Vic Cortese, DVM, Dip. ABVP, notes that 50 percent of PI calves will die within the first year of life, another 40 percent will die before two years of age, and only 10 percent make it to two years.
Before implementing testing procedures, it’s important for the veterinarian to know the goals of the producer. If the producer will take action and cull the PI animals, put biosecurity programs in place and vaccinate, then it’s worthwhile to test and cull.
Especially for small, closed herds, testing all calves, culling PIs (and PI dams) and continuing to test all new calves will eventually lead to a negative herd over time. “A lot of calf ranches are doing this,” says Cortese. “They test incoming calves for PI, and if they find any positives, they snip off two teats so when they are sold it indicates they are PI. Ideally, we would like them euthanized, but many places will just cut off teats and send them out.”
4. Testing animals
There are different tests available to find PI animals. One of them, PCR, still has some false negative and false positive problems, notes Cortese.
The microplate virus isolation test uses serum that is incubated in wells then put in an ELISA. This test, however, can be blocked by maternal antibody, so it’s unreliable in an animal under 3 months of age and can give a lot of false negatives. It also requires a confirmation test three weeks later.
The immunohistochemistry (IHC) “ear-notch” test will identify PI animals because they will have BVDV in higher levels of the skin than will acutely infected or MLV-vaccinated animals. “A single sample can tell whether a calf is PI,” says Cortese. This test is also not blocked by maternal antibodies.
Another advantage is that any piece of skin will do, such as a piece of belly skin in show animals.
Scott Smith, DVM, says two keys to preventing PI calves are to optimize the immunity in the breeding herd with effective, timely vaccination and to reduce potential exposure by testing new herd additions.
Skin samples must be read within a week because the sample will break down. Samples can be sent in empty serum tubes with a small amount of formaldehyde. This test is also the only test that can be done on dead animal skin to determine persistent infection. “If an animal is worth posting, I send in some skin, too, fresh or fixed, within a week,” says Cortese.
Veterinarians and their clients have to be careful, however, when labs indicate they have done an “ear-notch test.” Some labs take fluid from a sample and do an ELISA on it, which will not differentiate PI from an acute or MLV-vaccinated animal. It is not the same as the immunohistochemistry test. “You need to find out what test the lab is doing, especially if you’re dealing with expensive animals,” says Cortese. “Know which skin test they are doing – IHC or ELISA on fluids.”
Smith adds that it’s important that veterinarians realize there are two different types of ear-notch tests, and they should weigh the advantages and disadvantages of each one.
Use a BVDV vaccine that has known protection for Type I and Type II BVDV and fetal protection. “The goal of vaccination has changed,” says Cortese. “We know we can protect the cow and stop her from dying, but the difficult goal now is to protect the calf.”
For open heifers, Smith recommends two doses of a modified-live vaccine with both Type I and Type II strains two to four weeks apart. For potentially pregnant heifers with unknown vaccine history, he recommends a product with a killed BVDV component. “In milking cows, all of the herds I work with use an MLV product with both Type I and Type II,” he says. “Cows get a dose when first confirmed pregnant to obtain fetal protection against BVDV. A second dose is given at or just prior to dry-off.”
Smith also makes sure all bulls are tested for PI BVDV prior to entering the herd, and they are given two doses of an MLV 4-way with both Type I and Type II BVDV two to four weeks apart. In addition, all bulls are given a dose of this vaccine every six months.
Cortese says that if there is a BVDV outbreak with much disease and death loss, two things are occurring: a virulent strain of BVDV and, regardless of the history, somewhere the vaccination program broke down. “They either didn’t vaccinate them, mishandled the vaccine or something happened in their vaccination program.”
Smith adds that the two keys to preventing PI calves are to optimize the immunity in the breeding herd with effective, timely vaccination and to reduce potential exposure by testing new herd additions for PI status. “The best way to prevent acute BVDV infections in calf populations is early testing, identification and removal of PI calves,” he says. “A sound vaccination program will also reduce the amount of clinical BVDV in a calf population with circulating virus.”
What is BVDV?
BVDV belongs to the pestivirus group that includes Type I and II BVDV, hog cholera and border disease. Pesti-viruses, as a group, have the ability to cross-infect and cause disease in cloven-hoofed animals. “Fortunately, it has not learned how to jump to humans,” says Vic Cortese, DVM, PhD, Dipl. ABVP, “because BVDV has been one of the most common contaminants in human vaccines.”
All BVDVs are not alike. There are strains that are entirely respiratory in nature or reproductive in nature. Some strains are virulent, others less so. In the wild, normally all BVDV exists in a noncytopathic (NCP) state, meaning they leave surrounding cells intact. All cytopathic (CP) strains, which punch holes in other cells, arise from mutations of NCP BVDV.
It’s a common misperception that cytopathicity is related to virulence, but Cortese says that’s not the case. “Many virulent strains are NCP, and the CP strains usually occur in PI animals. Cytopathicity is not related to virulence.”
CP and NCP strains can only be differentiated in a lab – they cannot be distinguished by clinical signs.