BVD: can lead to diarrhoea and pneumonia in young calves

It can lead to reduced fertility in cows and many diseases in calves such as diarrhoea and pneumonia among others.

Cause

Disease due to BVD can manifest in many different ways on a farm and often plays a part in increasing the susceptibility of young animals, in particular, to other diseases.

The most important animals to consider when dealing with BVD are breeding heifers and cows. Most of the disease attributable to BVD relates to the stage of infection of the pregnant cow/heifer with BVD virus. The most significant is the infection of the cow/heifer at three months of gestation. The virus will cross the placenta into the calf at this stage and will infect the calf without causing any physical damage. However, that calf will be a carrier of BVD for the rest of its life, ie it will be “persistently infected” (PI) with BVD virus and will shed it in all its bodily secretions. It is these PI’s that spread and maintain infection in a herd.

Symptoms

Contrary to the name, BVD rarely causes diarrhoea except in cases of acute infections in naïve herds. One of the most common disease manifestations is that of immunosuppression. High levels of exposure to BVD virus, shed by PI animals, leads to increased calf disease in the form of diarrhoea and pneumonia. As well as an increased disease incidence, affected calves may not respond very well to treatment due to their immunosuppressed state.

In pregnant cows infected in the first two months of pregnancy, BVD leads to early embryonic deaths and problems with cows repeating. This can manifest as cows which were considered in calf, repeating. From mid-gestation onwards, BVD can lead to abortion. The virus can also lead to permanent damage to the developing calf in the uterus, with calves subsequently born with physical and neurological defects.

These defects include cerebellar hypoplasia, hydrocephalus, cataracts in the eye, blindness, small eyes and arthrogryposis. One of the most characteristic manifestations of disease due to BVD is that of BVD-Mucosal disease. This only occurs in PI animals over six months of age following virus mutation. Ninety percent of such cases occur before the animal’s second birthday.

With BVD-mucosal disease, animals may also be poor thrivers or may begin to lose weight over a few weeks/months and waste away. However, a significant proportion of PIs look perfectly healthy, in spite of all the damage they are doing in their herds. Diarrhoea is common, as well as ulceration in the oral cavity and between the claws of the feet, with resultant lameness. BVD-Mucosal disease is always fatal, although not all PI’s develop mucosal disease.

Treatment

Persistently infected animals cannot be treated and should be removed off the farm as soon as possible.

Prevention and control

The key to the control of BVD in a herd involves the protection of the pregnant cow in the third month of gestation. This involves the detection and removal of existing PI’s from the herd, as well as strict biosecurity. Ideally, a closed herd policy should be followed and boundaries well fenced.

In open herds, all purchased animals should be tested for BVD virus and held in quarantine from the rest of the herd until the result is known. Certainly, vaccination plays a very important role in the prevention of infection in open herds in particular. Ideally vaccination programmes for cows should be completed a month before the breeding season starts, however, vaccination should not occur later than two weeks pre breeding to ensure adequate protection.

However, it is vital to understand that vaccination without the removal of PI animals will not control BVD infection in a herd and disease will be on-going. Any delays in the removal of PIs prior to the commencement of breeding in the herd, poses a risk of further PIs in that herd.

The national BVD eradication programme, managed by AHI, has dramatically reduced the prevalence of PIs from 0.67% in 2013 to 0.05% in 2018.