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The virus is an elusive villain because there are 2 different biotypes that have different effects within body cells. These effects are called cyptopathic (making obvious changes in the cells and killing them) and noncytopathic. The latter category of virus does not destroy or change the shape of the cells. The main difference is that the cytopathic virus infects cells in the lymph tissues of the gut, while the noncytopathic virus infects a wider range of body cells—including respiratory tract and blood cells as well as lymphoid tissue—and also persists longer in the animal.

The confusing thing is that there are also 2 basic genetic types (Type I and Type II) of the virus, and both can be present in the same animal. They can also change their genetic composition during multiplication. This explains why there can be such variation in disease symptoms and differences in how an animal’s immune system deals with the virus. Each type also has both cytopathic and noncytopathic versions. Thus there are several forms of the disease, ranging from sub-clinical infections in which the animal shows no obvious signs, to a severe and highly fatal form called mucosal disease, characterized by diarrhea.

BVDV can affect digestive, respiratory, nervous, reproductive and immune systems. Non-cytopathic viruses cause more than 90 percent of BVD outbreaks and are always the cause of persistently infected cattle. BVDV can mutate or change and since there are several strains, the infected animal may or may not be able to mount a protective immune response or be protected by vaccination. Type I and Type II viruses can produce different disease signs.

The cytopathic biotype generally causes acute rather than chronic infection. The illness may be mild or severe, but it’s temporary. The animal sheds the virus for up to 15 days after infection, then clears it from the body. These animals are not a big a threat to herd health because they don’t continue to shed the virus. By contrast, persistently infected (PI) animals--infected before birth when the pregnant cow is exposed to the virus--shed larger numbers of virus particles in body secretions, and continue to do so throughout their lives.

Mucosal Disease
The most serious form of BVD was first called mucosal disease and the main symptom is diarrhea. This is a highly fatal impairment of the small intestine. The virus has a cytopathic effect, destroying the cells. Mucosal disease occurs only in cattle that were infected before birth with a noncytopathic version of the virus. These calves may seem normal at birth but have no immunity to the virus and are persistently infected (PI). If at some point they come into contact with a cyptopathic version of BVDV they cannot build immunity. They can’t develop immunity from vaccination, either. So they are vulnerable to severe effects of the cell-killing version of the virus if it’s a strain closely related to the persistently infecting virus. But not every combination of noncytopathic and cytopathic BVDV in a PI animal results in mucosal disase.

Cattle 6 months to 2 years of age are most likely to develop mucosal disease. Though only a small percentage of a herd may be affected, nearly all affected animals die. Most of these were persistently infected before birth with a strain of noncytopathic virus and then became exposed to an animal shedding a cytopathic BVD virus of the same strain. Mucosal disease often causes profuse watery diarrhea which may contain blood and intestinal lining. The diarrhea usually develops 2 to 3 days after the animal shows symptoms of illness (goes off feed and becomes weak and depressed, with fever). There are often lesions in the mouth (lips, tongue) or insides of the nostrils. Due to dehydration and emaciation, acute cases usually die in a few days or weeks. A few become chronic and may survive many months, becoming more and more emaciated.

Diarrhea in chronic cases may be continuous or intermittent and some animals develop chronic bloat. Lesions in the mouth are slow to heal. The animal may also be lame. Treatment is not recommended because even if there’s a slight chance for recovery, the animal will be infected for life. Most veterinarians recommend humane destruction. It’s important to work with your vet for proper diagnosis, to know whether it’s worth trying to treat the sick animal or not. Diarrhea from acute and temporary BVD infection caused by a cytopathic biotype of the virus is usually mild and followed by rapid recovery. But mucosal disease (which may be acute or chronic) is the result of dual infection with both cytopathic and noncytopathic biotypes and generally causes death in PI animals.

The Problem of Persistent Infections
If a cow is infected while pregnant, the virus usually crosses through the placenta to her unborn calf, even if the dam does not show signs of illness. Outcome of fetal infection depends on the age of the fetus at the time it’s infected. If infection occurs prior to 100 days of gestation, BVDV may cause birth defects or death of the fetus (absorption, mummification or abortion). The dead fetus may be aborted at the time of infection or up to several months later.

Calves infected during the third trimester (after 150 days) may be normal at birth, having developed immunity. If a blood test is taken, they have a titer to BVDV. Their immune systems were mature enough at the time of infection to create antibodies to protect themselves and fight off the virus.

The biggest problem is when fetuses are infected by a non-cytopathic virus before they’ve developed a competent immune system (before 140-150 days of gestation). These calves may end up persistently infected with BVDV because their immune systems do not recognize the virus as anything other than a part of their own body. The calf’s body therefore tolerates it as something normal rather than foreign and does not produce any immune response against it. The virus continues to multiply, infecting more cells, and being excreted from the animal. These calves may appear normal and healthy at birth, but continue to shed BVDV throughout their lives. They continually act as a source of infection for other cattle. These carriers of BVDV are the biggest threat to herd health, and the main source of spread within and between farms/ranches or in a feedlot.

