Dr.

ZAIDI Infectious diseases II (ENSV 2023-2024) Bluetongue

Bluetongue

I. Definition
Infectious, virulent, inoculable, specific pathology of ruminants, particularly severe in sheep. The
disease is caused by a virus from the Reoviridae family. It is transmitted by a blood-sucking
arthropod of the Culicoides genus. Clinically, it is characterized by a febrile condition followed
rapidly by a catarrhal infection of the mucous membranes, especially in the mouth, often
accompanied by congestive lesions of the hoof extremities and progressive emaciation.
Bluetongue is not a zoonosis.

II. Geographical Distribution

"Blue tongue" was prevalent in tropical and subtropical areas. It exists either in enzootic form (sub-
Saharan Africa or North America) or as periodic outbreaks (Maghreb). However, clinical disease is
actually reported in only a few countries, notably the United States and South Africa, where
improved sheep breeds are present. In Africa, local, hardy breeds show no symptoms, and only the
presence of antibodies indicates infection.
Furthermore, even disease-free countries are at risk of potential introduction due to the presence of
potentially vector species of Culicoides in their territory.
Africa has the largest number of serotypes (21), while there are only 5 in North America and 8 in
Australia.

III. Affected Species

Clinical disease primarily appears in sheep (improved, highly productive breeds). Inapparent forms
are the norm in cattle, which constitute the virus reservoir. In cattle and goats, clinical
manifestations, when they occur, are discreet. Dromedaries are also infected but exhibit few or no
symptoms.
The role played by wildlife in the epidemiology of the disease (dead end or reservoir) remains
controversial.

IV. Pathogen
The pathogen is a virus belonging to the Reoviridae family, Orbivirus genus, with a double-stranded
RNA genome. There are 24 recognized serotypes with cross-protection. The bluetongue virus is
small, with icosahedral symmetry and no envelope.
The genome is housed within an internal capsid composed of 32 capsomers (forming the
nucleocapsid), surrounded by an outer membrane.
The capsomers appear in the form of rings, which is why the genus is named Orbivirus, meaning
"ring" in Latin.
The genome consists of 10 double-stranded RNA fragments, with each fragment coding for a
protein.
Non-structural proteins (NS1, NS2, NS3): 3 proteins produced during virus multiplication in cells.
Genome fragmentation could explain the large number of serotypes encountered.
24 serotypes have been recognized worldwide, with complex antigenic relationships among them.

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❖ Pathogenicity:
Pathogenicity depends on numerous factors:
- Host-vector relationships
- Inoculated dose
- Environmental factors
It seems that serotypes do not have the same pathogenicity.
Moreover, strains of the same serotype isolated in different countries show variations in
pathogenicity.

❖ Virus Resistance:
Due to vector transmission, the virus's resistance in the external environment has no
epidemiological implication (studied to distinguish it from other viruses in the family). The
bluetongue virus is relatively resistant to heat (can survive for several years at room temperature, is
destroyed at 60°C after 30 minutes, and is not very resistant to -20°C.

V. Pathogenesis
After inoculation, the virus is drained to regional lymph nodes, where it multiplies, before
colonizing the lymphatic system, spleen, and lungs (silent phase), where it multiplies again. During
the subsequent viremia, the virus multiplies in monocytes, macrophages, and vascular endothelial
cells (invasion phase). Damage to endothelial cells, located in certain organs or tissues, leads to
capillary fragility with hemorrhages and edema, explaining all the lesions observed later (state
phase).

VI. Sources and Mode of Transmission

The virus is exclusively transmitted through blood-sucking vectors, all belonging to the Culicoides
genus. Two modalities are possible:
- Either animals in the viremic phase are introduced into a disease-free country or region, and local
Culicoides, provided vector species exist, transmit the virus.
- Or Culicoides themselves are carried by the wind to neighboring regions of infected countries.

