Last month we reviewed the BVD virus, its clinical symptoms and its devastating effects on productivity. This month we will assess the risk to your herd, diagnosing this disease, setting goals to control the disease and choosing the right tools to meet these goals.
BVDV-is this virus circulating in my herd? To answer this question, a producer must be aware of what is going on within his herd and keep accurate records to determine if his/her cattle are at a high or low risk of infection. Ask yourself this series of questions:
1. Does my herd have poor reproductive performance despite good nutrition (and fertile bulls if AI is not practiced)?
a. Is there a decrease in overall pregnancy rate and % pregnant after the first service?
b. Are there more abortions, stillbirths, and neonatal deaths than usual?
c. Are cows returning to heat more often than expected?
2. Do I see any physical abnormalities in the calves at birth such as weakness, inability to stand and nurse, eye defects, or cleft palate?
3. Does my herd have unexplained calf loss due to pneumonia or scours?
4. Do I introduce new cattle (including bulls) into the herd without testing for BVD?
5. Do animals from the neighbor’s herd come in contact (fence line) with my milking herd?
6. Is there a significant population of wild animals (such as deer) on my farm?
If you answered ―yes‖ to any of these questions, your herd is at risk for BVD (the more ―yes‖ answers, the higher the risk). Your goal should be to know with certainty if the virus is in your herd and, if found, work to eliminate it. If you answered ―no‖ to all of the questions, your herd is at low risk for BVD so your goal is to keep the herd free of the virus and minimize losses if it is introduced. In either case, the tools of diagnostic testing, vaccination, and biosecurity will all be needed to accomplish your individual herd goals.
Diagnostic testing for BVDV is important for two distinct reasons. The first reason is to find out if the virus is causing a clinical disease problem in your herd. Sending any aborted fetus and membranes, stillbirths and/or dead calves to a diagnostic laboratory will help to confirm the presence of the virus. The second, and perhaps most important reason to test, is to identify any persistently infected (PI) cattle in order to remove or isolate them before they spread the virus and infect other cattle. Recall from last month’s article that PI calves result when a cow is infected with the BVD virus between 42-125 days of gestation or if a PI cow has a calf. Once the calf is born, it is a virtual virus factory, churning out millions of virus particles in all of its body secretions throughout its life. Although many PI calves die at a young age, a small percentage will survive and look clinically normal. Therefore, it is crucial to get these PI animals away from any cows in the first trimester of pregnancy and stop the disease spread. The most commonly used sample for identifying PI cattle is skin, usually taken as an ear notch. Blood (serum) can also be used but not in young stock (calves less than 3 months old). If you suspect BVD virus is in your herd and you want to initiate testing, remember to:
1. Test early-before placing the calves in a group if possible to reduce virus spread and definitely before breeding them as heifers.
2. Test all calves-if positive, euthanize the calf and test the dam. Cull (+) dams.
3. Test any open cow if her calf died or was sold untested. Cull positives.
4. Test all bulls and replacement heifers (purchased or raised). Cull positives.
5. For Pregnant cows-test calf when it is born. If calf is positive, euthanize calf then test cow.
6. Purchased Pregnant Cows-Quarantine and test cow and, if negative, she can join the herd. However, bear in mind her calf could still be a PI. Therefore, when her calf is born, test her calf and if it is positive, euthanize this calf.
7. Any positive test in valuable animals can be confirmed by segregating the animal and retesting blood drawn at least 3 weeks after the first sample. True PI animals will still be positive after 3 weeks. Remember PIs are considered defective and there is a moral and ethical obligation to euthanize and dispose of these animals. They are still considered safe for human consumption.
To control BVDV, effective vaccines are available to combine with management practices in order to prevent/limit its introduction into the herd. For heifers, vaccines should be given 4-6 weeks prior to breeding to reduce the risk of reproductive problems and fetal infection. The lactating herd should receive at minimum an annual vaccination with at least 5 way viral protection. Dairy calves should be vaccinated after 4 months of age; ideally with 2 doses of a modified live vaccine given 4 weeks apart (observe label warnings). Boostering the vaccine to heifers at least 2 weeks before a stressful event such as transport or commingling is beneficial in certain management situations. Consult your veterinarian for specific recommendations for your herd. Other important management practices to reduce the risk of BVDV include:
1. Screen newly purchased cattle for the presence of the virus (submit ear notch or serum) and isolate cattle until the results are known. Remember that pregnant cows may be test negative for BVDV but the unborn fetus may be PI so you must test the cow and her newborn calf.
2. Show cattle should be isolated 3-4 weeks after returning to the farm.
3. Test new bulls before they are used (virus can live in the semen).
4. Prevent potentially BVDV contaminated objects (boots/vehicles/equipment) from entering the premise.
5. Limit wildlife interaction with the cows if possible.
6. Vaccinate the lactating herd annually to maintain immunity in the event of exposure.
7. Manage pastures to minimize fence line contact with other cattle, especially during early pregnancy.
Consult with your local veterinarian on the best way to detect BVDV on your farm as well as to assess the most biologically appropriate and cost-effective control measures. Successful control will result in improved productivity, performance, health, and ultimately economic return.
Source: Michelle Arnold, DVM, University of Kentucky Ruminant Extension Veterinarian