it is also helpful in describing our equine patients.
Experimental models of endotoxemia and bacterial sepsis in horses most
likely illustrate situations where SIRS predominates and situations where
early anti-inflammatory therapy may be beneficial. Because of their
susceptibility to endotoxemia, the large reservoir of bacteria and bacterial
products that constitute the equine large colon and the many clinical
situations that are associated with introduction of significant amounts of
endotoxins in the circulation, we do see peracute cases of endotoxemia in
horses, notably in acute colitis and surgical colic cases. These cases may be
good examples of pure SIRS leading rapidly to severe sepsis and, occasionally,
to septic shock. Neonatal foals also sometime present with fulminant bacterial
septicemia that rapidly progresses to septic shock. Conversely, cases with
a predominance of CARS may also be seen in equine medicine, such as some
hypoxic-ischemic and septic foals that linger for several days experiencing
repeated nosocomial infections or adult horses that are recovering from colic
46 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61

surgery or a severe colitis. Patients suffering from similar clinical conditions

may not be responding in the same way to the insult and thus may not benefit
from the same therapies. It is important to consider each patient as unique and
to be receptive to clinical findings that may indicate the state of the immune
system in a particular patient. In the future, diagnostic tests and, more
importantly, specific therapies may become available so that each patient
receives a treatment appropriate to its clinical state.

The inflammatory response

The pro-inflammatory response seen in horses with sepsis is principally
induced by the interaction of bacteria (or their products) with the
mononuclear phagocytes. The release of pro-inflammatory cytokines and
other mediators by the activated macrophages leads, directly or indirectly, to
the typical clinical signs seen in these cases, including fever, depression,
anorexia, tachycardia, tachypnea, and hypotension. (For reviews on the role
of cytokines in sepsis, see the articles by Dinarello and Oberholzer et al [7–9]).
Studies of experimental endotoxemia in horses have shown that, like in
other species, the earliest cytokine to appear in the circulation is tumor
necrosis factor (TNF) [10]. Increased circulating TNF can also be detected in
some clinical colic cases, and a marked increase in TNF level seems to be
associated with higher mortality [11]. TNF is responsible for induction of
interleukin (IL)-1, and these two molecules act synergistically to produce
most of the clinical signs associated with sepsis. Indeed, experimental animal
studies have showed that the clinical picture of septic shock can be
reproduced by the sole administration of TNF or IL-1 [12,13]. These two
cytokines induce the transcription of several genes involved in inflammation,
such as genes for other cytokines, phospholipase A2 (PLA2), cyclooxygenase
2 (COX-2), inducible nitric oxide (iNOS), endothelial adhesion molecules,
and chemokines. This leads to the production of several important mediators,
such as platelet-activating factor (PAF), prostaglandin E2 (PGE2), leuko-
trienes, and nitric oxide (NO), and to the activation of neutrophils, their
adhesion to the endothelium, and their emigration to injured tissues, with the
final outcome being inflammation, tissue destruction, and loss of function [8].
IL-6 is also an important cytokine that increases during sepsis. Several cell
types produce IL-6 after exposure to TNF and IL-1. IL-6 is a potent
stimulator of the hepatic acute-phase response and a growth factor for B cells.
Levels of circulating IL-6 seem to correlate better with mortality than TNF or
IL-1 levels in human studies and may be considered a prognostic indicator
[14]. IL-6 also increases in the blood of horses after experimental
administration of lipopolysaccharide (LPS) [15,16] and in the blood or
peritoneal fluid of clinical colic cases [17]. As is the case in human medicine,
IL-6 seems to be a better marker of disease severity and mortality in horses
because it may reflect the earlier extent of TNF and IL-1 release. Neverthe-
less, it is important to note that IL-6 is not a proinflammatory cytokine in
 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61 47

itself and that administration of high doses to animals or human beings does
not induce shock [7].

