Also Known As
Mucosal disease, Bovine viral diarrhea virus, BVDV
Description
This viral infection can affect cattle many ways. It can cause abortion, mummification of a fetus, birth defects, stillborn full-term calves, normal-looking calves with immune deficiencies, acute or chronic illness. It is also an indirect cause of many other diseases due to adverse effects on the immune system.
An estimate a few years ago stated that 80% of cattle in the U.S. have been exposed to BVDV and that 70 to 90% of infections go undetected, without visible signs. The only clue that there might be BVDV in a herd may be poor reproductive rate due to pregnancy losses, or a higher-than-normal rate of sickness in calves.
First descriptions of this disease in North America were in the 1940’s—outbreaks of highly fatal diarrhea (hence the name), digestive tract ulcers, nasal discharge and abortions. Many veterinarians feel this disease should be renamed, since diarrhea is just one aspect of the disease, and less common now with vaccination. In most herds today you won’t find an animal with diarrhea, but you might find BVDV.
Two different biotypes have different effects within body cells. These effects are either cytopathic (making obvious changes in cells and killing them) or non-cytopathic. The latter 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 non-cytopathic virus infects a wider range of body cells—including respiratory tract and blood cells as well as lymph tissue—and also persists longer in the animal.
The confusing thing is that there are also 2 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 is why there can be such variation in disease signs, and differences in how an animal’s immune system deals with the virus. Each genetic type also has both cytopathic and non-cytopathic versions. Thus there are several forms of the disease, ranging from sub-clinical infections with no obvious signs, to the 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% of BVD outbreaks and are the cause of persistently infected (PI) 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. Illness may be mild or severe, but temporary. The animal sheds the virus for up to 15 days after infection, then clears it from the body. These animals are not as 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, throughout their lives.
Pregnant cows that become infected may abort, or give birth to persistently infected (PI) calves, or calves that are stunted (low birthweight). Infection between 50 and 150 days (second trimester) may result in birth defects. Lung development may be incomplete in the newborn calf. Skeletal defects may include a too-short jaw or fused joints. Another defect sometimes seen is no hair, less than normal amount of hair, or curly hair.
Between 100 and 150 days’ gestation is when the fetus’ nervous system is in final stages of development; defects involving the nervous system include incomplete brain development (calves may be uncoordinated 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.
If pregnant animals have an acute infection and recover, they may abort 2 to 4 weeks after exposure, especially if they are in the second trimester. Exposure in the first trimester may result in early embryonic death. Open cows, if exposed, may fail to conceive when bred, and return to heat. Some cows, if exposed between approximately 60 and 120 days of pregnancy, may not lose their fetus, but rather may go on to deliver a persistently infected (PI) carrier calf.
The signs of BVD vary, depending on immune status of the animals, and the strain of the infecting virus. Incubation period is three to five days. If susceptible (non-vaccinated) animals are infected with a virulent strain of the virus, the disease often appears as an acute, severe illness. Some infected animals die, while others recover, usually within one or two weeks. Occasionally an animal dies very quickly, before other signs are apparent.
Mucosal Disease
The most serious form of BVD was first called mucosal disease and the main sign is diarrhea. This is a highly fatal impairment of the small intestine because the virus has a cytopathic effect, destroying the cells. Mucosal disease occurs only in cattle that were infected before birth with a non-cytopathic 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 cytopathic version of BVDV they cannot build immunity—and they can’t develop immunity from vaccination. 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 non-cytopathic and cytopathic BVDV in a PI animal results in mucosal disease.
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 non-cytopathic 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. Diarrhea usually develops 2 to 3 days after the animal shows signs of illness (off feed, weak and depressed, with fever). There are often lesions in the mouth (lips, tongue) or insides of the nostrils, which are slow to heal. Due to dehydration and emaciation, acute cases usually die in a few days or weeks. Some 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. The animal may also be lame.
By contrast, 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 non-cytopathic biotypes and generally causes death in PI animals.
When cattle are well vaccinated (as is more common today than when mucosal disease was a big problem) and not heavily stressed, the BVDV infection is usually much less severe, and often the only visible signs are sporadic abortions and/or repeat breeding. However, vaccinated cows exposed to the virus between approximately 60 and 120 days of pregnancy may still occasionally produce persistently infected carrier calves.
