Summary of the Current Feline Guidelines for the

Prevention, Diagnosis,
and Management of
Heartworm (Dirofilaria immitis)
Infection in Cats 1

Prepared for and approved by the Executive Board

of the American Heartworm Society2

EPIDEMIOLOGY of less than six adult worms. Typically only one or

two worms are present and worms in approximately
Heartworm infection has been diagnosed around the one third of infections are single sex. Cats with
globe, including all 50 of the United States, and is only a few worms are still considered to be heavily
considered at least regionally endemic in each of the infected in terms of parasite biomass, however,
contiguous 48 states and Hawaii. The relocation of because of their relatively small body size.
microfilaremic dogs and expansion of the territories
of microfilaremic wild canids in other areas of the Host preference by some of the most abundant
US continue to be important factors contributing to vectors does favor the dog and may contribute
further dissemination of the parasite. Environmental to the lower prevalence of infection in cats. The
changes created by humans, such as the formation of Culex spp mosquito, which is the most common
“heat islands” due to urban sprawl, and changes in species in many urban areas, feeds on both cats
natural climatic conditions have increased heartworm and dogs without preference. The true prevalence of
infection potential by creating microenvironments heartworm infection in cats is probably understated
that support development of heartworm larvae in due to diagnostic limitations and the greater tendency
mosquito vectors during colder months, thereby of cats to exhibit only transient clinical signs or
lengthening the transmission season. die without confirmation of infection. Necropsy
surveys of shelter cats have placed the prevalence
BIOLOGY OF FELINE HEARTWORM INFECTION
of adult heartworm infections at 5% to 15% of the
Significant differences exist between feline rate in unprotected dogs in a given area. Circulating
heartworm disease and its classical canine microfilariae are seldom found in infected cats. When
counterpart. Although cats are susceptible hosts, microfilaremia does develop in cats, they appear only
they are more resistant to infection with adult about one week later than in dogs (195 days post
Dirofilaria immitis than are dogs. Most heartworm infection at the earliest), and seldom persist beyond
infections in cats are comparatively light and consist 228 days post infection. Heartworms transplanted
________________________________________________________________________________________________________________________
1
These recommendations, revised in January 2014, supersede previous editions and are based on the latest information
presented at the 2013 Triennial Symposium of the American Heartworm Society (AHS), new research, and additional clinical
experience, and have been peer reviewed by independent experts. The complete version of the 2014 Feline Guidelines can be
found at the American Heartworm Society’s website, www.heartwormsociety.org.
2
Officers: Dr. Stephen Jones, President; Dr. Wallace Graham, Past President; Dr. Cristiano von Simson, Vice President; Dr.
Robert Stannard, Secretary-Treasurer; Dr. Doug Carithers, Editor; Dr. Patricia Payne, Dr. Chris Rehm, Dr. Charles Thomas
Nelson, Dr. Martha Smith-Blackmore, Dr. Elizabeth Clyde and, Dr. Bianca Zaffarano Board Members; Dr. Matthew Miller,
Symposium Chair; Dr. Clarke Atkins, Symposium Co-Chair; Dr. John McCall, Co-Editor; Dr. Mike Loenser and Dr. Tony
Rumschlag, Ex Officio Members.
2014 Feline Heartworm Guidelines Summary 1
from cats are capable of resuming production of resembling the characteristic heartworm arteritis
circulating microfilariae in dogs; thus, it appears found in dogs also develops in the major lobar and
feline infections become occult due to host immune- peripheral pulmonary arteries of cats but usually
mediated clearance of the microfilariae and perhaps fails to cause sufficient obstruction to produce
a reversible suppression of microfilariae production. clinically significant pulmonary hypertension.
There are other indications that the cat is an imperfect
PHYSICAL DIAGNOSIS
