When BVD finds your dairy herd

Don’t discount Bovine Viral Diarrhea (BVD) as a cause for unthrifty calves and poor reproductive performance just because you vaccinate. Vaccination programs are regularly challenged by constant evolution of the virus and exposure to new strains from sources outside your control.

Rather than assuming the absence of BVD, consider the many potential breaches in biosecurity, even in a traditionally closed herd. Heifers raised off-site? Have 4-H projects? BVD is caused by a virus, a tiny particle that can ride a sneeze just like the common cold. This article will help you evaluate BVD as a cause for problems in your calf and reproductive programs. After determining your herd’s risk of harboring BVD, you can use the testing schemes below to confirm or contradict your suspicions.

Suspecting BVD
When addressing BVD, it’s important to first consider if your herd is at low or high risk for harboring the virus. The accompanying flow chart contains observations common to high risk herds. Considering persistently infected (PI) calves rarely survive their first year, the clinical signs of BVD are typically quite noticeable. Infection with BVD affects the respiratory and immune systems, leaving calves sickly and susceptible to chronic respiratory disease. Fever, coughing and nasal discharge, common to many calf maladies are also indicative of BVD.

In addition to clinical symptoms, consider coincidental changes in management that are consistent with the introduction of BVD. Most BVD outbreaks occur from the introduction of untested animals during expansion, or maybe even a neighbor’s expansion. Outbreaks of BVD are sometimes regional, reflecting the introduction of a new strain of virus in the local vicinity and the popularity of a single source of vaccine that fails to provide adequate protection. In general, high frequency of cattle movement without particular attention to BVD places many producers at high risk.

Testing plans for high risk herds
In high risk herds, screening is critical to detect BVD before it turns into an outbreak. Screening for BVD is a two pronged approach (see chart); one for the milking herd and another for calves. Used properly, screening will eliminate BVD concerns during expansion and keep the herd free of PI animals that propagate BVD by maintaining a reservoir of virus as a source for future infections.

Screen the milking herd
Bulk tank testing at regular intervals can detect the introduction of a single PI cow in a herd of 250 cows; subgroups of 250 can be used to screen larger herds. Such screens are vital as PI cows can look healthy, but shed the BVD virus continuously.

Finding the PI cow(s) after a positive bulk tank test used to be expensive and time consuming. Now, it can be done on DHI milk samples, greatly reducing labor and testing costs. As dry cows reenter the milking string, or with the addition of new replacements, bulk tank screening can be repeated to assure the continual absence of PI animals in the milking herd.

Dealing with a PI cow is simple, cull her. Dealing with the situation is more complicated. Where did she come from, how long has she been in the herd, how many of her herd mates are on the farm, and where are her daughters (PI dams always produce PI calves) are just a few of the questions that should be considered when evaluating the herd’s biosecurity program.

Screen calves
When BVD is suspected, a calf screening program should also be implemented. As part of a calf rearing program, ear notches can be aseptically taken from the tip of the ear with notching pliers and sent to the lab for analysis. A negative test result not only clears the calf of PI, but also clears her dam. By eliminating that pair from any future testing, screening the milking herd can focus only on untested cows.

A positive test result on a calf should result in quarantine and a retest in two to three weeks to differentiate acute infection from PI since most animals fully recover from acute infection. Another positive result upon retest should seal the calf’s fate and indicate her dam on the suspect list for subsequent testing, either by ear notch or individual milk sample.

Screening calves and bulk tanks should continue until you are confident the risk of BVD has been minimized. Prevent reintroduction and potential spread of BVD into low risk herds by testing new additions, limiting cattle movement and maintaining a proper vaccination program.

Summary
It’s a small world when it comes to BVD. Like the common cold, new strains of BVD are continually evolving, passing from cow to cow and region to region.

Complete isolation is improbable. Expansions, heifer growers, cattle shows and your neighbors could all be sources of new BVD infections. When new strains get past your first line of defense (vaccination), it is critical to find and remove PI animals from the herd.

Don’t leave your herd’s health to chance. Assess your risk, and implement a screening program using these efficient diagnostic tools to keep losses from BVD to a minimum.