Signs
- High fever (105-107 degrees)
- Bloody diarrhea
- Lack of appetite
- Mouth ulcers
- Respiratory disease/pneumonia
- Bloat
- Lameness
- Abortion or early embryonic death in pregnant animals
- Birth defects in calves: short jaw,fused joints, no hair, blindness or cataracts,incomplete brain development,(incoordination, inability to stand)
Note: Not all animals with BVD have all signs
Causes
Contact with the BVD virus may cause disease in susceptible animals that have poor immunity. Cattle become exposed through contact with recently infected or persistently infected (carrier) animals that are shedding the virus. Cattle may also become infected via contact with contaminated fomites (inanimate objects with the virus on them), such as water buckets, calf feeders, feed bunks, IV equipment, nose leads, clothing, people who have handled infected animals, cattle trucks, etc.
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 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.
Cattle of all ages are susceptible to acute infection, but antibodies in colostrum from immune dams can prevent infection in young animals so BVD is seldom seen before 3 months of age. PI cattle (carriers) only develop in utero--if the dam is exposed to BVDV at a certain stage of pregnancy. An animal cannot become persistently infected after it is born.
If a cow is infected while pregnant, the virus usually crosses through the placenta to her unborn calf, even if the dam shows no signs of illness. Outcome of fetal infection depends on age of the fetus at the time it’s infected. If infection occurs prior to 100 days’ gestation, BVDV may cause birth defects or kill the fetus (absorption, mummification or abortion). The 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 big problem is when fetuses are infected by a non-cytopathic virus before they’ve developed a competent immune system (before 140-150 days’ gestation). They 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 immune response against it. The virus continues to multiply, infecting more cells, and is continuously excreted from the animal.
These calves may appear normal at birth, but continue to shed BVDV throughout their lives, acting as a source of infection for other cattle. These carriers are the biggest threat to herd health, and the main source of spread within and between farms/ranches, at a sale barn or in a feedlot.
The PI animal sheds high numbers of virus particles in nasal discharges, saliva, semen, milk, urine and tears. Feces also contain virus particles, but in lesser numbers. Acute BVD infections, by contrast, are less likely to spread the virus as much because the usual shedding period is only 7 to 15 days. The sick animal mounts an immune defense, clears the virus, and is no longer a source of infection.
A PI heifer 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 virus.
PI cattle are 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. Some might not be selected as replacements if they don’t grow as well as the other calves.
Diagnosis
BVDV infection is diagnosed on the basis of clinical signs plus confirmation through necropsy findings and laboratory tests of blood samples. If blood is drawn during acute phase of the disease, a laboratory can often isolate the virus from white blood cells. If two serum samples are obtained, one in the acute phase and one a few weeks later, a rise in serum antibodies between the two samples also confirms BVDV infection.
When abortion is the only sign, diagnosis may be more difficult. Your veterinarian should submit the aborted fetus and placenta, in addition to serum samples from the dam, to the laboratory for testing.
The persistently infected (PI) animal is usually easy to detect because it sheds so much virus that a viral antigen is readily found. Antibodies from colostrum may neutralize enough of the virus in a persistently infected calf to give a “false negative” result, however, in a young calf. Other test methods, including skin notch testing and whole blood viral DNA detection can be utilized in baby calves and older animals.
A small skin sample can be checked for presence of the virus. An ear notch is easy to collect; a ¼ inch square piece (size of a pencil eraser) is taken from an ear, using a pig ear-notcher or a hole punch. The sample is placed in a labeled container (such as a sandwich bag or blood vial) and sent to a diagnostic lab. 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 high 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.
Prevention
Prevention is based on maximizing immunity (with proper vaccination) and minimizing exposure of the herd (or susceptible animals in the herd) to the virus by avoiding contact with other cattle and by eliminating any PI cattle already in the herd, by testing.
The first year of testing, all calves, heifers and bulls should be tested before breeding season (and any cow that 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 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.
BVD vaccines are an important part of prevention, and available in two forms--modified live virus (MLV) and killed. Both have advantages and disadvantages.
An advantage of MLV vaccines is that they stimulate the entire immune system (both cell-mediated and humoral immunity). It is generally recommended that every animal receive a modified-live BVD vaccine at least once in its lifetime, preferably at 3-6 months of age. A disadvantage of MLV BVD vaccines is there may be label restrictions against using them on some or all pregnant animals.