The anti-inflammatory response

Although an excessive proinflammatory response (SIRS) has long been
viewed as the basis for development of severe sepsis and its consequences
[18], it is now recognized that clinical situations associated with an excessive
anti-inflammatory response are frequent and most likely also associated
with an adverse outcome in sepsis. Some authors [19] have proposed that the
normal body response to stress is usually to prevent systemic inflammation
by activating several anti-inflammatory mechanisms, with the goal being to
allow inflammation to occur locally while preventing its systemic spread.
As a counterbalancing force to the secretion of proinflammatory
cytokines, several anti-inflammatory molecules are also secreted during
sepsis, including IL-4, IL-10, IL-11, IL-13, transforming growth factor-b,
soluble TNF receptors, and IL-1 receptor antagonist. The anti-inflammatory
cytokines are important because they suppress the production of IL-1, TNF,
chemokines, and vascular adhesion molecules, thereby ensuring that the
inflammatory response of the host does not become overwhelming. In
addition to the effects of the anti-inflammatory cytokines, the role of the
nervous system in the body’s anti-inflammatory response is now being
recognized [20]. Inflammatory products found in peripheral tissues can
stimulate an afferent signal through the sensory vagus nerve that is relayed to
the nucleus tractus solitarius and results in efferent motor vagus stimulation
and inhibition of cytokine synthesis in peripheral tissues. This inflammatory
reflex seems to take effect through nicotinic acetylcholine receptors of tissue
macrophages [21]. Finally, the body’s anti-inflammatory response also in-
volves other mechanisms, such as the release of glucocorticoids and sym-
pathetic mediators. The interrelation between pro- and anti-inflammatory
mediators during sepsis is thus quite complex and tightly regulated and
probably differs between different patients and different diseases. A proper
balance between these two arms, along with a response appropriate to the
initial insult, is probably key in allowing re-establishment of homeostasis.

Dysregulation of coagulation during sepsis

The innate immune response and the coagulation cascade are phyloge-
netically and functionally related so that proinflammatory cytokines have
procoagulant effects and activation of the clotting system promotes
inflammation [22]. Although the dual activation of inflammation and
coagulation is effective in controlling localized infection, widespread systemic
activation of these two systems may be detrimental to the host.
Three major mechanisms seem to contribute to the dysregulation of
coagulation during sepsis [23]. First, some cytokines (especially IL-6) strongly
48 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61

induce the expression of tissue factor (TF) on the surface of macrophages and
endothelial cells, leading to the generation of thrombin through the extrinsic
pathway of coagulation. Second, the natural mechanisms of inhibition of
coagulation (antithrombin, protein C system, and TF pathway inhibitor) are
impaired during sepsis. Finally, the fibrinolytic system, although activated
early during sepsis, is soon strongly inhibited through the sustained release of
plasminogen activator inhibitor type 1. In addition to these three mecha-
nisms, endothelial dysfunction (or damage) may contribute to the coagulation
complications of sepsis. Under physiologic conditions, the endothelial cells
exert several functions, such as prevention of coagulation; orchestration of
leukocyte emigration; production of chemoattractant compounds; and re-
gulation of the microcirculation, blood pressure, and vasopermeability [24].
After stimulation by cytokines or interaction with inflammatory molecules,
the endothelial cells become activated and many of their physiologic
functions are dramatically modified. For instance, the endothelium loses its
normal anticoagulant properties to become a procoagulant surface. Addit-
ionally, the endothelial cells express adhesion molecules along with increased
amounts of vasoactive compounds and mediators, such as IL-6, chemokines,
PAF, prostaglandins, and complement factors.
Alteration of coagulation is often seen in horses with colic or colitis [25–27]
as well as in neonatal septic foals [28] and is reflected by several alterations in
hemostasis parameters, including increased PT, PTT, fibrinogen degradation
products (FDPs), and plasminogen activator inhibitor type 1 as well as
decreased platelets, ATIII, and protein C. These alterations, along with
endothelial dysfunction, may lead to clinically significant complications, such
as large vessel thrombosis [29], disseminated intravascular coagulation
[30,31], and, possibly, MODS and leaky capillary syndrome, which each
contribute to an adverse outcome. The pathogenesis of these pathologic
changes in horses seems to be quite similar to what is seen in other species,
including increased expression of TF [32,33], decreased natural anticoagu-
lants [27,28,34], and increased plasminogen activator inhibitor type 1 [25].

