Wednesday, 23 February 2011 13:17

Wading through the murky depths of Pestivirus

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At this time of the year, in seasonal dairy practices, we are starting to get into pregnancy testing.

Often, the results are not what was hoped for – or anticipated. It is natural for farmers to want answers when pregnancy rates are worse than expected.

Bovine Pestivirus Disease is potentially one of the most significant viral diseases affecting reproduction in cattle and it has received a lot of press recently.

Pestivirus infection is a potential time bomb ticking away in many dairy herds. It has the potential to devastate the reproductive performance of herds all over Australia.

Herds that have had no previous exposure to the virus are at the greatest risk of a reproductive disaster. However, those herds which are endemically infected with the virus can also suffer losses in many different forms.

I have heard of farmers who – faced with poor reproductive performance – have blood tested or milk tested and found antibodies to Pestivirus in the milking herd and been convinced they have found the “answer” to their woes.

However, after vaccinating their herd, they have experienced a similarly poor reproductive performance the following year.

Why does this happen? Because the mere presence of Pestivirus antibodies only tells the cattle have been exposed to the virus at some time during their life.

The presence of antibodies in the herd may indicate chronic infection, which is less likely to cause catastrophic reproductive losses. Or a recent infection – which could explain pregnancy loss, stillbirth, congenital deformities and the birth of Persistently Infected (PI) calves.

The virus can cause early embryonic deaths – with any delayed returns to oestrus infecting the developing foetus causing birth defects or abortion. When a foetus is infected – before it has developed its own immune system and survives – the phenomenon of a Persistently Infected (PI) calf can occur.

This happens because as the immune system of the growing foetus develops, it never identifies the virus as being “foreign” and so never produces an antibody response to it. Therefore, PI cattle will usually test negative on an antibody test, turning them into a likely “sleeper agent” of destruction.

The “ear notch” is the best test for identifying PI calves, and eliminating them from the herd.

Whenever I am asked to investigate a possible Pestivirus “outbreak”, I usually start with a history of the reproductive problem. In order to rule Pestivirus – in or out – as a potential cause of the reproductive loss, it is important to determine at what age the infection is occurring in the herd.

By testing groups of weaners, yearlings, springing two-year-olds as well as the cow herd, it is possible to determine whether there is a recent Pestivirus infection, or if the virus may be endemic in the herd.

The most dangerous time for a heifer or cow to become infected is at – or soon after – joining. That is why whenever I identify a herd that has no antibodies – especially in the young stock – I shudder at the potential for disaster should the virus be introduced to the herd at a vulnerable time.

The two main strategies for reducing this risk is to either undertake a comprehensive vaccination program, which must be ongoing, or redouble the efforts to maintain a completely closed herd.

PI calves are often poor doing calves, and many die when young.

However, some appear perfectly normal but they are like walking virus machines, infecting others that they come into contact with.

It is usually by buying in, or the straying of a transiently (recently) infected or PI animal into a negative herd, which can cause catastrophic losses

I could never hope to cover what is one of the most complex diseases that cattle veterinarians deal with in such a short article.

If you are looking for any information on Pestivirus disease, prevention strategies or advice on farm biosecurity, contact your local dairy veterinarian for unbiased scientific advice

Rob Bonanno is president of the Australian Cattle Veterinarians Association and a director of the Shepparton Veterinary Clinic.

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