NADIS (National Animal Disease Information Service) is a network of 59 veterinary practices and six veterinary colleges monitoring diseases in cattle, sheep and pigs in the UK.

This month Phil Scott DVM&S, DipECBHM, CertCHP, DSHP, FRCVS offers advice on bovine virus diarrhoea (BVD).

It is estimated that more than 90per cent of UK herds have had exposure to bovine virus diarrhoea virus (BVDV).

Losses result from reduced fertility, poor production and increased susceptibility to other infections especially in young calves. Major financial losses result where infection is introduced into a group of susceptible breeding cattle resulting in poor reproductive performance and the birth of calves with persistent virus infection.

Causes and background to disease The main transmission route is by direct contact with cattle persistently infected with BVD virus. Only one persistently infected animal introduced to a susceptible herd can cause significant financial loss.

Early clinical signs to watch out for Cattle exposed to BVD virus show transient infection producing protective antibodies within three to four weeks. BVD virus infection may temporarily lower immunity to other infectious diseases such as salmonella, respiratory infections, and coccidia exacerbating these clinical infections particularly in young calves. However, BVD virus is most important when it infects susceptible breeding cattle during early pregnancy causing embryonic death and return to oestrus, foetal death/abortion, mummification of the foetus, birth defects of the nervous system and eyes, weak/premature calves, and live persistently infected calves.

Pregnant animal Virus infection before 110 days of pregnancy results in the birth of a live, persistently infected calf. This is caused by failure of the developing immune system of the foetus to function properly before 110 days. After birth these calves carry the virus for life and act as potential transmitters of BVDV infection. Virus infection may also lead to various defects of the developing foetus' eyes and brain. These calves may be born blind and lack co-ordination, respectively.

Virus infection after 150 days gestation has little effect with live calves born at full term. Abortion can occur following infection at any stage of pregnancy but this is not common.

BVD virus can be spread in semen of persistently infected bulls or in bulls experiencing acute BVD with transient virus infection. BVDV will lead to low pregnancy rate due to embryonic death or later foetal death/abortion. Bulls are vigorously tested for BVD before entering AI studs.

Mucosal disease Mucosal disease occurs when persistently infected animals become superinfected with cytopathic BVD virus. The cytopathic BVD virus arises from changes in the BVD virus within the PI animal. Most cases of mucosal disease occur in six to 12 month-old calves with a sudden onset of depression, salivation, fever and anorexia. Ulcers appear in the mouth and on the muzzle. There are purulent discharges from the eyes and nostrils and there is profuse diarrhoea with shreds of gut mucosa/blood present during the terminal stages. There is rapid weight loss followed by death within five to ten days.

Diagnosis - Acute BVD infection Paired blood samples three to four weeks apart to demonstrate rising antibody levels to this virus.

Persistent infection PI calves may be clinically normal but are commonly stunted calves due to their susceptibility to bacterial infection such as pneumonia. Two positive samples taken three to four weeks apart confirm persistent virus infection.

Treatment Acute BVD - treatment of any concurrent infections if present.

Persistent infection - PI animals should be disposed of as they act as a source of BVD infection.

Control and Prevention In many infected herds disease/losses are partly controlled by PI calves acting as "natural vaccinators". When most adult animals in the herd are immune disease losses are not so obvious to the farmer. However, losses can be catastrophic if susceptible breeding females are introduced to the herd.

Vaccination - three inactivated BVD vaccines are available in UK. Initial vaccination comprises two doses three to four weeks apart before first service followed by booster vaccination at 12 months intervals. If all breeding females are vaccinated then this will control disease by preventing BVD infection of the developing foetus during pregnancy and production of PI calves.

BVD eradication is possible following whole herd blood testing and elimination of all PI carrier animals. If farmers go for eradication then strict herd biosecurity measures must be maintained to prevent re-introduction of virus infection as the herd will soon become naïve and fully susceptible to infection.

Welfare implications Cattle with mucosal disease must be euthanased immediately upon diagnosis. Calves born with eye and brain defects due to virus infection during their development should be culled.

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