The role of the innate immune system

In their battle against invading microorganisms, mammals are armed with
three main defense mechanisms. The first line of defense consists of numerous
surface barriers, such as intact skin and mucous membranes, a mucus layer,
enzymes, and antimicrobial peptides, which usually prevent most of the
microorganisms from invading the body. When, however, microbial invasion
does occur, two other components of the immune system come into play: the
innate and adaptive responses [35–37].
The adaptive immune system, a relative newcomer on the evolutionary
scale, uses T and B lymphocytes to generate receptors (B-cell receptors or
immunoglobulins and T-cell receptors) that are specific for the encountered
antigens and help in its neutralization or in the destruction of infected cells.
 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61 49

The adaptive response also allows the production of memory T and B cells
that permit a more efficient and rapid response in case of re-exposure to the
same pathogen. The B- and T-cell receptors are generated randomly by
somatic cell DNA rearrangement, a fact that accounts for their fantastic
diversity. The inherent characteristics of the adaptive response also carry
some disadvantages, however. First, because they are somatically generated,
the T- and B-cell receptors cannot be transmitted to descendants and have
to be reinvented at each generation. Moreover, because the T- and B-cell
receptors are randomly generated, there is a chance that they may bind to
self-antigen or to antigen of innocuous microbes. Finally, this response,
although necessary for efficient resolution of infection, takes several days to
develop fully, which leaves time for microbial mutations to occur faster than
the immune response and would leave the body defenseless during the first
days of infection if it were not for the other arm of the immune system.
The innate immune system was the first to appear during evolution, and it
is shared by distantly related entities, such as plants, invertebrates, and
vertebrates. The main effector mechanisms of innate immunity are anti-
microbial peptides, phagocytes, and the alternative complement pathway. To
recognize invading organisms, the innate immune system uses pattern
recognition receptors (PRRs) to detect specific pathogen-associated molec-
ular patterns (PAMPs) of invading organisms. The PRRs are encoded by
specific host genes and can be transmitted from generation to generation,
allowing for years of natural selection for survival advantage. Therefore, the
PRRs have become highly specific for detecting molecular patterns of
invading pathogens that are not found on the host cell. Moreover, because the
PAMPs are essential for microbial survival or virulence, these patterns have
been conserved through evolution and are often shared by a broad range of
organisms. Finally, the innate immune system is ready to work the minute it
recognizes invading pathogens, allowing for control of their replication and
giving time for and directing the development of the adaptive response.
The PRRs can be functionally divided in three categories [37]: secreted
PRRs (eg, mannose-binding lectin [MBL]) act as opsonins by binding the
microbial cell wall and helping in recognition by the complement system or in
phagocytosis; endocytic PRRs (eg, macrophage mannose receptor) are
present on the surface of phagocytic cells, where they bind microbial
components and mediate the uptake and delivery of the pathogen to the
endosome; and signaling PRRs, on binding PAMPs, activate signal trans-
duction pathways that lead to the transcription of several genes that are
important for immunity. The recently recognized Toll-like receptors (TLRs)
belong to this category of PRRs and are now viewed as important activators
of the innate immune response and may be important players in the
pathogenesis of sepsis.
The TLRs possess a leucine-rich repeat extracellular domain, a trans-
membrane domain, and an intracellular domain similar to the intracellular
domain of the IL-1 receptor (called Toll-interleukin receptor [TIR] domain).
50 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61

These receptors were named because of their similarities to Drosophila Toll

(dToll), which was first recognized to be important for normal development of
the fruitfly. Further studies have found that adult flies carrying mutations in
dToll were susceptible to fungal infection, thereby linking the innate immune
response to dToll [38]. The first homolog of dToll, now named Toll-like
receptor 4 (TLR4), was discovered in human beings as a molecule capable of
inducing activation of the nuclear factor-jB (NF-jB) signaling pathway [39].
An important breakthrough concerning the TLRs came shortly afterward,
when they were found to be of paramount importance in LPS signaling.
Although it had been accepted for some time that circulating LPS was
transferred to macrophage-bound CD14 by lipoprotein-binding protein
(LBP), it was not known how this signal was transduced to the inner cell,
because CD14 lacks an intracytoplasmic domain. The answer to this problem
came with the cloning of the Lps locus in mice (reviewed in the article by Roy
and Malo [40]). It was indeed recognized for several years that some strains of
mice were hyporesponsive to the administration of LPS, with a median lethal
dose (LD50) 28 to 40 times higher than normally responsive strains.
Conversely, these mice were also found to be far more susceptible to some
gram-negative infections (eg, Salmonella Typhimurium). This phenotype was
controlled by a single gene localized through linkage analysis to the mouse
chromosome 4. Twenty years later, using a positional cloning approach, two
independent groups of investigators [41,42] identified the mouse homolog of
TLR4 as the gene for the LPS hyporesponsiveness in these strains of mice.
Mutations within this gene render the affected strains hyporesponsive to LPS
but hypersusceptible to some gram-negative bacteria, a finding that was later
confirmed through examination of Tlr4 knock-out mice [43].
Since then, 10 human TLRs have been cloned, and most seem to be in-
volved, alone or in cooperation with other TLRs, in innate immune recog-
nition of invariant molecular patterns associated with invading pathogens
[44]. For instance, TLR2 is important for recognition of gram-positive
peptidoglycans and lipoproteins, TLR3 is involved in viral double-stranded
RNA recognition, TLR5 binds microbial flagellin, and TLR9 recognizes
CpG motifs, which are found in greater abundance in microbial DNA. After
interaction between the PAMPs and the TLRs, in cooperation with several
other molecules forming an activation complex [45], a series of intracellular
events occurs and leads to the activation of NF-jB and mitogen activated
protein (MAP) kinases, resulting in the induction of several genes involved in
the host response to infection, such as proinflammatory cytokines, chemo-
kines, costimulatory molecules, and the major histocompatibility complex
(MHC). The innate immune response is thus the main initiator of systemic
inflammation during infection, and its excessive or defective activation may
result in an unbalanced immune response ultimately leading to severe sepsis
and septic shock.
Although most studies have focused on human or rodent TLRs, equine
TLR4 has recently been cloned (GenBank Accession Number AY005808),
 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61 51

