BLUETONGUE
Aetiology Epidemiology Diagnosis Prevention and Control References
AETIOLOGY
Classification of the causative agent
Virus family Reoviridae, genus Orbivirus with 20 recognised species in the genus. The bluetongue virus
(BTV) species contain 24 recognised serotypes and are related to the viruses in the epizootic hemorrhagic
disease (EHD) serogroup.
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/Disinfectants: Inactivated by ß-propiolactone; 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
Non-contagious by casual contact
Some midges of the genus Culicoides (insect host) transmit BTV among susceptible ruminants;
these insect hosts having become infected by feeding on viraemic animals (the vertebrate host)
o
replication period in the insect’s salivary gland of 6–8 days
o
infected midges infective for life
Midges are the only significant natural transmitters of BTV; thus distribution and prevalence of the
disease is governed by ecological factors (i.e. high rainfall, temperature, humidity and soil
characteristics)
o
in many parts of the world infection has a seasonal occurrence
BTV does not establish persistent infections in ruminants thus survival of the agent in the
environment is associated with insect factors
Morbidity in sheep can reach 100% with mortality between 30 and 70% in more susceptible breeds;
mortality in wild deer and antelopes can reach 90%
o
BTV serotype 8 in Europe saw higher numbers of cattle affected however mortality remained
below 1%
Hosts
BTV vertebrate hosts include domestic and wild ruminants; sheep, goats, cattle, buffaloes, deer,
most species of African antelope and other Artiodactyla such as camels
o
the roll of non-ruminant species in the disease in the wild is not known
o
variation in sheep breed susceptibility
Cattle, goats, dromedaries, wild ruminants: generally inapparent infection
Transmission
Biological vectors: Culicoides spp.
Sources of virus
Infected Culicoides
Blood
Semen
Occurrence
Globally the distribution of BTV is directly associated with the presence of competent vectors and their
habitats (episystems). BTV activity can be found on all continents except Antarctica; though different
serotypes and strains cause markedly variable disease.
For more recent, detailed information on the occurrence of this disease worldwide, see the OIE World
Animal Health Information Database (WAHID) interface
[http://www.oie.int/wahis/public.php?page=home] or refer to the latest issues of the World Animal
Health and the OIE Bulletin.
DIAGNOSIS
Incubation period is usually 5
–10 days. Subclinically infected cattle can become viraemic 4 days post-
infection.
Clinical diagnosis
Disease outcome of infection ranges from inapparent, in the vast majority of infected animals, to fatal, in a
proportion of infected sheep, goats, deer and some wild ruminants. As with many diseases, severity will
depend on factors related to agent, host, and environment.
Acute form (sheep and some species of deer)
Pyrexia up to 42°C, excessive salivation, depression, dyspnoea and panting
Initially clear nasal discharge becomes mucopurulent and upon drying may form a crust around the
nares
Hyperaemia and congestion of the muzzle, lips, face, eyelids and ears; leading to oedema
Ulceration and necrosis of the mucosae of the mouth
Tongue may become hyperaemic and oedematous; later cyanotic and protrude from the mouth
Extension of hyperaemia to coronary band of the hoof, the groin, axilla and perineum; lameness due
to coronitis or pododermatitis and myositis
Torticolis in severe cases
Abortion or birth of malformed lambs
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 for certain serotypes
Lesions
Congestion, oedema, haemorrhages and ulcerations of digestive and respiratory mucosae (mouth,
oesophagus, stomach, intestine, pituitary mucosa, tracheal mucosa)
Severe bilateral broncholobular pneumonia (when complications occur); in fatal cases, lungs may
show interalveolar hyperaemia, severe alveolar oedema and the bronchial tree may be filled with
froth
Thoracic cavity and pericardial sac may contain large quantities of plasma-like fluid; distinctive
haemorrhages found at base of pulmonary artery
Congestion of hoof laminae and coronary band
Hypertrophy of lymph nodes and splenomegaly
Differential diagnosis
Contagious ecthyma
Foot and mouth disease
Vesicular stomatitis
Malignant catarrhal fever
Bovine virus diarrhoea
Infectious bovine rhinotracheitis
Parainfluenza-3 infection
Sheep pox
Photosensitisation
Pneumonia
Polyarthritis, footrot, foot abscesses
Plant poisonings (photosensitisation)
Peste des petits ruminants
Coenurosis (Oestrus ovis infestation)
Epizootic haemorrhagic disease of deer
Laboratory diagnosis
Samples
Living animals: blood in heparin
