Clinical Signs of Bluetongue Infection

All ruminants can be infected by the Bluetongue virus (BTV). Not all infected animals show clinical signs. The most severe clinical signs are usually seen in sheep. Even in sheep, a relatively low proportion of viraemic sheep develop clinical signs.

Severity of clinical signs can vary from asymptomatic or mild to severe depending on:

• Virus serotype
• Infecting dose of the virus
• Age, condition, and resistance of the animal
• Breed - some breeds e.g merino are particularly susceptible.

General Clinical Signs

Bluetongue virus infection is manifested as a vascular disease affecting several organ systems and often presents as a reproductive syndrome. BTV infection may resemble Foot-and-Mouth-Disease, especially in the later stages.

Clinical signs of bluetongue are:

• High fever (40-41°C) for 2-14 days
• Superficial hemorrhages (bleedings) , open sores on the tongue, mouth and nostrils. Causing food refusal and subsequent dehydration and weight loss.
• Hyperaemia (redness) of the skin, face, neck, and possibly body especially in areas more exposed to sunlight.
• Oedema of udder and head
• Lameness accompanied by an engorged reddish-blue area around the base of the claws and coronary band.
• Loss of condition and muscular weakness
• Loss of wool in sheep
• Depression
• Dyspnoea (difficulty breathing)
• Collapse, asphyxiation and death in late cases

Reproductive signs

BTV infection of pregnant cattle can cause infertility, foetal mummification, abortion and still born calves

In sheep, cases of abortion and foetal malformations due to BT occur sporadically. Abortions are rare compared to congenital abnormalities; however many abortions will be non-specific and a direct stress-related consequence of the systemic infection of the ewe.

Oedema and hyperaemia (swelling and redness) of the udder caused by severe sunburn-like lesions.

Lesions in the mouth of a cow infected with BTV

Lesions on the muzzle and a mucopurulent (snotty) nasal discharge.

Lachrymation, hyperaemia and oedema (weeping, redness and swelling) of conjunctiva.