host for heartworms. Aberrant migration occurs more
frequently in cats than in dogs. Although uncommon, Many cats tolerate their infection without any
heartworms are found disproportionately often in noticeable clinical signs or with signs manifested
the body cavities, systemic arteries, and central only transiently. Clinical signs associated with
nervous system of cats. In addition, the life span of feline heartworm disease may be only a vague
the parasite in cats is thought to be 2 to 3 years or malaise or can comprise predominantly respiratory,
even longer, which is considerably shorter than that gastrointestinal (e.g., emesis), or occasionally
in dogs. Nevertheless, heartworms are capable of neurologic manifestations, chronically or acutely.
causing severe disease in cats. Signs of chronic respiratory disease, such as
PATHOPHYSIOLOGY persistent tachypnea, intermittent coughing, and
increased respiratory, effort are most common. A
The clinical importance of heartworms is amplified systolic heart murmur may be present in cats when
in cats because even a small number of heartworms worms reside in the right atrioventricular junction
are potentially life threatening. Although live adult interfering with tricuspid valvular function. Anorexia
worms in the pulmonary arteries cause a local and weight loss occur in some cats. Intermittent
arteritis, some cats never manifest clinical signs. vomiting unrelated to eating is reported frequently,
When signs are evident, they usually develop during and in endemic areas when no other cause is evident
two stages of the disease: 1) arrival of heartworms should raise suspicion of heartworm infection. A
in the pulmonary vasculature and 2) death of adult peracute syndrome of signs including respiratory
heartworms. The first stage occurs approximately distress, ataxia, collapse, seizures, hemoptysis, or
3 to 4 months post infection with an acute vascular sometimes sudden death may occur.
and parenchymal inflammatory response to the
DIAGNOSTIC TESTING
newly arriving worms and the subsequent death of
most of these same worms. This initial phase is often Heartworm infection in cats is a more elusive
misdiagnosed as asthma or allergic bronchitis but in diagnosis than in dogs and can be easily overlooked.
actuality is part of a syndrome known as heartworm- A conscious awareness of its existence is critical.
associated respiratory disease (HARD). Clinical Establishing a definitive antemortem diagnosis
signs associated with this acute phase subside as frequently entails application of multiple diagnostic
the worms mature but demonstrable histopathologic tests, some of which may need to be repeated. Of
lesions, most notably occlusive medial hypertrophy these, heartworm serology, thoracic radiography,
of the small pulmonary arterioles, are evident and echocardiography are the most useful methods
even in those cats that clear the infection. Once of clinical confirmation.
the pulmonary infection is established, evidence
suggests that live heartworms are able to suppress Microfilariae. Cats are seldom microfilaremic when
immune function. This allows many cats to tolerate examined. Since few microfilariae are ever present,
their infection without apparent ill effects—that the chances of finding them are improved by
is, until the mature worms begin to die, which using concentration techniques (modified Knott or
initiates the second stage of disease expression. millipore filter).
The degenerating parasites can cause pulmonary
Serology. Interpretation of antibody and antigen test
inflammation and thromboembolism, which often
results is complicated and a thorough understanding
leads to fatal acute lung injury. Such reactions in cats
of the limitations of both tests is necessary in order
can occur even in single-worm infections.
to use these assays with confidence. In dogs, the
Caval syndrome occurs rarely in cats compared with antigen test is the “gold standard” but because
dogs as infections are usually light, but even one or infections consisting of only male worms or
two worms can cause tricuspid regurgitation and symptomatic immature infections are more common
resultant heart murmur. Arterial intimal proliferation in cats, none of the presently available antigen tests

