Blue Tongue (BT)

Syn: Sore mouth, sore muzzle, ovine catarrhal fever, Pseudo FMD, Stiff lamb disease

• It is infectious non-contagious arthropod borne virus disease of domestic and wild animals.
• It is characterized by inflammation of mucus membrane, congestion, swelling, rhinitis and inflammation of coronary band of foot.
• This is predominantly a disease of sheep but occasionally cattle and goats are affected.
• In cattle, disease mostly remain in sub-clinical form.

Etiology

• Orbivirus of Reoviridae family
• Virus persist for long period in meat and offal and killed by 3% NaOH
• Virus can persist for a long time in meat and blood. It can thrive even upto 7 years in stored chick embryo kept at 6°C.
• Virus is 65-80 nm in diameter, RNA virus

Epidemiology

• BTV spreads through species of Culicoides insects.
• Onset of monsoon spreads the disease.
• Disease is distributed worldwide, with outbreaks occurring in various regions including Europe, Asia, Africa and Americas.
• Mortality ranges from 2-90%.

Transmission

• Culicoides midges’ bites
• No transmission by direct and indirect contact
• Rarely virus in semen and contaminated semen may infect recipient cows.
• Blood also act as infective materials.
• Vertical transmission in cattle

Pathogenesis:

• Following entry of virus, they multiply in blood stream leading to primary viremia.
• Virus then localizes in endothelial lining of blood vessel
• Virus gets attached with RBCs. Characteristic lesion appears due to destruction of blood vessels.

Clinical Findings:

Incubation period of disease ranges from 1-10 days

Sheep:

• Fever around 106°F that lasts for 1-2 days
• Loss of body condition, emaciation, loss of wool in severely affected ones
• Inflammation, ulcers and necrosis in around mouth, i.e. gums, cheeks, tongue
• Edematous lips, dental pads, face, ears, mandibular space
• Tongue becomes cyanotic/bluish/purplish blue and lenticular shaped ulcers appears in tongue
• Frothy salivation, nasal discharge
• Reddening of coronary band causing lameness
• Abortion and congenital malformation
• Anorexic and increased respiration
• Death may occur due to aspiration pneumonia

Cattle:

• Sub-clinical stage and remain source of infection for sheep
• Fever, salivation, congestion, swelling and ulcers inside mouth
• Hydrocephaly in calves

PM Findings:

• Small, hyperemia, edema, cyanosis and skin thickening
• Petechiation, erosion and ulceration of buccal mucosa
• Edema of skeletal and cardiac muscles of heart
• Swelling and necrosis of muscles with enlarged LN
• Hyperemia and edema of abomasal mucosa
• Swelling and congestion of spleen and liver
• Hemorrhage in coronary band

Diagnosis:

• Based on clinical findings
• Based on PM findings
• Hematological examination: Leukopenia
• Serological test: CFT, ELISA, AGDT, VNT, FAT
• Animal inoculation test: Suspected blood or tissue suspension is used at height of temperature for inoculation.
• Isolation of virus in chicken embryo cell

Differential Diagnosis:

1. FMD:

• Highly contagious disease and spread by direct contact
• Absence of cyanotic changes in tongue
• Pink line appears on coronet in blue tongue which is absent in FMD
• No pneumonia

2. MCF:

• Extensive erosion in buccal cavity, tongue, hard palate
• Presence of corneal opacity

3. Photosensitization:

• Dermatitis of light skin
• Absence of cyanosis of tongue
• Reddening and edema of skin

Treatment and Control:

• Broad spectrum antibiotics for secondary bacterial infection.
• Avoid grazing in areas where Culicoides available along with Culicoides control
• Polyvalent vaccine 2 ml SC route, Booster dose applied at 1 month
• Attempt should be made to control vector population by spraying insecticide and good water management.
• Contact with wild ruminants should be avoided.
• Quarantine of sick animals should be made.
• Import of animals from disease prevailing countries should be avoided.
• Strict regulation is to be followed to prevent entry of diseased animals from endemic zones.