PIs and Trojans endanger BVD eradication efforts

Since BVD testing of calves began, there have been 5.62 million tests results.
PIs and Trojans endanger BVD eradication efforts

The key to eradicating Bovine Viral Diarrhoea from the national herd is to identify and remove persistently infected (PI) cattle.

This can be done cost-effectively by testing ear punch samples collected by farmers, as part of the official calf identity tagging process.

Three years of this testing of calves began on January 1, 2013, accompanied by a ban on sale of calves without a negative test result.

PI cattle are the main source of infection within herds, and the main means of spread between herds.

Calves become persistently infected when their mother is exposed to the virus during the second to fourth month of pregnancy (or if the mother is PI). While apparently normal at birth, PI calves often become ill-thrifty and die before reaching slaughter weight.

During this time they remain a source of infection for other cattle, which may lead to the birth of further PI calves. Eradicating BVD virus will save Irish farmers about €102 million per year, which is the estimated cost of ill-health in herds caused by BVD infection.

March 1, 2015 was the deadline for herdowners to dispose of any PI animals born in 2012, 2013 and 2014.

Those who missed that deadline cannot avail of PI disposal grant aid, and may also be ruled out of the proposed Beef Data Genomics Programme and successor programmes (see details of the new BVD eradication rules on page 19).

In other BVD eradication news, the Animal Health Ireland section directing the scheme has now issued guidance on measures to minimise the risk of introducing a Trojan animal into your herd.

What is a Trojan animal?

A Trojan animal refers to a pregnant animal that is not itself persistently infected (PI) with BVD virus, but which is carrying a PI calf.

How do animals become Trojans?

Trojan animals typically occur when heifers or cows that have not previously been in contact with BVD virus are infected in early pregnancy (between approximately 30 and 120 days of gestation).

As a result, the dam becomes transiently infected (TI) with BVD virus, which crosses the placenta to infect the unborn calf.

The immune system of the dam responds to the transient infection, resulting in the dam becoming virus negative and antibody positive, typically within two or three weeks. However, at this stage of pregnancy, the immune system of the calf has not yet developed, resulting in it becoming persistently infected.

Laboratory tests to identify Trojan animals are not available, so the safest approach to protecting your herd is not to introduce pregnant animals.

If I must purchase pregnant animals, how can I minimise the risk of introducing a trojan animal?

Discuss all aspects of herd biosecurity with your own veterinary practitioner.

Include pre-purchase blood tests for antibodies to BVD virus (an animal that is negative for both virus and antibodies is very unlikely to be a Trojan).

Buy lower risk animals. Purchase animals from herds where the risk of pregnant animals contacting BVD virus has been minimised.

The risk should be lowest in herds that have acquired negative herd status (NHS).

Herds have NHS if they complete at least three years of tissue tag testing on calves born into the herd; they have negative BVD status for every animal currently in the herd; and had no animal deemed to be persistently infected with BVD virus in the 12 months preceding.

Herds that have contained one or more PI animals in the previous year present a high Trojan animal risk, particularly where these PI animals have been in the herd after the breeding date of the purchased animals.

To further minimise the risk of introducing a trojan animal, avoid contact with other cattle.

Minimise opportunities for contact of pregnant stock with cattle of unknown status during purchase and transport.

Quarantine pregnant stock on arrival. Isolate purchased pregnant animals until calved, and tested with negative results.

Pregnant animals being introduced following their return from contract rearing or from other herds, should be treated in the same way as purchased animals.

For further information on herd biosecurity, see Animal Health Ireland’s www.biosecurity.ie website.

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