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Beyond what you see: Tracking down BVD PI cattle

Bruce Hoffman Published on 24 April 2012

You continue to see it in articles and hear about it at meetings … Bovine Viral Diarrhea (BVD). They say the virus causes all kinds of problems and costs ranchers of money.

Really? I don’t see it on my ranch and besides, I vaccinate, I’m safe. Calves have never been higher; life is good. Consider … it may be good, but it could be even better without BVD.

Data that I have reviewed from 2011 suggests that the appearance of PIs (persistently infected) in U.S. cattle herds is increasing.

Factors such as herd expansion and the transfer of animals due to drought conditions may be contributing to this increase over the five previous years’ data.

The alarming issue with this trend is that, due to the characteristics of the virus, the PI calf will shed the bug its entire life and becomes the main reservoir for the spread of the disease.

In response to the concerns of this data, the following is a review of some of the most common themes that I have heard in seven years of identifying and eliminating BVD from beef herds in the U.S.

I vaccinate my herd …

Truth is, so do the majority of the operators in whose herds we’ve identified BVD PI cattle. The unfortunate reality is that no vaccination program completely protects against the formation of a PI calf.

Vaccination is one leg of protection in a BVD control plan, but it should not be your only defense. If a PI is in the herd, the huge amounts of virus that the animal sheds can overwhelm the immunity of susceptible cattle. Herds that are nutritionally challenged or experiencing drought conditions may be the most at risk.

I don’t see cattle with diarrhea …

Since the BVD disease was first identified in 1948, the clinical signs have changed. Thanks to the increased use of vaccines, clinical disease has been reduced.

Unfortunately, the disease has not been eliminated because of the development of the PI, which serves as a reservoir for new infections.

At present, most practicing veterinarians have not witnessed the classical BVD problems seen in the ’70s and ’80s that resulted in high sickness, abortion storms and death loss.

Today, we see BVD being involved as a silent partner along with conditions like respiratory disease, Mycoplasma and scours. The virus is a pro at weakening the animal’s immune system, offering other bugs an easy entrance for attack.

If I can’t see it, how do I know I have it?

Typically, we only see small changes or conduct preventative screening to identify a problem. Weak calves, stillborn, big-headed calves and scours can all be indications that BVD may be infecting your cattle.

There are many tests to identify a PI that use a simple ear notch or blood sample. I like to remind producers that an ear notch from a dead calf can also be used.

Further, current PCR (a technique which is used to amplify the number of copies of a specific region of DNA) methods can detect the virus in notches that have been frozen for up to 30 days.

This economic option allows you to collect and freeze notches from newborn calves when you catch and tag them. Once you have collected a sufficient amount of samples, the entire collection can be sent to the diagnostic lab in one shipment.

When is the best time to test? Who should I test?

Ideally, you want to test and remove PIs before the bull is turned out. Since PIs are formed during the first trimester of pregnancy, having a PI shedding in the herd during breeding only means the problem will persist into the next calf crop.

We see most herds tested at branding or during spring vaccination time. If you do wait to test until weaning and find one, you will need to test again the following spring, as the exposure to the virus may have generated more PIs.

The best animals to test for a complete herd screen are breeding bulls, yearling heifers (pre-breeding) and newborn calves.

Remember, a negative test from a calf means the dam is also negative, so if the cow has a calf she does not need to be tested. If an open cow stays in the herd, a sample should be taken.

How can I protect myself once my herd is clean?

The biggest element in protecting your herd is keeping PI cattle out. This can be easily done through a couple of management strategies.

The first is what I have always phrased “ask if it is a PI before you buy.” I think all seedstock sellers should assure their clients that a valuable heifer or bull has been tested prior to sale.

If they don’t test, ask why. You may be confident in their answer or you may want to have it tested yourself.

Herd expansion can involve purchasing heifers or cows from unknown, untested sources. If you can’t get them tested before they arrive, get it done yourself ... don’t take the chance.

Remember that when you buy bred animals, the calf can be a PI without the dam being a PI. I recommend that the calves be tested from purchased bred animals to assure there was no BVD exposure leading to PI calves at the location of breeding.

Again, the goal here is to not let a PI into your herd. Consult your vet if you are unsure how to set up a program.

What about the expense and hassle involved?

There are multiple ways to accurately identify if the BVD virus is causing problems on your operation. The PCR method mentioned above has the capability to identify very small amounts in batched samples.

It allows us to screen herds for the virus and can cost as little as $1 per head. A new product, a rapid, on-farm test allows for a PI animal to be identified without having to send a sample to a lab.

This is great for those grafted calves, new arrivals or “ain’t doing right” calves that, many times, you are concerned about but don’t necessarily want to bother with in terms of preparing a sample for shipment to a distant lab.

Keep your eyes open to potential problems associated with BVD. Ask your veterinarian to help you design a program that includes vaccination, surveillance testing and biosecurity to identify BVD and also keep it out of your herd.

With the price of calves, can you afford not to be vigilant?  end_mark

Bruce Hoffman

Animal Profiling International