This producer was specifically referring to what we call a “persistently infected” animal (or “PI”). PIs are animals that have been infected with pestivirus since before birth, and they never clear the virus from their body. As you can imagine, this can have some negative effects.

Pestivirus is a complex disease, and I’ll try to explain some more about how it works over the next few weeks in this column. For a start, let’s look at how PIs are created and why they’re important in the spread of pestivirus.

How is a PI created? It starts with a pregnant cow – specifically, one with a foetus of about 40 to 120 days gestation. Then, that cow must be exposed to a source of pestivirus (another animal which is infected and spreading the disease, just like people spread a cold or flu virus).

In this situation, the foetus can become infected while still inside the mother. And there’s a catch. Because the foetus’s immune system is still developing, it doesn’t recognise the virus as something which needs to be eliminated from the body. For this reason, the calf is still infected with pestivirus when it is born, and will remain infected for life.

All calves of PIs are also PIs themselves. PI calves can look normal, but often they’re somewhat stunted, ill-thrifty, or have rough coats. They’re also more susceptible to other diseases, due to a weakened immune system.

If you have a poorly growing animal which is the only one to get pink-eye, or the only one to get really bad ringworm or lice, then it could be a PI. A simple lab test can confirm this.

There’s a bigger reason why having PIs around is risky. PIs spread huge amounts of pestivirus and so can easily infect other animals. When other animals get exposed, they don’t become PIs (that only happens to calves infected before birth), but if they’re pregnant and not immune they can abort or give birth to deformed or PI calves. I’ll discuss the reproductive issues associated with pestivirus more next week.