The safest approach in protecting a cattle herd from a trojan animal is to not introduce pregnant animals to the herd, Animal Health Ireland (AHI) says.

“A trojan animal refers to a pregnant animal that is not itself persistently infected (PI) with Bovine Viral Diarrhoea (BVD) virus but which is carrying a PI calf,” AHI says.

AHI says that 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. It says that 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 2-3 weeks. However, at this stage of pregnancy the immune system of the calf has not yet developed, resulting in it becoming persistently infected,” AHI says.

AHI says that currently laboratory tests to identify trojan animals are not available and it has the following advice for farmers who must purchase pregnant animals on minimising the risk of intorducing a trojan animal to the herd.

AHI is advising farmers to discuss all aspects of herd biosecurity with their veterinary practitioner and include the performance and interpretation of pre-purchase blood tests for antibodies to BVD virus. AHI says that an animal that is negative for both virus and antibodies is very unlikely to be a trojan.

Furthermore it tells farmers to buy lower risk animals and to purchase animals from herds where the risk of pregnant animals contacting BVD virus has been minimised.

“This should be lowest in herds that have acquired negative herd status (NHS). Herds that have contained one or more PI animals in the previous year present a high risk, particularly where
these PI animals have been in the herd after the breeding date of the purchased animals.

“Avoid contact with other cattle. Minimise opportunities for contact of pregnant stock with cattle of unknown status during purchase and transport. Quarantine on arrival. Isolate purchased pregnant animals until calved and tested with negative results,” AHI says.

AHI says that pregnant animals being introduced following their return from contract rearing or other herds should be treated in the same way as purchased animals.

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