The PI animal sheds high numbers of virus particles in nasal discharges, saliva, semen, milk, urine and tears. Feces contain virus particles, but in lesser numbers. Acute BVD infections, by contrast, are less likely to spread the virus because even though the animal is ill the usual shedding period is only 7 to 10 days. Then the sick animal mounts an immune defense, clears the virus, and is no longer a source of infection.

If you keep a PI heifer in your herd, however, she sheds BVDV for the rest of her life, and all her calves become infected during her pregnancies. This is one of the main ways BVDV persists within a herd, and these PI cattle are always at risk for mucosal disease if they encounter a cytopathic form of the virus.

PI cattle are also at risk for other diseases, since BVDV in their bodies hinders the immune system. PI calves have higher incidence of scours and pneumonia, and higher mortality rate. They may be unthrifty and won’t gain well. Heifers may not be selected as replacements just because they don’t grow as well as the other calves.

Diagnosis and Testing for BVDV
To determine if your herd has a BVD problem or a risk for problems, look at your current pregnancy rates and death loss and also whether you’ve added new animals that might have brought the virus to your herd. Tests can be done, to find and remove any PI animals before the next breeding season, so no females will be exposed during breeding and pregnancy. This will eliminate the risk of having any PI calves born in your herd.

The point to remember is that PI animals are created before birth, when the dam encounters BVDV between 40 and 125 days of gestation. Once a calf is PI it will always be PI. If a calf is not PI at birth, it can never become PI. After any animal has tested negative for BVDV, it never needs to be tested again for PI status. The key to eliminating PI animals is to protect the cow herd from BVDV during pregnancy, so no fetuses ever become exposed.

If BVDV is present, it can usually be isolated from a blood sample, from tissue of an aborted fetus, or from an infected weak or unthrifty calf. You might decide to test a cow because she aborted or gave birth to a weak or abnormal calf. To screen a herd, most testing programs focus first on finding any PI calves in the new calf crop, before the next breeding season—before an infected calf can spread the virus to a pregnant cow. If a calf is found to be PI, its dam can be checked, too.

A small skin sample can be checked for presence of the virus. Since PI cattle have high levels of virus in their bodies, it can be readily detected. An ear notch is easy to collect as a sample. A ¼ inch square piece (about the size of a pencil eraser) is taken from an ear, using a pig ear notcher or a large diameter hole punch. The sample is placed in a labeled container such as a sandwich bag or blood vial and sent to a diagnostic lab. Your vet can collect the ear notches if you wish. Samples must be kept cool but don’t need to be frozen. Any animal that tests positive should be checked again 4 weeks later to see if it had an acute infection at the time of testing or is persistently infected. Virus level will be dropping by the second test if the animal had an acute infection (and the animal will recover and get rid of the virus) but virus level will remain at a high level if the animal is PI. All PI animals should be culled—making sure they go to slaughter rather than ending up in someone else’s herd.

The first year of testing, all calves, heifers and bulls should be tested before breeding season (and any cow that may have lost a calf). If tissue samples are taken from suckling calves before the cows are rebred, you can then test the dam of any calf of any calf that tests positive. If a calf tests negative, the dam is ok. All PI calves and their dams should be removed from the herd before they come into contact with any pregnant animals. This eliminates the risk of passing the virus to a fetus. If you can keep BVDV from reaching a fetus, there will be no more PI calves produced. In subsequent years, only the young calves in a herd and any newly purchased animals need to be tested. An individual only needs to be tested once. If it is negative, it will never become PI.

In addition to testing live animals, it pays to necropsy any calves that die of scours, pneumonia or any other cause, along with any stillborn calves or aborted fetuses. If a calf is PI, this means the dam was infected. The best way to eliminate BVDV is to check all animals in the herd at the beginning of an eradication program, cull any shown to be PI, then keep the herd on a good vaccination program. If you can keep the virus from reaching a fetus—making sure all pregnant cows have strong immunity so they don’t get BVD while pregnant—your herd will never produce a PI calf. No new animals should be added unless they are tested to make sure they are free of BVDV.

Vaccination
There are several different killed and modified live virus vaccines containing various strains and concentrations of BVD virus. Health programs using vaccination are aimed at preventing infection of the fetus, and this goal is harder to achieve that just protecting an animal against acute BVD infection. Calfhood vaccination (if properly boostered to stimulate adequate immunity, then boostered annually with modified live virus vaccine or semi-annually with killed vaccine) will usually protect the animal itself from BVDV unless it was born persistently infected.