and TLR4 and TLR2 have been cloned in other domestic animal species
[46]. Although it is expected that the main principles underlying the function
of these TLRs in innate immune recognition will hold true for veterinary
species, differences in ligand-binding specificities and signaling disparities
will most likely be uncovered. Functional studies involving equine TLRs are
thus needed if we want to understand the basis of the innate immune
response in this species. The unraveling of the TLR ligand specificities and
pathways may facilitate an understanding of species differences in sepsis
susceptibility or identification of new therapeutic targets.

Host genetic factors

The host genetic background is certainly an important factor in
determining the susceptibility to infectious diseases and their associated
complications [47]. Twin and adoptee studies demonstrated years ago in
human beings that genetic factors influence the susceptibility to death from
infectious diseases [48–50], and several genes have now been associated with
increased susceptibility to various infectious diseases in people and domestic
animals. In some cases, the genetic influence results from single gene defects
with a strong effect and lead to severe susceptibility to infectious diseases early
in life, such as the mutation in the catalytic subunit of the DNA-dependent
protein kinase causing severe combined immunodeficiency disease (SCID) in
Arabian foals [51]. In other cases, the genetic defect(s) may lead to sus-
ceptibility to infectious diseases only in a specific infectious context, in the
presence of other predisposing genes, or in the presence of some environ-
mental influences. Examples of such gene defects include polymorphisms in
cytokine, cytokine receptors, and PRR genes. For instance, in human beings,
pathogenic mutations in genes coding for the interferon-c (IFNc) receptors,
IL-12 p40 subunit, and IL-12 receptor lead to the syndrome of Mendelian
susceptibility to mycobacterial disease [52,53], a condition characterized by
increased susceptibility to poorly virulent mycobacterial species. The affected
patients rarely develop other infectious diseases with the exception of
Salmonella infections, which are found in almost half of the cases. Variants in
the coding region of MBL lead to decreased circulating levels of MBL, and
homozygosity for such variants have been associated with susceptibility to
invasive pneumococcal disease [54]. More recently, polymorphisms in human
TLR4 have been associated with LPS hyporesponsiveness [55] and may
confer increased susceptibility to gram-negative bacteria [56,57]. With the
development of the horse genome project and with the use of comparative
genomics, it is possible that genetic polymorphisms affecting the response of
horses to infection or the development of severe sepsis and septic shock may
be found in the future. Ideally, this could lead to improvement of host
resistance through breeding selection or to early identification of susceptible
animals so that appropriate prophylactic or therapeutic interventions could
be undertaken.
52 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61

Treatment of sepsis in horses

Although the search for mortality-reducing therapy for sepsis continues,
the clinician should not forget that the most important considerations in
treating septic patients are early diagnosis, treatment of the primary disease
process, and supportive care. This may be especially true in equine medicine,
given the frequent financial restrictions and the lack of large-scale clinical
trials to evaluate potential therapies. Frequent thorough physical examina-
tion of patients, adequate fluid resuscitation, continuous monitoring, sup-
port of cardiovascular function with inotropes and vasopressors, use of
appropriate antimicrobial therapy, and a thorough search for existing or
newly developing infectious foci remain the attending clinician’s best chances
for a successful outcome. In addition to basic patient care, some therapeutic
interventions most likely have their place and may reduce mortality in equine
sepsis patients, although formal proof that such treatment exists has not been
obtained so far in equine medicine. In the following sections, some of the
therapeutic interventions that have been investigated in horses are briefly
presented.

