Scours, pneumonia, then a bout of coccidiosis at weaning; she is just a “poor doer” is often what I hear. Even the best calf-management programs have some challenging disease cases. Often, individual calves that are unthrifty and have multiple diseases are a result of colostrum feeding issues (passive transfer) or a difficult birth. But let’s not forget the calf persistently infected with bovine viral diarrhea (BVD).
There are many other signs of disease that should also make us consider BVD infection. Abortions, pneumonia, diarrhea, early embryonic death and overall immune suppression are other commons findings with BVD infection.
In most herds, a complete vaccination program, including BVD vaccinations, will prevent disease from this virus. Certain vaccines also aid in preventing the reproductive losses we can see from BVD. This, along with the fact that the incidence of PI infection is less than 2 percent, often puts BVD lower on the list of diseases to consider.
ID the PI’s
Persistent infection with BVD occurs in the developing calf when a cow is infected with the virus before 125 days of gestation. Prior to this time, the fetal immune system cannot recognize the virus as foreign. The virus then incorporates into the fetus.
When the immune system develops, it takes a survey of what is already there and assumes it is part of the calf or “self.” The immune system develops without attacking the virus because it was already part of the calf. In some instances, the virus can also kill the fetus and cause an abortion or cause a developmental defect, such as a malformation of the brain or eyes.
The major concern with BVD PI animals is that these calves shed virus continuously from all of their secretions and excretions (saliva, mucous, urine, manure and eventually milk) exposing other cattle to the virus. In many cases, the PI BVD calf is unthrifty and susceptible to other diseases. But, in other cases, this PI calf may appear completely normal.
How do PI calves get into a herd? Most commonly, it happens via the purchase of herd replacements that have not been tested for BVD.
For example, a springing heifer may be purchased that is a PI animal, but appears normal and healthy. This is possible, but less likely, given many PI animals succumb to other diseases prior to entering the adult herd.
The other common means is that the purchased heifer carries a PI calf. This heifer will test negative for BVD because she was infected at some point during gestation; the disease ran its course (which could have been very mild in a vaccinated animal) and the virus cleared. But it was incorporated into the developing fetus, creating a PI offspring. Think of these springing heifers as the Trojan horse that allows an infected animal to enter the herd.
Prevention is key
Discuss a BVD vaccination and control program with your veterinarian. The specifics of a program will vary depending on your herd history and if you are a “closed” herd or not. It is important to have a complete vaccination program which includes the use of modified-live BVD vaccines prior to breeding in both heifers and adult cows.
It is also important to have a BVD testing program which not only involves testing purchased replacements but also the testing of any offspring that will remain on the farm soon after birth. We are often good at testing replacements prior to arrival or soon after purchase as this is a simple procedure. But testing the offspring is often the weak link in the protocol that allows the Trojan horse success in spreading disease.
Although the incidence of this disease has declined dramatically due to good vaccination programs, don’t forget to consider BVD infection especially in those “poor doer” calves.
Mark J. Thomas is a veterinarian and partner in Countryside Veterinary Clinic, LLP in Lowville, N.Y.