Mucosal disease

BOVINE VIRUS DIARRHOEA / MUCOSAL DISEASE

DEFINITION

Bovine virus diarrhoea (BVD) and mucosal disease (MD) are two conditions caused by the same virus, BVD virus. BVD is an acute transient, often asymptomatic condition which occurs following infection of susceptible calves or adult cattle. There may be transient diarrhoea but mortality is negligible. Mucosal disease is, in contrast, a sporadic condition of low morbidity but high fatality. Clinically apparent illness may be of short duration but can be prolonged. Diarrhoea is a major feature, mouth lesions are almost invariably present and interdigital ulceration, lameness and skin lesions are common. Mucosal disease occurs in animals persistently infected with BVD virus as a result of infection prior to birth; these persistently infected animals produce no antibody against the virus. Infection with BVD virus can also result in embryonic deaths, abortions and congenital abnormalities.

INCIDENCE

Infection with BVD virus is common in British cattle. Over 60% of adult cattle are seropositive. Acute outbreaks of diarrhoea in adult animals (BVD) are rare. Cases of mucosal disease occur sporadically, usually in animals 6-18 months of age although cases do occur outside this age range. The introduction of infection into a previously unexposed group of pregnant cattle can result in significant losses in calves both pre and post-natally.

The virus can cause an inapparent infection of animals and may persist in a variety of tissues following infection of the foetus.

PATHOGENESIS

Of critical importance to the outcome of infection with BVD virus is the gestational status of the animal infected and, in pregnant animals, the age of the foetus, as well as the strain of virus involved.

Infection of previously unexposed, non-pregnant animals results in transient illness of 1-2 weeks duration. Most cases are subclinical but occasionally there is a transient diarrhoea. Infected animals develop neutralising antibody and virus is eliminated from the body.

Infection of previously unexposed pregnant animals with virus results in similar effects to those above in the cow but transplacental spread of virus to the calf also occurs with consequences which vary most importantly according to the stage of gestation.

At any stage of gestation, virus infection may result in foetal death. This will be apparent variously as early embryonic death with return to service and apparent infertility, as obvious abortion or as stillbirths; there may also be production of weak or undersized calves. Animals infected between 100 and 150 days of gestation, may have abnormal brain and nervous system development.

Infection occurring in the foetus before the development of immune competence at about 120 days may not result in foetal death or gross or even microscopic abnormalities. However, the calves produced are persistently infected with BVD virus. Virus is present in many organs and tissues and remains so for life. No antibody is produced against the virus and it appears that the presence of virus early in foetal life results in a failure to recognise it as ‘non-self* and therefore there is development of a specific immunotolerance with persistent infection and no antibody production.

The production of persistently infected calves is important for two reasons. Firstly, these animals continually excrete large amounts of virus in a variety of secretions throughout life and are an important source of infection for other animals, especially as they may be clinically normal. Transmission can occur by ingestion, inhalation, transplacentally or, with persistently infected bulls, venereally. Secondly, only persistently infected animals may, subsequently, succumb to mucosal disease.

Not all animals persistently infected with BVD virus will develop mucosal disease. A number survive to adulthood and may become pregnant and any calves produced will also be persistently infected. Persistently infected calves may also be stunted and are more likely to succumb to other common diseases such as calf diarrhoea or pneumonia.

MUCOSAL DISEASE

Course

Short lived acute cases of mucosal disease lasting only 1-2 weeks before death do occur. Chronic cases may last for 1 to 2 months or longer with successive crops of lesions developing and fading. Virtually all animals with recognisable mucosal disease will eventually die.

CLINICAL SIGNS

Depression is common. Inappetance is common and sometimes there is complete anorexia. The animal is usually wet around the lips and muzzle, and saliva tends to be held in the mouth, possibly because swallowing causes discomfort. Diarrhoea is commonly present, often continuous but sometimes intermittent. Oral lesions are usually diffuse and any part of the oropharynx may be affected. Vesicles are seen. Gums are often reddened and numerous small irregular shaped shallow ulcers appear. These superficial erosions and ulcers often become confluent and covered with a thin light-coloured diphtheritic membrane. Handling of the tongue can cause some pain.

Mucosal disease virus has not been clearly associated with respiratory tract signs .Erosions and ulcerations on the nose may develop a diphtheritic membrane. The nostrils usually have a profuse discharge. There is often crusting at the nose-skin junction. Eyes may be congested with moderate discharge.

Ulceration of the skin of the interdigital cleft is common and the ulcer is usually quite large and irregular in outline. Sometimes there is under-running of the horn and lameness associated with laminitis. In longstanding or recovered cases there may sometimes be thickening of the coronary band and parallel marks on the hoof associated with previous laminitis. There is marked loss of condition.

If pregnant animals are affected they may abort.

DIAGNOSIS

Diagnosis of cases of mucosal disease should be relatively easy on the basis of clinical and/or pathological findings. In such clinical cases, if confirmation is required, examination of blood samples should reveal a persistent viraemia and an absence of neutralising antibody. Virus may also be detected in nasal or ocular discharges. At post mortem, virus may also be detected in lymph nodes, alimentary tract lesions and other tissues.

Fully susceptible animals, both virus and antibody negative, will usually seroconvert within 3 weeks of infection. It should be apparent that within a herd both virus detection and antibody analysis would be required to differentiate between immune, susceptible and persistently infected animals which might succumb to mucosal disease in the future and are a source of risk to other stock.

CONTROL

Control of BVD-MD within a herd may not be justifiable on the grounds of cost and can only be done effectively on the basis of knowledge of virus and antibody status of animals in the herd.

In fattening stock, detection of persistently infected animals would allow elimination of those likely to die from mucosal disease. In an infected breeding herd, the main object of control should be to ensure that all animals are immune before they are bred: this should prevent foetal losses, and production of weak calves or calves likely to die from mucosal disease. Vaccination can be effective in controlling the problem.

In known BVD-MD free herds it is important that infection should not be introduced into groups of susceptible breeding animals. This means that bought-in stock should preferably be both virus and antibody free or, at the least, virus negative and antibody positive. It should be noted that both tests are essential in assessing status - antibody analysis alone is unsatisfactory.

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