Bluetongue is also called; Sore mouth or sore muzzle, ovine catarrhal fever.
• Bluetongue (BT) is an infectious, non-contagious acute arthropod-borne virus, primarily affecting sheep characterized by catarrhal inflammation of the mucous membrane of buccal mucosa and gastrointestinal tract.
• 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 / Cause
• Bluetongue virus (BTV) is the double-stranded RNA virus, species of the Orbivirus genus, family Reoviridae.
• The distribution of the disease is dependent on the presence of reservoir and amplifying host such as cattle and on suitable species of Culicoides being present in large numbers to effect transmission to sheep.
• It is basically a disease of sheep but amongst sheep, susceptibility varies in different age groups.
• Young sheep within the age group of one year are more prone to infection.
• Goats, cattle, and wild ruminants exhibit milder symptoms and may act as non-clinical carriers.
• Horses, dogs, cats, ferrets, rabbits, and guinea pigs are not susceptible to natural infection.
• The disease spread through the blood-sucking midges of the genus Culicoides.
• Besides mosquitoes and other ectoparasites like sheep ked, Melophagus ovinus, may transmit the disease mechanically.
• Wild animal reservoirs play an important role in maintaining the infection during the interepizootic period.
• Stress factors like a lower plane of nutrition, worm burden, inclement weather, and fatigue due to transportation are predisposing factors for the disease.
• Incubation period one week. The disease has been arbitrarily divided into three forms.
a. High rise of temperature, nasal discharge, salivation, and lacrimation.
b. Hyperemia, Petechiation, and swelling of the buccal mucosa, dental pads, and tongue.
c. Later hyperaemic regions become “cyanotic” or “purplish-blue”.
d. Extensive necrosis of the dental pad.
e. Muzzle turns dry show burnt appearance.
f. Cyanotic and bluish appearance of the tongue.
g. Bloodstained diarrhea occurs when the intestinal mucosa is involved which is always fatal.
h. There may be involvement of sensitive laminae and breaking of wool fibers.
i. Heavy mortality occurs in early stages and chronically affected animals become thin and emaciated. Morbidity is 50%.
Subacute form or subclinical form
a. Common in cattle and generally passed unnoticed
a. Abortion of pregnant ewes, which results in the loss of the entire breeding season. Young animals and Merino sheep are particularly susceptible.
• Tongue: Blue and gangrenous
• Leg: Coronary band show congestion and hemorrhage
• Lungs: Pneumonic changes
• Based on history, clinical findings, seasonal occurrence.
• Samples to collect-unclotted blood from febrile animals or fresh spleen and lymph node collected at post mortem.
• Virus isolation in unweaned mice or growing in susceptible cell cultures or in embryonated eggs (i/v or yolk sac routes).
• The virus propagated in primary cultures of calf kidney, adrenal and testis, HELA cell cultures, and also BHK21.
CPE within 24-72 hours, which consists of foci of enlarged and retractile cells.
• Other tests – CFT, SNT, Plaque inhibition test, Microgel diffusion test, FAT, AGID, ELISA.
• Foot and mouth disease
• Malignant catarrhal fever
Prevention and control
• The recovered sheep are immune for 6 months. Avoid grazing of animals in areas where there is a lot of vectors.
• Vector control by applying insecticides and good water management.
• Quarantine of the newly purchased animals.
• Live, attenuated vaccines are used as monovalent strain or quadrivalent vaccines.
• Do not use live vaccines in pregnant sheep.
• Vaccinate sheep annually, and several weeks before service and before the start of the rainy season.
• Lambs vaccinated at 6 months of age and booster vaccination at once a year.