Bluetongue is a non-contagious, insect-borne viral infection primarily impacting domestic and wild ruminants globally. It causes considerable economic losses within the livestock industry.
Understanding Bluetongue
Bluetongue is caused by the bluetongue virus (BTV), a member of the Orbivirus genus within the family Reoviridae. There are at least 28 identified BTV serotypes, with disease severity varying by strain.
Sheep are often the most severely affected ruminants, experiencing higher rates of clinical signs and mortality. Cattle and goats, though susceptible, frequently show subclinical or mild symptoms, yet they can serve as significant virus reservoirs.
Transmission Pathways
The bluetongue virus is primarily transmitted through the bites of infected midges, specifically certain species from the Culicoides genus. These tiny insects become infected when feeding on an infected animal’s blood, then transmit the virus to other susceptible ruminants through subsequent bites. The geographical distribution of these midge species largely dictates where the disease occurs.
The virus replicates within the midge’s body, with warmer conditions accelerating this process. Less common transmission methods include transplacental transfer to a fetus, contaminated needles, or semen from infected bulls and rams. These routes are not significant for widespread transmission. Direct animal-to-animal contact typically does not spread the disease.
Signs of Infection
Clinical signs of bluetongue infection vary by animal species, breed, age, and BTV serotype. Symptoms are most pronounced in sheep, often appearing after a 4 to 12-day incubation period. Initial signs include high fever, listlessness, and reluctance to move.
As the disease progresses, affected sheep may develop excessive salivation, a thick nasal discharge, and swelling of the face, lips, and jaw. The tongue can swell and appear bluish due to lack of oxygen, giving the disease its name.
Lameness is common, resulting from inflammation of the coronary band (above the hoof) and muscle damage. Other signs include ulcers in the mouth and nose, difficulty breathing, and abnormal wool growth. Pregnant ewes may experience abortions or deliver malformed offspring.
Prevention and Control
Controlling bluetongue involves a multi-faceted approach, with vaccination programs playing a primary role in preventing outbreaks and minimizing losses. Various vaccine types exist, important in endemic or high-risk regions. However, vaccines are serotype-specific, meaning one vaccine may not protect against others.
Biosecurity measures are also crucial, focusing on controlling the midge vector population. This involves using insecticides or housing animals during peak midge activity. Strict movement restrictions on animals from affected areas prevent virus spread. Surveillance programs monitor disease presence and spread, allowing for early detection and rapid response.
No Threat to Humans
Bluetongue is exclusively an animal disease and does not affect humans. There is no evidence the virus causes illness in people, nor is it transmitted through contact with infected animals, their wool, or consumption of meat or milk products. Therefore, there is no public health risk.