Antiendotoxin interventions
Because of the suspected importance of endotoxins in equine sepsis,
therapeutic approaches directly targeting circulating LPS have been de-
veloped. One such approach consists of transfusing hyperimmune plasma-
containing antibodies against Re mutant S typhimurium or J5 mutant
Escherichia coli. In studies using experimental models of sublethal endotox-
emia in horses, prior administration of hyperimmune plasma did not provide
any benefit [58,59]. In a small double-blind clinical study of horses with
gastrointestinal diseases, however, a significant reduction in mortality was
found with the administration of hyperimmune J5 plasma [60]. In human
studies, none of the large randomized clinical trials have shown beneficial
results [61]; however, endotoxemia may be a more common cause of sepsis in
horses compared with people. These days, hyperimmune J5 plasma is
frequently administered to horses with clinical evidence of endotoxemia.
Because of the high cost of hyperimmune J5 plasma and the potential
complications associated with its administration, large-scale controlled
clinical trials with strict entry criteria for patient selection are needed before
clear recommendations can be made regarding its use in clinical cases.
An alternative approach to the use of expensive hyperimmune
antiendotoxin plasma is the antimicrobial drug polymyxin B, which is
known to bind circulating endotoxin and thus to prevent its binding to
cellular receptors with subsequent activation of inflammation. Experimental
studies have shown that treatment with polymyxin B before endotoxin
challenge resulted in improved clinical signs and a decreased level of
circulating proinflammatory cytokines [58,62]. Critical evaluation of the
 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61 53

potential benefits of polymyxin B in clinical cases, where the treatment is

administered well after the challenge, are still awaited, however.

Anti-tumor necrosis factor therapy

The use of anti-TNF antibodies in experimental endotoxemia in horses
has proven beneficial when large doses of murine monoclonal antibodies
were given just before LPS challenge [63]. When lower doses of rabbit
polyclonal antibodies were given 15 minutes after LPS challenge, no
significant benefits were found, however [64]. Large randomized clinical
trials in human patients have failed to show any significant benefits in
survival for sepsis patients receiving anti-TNF antibodies [14,65]. Because of
the early role of TNF in sepsis, because TNF is only one of the several
mediators involved in sepsis, and because not all patients may indeed have
increased circulating TNF, it is unlikely that monotherapy targeting of TNF
will prove to be beneficial in all clinical sepsis patients.

Nonsteroidal anti-inflammatory drugs

One important effect of TNF and IL-1 is the induction of PLA2 and COX-
2 and thus the production of eicosanoids, which are responsible for several
pathologic changes during inflammation. Blocking the production of some of
these mediators by using nonsteroidal anti-inflammatory drugs (NSAIDs)
becomes a logical approach in the treatment of sepsis and is supported by
experimental data in horses. Administration of flunixin meglumine (1.1 mg/
kg) before administration of sublethal doses of endotoxin dramatically
reduces the effects of experimentally administered LPS [66]. Furthermore,
pretreatment with a lower dose of flunixin meglumine (0.25 mg/kg) was
effective in preventing eicosanoid production and suppressing increases in
blood lactate without completely alleviating the clinical signs of endotoxemia
[67]. When comparing flunixin meglumine with phenylbutazone, it seemed
that the former was more effective in controlling the clinical signs of
endotoxemia and the production of prostaglandins and thromboxane [68].
Because of these studies, it is now extremely common for equine
practitioners to treat suspected endotoxemia cases with a low dose of
flunixin meglumine (0.25 mg/kg three times daily). There are no controlled
trials that have looked at the effect of such therapy in clinical cases;
therefore, it is not known whether this approach is beneficial. Here again,
large controlled clinical studies are needed to determine in which situation,
at what dose, and for how long flunixin meglumine may be beneficial in
clinical sepsis cases. In a large double-blind, placebo-controlled, randomized
clinical trial in human sepsis patients, treatment with ibuprofen led to
decreased thromboxane and prostacyclin concentration, fever, heart rate,
oxygen consumption, and lactic acidosis but did not prevent shock or acute
54 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61

respiratory distress syndrome or improve survival [69]. Despite lack of

evidence of efficacy in improving survival, judicious use of NSAIDs most
likely has its place in the treatment of sepsis in some horses, at least for pain
control and for improving patient appetite and well-being.