Freshly dead animals: spleen, liver, red bone marrow, heart blood, lymph nodes
Aborted and congenitally infected newborn animals: pre-colostrum serum plus same samples as for
freshly dead animals
All samples have to be preserved at 4°C, and not frozen
Paired sample sera
Procedures
Isolation of the agent
Inoculation of sheep
Intravascular inoculation in 10
–12-day-old embryonated chicken eggs
Identification of the agent (a prescribed test for international trade)
Virus isolation
o
performed in: embryonated chicken eggs, cell culture or sheep
o
same diagnostic procedures are used for domestic and wild ruminants
Immunological methods
o
Serogrouping of viruses by
Immunofluorescence
Antigen capture enzyme-linked immunosorbent assay
Immunospot test
o
Serotyping by virus neutralisation via
Plaque reduction
Plaque inhibition
Microtitre neutralisation
Fluorescence inhibition test
Reverse-transcription polymerase chain reaction (a prescribed test for international trade)
Real-time reverse-transcription polymerase chain reaction tests
Serological tests
Complement fixation
o
largely replaced by the AGID test
Agar gel immunodiffusion (an alternative test for international trade)
o
simple to perform and the antigen used in the assay is relatively easy to generate
o
one of the standard testing procedure for international movement of ruminants
o
one of the disadvantages of the AGID used for BT is its lack of specificity in that it can
detect antibodies to other Orbiviruses, particularly those in the EHD serogroup
o
AGID positive sera may have to be retested using a BT serogroup-specific assay
o
lack of specificity and the subjectivity exercised in reading the results have encouraged the
development of ELISA-based procedures for the specific detection of anti-BTV antibodies
Competitive enzyme-linked immunosorbent assay (a prescribed test for international trade)
o
BT competitive or blocking ELISA was developed to measure BTV-specific antibody without
detecting cross-reacting antibody to other Orbiviruses
o
specificity is the result of using one of a number of BT serogroup-reactive MAbs
Indirect ELISA
o
shown to be reliable and useful for surveillance purposes for bulk milk samples
For more detailed information regarding laboratory diagnostic methodologies, please refer to Chapter
2.1.3 Bluetongue in the latest edition of the OIE Manual of Diagnostic Tests and Vaccines for
Terrestrial Animals under the heading
“Diagnostic Techniques”.
PREVENTION AND CONTROL
Sanitary prophylaxis
No efficient treatment
Disease-free areas:
o
animal movement control, quarantine and serological survey
o
vector control, especially in aircraft
Infected areas:
o
vector control
Medical prophylaxis
Both live attenuated and killed BTV vaccines are currently available; attenuated vaccines are
seroype specific
o
vaccine serotype must be same as those causing infection
o
attenuated vaccines can be transmitted to unvaccinated animals and could reassort with
field strains; resulting in new viral strains
Recombinant vaccines are under development
For more detailed information regarding vaccines, please refer to Chapter 2.1.3 Bluetongue in the
latest edition of the OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals under the
heading “Requirements for Vaccines and Diagnostic Biologicals”.
For more detailed information regarding safe international trade in terrestrial animals and their
products, please refer to the latest edition of the OIE Terrestrial Animal Health Code.
REFERENCES AND OTHER INFORMATION
Brown, C. & Torres, A., Eds. (2008). - USAHA Foreign Animal Diseases, Seventh Edition.
Committee of Foreign and Emerging Diseases of the US Animal Health Association. Boca
Publications Group, Inc.
Coetzer, J.A.W. & Tustin, R.C. Eds. (2004). - Infectious Diseases of Livestock, 2nd Edition. Oxford
University Press.
Fauquet, C., Fauquet, M., & Mayo, M.A. (2005). - Virus Taxonomy: VIII Report of the International
Committee on Taxonomy of Viruses. Academic Press.
Kahn, C.M., Ed. (2005). - Merck Veterinary Manual. Merck & Co. Inc. and Merial Ltd.
Saegerman, C., Reviriego-Gordejo, F. & Pastoret, P.-P. Eds. (2008).
– Bluetongue in Northern
Europe. OIE, Paris.
Spickler, A.R., & Roth, J.A. Iowa State University, College of Veterinary Medicine -
http://www.cfsph.iastate.edu/DiseaseInfo/factsheets.htm
World Organisation for Animal Health (2012). - Terrestrial Animal Health Code. OIE, Paris.
World Organisation for Animal Health (2012). - Manual of Diagnostic Tests and Vaccines for
Terrestrial Animals. OIE, Paris.
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The OIE will periodically update the OIE Technical Disease Cards. Please send relevant new references and
proposed modifications to the OIE Scientific and Technical Department (scientific.dept@oie.int). Last updated
April 2013.