What is a PI?

Persistently infected (PI) animals are the PRIMARY cause of catastrophes!​

PI stands for Persistently infected. PI cows are lifelong carriers of the BVD virus and shed hundreds of thousands of virus particles. Enough to overwhelm the immune system of animals they come into contact with (and even enough to overwhelm some vaccinated animals). PI’s propagate the BVD virus and identification and management of PI’s is crucial to controlling BVD.

You cannot develop a persistent infection. Cows are born as PI’s and then are PI’s for life. 

There are two ways a PI calf can be created:

1. PI cows will always give birth to PI calves (under 10% of PI calves are created this way)

2. Infection of a pregnant cow with BVD at about 6 weeks – 4 months of gestation resulting in the birth of a PI calf. (Route of over 90% of cases).


During this stage of development the foetus is ‘identifying itself’, recognising its own cells and organising them - “These cells are a part of me, they are lung cells”. If the dam is infected at this time the foetus identifies the BVD virus as a part of itself. This means the foetus does not (and will not ever) produce antibodies to fight the virus (this is why PI’s shed such high levels of the virus because their body produces the virus with no attempt to contain or fight it). “This is BVD, this is a part of me, I’m not going to attack me!”. As a result PI animals will test positive for BVD antigens (the virus itself) but negative for BVD antibodies (what fights the virus).


Trojan is a term used to describe pregnant cows who themselves are not PI’s but due to a transient BVD infection during pregnancy are carrying a PI calf.

BVD infections causing PI calves can occur on or off farm. It’s important to be aware that if tested, despite carrying a PI calf, a trojan cow herself could test negative for BVD antigens (the virus itself) as her infection has passed. A pregnant cow grazed off farm or a pregnant cow introduced to the herd could easily be brought on farm carrying a PI calf. This is why all calves should be tested for BVD.