Treatment of sepsis in human patients: recent successes

For several years, despite enormous progress in the understanding of the
pathogenesis of sepsis, little progress was made in finding therapies that
could improve mortality from sepsis [70]. The situation has recently changed
with four human studies now showing significant reduction in mortality
using simple strategies, each targeting a different aspect of the pathophys-
iology of sepsis.

Early goal-directed therapy

In the progression from SIRS to severe sepsis and septic shock, imbalances
between oxygen delivery and oxygen needs develop because of circulatory
abnormalities, leading to global tissue hypoxia or shock [71]. In critically ill
patients, there seems to be a narrow window where therapeutic interventions
can stop the progression toward shock and death [72]. The importance of
early goal-directed intervention has been emphasized by a recent study of
early goal-directed therapy in 263 patients with severe sepsis or septic shock
[71]. By assigning patients as early as possible in the course of their disease to
a strict hemodynamic support protocol aimed at maintaining central venous
pressure between 8 and 12 mm Hg, mean arterial pressure between 65 and 90
mm Hg, and central venous oxygen saturation above 70%, the authors
showed that in-hospital mortality could be significantly reduced from 46.5%
in the standard therapy group to 30.5% in the early goal-directed therapy
group.
No such studies have been completed in the horse, and, most particularly
in the adult horse, invasive monitoring is not routinely performed or easily
accomplished. Nevertheless, it may be worthwhile for similar horse-adapted
studies to be undertaken in adults and foals so that precise recommenda-
tions may be given if such an approach turns out to be beneficial for horses
as well. In particular, the value of rapid correction of fluid deficits and the
value of monitoring and correcting abnormalities in blood pressure, blood
lactate, central venous pressure, and central venous oxygen saturation
should be investigated. It is common for the equine internist to extrapolate
from human critical care guidelines; however, especially for adult horses, it
would be of great value if guidelines were published for the resuscitation of
severely ill equine patients. Such guidelines would offer a simple approach
that could most likely be easily and cost-effectively implemented in equine
intensive care units.
 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61 55

Recombinant human activated protein C

The protein C pathway functions as an important mechanism to maintain
a proper balance between pro- and anticoagulant activities [73]. Protein C is
converted to its active counterpart, activated protein C (APC), after binding
of thrombin to endothelial thrombomodulin. APC inhibits the activated
forms of factors V and VIII, thereby blocking the generation of thrombin. In
addition to its antithrombotic activities, APC can modulate antiapoptosis
and cell survival pathways in endothelial cells and has anti-inflammatory and
profibrinolytic properties, making it an excellent candidate for sepsis therapy.
The potential effect of recombinant human activated protein C (rhAPC)
on the rate of death in patients with SIRS and organ failure caused by acute
infection was evaluated in a randomized, double-blind, placebo-controlled,
multicenter trial [74]. A significant reduction, by 19.4%, in the relative risk of
death was achieved by the administration of rhAPC. A possible increase in
the risk of severe bleeding was associated with treatment, however, although
severe bleeding seemed to occur mainly in patients with a predisposition to
bleeding and only during the infusion period. In this trial, rhAPC was
associated with decreased D-dimer and IL-6 concentrations, consistent with
its known anticoagulant and anti-inflammatory biologic effects.
Similar to what has been found in human sepsis patients, protein C
decreases in the plasma of septic foals and horses with colic [27,28,34]. To the
author’s knowledge, however, therapeutic administration of APC has not
been evaluated in the treatment of sepsis in horses. It is likely that financial
constrains will limit the use of this molecule in horses for the foreseeable
future; however, it is certainly warranted that the significance of decreased
protein C in equine sepsis and the potential benefits of administration of APC
be investigated.

