by John Maday
How is your calving season going? Have you noticed a drop-off in calving percentage or an increase in stillbirths, abortions, open or late-calving cows? Any of those reproductive problems could suggest a problem with bovine viral diarrhea (BVD), and working with your veterinarian to institute a prevention program, before breeding season, could help prevent future losses.
University of Tennessee Extension veterinarian Lew Strickland says infections with the BVD virus (BVDV) are classified into three clinical syndromes;
- Acute infections can result in fever, depression, diarrhea, respiratory disease and reproductive problems. Some animals will show no outward signs of illness and most animals will recover.
- Fetal infections occur when a cow or heifer experiences an acute BVDV infection during breeding or gestation. Outcomes include infertility, embryonic death, abortion, stillbirths and persistently infected (PI) calves.
- PI calves are created when a fetus is exposed to BVDV during the first half of gestation. The fetus might be aborted, but some PI calves survive for several years and constantly shed BVDV via saliva, mucus, tears, milk, feces and urine.
Julia Ridpath, PhD, recently retired after studying BVDV and related viruses at the USDA’s National Animal Disease Center (NADC) in Ames, Iowa. She has authored or co-authored more than 200 research papers, mostly on BVD and related diseases in cattle. She says three species of pestiviruses cause BVD disease worldwide; BVDV1, BVDV2 and HoBi-like viruses. BVDV1 and BVDV2 are the pathogens prevalent in the U.S.
Ridpath stresses effective BVDV control requires three components; diagnosis, vaccination and biosecurity.
There are many challenges to diagnosis because there is no single “typical” BVD presentation. Cases can be from mild to severe, and accurate diagnosis required laboratory testing, ideally on two samples submitted about three weeks apart. Diagnosis, removal, and isolation of PI cattle serves as a critical step in a BVDV-control program.
While BVD receives deserved attention as a reproductive disease, Ridpath notes that in young calves BVDV can disrupt maturation of the immune system,
potentially with long-term health effects.
Current vaccines available for protection against BVDV1 and BVDV2 include modified-live virus (MLV) and killed-virus vaccines. Typically MLV vaccines require a single dose while killed vaccines need two doses. Ridpath points out some vaccines are labeled for preventing clinical disease following acute infection, some for prevention of PI calves and some for use in pregnant cattle.
Ridpath stresses that while vaccination is effective in increasing herd immunity against BVDV, no vaccine is 100% effective. Vaccines sometimes appear to fail, primarily for three reasons:
- .Currently approved vaccines contain BVDV1a and BVDV2a viruses, but another sub-type, BVDV1b has become increasingly prevalent in U.S. herds, Ridpath says. When PI calves turn up in vaccinated herds, diagnostic labs frequently have found the 1b subtype. Manufacturers are working to incorporate the 1b strain into their vaccines.
- Problems with the animal being unable to respond are more common than problems with the vaccine. This includes animals sick or stressed at the time of vaccination, interference from maternal antibodies in young calves and inadequate nutrition.
- Management problems also affect vaccine efficacy. These include continuous exposure to a PI animal, insufficient time between vaccination and exposure and failure to follow label directions.
In developing biosecurity programs, consider the potential for the virus entering a herd through embryo transplant, vehicles or equipment. Mostly, though, BVD spreads via cattle.
While the prevalence of PI cattle remains small, the possibility of importing one can be significant, especially in large operations. If the overall incidence is 1% and 1,000- cow operation imports 100 bred heifers each year, the likelihood of bringing in a PI animal becomes quite high, unless the heifer supplier employs effective biosecurity and testing programs.
Importing a heifer that delivers a PI calf is one obvious risk, but the disease could move in the other direction. Ridpath says some ranches can have low-grade BVD infection circulating in their herd, with cows developing some tolerance to the disease. Reproductive losses might remain low enough to avoid attention. When the operator imports replacement heifers with naïve immune systems they can see disastrous abortion rates in those heifers.
- BVDV control points
The cow-calf stage represents best control point for BVD, through vaccination and removal of PI calves. Any imported cattle should be tested to prevent the introduction of a PI animal, and all calves should be tested for PI status. Ranchers should test the dams of any calf that tests positive for BVDV and any cow or heifer that does not deliver a calf. Ridpath also encourages producers to isolate any “visitor” cattle. There is no such thing as benign BVD,” she says.
- Disposition of PI calves
The necessity for removing PI calves leads to the question of what to do with them. Producers facing tight margins might be reluctant to euthanize or slaughter PI calves that appear healthy, and the potential loss of revenue can even discourage some producers from testing.
Ethically, options include euthanasia or shipping directly to slaughter. In some cases, producers can limit losses by isolating and feeding PI calves to slaughter weights. That option entails considerable risk for sickness and death loss. Legally, policies vary between states, but producers who market PI calves without clearly identifying their health status could be held liable if calves spread the virus in the next production stage.
- Control programs
Regional efforts focused on education, management and testing could help reduce the impact of the disease, Ridpath says. Kentucky has developed a verification program that documents participants’ BVD-control programs and potentially market their calves as BVDV-free. Participants sign an agreement to dispose of PI calves, either with euthanasia or transparent marketing through approved channels. The program includes indemnity payments, covering 50% to 100% of a calf’s value.
Ridpath encourages producers to work with their veterinarians and use the “BVD Consult” website (www.bvdconsult.com) to develop control strategies for their operations.
Source: Drovers March 2017