A confusing picture

The uptake of Bovilis® BTV8 vaccine in the UK has so far been good however momentum must not be lost.  Vets and farmers need to be extra vigilant as this is the time of year the disease may appear in unvaccinated stock.  As such Intervet/Schering-Plough Animal Health is not only encouraging the uptake of vaccine, it is also making people aware of the long list of differential diagnosis that the disease can be confused with.

Despite its name, hyperaemic purple colouration of the tongue is reported in only 20% of cases of bluetongue in sheep and fewer than 6% in cattle. Furthermore, many of its other symptoms are similar to diseases such as foot and mouth disease (FMD), orf and mucosal disease, making it hard to detect.

Yet early recognition by stockmen is crucial if vets are to make a prompt diagnosis. Fortunately, as the BTV8 virus continues its third year in countries such as Germany and the Netherlands, more practitioners are reporting a range of clinical signs which are helping to build a clearer picture – and therefore help with differential diagnosis.

According to Dr Daan Dercksen from the Dutch Animal Health Service, many symptoms of BTV-8 in sheep are also like sunburn, liver damage and the acute form of cobalt and vitamin B12 deficiency. A blue tongue was only reported in about 15% of cases, he told a recent Intervet BTV symposium. Dr Derckson also pointed out that clinical signs ranged from mild to severe, with nothing in between. Often, symptoms were so mild in sheep and goats that it would take a very alert farmer to spot them.

“In 2007, we saw more clinical signs: morbidity was about four times as high as 2006. A lot more rams were infected and the high fever led to a rapid drop in semen quality which took between two and four months to get back. A limited number went sterile,” he said.

“There was congestion of the buccal and nasal mucosae and coronary bands. Both cows and sheep had the appearance of a burnt muzzle, swollen heads and salivation (though not as bad as in FMD), and what appeared to be bottle jaw. In the acute phase, animals had a high temperature and were unable to stand.”

Considering relatively recent outbreaks of FMD in the UK, this disease is the most important differential diagnosis in cattle and sheep. Belgian vets from the Faculty of Veterinary Medicine in Ghent* state that in sheep, FMD symptoms are relatively mild, most remarkably transient lameness. In comparison the classic sign of BTV is a swollen face – absent in FMD. In cattle, however, there can be confusion, particularly when there is excess salivation.

However, lacrimation occurs regularly in BTV cases, but not is an FMD symptom. “For FMD, the initial stage of lesions is typically the vesicular form, whereas the presence of vesicles in BTV has not been observed.” Also typical of FMD is the fast spread of disease within the herd with the majority of animals suffering from clinical signs. In bluetongue outbreaks, only a small number of animals are affected.

SHEEP

Orf

This condition can cause swelling of lips and muzzle which could be confused with bluetongue. However, orf lesions are papilomateous and occur on the coronary band, vulva and preputium, BTV lesions observed have not been papilomateous.

Haemonchosis (and chronic liver fluke infestations)

Cases with ‘bottle jaw’ may be confused with swollen head symptom of BTV. Haemonchosis is typically accompanied by severe anaemia however, in BTV the mucosal membranes are usually hyperaemic often with petechiae and haemorrhagic lesions.

Lameness

All causes of lameness with concomitant swelling and lesions of the coronet, may be confused with BTV.

Cobalt/vitamin B12 deficiency

A typical clinical sign is a swollen face (usually chronic cases) which may be confused with BTV.

Photosensitisation and facial eczema

BTV often presents as a case of photosensitisation or severe facial eczema however, with photosensitisation and eczema, there are no lesions inside the oral cavity.

CATTLE

IBR

This is primarily a respiratory disease with lameness or teat lesions as clinical signs unlike BTV.  Also, vesicles and ulcers are not normally associated with IBR infection.

Mucosal disease (original paper refers only to mucosal disease, not BVD)

The most common clinical sign of mucosal disease is diarrhoea, but this is not present in BTV cases.  In addition, mucosal disease is frequently fatal, whereas BTV commonly gradually heals.

Photosensitisation

As for sheep above.

Look on www.bluetongue-info.co.uk for more information.

*Clinical Symptoms Caused by Bluetongue Virus Serotype 8: Experiences from the Outbreak in Belgium

Van Aert, M., Laureyns, J., de Kruif, A., Opsomer, G.,

Department of Reproduction, Obstetrics and Herd Health, Ambulatory Clinic, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, B-9820 Merelbeke, Belgium