BLUETONGUE
Aetiology Epidemiology Diagnosis Prevention and control References
AETIOLOGY
Classification of the causative agent
Virus family Reoviridae, genus Orbivirus. 24 serotypes have been identified.
Resistance to physical and chemical action
Temperature: Inactivated by 50°C/3 hours; 60°C/15 minutes pH: Sensitive to pH <6.0 and >8.0 Chemicals: Inactivated by ß-propiolactone Disinfectants: Inactivated by iodophores and phenolic compounds Survival: Very stable in the presence of protein (e.g. has survived for years in blood stored at 20°C) EPIDEMIOLOGY
- Mortality rate normally low in sheep but up to 10% in some epizooties
- Noncontagious
Hosts
- Sheep: disease; variation in breed susceptibility
- Cattle, goats, dromedaries, wild ruminants: generally inapparent infection
Transmission
- Biological vectors: Culicoides spp.
Sources of virus
- Infected Culicoides
- Blood
- Semen
Occurrence
The virus is present in a broad band of countries extending appriximately between 40°N and 35°S. The bluetongue virus has been shown by serology to be present in regions where the Culicoides vector is present (e.g. Africa, the Americas, Australia and some countries of southern Asia and Oceania). However, clinical disease with confirmation by virus isolation has been observed in a few countries only.
For detailed information on occurrence, see recent issues of World Animal Health and the OIE Bulletin
DIAGNOSIS
Incubation period is 5-20 days
Clinical diagnosis
Acute form (sheep and some species of deer)
- Pyrexia up to 42°C, depression
- Inflammation, ulceration, erosion and necrosis of the mucosae of the mouth
- Swollen and sometimes cyanotic tongue
- Lameness due to coronitis or pododermatitis and myositis
- Abortion
- Complications of pneumonia
- Emaciation
- Either death within 8-10 days or long recovery with alopecia, sterility and growth delay
Inapparent infection
- Frequent in cattle and other species (cf. epidemiology)
Lesions
- Congestion, oedema, haemorrhages and ulcerations of digestive and respiratory mucosae (mouth, oesophagus, stomach, intestine, pituitary mucosa, tracheal mucosa)
- Congestion of hoof laminae and coronary band
- Hypertrophy of lymph nodes and splenomegaly
- Severe bilateral broncholobular pneumonia (when complications occur)
Differential diagnosis
- Contagious ecthyma
- Foot and mouth disease
- Photosensitisation
- Pneumonia
- Polyarthritis, footrot, foot abscesses
- Plant poisonings
- Peste des petits ruminants
- Coenurosis
- Epizootic haemorrhagic disease of deer
Laboratory diagnosis
Procedures
Isolation of the agent
- Inoculation of sheep
- Intravascular inoculation in 10-12-day-old embryonated chicken eggs
Identification of the agent
- Plaque reduction serum neutralisation (for serotyping - many cross-reactions)
Serological tests
- Competitive ELISA
- Agar gel immunodiffusion
(prescribed tests in the Manual)
- Virus neutralisation
- Complement fixation
Samples
Isolation and identification of the agent
- Living animals: blood in heparin
- Freshly dead animals: spleen, liver, red bone marrow, heart blood, lymphnodes
- Aborted and congenitally infected newborn animals: precolostrum serum plus same samples as for freshly dead animals
- All samples have to be preserved at 4°C, and not frozen
Serological tests
- Paired sample sera
PREVENTION AND CONTROL
Sanitary prophylaxis
- No efficient treatment
- Disease-free areas:
- quarantine and serological survey
- vector control, especially in aircraft
- Infected areas:
- vector control
Medical prophylaxis
- Vaccination with modified live virus vaccine. Serotypes incorporated into the vaccine must be the same as those causing infection in the field.
REFERENCES AND OTHER INFORMATION
- Reference experts and laboratories
- Classified as an OIE List A disease (A090)
- Chapter 2.1.9. in the Manual of Diagnostic Tests and Vaccines for Terrestrial Animals.
- Terrestrial Animal Health Code
- Other references - see the Index
- World Animal Health .
- Current Animal Health Status (Disease Information)
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Contact : scientific.dept@oie.int