Blue Tongue Disease in Sheep

Background

Blue Tongue Disease is a viral disease affecting sheep and other ruminants, caused by a virus transmitted by insects. Currently, 25 serological types of the Blue Tongue virus are known worldwide. In Israel, 9 virus types have been diagnosed to date.

The disease’s name is derived from one of the clinical manifestations caused by the virus in sheep.

Transmission

In field conditions, the Blue Tongue virus is transmitted between sheep and cattle. In goats, natural infection is rarer. Typically, cattle serve as carriers and breeding sites for the virus that infects sheep. Cattle are more attractive to the insect species (midges) that serve as biological vectors of the virus and therefore serve as the main carrier.

Susceptibility to Disease

All sheep breeds are susceptible to the disease, but susceptibility varies between breeds:

• European and Merino breeds: Highly susceptible to the disease; if there is morbidity, it may result in death
• African breeds: Relatively resistant to the disease, usually showing only mild fever

Additionally, there is different susceptibility at different ages, and there is variation in susceptibility between different individuals from the same species and even from the same flock.

Disease Characteristics

After the virus enters the skin, it multiplies in the draining lymph nodes and spreads via mononuclear cells to secondary multiplication sites in lymph tissue and lungs. Viremia typically appears on day three. The peak is accompanied by high fever and occurs 6-7 days after infection.

The result of viremia is the settlement of the virus in blood vessel endothelium, causing damage and necrosis in cells. Endothelial damage causes increased blood vessel fragility, leading to tissue necrosis. Changes manifest in edema, blood congestion, hemorrhages, inflammation, and necrosis.

Symptoms

The disease is characterized by:

• High fever (40.5° - 42°C)
• Prominent edema in the head area
• Hemorrhages in the oral cavity and hooves
• Muscle inflammation
• Light sensitivity

The duration of the disease depends on the severity of clinical signs. In mild cases, the disease is rapid; in severe cases, the disease will cause death within a week of infection.

Clinical Signs

Incubation Stage

• No clinical signs (lasts 4-6 days)

After Incubation Period

• Body temperature rises (40.5° - 42°C)

48 Hours After Temperature Rise

• Mucus and nasal discharge appear, accompanied by redness of nasal and oral mucous membranes
• Blood spots may appear in nasal discharge, which then becomes foamy
• The discharge causes crusts in the nostril area, making breathing difficult
• Breathing rate increases and the sheep breathes with open mouth

Edema

• Edema develops in lips, face, eyelids, and ears
• May progress to other areas such as the chest and neck
• In peracute cases where lung edema develops, significant breathing difficulties and foam from the nose are observed

Ulcers

• Ulcers may develop on gums, tongue, and around the incisor teeth
• Also in the area of molar teeth
• They cause bad breath from the mouth
• Pain from ulcers causes excessive lip and tongue movement, resulting in bloody foam formation and smearing of water and food on lips
• May progress to difficulty swallowing

Hyperemia

• Hyperemia with punctate hemorrhages on papillae at tongue tip
• Tongue swells and becomes edematous, protruding from the mouth
• Hyperemia in lip skin may progress to entire face and body
• Extensive dermatitis develops
• Several weeks later, breaks in wool can be observed, which may cause wool shedding

Leg Defects

• Initially, hyperemia appears in the horn area
• Then bleeding appears under the thin horn, at the border between skin and horn of the hoof
• Appears as a prominent line of bleeding in red to black color
• Hemorrhages cause significant pain, and as a result, sheep refuse to walk (walk on knees)
• In further deterioration, horn separation may occur, creating a clear crack on the hoof

Eyes

• Another sign is eye inflammation
• Inflammation manifested by tearing from the sheep’s eyes

Twisted Neck

• If the virus affects skeletal muscles, some sheep will exhibit twisted neck to one side

Treatment and Prevention

There is no specific treatment for Blue Tongue Disease. Treatment is primarily supportive care:

• Antibiotics to prevent secondary infections
• Anti-inflammatory medications
• Proper nutrition and hydration
• Isolation of affected animals

Prevention is achieved through:

• Vaccination programs
• Vector control (managing midge populations)
• Avoiding movement of animals during high-risk periods
• Quarantine of new animals

Reporting

Blue Tongue is a notifiable disease in many countries. If you suspect your animals have this disease, contact your veterinarian immediately.