2 American Heartworm Society

can be relied upon to rule out heartworm disease in may provide strong supportive evidence of feline
cats. The current generation of heartworm antigen heartworm disease. It is valuable for assessing the
tests identify most “occult” infections (adult worms severity of disease and monitoring its progression
present but no circulating microfilariae) consisting or regression as well as eliminating other differential
of at least one mature female worm and are nearly diagnoses. The most characteristic radiographic
100% specific. Necropsy surveys of shelter cats features of heartworm disease in cats, as in dogs, are
have shown that 50% to 70% of infected cats have a sometimes subtle enlargement of the main lobar
at least one female worm. Detectable antigenemia and peripheral pulmonary arteries, characterized
develops at about 5.5 to 8 months post infection. by loss of taper, and sometimes tortuosity and
False-negative test results occur most commonly truncation in the caudal lobar branches. These
when infections are light, female worms are still vascular features are visualized best in the
immature, only male worms are present, and/or the ventrodorsal view and are sometimes visible only
test kit instructions have not been followed. There in the right caudal lobar artery, where heartworms
are also documented cases of antigen-antibody are found most often. Enlargement of the main
complexes interfering with antigen testing resulting pulmonary artery segment may occur in heavily
in false-negative tests. Heating the sample test-tube infected cats but is not a reliable marker, since most
in a warm water bath to 104°C for 10 minutes will cats do not develop pulmonary hypertension and
break these complexes down, releasing any antigen, because the main pulmonary artery is obscured by
resulting in more accurate test results. Heartworm the cardiac silhouette. The cardiac silhouette itself is
test results should only be recorded as positive or no seldom enlarged. A bronchointerstitial lung pattern
antigen detected (NAD) and should not be written as that may clear spontaneously within a few months
“negative.” is a common secondary feature suggestive of but
not unique to feline heartworm disease. In some
Antibody tests have the advantage of being cases, thoracic radiographs provide no evidence
able to detect infection by both male and female of infection. Radiographic features suggestive of
worms, as larvae of either sex can stimulate a heartworm disease can be found in about half
detectable immune response as early as 2 months of the cats suspected of being infected based on
post infection. Antibody tests do not offer an historical and physical signs. In addition, about half
indication of the continued existence of an infection, of those cats with pulmonary arterial enlargement
however, just that an infection occurred. Initial indicative of heartworm disease are antibody
research reported the sensitivity and specificity positive. Temporal differences in the development
of the feline antibody tests to be as high as 98% of the parasite, host immune responses, and
in cats experimentally infected with adult worms. spontaneous regression of lesions may account for
Necropsy surveys of naturally infected cats from discrepancies between radiographic, clinical, and
shelters, however, have indicated a lower sensitivity serologic findings. Infection with Toxocara cati and
ranging from 32% to 89%. The different antibody Aelurostongylus spp can cause similar radiographic
tests vary in their sensitivity to each stage of larval patterns and must be considered in a differential
development; thus, discordant results between diagnosis.
test methods are common. Limited evidence from
several studies suggests that the antibody level in Echocardiography. The chambers of the right side of
cats decreases with time as the parasite matures the feline heart can be thoroughly interrogated by
and that heartworm-infected cats with clinical two-dimensional echocardiography. Images of the
signs are more likely to be antibody positive than main pulmonary artery and a long segment of the
infected asymptomatic cats. Correct interpretation of proximal right and a very short portion of the left
antibody tests results requires additional information pulmonary arteries can also be obtained. Although
and thoughtful analysis. heartworms are found most often in the main and
right lobar branch of the pulmonary artery, it is
Since both juvenile and adult worms are capable necessary to methodically interrogate all of these
of causing clinical disease in the cat, both antibody locations because worms in a typical light infection
and antigen tests are useful tools and when may occupy only one site. An adult heartworm
used together increase the probability of making is relatively long compared with the length of
appropriate diagnostic decisions. the pulmonary arteries in cats. Therefore, there
Thoracic Radiography. Thoracic radiography is a better chance in cats than in dogs of finding
heartworms extending from peripheral branches into
2014 Feline Heartworm Guidelines Summary 3
proximal segments where they can be visualized. An promptly with supportive therapy appropriate
experienced sonographer has a very good chance of for treating shock; this may include intravenous
making a definitive diagnosis in cats that are actually corticosteroids, balanced electrolyte solutions,
infected with adult heartworms, particularly when bronchodilators, and oxygen via intranasal catheter
there are several worms. In suspected cases, the or closed cage. Diuretics are inappropriate,
high specificity of this examination generally allows even for infected cats with severe interstitial or
for confirmation of heartworm infection of at least 5 patchy alveolar lung patterns. Aspirin and other
months’ duration. Quantification of worm burden is nonsteroidal anti-inflammatory drugs (NSAIDs) have
difficult. failed to produce demonstrable benefit.

