Blue Tongue (BT)

Blue Tongue (BT) also known as Sore mouth or sore muzzle and Ovine catarrhal fever in animals.

Blue Tongue (BT) is a infectious non-contagious arthropod borne viral disease of sheep, cattle, goat and other wild ruminants, clinically characterized by fever, oculonasal discharge, hyperaemia and edema of lips and lameness.

This disease frequently involves the udder, coronary band of the foot and sensitive laminae of the hoof. There is epithelial desquamation but no vesicle formation occurs.

Etiology

• Blue Tongue (BT) is caused by Blue tongue virus, genus Orbivirus belongs to family Reoviridae.
• The virus contains 25 serotypes and has considerable heterogeneity when ruminant or vector infected with more than one strain reassortment of gene segment may occur.
• Infection with one serotype may not be cross protective against others.
• The virus is inactivated at pH < 6.0 and susceptible to 3% sodium hydroxide solution and organic iodides.
• The prevalence of serotype in Asia is serotype 1,4,7,9,10,12,16,17,20,21 and 23.
• The prevalence of serotype in India serotype 1, 2, 3, 4, 6, 9, 10, 12, 15, 16, 17, 18, 21 and 23.

Epidemiology

• Blue tongue virus infects domestic livestock population in all tropical and subtropical countries and occurs in continents of Asia, Africa, North America, South America and Australia.
• The distribution and intensity of infection is determined by climate, geography and altitude, which influence the occurrence and activity of culicoides vector.
• Morbidity rate- 50-75% and Mortality rate upto 30%.

Host affected

• Bluetongue virus can infect many domesticated and wild ruminants including sheep, goats, cattle, water buffalo, African buffalo, bison  and  various cervids.
• Cattle are major reservoir host for sheep.
• All breeds and all age group of sheep are susceptible, but Merino  and British breeds are more susceptible than others.

Transmission

• Blue Tongue (BT) is mainly transmitted by biting midges in the genus Culicoides,which are biological vectors.
• Mechanical transmission occur through sheep keds (Melophagus ovinus), cattle lice, ticks and other mosquitoes.
• Bluetongue virus can also spread mechanically on surgical equipment and needles.
• Venereal  transmission through semen of infected bull.

Pathogenesis

Clinical manifestation

• Incubation period is upto 10 days.
• Fever (40.5-41^oC) persist for 5-6 days.
• Nasal discharge, salivation with reddening of buccal and nasal mucosa noticed after 48 hrs of rise of temperature.
• Nasal discharge is mucopurulent and often blood stained.
• Swelling  and edema of the lips, gums, dental pad and tongue.
• Facial swelling and drooping of ears.
• Conjunctivitis and lacrimation.
• Excoriation of buccal mucosa follows blood stained saliva and offensive odour in the mouth.
• Hyperaemia and ulceration are common at the commissures of the lips, buccal papillae, around the anus and vulva.
• Increased respiratory rate upto 100/min.
• Diarrhoea and dysentery  is rarely noticed.
• Appearance of dark red to purple band in the skin just above the coronet due to coronitis appear only in some animals.
• Lameness.
• Wry neck due to twisting of  head and neck to one side due to direct action of virus on muscle tissue, which occur on 12^th day of infection.
• Loss of  wool.
• Long convalescence period.
• Cracking of hooves.

Cattle

• The symptoms are mostly inapparent.
• Fever (104-106^oF).
• Stiffness and laminitis in all four limbs.
• Excessive salivation.
• Edema of the lips, inappetence, nasal discharge  and fetid breath.
• Severe coronitis, sometime cause the sloughing of hooves.
• Photodermatitis.
• Congenital deformity in early pregnancy.

Goats

• Mild to moderate fever.
• Hyperaemia of mucosa and conjunctivitis.

Necropsy Finding

• Distinct haemorrhage at the base of the pulmonary artery.
• Focal necrosis of papillary muscles of left ventricle.
• Pulmonary edema, which may be accompanied by pleural and pericardial effusion is a common cause of death in fatal cases.

Sample Collection

• Whole blood and paired sera sample from live animals.
• Spleen, lymph node and bone marrow from dead animals.

Diagnosis

• Based on clinical symptoms and necropsy finding.
• Isolation of virus in embryonated hens egg by intravenous route of inoculation at 15 days  of embryonation.
• Isolation and identification of virus in cell culture.
• Reverse transcriptase-polymerase chain reaction (RT-PCR) tests are widely used to identify viral RNA in clinical samples, and can also identify the serotype.
• AGID is used for detection of antibodies in serum.
• Competitive ELISA is more sensitive and more specific than AGID for detection of antibodies in serum.
• Marked leukopenia due to lymphopenia in severe cases.
• Increased level of creatine kinase due to skeletal myopathy.

Differential diagnosis

• Foot and mouth disease
• Epizootic haemorrhagic disease
• Contagious ecthyma and sheep pox

Treatment

• Cleaning of mouth lesions with mild antiseptic solution.
• Fluid and electrolyte therapy.
• Antibiotics to be  administered to control secondary infection.
• Protection of animals from hot sunlight.

Prevention and control

• Vaccination of Lambs and kids above 3 months of age.
• Live attenuated vaccines should not be used in pregnant ewes less than 8 weeks of pregnancy, because risk of congenital deformity in lambs.
• Currently pentavalent inactivated  blue tongue vaccine (contain serotype 1, 2, 10, 16 & 23) is widely used and safe to use in pregnant animals.
• Rams should be vaccinated before mating time.
• Annual revaccination of animals one month before the expected occurrence of disease.
• Reducing the risk of exposure by spraying of insecticide around the shed at night.
• Avoid stabling or resting of animal in low lying damp pasture.