It’s harder to obtain 100 percent protection for the fetus if the dam is exposed to the virus. She may have enough immunity from previous exposure or vaccination that she won’t get sick, but the virus can still, in some cases, infect the fetus. If the cow is exposed to BVDV in early pregnancy, the damage is done. Annual fall vaccination of pregnant cows (using a killed vaccine that’s safe for pregnant cows) will not correct a problem that’s already occurred. The best protection against BVDV is to eliminate all sources of infection within a herd, keep up a regular vaccination program using modified live virus vaccine ahead of breeding or an oil-based killed virus vaccine (so cows have strongest possible protection in early pregnancy) and never let cattle become exposed to infected animals.

In a herd that’s already experienced BVD problems, starting a vaccination program may not show immediate results unless you also test for and eliminate PI animals. Those individuals can’t produce immune response to vaccination and continue to carry and shed the virus. Without testing, it may take 3 years of vaccinating and culling (removing any cows that are late breeders or those that produce unthrifty calves) to rid a herd of BVDV.

Modified live virus (MLV) vaccine contains the virus itself but it has been altered so it won’t cause disease. It retains some of its original characteristics, however, so the animal’s body will recognize it and mount a strong immune defense, just as it would do if confronted with the actual disease. It thus produces a stronger and longer-lasting immunity that some of the killed vaccines. Oil based killed vaccines give good results, however, and according to Dr. Ron Skinner (a veterinarian and seedstock producer near Hall, Montana, who has done extensive research on BVDV during the past 30 years), the oil based killed vaccines produce better vaccine titers than either the modified live vaccines or the aluminum hydroxide based killed vaccine—and are also safer than modified live virus. He has seen several “wrecks” with modified live vaccines in which the vaccines were flawed and actually gave the cattle BVD.
In many herds, an annual booster (after the initial series of shots in a young animal) with modified live vaccine is enough to keep cows protected. Because the modified live vaccine is so potent, however, it should only be given to non-pregnant cows. Otherwise there’s risk that giving it to a pregnant cow may infect her fetus, unless you are vaccinating her during the last trimester. You’d have to be sure of the cow’s stage of pregnancy (such as by rectal palpation) to be safe using this vaccine, and only do it on the advice of your vet because this would be off-label use. There’s no reason to use it in pregnant cows now, however, since today there are better killed virus vaccines (oil based) that some vets feel are just as effective as modified live virus vaccines.

Label directions for MLV vaccines state that they be given only to non-pregnant animals and to calves not nursing pregnant dams. There’s always some risk that a vaccinated calf still nursing its dam (vaccinated pre-weaning) might pass the virus to the cow and infect her fetus. If you vaccinate your calves pre-weaning, most vets recommend using killed vaccines, even though they require a booster later.

MLV vaccines are commonly given to young animals (after weaning) and once a year to the cow herd after calving and before rebreeding—preferably at least 3 weeks before breeding—to ensure that strong immunity has developed before pregnancy begins. MLV vaccine given at the time of breeding or soon after (to cows with no prior immunity) may result in death of the embryo. And there are a few cows that when vaccinated with MLV vaccine pre-breeding will carry the virus in their ovarian tissue (around the ovary) for 60 to 90 days, and this may hinder their ability to become pregnant. Some veterinarians recommend use of killed vaccines, instead, for these reasons.

A killed vaccine cannot replicate; the virus can’t reproduce in body tissues and there’s no chance for it to cause disease. It’s safe for pregnant cows or for calves nursing pregnant dams. But some types of killed vaccine don’t give as strong nor as long-lasting protection (because they don’t stimulate the immune system as much). Most vets recommend giving killed vaccine twice a year to cows. Calfhood vaccinations also require more boosters. Most killed vaccines need 2 doses at least 2 weeks apart to confer immunity.

Some veterinarians feel that the most protection can be gained by giving MLV vaccine to cows after calving (and at least 3 weeks before rebreeding), and then using a killed vaccine in the fall on pregnant cows as a booster. This may give more immunity through winter, and better levels of antibodies in colostrum at the next calving.

Calfhood Vaccination for BVD
Many stockmen vaccinate calves pre-weaning with BVD-IBR vaccines to develop immunity before calves are stressed. Killed vaccine is generally used pre-weaning so dams are not at risk, even though the IBR portion of the vaccine doesn’t give calves as much (nor as long lasting) protection as the BVD portion and must be boostered. Some stockmen use MLV vaccines for calves pre-weaning even though it’s not recommended on the label. Working with their vets on a total herd health program, they’ve found that if the herd has strong immunity (due to annual vaccinations kept up to date) there’s little risk of cows being affected by their calves’ vaccinations. Thus some ranchers, on the advice of their vets, use MLV vaccine in pre-weaning programs for calves, without need for an additional booster. If immune status of the herd is in question, however, MLV vaccine should not be given to calves until after they are weaned. Keep in mind, however, that even though combination IBR-BVD vaccines generally give calves good protection against BVD they may be inadequate to protect against IBR. There can still be latent carriers, and neither the modified live nor killed products can solve that problem.