Necropsy Confirmation. A complete necropsy should Adulticide (melarsomine dihydrochloride) use is

be performed in any cat suspected of dying of not recommended for use in cats due to insufficient
heartworm disease or in which the cause of sudden experience and preliminary data which suggest that
death is unexplained. A thorough search of the vena melarsomine is toxic to cats at doses as low as 3.5
cavae, right side of the heart, and pulmonary arteries mg/kg. Ivermectin administered at 24 µg/kg monthly
must be performed because one or two worms given for 2 years has been reported to reduce
easily can be overlooked, particularly if immature, worm burdens by 65% as compared with untreated
dead, and fragmented. Special attention should be cats. Because most cats have small worm burdens,
paid to examining the most distal aspects of the it is not worm mass alone that is problematic but the
pulmonary arteries as dead worms would be forced “anaphylactic” type reaction that results when the
and compressed to this position by blood flow. worms die. To date, there are no studies that indicate
Heartworms occasionally are restricted to ectopic any form of medical adulticidal therapy increases the
sites, so the systemic arteries, body cavities and, if survival rate of cats harboring adult heartworms.
neurologic signs were present, the brain and spinal
canal should also be examined thoroughly. Surgical Options. In principle, it is preferable to
remove heartworms rather than destroy them in
TREATMENT situ. This can be accomplished successfully by
introducing brush strings, basket catheters, or loop
Medical Options. If a cat displays no overt clinical
snares via right jugular venotomy or, alternatively,
signs despite radiographic evidence of pulmonary
after left thoracotomy, alligator forceps can be
vascular/interstitial lung disease consistent with
inserted through a right ventricular purse-string
heartworm disease, it may be prudent to allow
incision. Heartworms first should be identified
time for a spontaneous cure to occur. The course
ultrasonographically in locations that can be
of infection can be monitored at 6- to 12-month
reached with these inflexible instruments. Although
intervals by repeat antibody and antigen testing
it may not be possible to retrieve every worm, the
and thoracic radiography. In those cats destined
surgical option may be a reasonable alternative
to recover, regression of radiographic signs and
to symptomatic support or adulticide treatment of
especially seroconversion of a positive antigen test
cats that are heavily infected and/or are in critical
to negative status provide evidence that the period
condition. Surgery is specifically indicated in those
of risk probably has passed.
few cases that develop caval syndrome. Care must be
Prednisone in diminishing doses often is effective taken to remove the worms intact; partial or complete
medical support for infected cats with radiographic traumatic transection of a worm almost invariably
evidence of lung disease whether or not they appear results in acute circulatory collapse and death.
ill. Prednisone also should be initiated whenever ADDITIONAL CONSIDERATIONS FOR ADULTICIDE
antibody- and/or antigen-positive cats display clinical THERAPY
signs. An empirical oral regimen is 2 mg/kg body
weight/day, declining gradually to 0.5 mg/kg every Wolbachia. Most filarial nematodes, including
other day by 2 weeks and then discontinued after D immitis, harbor obligate, intracellular, gram-
an additional 2 weeks. At that time the effects of negative bacteria belonging to the genus Wolbachia
treatment should be reassessed based on the clinical (Rickettsiales). These bacteria have been implicated
response and/or thoracic radiography. This treatment in the pathogenesis of filarial diseases, possibly
may be repeated in cats with recurrent clinical signs. through their endotoxins. Recent studies have
shown that a major surface protein of Wolbachia
Cats that become acutely ill need to be stabilized
4 American Heartworm Society
(WSP) induces a specific IgG response in hosts a safeguard for missed doses. Heartworm prevention
infected by D immitis. It is hypothesized that can be achieved in cats with monthly doses of
Wolbachia contributes to pulmonary and renal either ivermectin or milbemycin oxime orally,
inflammation through its surface protein WSP, or topical moxidectin or selamectin. Preventives
independently from its endotoxin component. should be started in kittens at 8 weeks of age and be
Studies to determine the effects of suppressing administered to all cats in heartworm-endemic areas
Wolbachia populations with doxycycline prior to during the heartworm transmission season. The
adulticide therapy to determine the clinical utility of individual minimum monthly prophylactic dose of
this therapeutic approach in cats are in progress. ivermectin is 24 µg/kg, milbemycin oxime 2.0 mg/kg,
moxidectin 1.0 mg/kg, and selamectin 6 mg/kg body
Surveillance of Infected Cats. Serologic retesting weight. Administration of these drugs in cats is not
at 6- to 12-month intervals to monitor infection precluded by antibody or antigen seropositivity.
status is recommended for all infected cats whether
or not they have clinical signs that are treated Seroepidemiologic data for most communities is
empirically or are given medical/surgical adulticide presently meager; thus, it behooves veterinarians
therapy. Once adult heartworm infection has been to become familiar with the local risk potential by
diagnosed, monitoring will be most informative if testing cats before initiating heartworm preventives
both antibody and antigen testing are performed. to establish a serologic benchmark. Although testing
An annual retest may be adequate for asymptomatic cats before starting them on heartworm preventives
cats. Spontaneous or adulticide-induced elimination is recommended, it is less useful than doing so
of infection in antigen-positive cats ordinarily will be in dogs. This apparent contradiction reflects the
followed within 4 to 5 months by disappearance of differences in testing methods and test performance
detectable antigenemia. Once cats become antigen in the two hosts. Pretesting (screening) dogs is limited
negative and are clinically normal, further antibody to documenting either heartworm antigenemia or
retesting becomes optional because antibody may circulating microfilariae, both of which are specific
persist for an indefinite period after the parasites indicators of adult worm infection. Many, if not
are gone and because continued exposure, even most, cats that are antibody positive have only been
with preventive therapy, will result in a positive transiently infected to the 4th larval stage. Evidence of
test. Radiography and ultrasonography also may exposure of a cat to at least 4th stage larvae confirms
be useful for monitoring cats with pulmonary the potential risk of developing HARD and reinforces
vascular and/or parenchymal lung disease or those justification for recommending preventives. The
in which heartworms have been identified on preferred method for screening includes the use of
echocardiography. both an antigen and an antibody test.