Intensive insulin therapy

Hyperglycemia and insulin resistance are common in human critical care
patients, even in those with no prior history of diabetes. A prospective,
randomized, controlled study was performed to evaluate whether intensive
insulin therapy aimed at maintaining normoglycemia in patients admitted to
a surgical intensive care unit could be beneficial [75]. Patients were ran-
domized to receive either intensive insulin therapy (to maintain normo-
glycemia [ie, blood glucose between 80 and 110 mg/dL]) or conventional
therapy (insulin started only if blood glucose exceeded 215 mg/dL and used to
maintain blood glucose between 180 and 200 mg/dL). A significant reduction
in mortality was found among patients randomized to the treatment arm; the
reduction in mortality was more pronounced in patients that remained in the
intensive care unit for more than 5 days and in patients that had multiple
organ failure and a proven septic focus. In addition, intensive insulin therapy
was associated with a reduction in blood stream infections, acute renal
56 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61

failure, need for transfusion, critical illness polyneuropathy, and prolonged

mechanical ventilation and intensive care. The mechanisms underlying the
beneficial effects of intensive insulin therapy remain obscure but could be
related to an anti-inflammatory effect of such treatment [76].
Hyperglycemia and insulin resistance are seen in equine critical care
patients, although most commonly found and monitored in critically ill
neonatal foals. Although insulin therapy is common in some equine neonatal
intensive care units, the goal of such therapy is usually to avoid gross
hyperglycemia and not to maintain normoglycemia tightly. As of yet, no
published data are available regarding the frequency and importance of
glucose metabolism alterations in acutely ill equine patients, and, evidently,
no clinical trials have been performed to evaluate the potential efficacy of
tight glucose regulation in horses. Therefore, it may be worthwhile designing
clinical trials to evaluate the use of insulin therapy in critically ill foals. At the
least, clinicians should be aware of the potential importance of strict glucose
control so that adequate monitoring and, if needed, adequate treatment can
be provided to critically ill equine patients.

Low-dose corticosteroid therapy

The activation of the hypothalamic-pituitary-adrenal axis during acute
illness is an important protective mechanism aimed at harnessing the acute
inflammatory response, and critically ill patients should normally present
with increased plasma cortisol concentrations [77,78]. This is not always the
case, however, because some patients may instead present with functional or
relative adrenal insufficiency, which could contribute to a poor outcome [79].
Although several well-conducted randomized and controlled trials have
shown that indiscriminate use of large doses of steroids should be avoided in
severe sepsis [80,81], the question has arisen as to the potential benefits of low
doses of corticosteroids in patients with septic shock and demonstrated
adrenal insufficiency. In a multicenter, placebo-controlled, randomized,
double-blind trial, low-dose therapy with hydrocortisone and fludrocortisone
was shown to significantly reduce 28-day mortality and duration of vaso-
pressor administration in patients with septic shock significantly, especially
those with documented adrenal insufficiency [82].
Adrenal insufficiency may be seen in premature foals [83], and it has been
reported in one neonatal, full-term, critically ill Paint foal [84]. In this case,
a diagnosis of adrenal insufficiency was made on the basis of clinical signs,
electrolyte abnormalities, low basal cortisol concentration, lack of response
to exogenous adrenocorticotropin, and response to treatment with prednis-
olone. Here again, the real importance of adrenal insufficiency in acutely ill
horses and the potential benefits for these patients of supplemental therapy
remain completely unknown. Further studies are needed to evaluate the
incidence of adrenal insufficiency and the potential benefits of corticosteroid
therapy in horses before any recommendation can be made, especially in view
 M.-F. Roy / Vet Clin Equine 20 (2004) 41–61 57

of the ongoing controversy linking administration of corticosteroids to

laminitis in adult horses [85].

Summary
Sepsis develops in horses when the host response to the invading pathogens
is not properly balanced according to the severity of the insult. Several clinical
conditions frequently encountered in equine practice may be associated with
the development of sepsis and have the potential to progress to more severe
forms, such as severe sepsis, MODS, and septic shock. Consequently, it is
important for equine practitioners to be aware of the manifestations,
pathophysiology, and treatment of sepsis. Although enormous progress has
been made in recent years in our understanding of the pathophysiology of
sepsis, more work remains to be done in improving basic critical care
guidelines and basic monitoring in equine intensive care units and in critically
evaluating potential equine sepsis therapy. Fortunately, we can learn from the
important advances made recently in the treatment of human sepsis patients;
hence, rapid progress may be expected in a near future, especially as more and
more veterinarians show interest in the discipline of equine critical care. With
the completion of several genome projects and the availability of high-
throughput genetic techniques, one hopes that we will further refine our
understanding of the events underlying the development of severe sepsis and
septic shock, which could lead to more appropriate therapeutic intervention
targeted to each individual according to the state of the immune response in
that horse.

Acknowledgment
The author thanks Dr. Renaud Léguillette for critical reading of the
manuscript.

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