VII. Symptomatology
1. In sheep:
- Acute form: Incubation of 2 to 18 days. High fever (up to 42°C), anorexia, depression, followed
by intense congestion of the oral and nasal mucous membranes with profuse hypersalivation,
tearing, and abundant serous nasal discharge (hence the name of the disease), edema of the lips and
tongue, which can extend to the head. Cyanosis of the tongue is frequent but not constant (hence the
name in English). In this case, anorexia is total, and after a few days, ulcers appear throughout the
oral cavity (gums, lips, etc.). At this stage, the animal keeps its mouth open with tongue prostration.
Subsequently, the animal experiences difficulties in movement and lameness due to hoof
involvement, torticollis, stiffness with muscle involvement (degenerative myositis). Sometimes, we
observe pulmonary or digestive complications (microbial complications). Death occurs after about a
week due to lung edema.

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- Subacute form: Some of the above-mentioned symptoms are observed but in a milder form.

- Inapparent form: Observed in hardy breeds in Africa or South America. Only the presence of
antibodies indicates infection. The course can lead to death due to bacterial complications or
recovery after a long convalescence period.

2. In cattle:
- Bluetongue generally goes unnoticed. In some cases, hyperthermia, transient acceleration of
respiratory rate, exudative dermatitis, erosions in the mouth, and hypersalivation can be observed.
Abortion or congenital malformations are rarely seen.

3. In goats:
- Bluetongue leads to lung diseases or weakness states.

VIII. Lesions:
Characterized by a:
- Congestive
- Edematous
- Hemorrhagic (pulmonary artery wall) pattern.

IX. Diagnosis:
1. Epidemiological and Clinical Diagnosis:
- Very high mortality rate
- Febrile condition + cyanotic ulcerative and then necrotic stomatitis (blue tongue)
- Crown congestion and lameness.

2. Differential Diagnosis:
In Sheep:
- Foot-and-mouth disease: Vesicles, low mortality, especially in cattle, can occur at any time of the
year.
- Contagious ecthyma: Skin eruptions, crusts, not fatal.
- *Orf*: Generalized eruptions, indelible scars, palpebral edema, clavicle.

In Cattle:
- Malignant catarrhal fever: Contagious, lymphadenopathy, bilateral corneal opacity, and mammary
ulceration.
- Bovine viral diarrhea-Mucosal disease complex: Diarrhea, abortion, deep tongue ulcers.
- Foot-and-mouth disease: Extensive superficial tongue ulcers, presence of all three localizations, no
hemorrhagic syndrome.
- Infectious bovine rhinotracheitis (IBR): Respiratory involvement, abortion, ulcerative lesions
referred to as "geographic map" on the tongue.
- Papular stomatitis: large papules on the muzzle and gingival margin, characteristic halo, no hoof
involvement.

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3. Laboratory Diagnosis:
Of paramount importance and allows:
- Precise and rapid confirmation of clinical suspicion (serology).
- Early identification of viral type.
- Serological diagnosis (antibodies - 10 days after the first viremia).
- Molecular diagnosis (PCR - segmented genome).
- Viral isolation (blood, spleen - culture on embryonic eggs - inoculation in mice).

X. Prophylaxis
1. Sanitary Prophylaxis
In disease-free countries, it is appropriate to:
- Prohibit the importation of animals from infected countries.
- Avoid the introduction of blood-sucking insects through insect control measures.

In infected countries:
- Slaughter of sick animals.
- Carcass destruction.
- Insect control measures.

During clinical suspicion:

- Report the suspicion.
- Seek diagnostic assistance.
- Validate the nature of the samples and their sending modalities.
- Specify measures to limit the risk of spread by instructing the breeder to:
- Restrict animals on the farm: no entry or exit of susceptible animals.
- Treat susceptible animals with external insecticides (baths in insecticidal solutions).
- If possible, bring susceptible animals indoors at dusk to avoid insect bites.

Management upon confirmation:

In the outbreak area:
- Slaughter of sick animals.
- Carcass destruction.
- Farm and surroundings sanitation.

Peripheral measures:
- Conservative measures in farms and veterinary surveillance (20 km radius).
- Veterinary surveillance of farms and restrictions on animal movement outside designated zones
(50 km radius - protection zone, 100 km radius - surveillance zone).

2. Medical Prophylaxis
Two types of vaccines:
- Live attenuated vaccines: 24 serotypes.
- Inactivated vaccines: serotypes 1, 2, 4, 8.
- Vaccination is prohibited in Algeria (risk of genetic reassortment between the vaccine strain and
the wild strain).