CHEMOPROPHYLAXIS Microfilaremia in cats is uncommon, transient, and

typically below concentration levels that might
Monthly heartworm preventives are a safe and trigger an adverse reaction to microfilaricidal
effective option for cats in areas where heartworm preventive drugs; thus pretesting for microfilariae
infection is considered endemic in dogs and exposure is unnecessary. Furthermore, antibody retesting
to infective mosquitoes is possible. So-called of cats already committed to prevention provides
“indoor” cats may also be at risk. When monthly no assurance of efficacy because sensitization
heartworm prevention is elected, it should at least from repetitive aborted precardiac larval infections
be administered for 30 days following the estimated is possible in cats that are repetitively exposed.
seasonal onset of transmission and continued within Therefore, the primary reasons for heartworm testing
30 to 90 days after that period has ended. Advantages cats are 1) to establish an etiologic diagnosis in those
of year-round administration of heartworm preventive individuals that, based on other clinical evidence, are
include 1) activity against some common intestinal suspected to be infected, 2) to monitor the clinical
parasites and in the case of selamectin and topical course of those that have already been diagnosed
moxidectin + imidacloprid, external parasites, with feline heartworm disease, and 3) to establish a
2) increased compliance, and 3) retroactive efficacy as baseline reference prior to initiating preventives.
© 2014 American Heartworm Society | PO Box 8266 | Wilmington, DE 19803-8266 | E-mail: info@heartwormsociety.org

2014 Feline Heartworm Guidelines Summary 5