Best protection against BVDV for young heifers or bulls going into the herd requires 3 vaccinations (preferably MLV or oil based killed products) before breeding age. The first injection can be given at 1 to 2 months of age before the dam is bred again (such as at branding time), the second at weaning age, and another booster just before the animal’s first breeding season. This gives a heifer the strongest protection against becoming infected in early pregnancy. After that, annual pre-breeding vaccination with MLV is usually adequate for the rest of her life.

A 1992 study showed that use of killed IBR-BVD vaccine at 1 to 3 months of age resulted in good immunity to BVD when boostered just before weaning. First dose of a killed vaccine generally gives no protection, but boostering an oil-based vaccine 30 days later (optimal time for a booster, for best immunity) stimulates the calf to build immunity. Stockmen who use MLV vaccine on young calves instead of killed vaccine found that calves develop immunity after only one dose—and have less calfhood illness (such as scours, pneumonia, diphtheria, pinkeye, ruptured stomachs from abomasal ulcers, etc.)

If calves are not vaccinated until after bulls have been put with the cows, killed vaccine is much safer, but water based or aluminum hydroxide based adjuvants won’t give the calves any protection until they receive a booster at or before weaning. You do get some protection with one shot, using an oil-based killed product, according to Dr. Skinner. Even if you use MLV vaccine in young calves, a booster at weaning time is essential because immunity in the young calf may last only a few months.

Note, much of this material will appear in my upcoming book on Cattle Health Care, to be published by Storey in late 2008, and was checked for accuracy by Ron Skinner, DVM, at Hall, Montana.

WAYS TO PREVENT INTRODUCTION OF BVDV INTO YOUR HERD
Any cattle coming to your place or added to your herd (even home-raised heifers) should be tested to see if they are PI, and kept away from the breeding herd until test results are received. One of the primary ways BVDV is introduced or perpetuated on farms is by bringing in new animals (purchased bulls, cows, heifers, pairs, stocker cattle, calves to graft onto cows that lost calves) without testing them. PI cattle can be hard to recognize because they rarely show visible signs, yet continually shed the virus.

Any cow or heifer that conceived a calf somewhere else (bred on the range or in a communal pasture, or purchased as a bred animal) should be isolated until her calf is born and tested. Purchased pregnant animals can be PI negative themselves and still carry a PI fetus if the dam was infected during pregnancy; she may have recovered from the infection that affected her fetus and she’ll test negative. But her calf will be PI and can expose your cattle to BVDV after it’s born.

Avoid using communal pastures and try to minimize or eliminate fenceline contact between your herd and other cattle during the time of year your cows are in early to mid pregnancy. Make sure your females are adequately vaccinated to provide a high degree of protection in case they do become exposed to BVDV during pregnancy.

BVDV CAN BE SPREAD BY BREEDING, AND BY PREG-CHECKING
Semen from infected bulls may contain the virus. Reduced conception rates may occur in otherwise healthy cows that were bred to (or inseminated with semen from) PI bulls. Bulls affected with acute BVD (as opposed to being PI) may shed the virus in their semen for a while after being infected. Infection at the time of breeding may result in reduced pregnancy rates due to embryo or fetal loss.

BVDV can also be spread from infected females to susceptible females via rectal palpation. A veterinarian using the same obstetrical glove on multiple animals when preg-checking rectally may spread the virus from one animal to another. Some stockmen are now using a blood test to check for pregnancy rather than using rectal palpation, not only because pregnancy can usually be determined sooner, but also because it’s safer for the cow and her fetus and has no risk for spreading disease from animal to animal.

BIRTH DEFECTS DUE TO BVDV
Infection at any stage of gestation may retard fetal growth, resulting in poor bone growth and low birth weight. Infection between 50 and 150 days (roughly the second trimester) may result in birth defects rather than killing the fetus. Lung development may be incomplete. Skeletal defects that may occur include a jaw too short or fused joints. Another defect sometimes seen is no hair, or less than normal amount of hair, or curly hair. Between 100 and 150 days is the stage when the fetus’ nervous system is in final stages of development; defects involving the nervous system include incomplete brain development (calves may be incoordinated and have trouble standing up), water on the brain, and other brain problems. Defects involving the eyes include cataracts, opaque cornea, inflammation of the optic nerve, atrophy or abnormality of the retina, and varying degrees of blindness in the newborn calf.

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