An advantage of killed BVD vaccines is that they can be used on all open and pregnant animals, and the entire herd can be vaccinated at any time. Disadvantages of killed BVD vaccine include a shorter duration of immunity and reduced ability to stimulate cell-mediated immunity. Killed vaccines must be administered twice (two or three weeks apart) if the animal is being vaccinated for the first time. Only one vaccination is required thereafter at 4 to 6 month intervals.
Both types of vaccines, if administered properly and according to label, will provide sufficient immunity to prevent acute BVD. To maintain this level of immunity, the MLV vaccine needs to be given at least annually, and the killed vaccine needs to be given every 4-6 months.
Both types of vaccines can also help prevent transmission of BVD virus to the fetus and therefore prevent abortions and/or birth of PI calves. No vaccine can give 100% protection, however. It is still possible to occasionally have BVDV-induced abortions and/or persistently infected calves in a vaccinated herd exposed to a field strain of BVD.
If new animals will be entering the herd, vaccinate them at least a week prior to their coming, rather than waiting until they arrive in the herd.
There are several killed and MLV vaccines available, containing various strains and concentrations of BVD virus. Consult your herd health veterinarian to help you determine which vaccines to use, and when, in your own herd situation. Health programs using vaccination are aimed at preventing infection of the fetus, and this goal is harder to achieve than 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% 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 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 (so cows will 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; they 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 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 produces a stronger and longer-lasting immunity than some of the killed vaccines. Oil-based killed vaccines give good results, however, and produce better vaccine titers than either the MLV vaccines or the aluminum hydroxide based killed vaccine—and are also safer than MLV vaccine.
In many herds, annual booster (after the initial series 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, unless they have been previously vaccinated that year (such as pre-breeding) and already have some immunity. There’s no reason to use it in pregnant cows, however, since today there are better killed virus vaccines.
MLV vaccines are commonly given to young animals and once a year to the cow herd after calving and before rebreeding—preferably at least 3 weeks before breeding—to ensure strong immunity 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 also needs to be repeated more often. 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.
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 (MLV vaccine in the calf may pass the vaccine virus to the dam), 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. Working with their vets on a total herd health program, they’ve found that if the cow herd has strong immunity (due to annual vaccinations kept up to date) there’s very little risk of cows being affected by their calves’ vaccinations. Thus some ranchers 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 weaning, to prevent risk to the cows.
Keep in mind that even though combination IBR-BVD vaccines generally give calves good protection against BVDV 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. 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.
To determine if a herd has a BVD problem or risk for problems, look at current pregnancy rates and death loss and also whether new animals have been added, that might have brought the virus. 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.
BVDV can usually be detected in 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.
Once BVD occurs in the herd, all calves born over the next 12 months should be tested for persistent infection. If any carriers are found, they must be removed from the herd. It is best to test calves for persistent infection at birth or as soon as possible thereafter.
If BVDV gets into a non-vaccinated or improperly vaccinated herd, it will spread from animal to animal. Thus it is important to maintain a strong BVDV vaccination program that will minimize this type of transmission and allow containment of the virus before it infects a large portion of the herd.
Common ways BVDV comes into a herd
Purchase of animals through an auction market. They may be exposed to infected animals as they go through the market. Also they may be stressed, which lowers immunity. Thus they might be incubating and/or shedding the virus when they arrive. The fetuses of pregnant animals are at risk to become persistently infected or for abortion.
Introducing animals into the herd without first spending a week or two in isolation. If new additions are placed in isolation, animals incubating disease will become apparent before they expose the herd. A common history associated with herd outbreaks of BVD is that new animals entered the herd a week or so before the first case appeared.
Failure to maintain adequate vaccination against BVDV. If BVDV gets into a non-vaccinated or improperly-vaccinated herd, it will spread from animal to animal. Thus it is important to maintain a strong vaccination program that will minimize transmission before it infects a large portion of the herd.
Failure to test new additions to make sure that they are not BVDV carriers. BVDV carriers shed so much virus that they could overwhelm good vaccination programs.
Contaminated semen. If natural service is used, semen can be infected with BVDV, if the bull was recently infected or is persistently infected. It is unlikely for semen from certified artificial breeding establishments to contain the virus, since these bulls must be tested for BVD infection.]
Treatment
There is no specific treatment for BVD—either subclinical BVD or acute mucosal disease. Antibiotics are ineffective against viruses. Supportive therapy (fluids and anti-inflammatory agents) can be helpful for acutely infected animals to help them fight the infection. PI animals, whether diseased or apparently healthy, should be found through testing and removed from the herd. There is no treatment that will cure them.
Treatment of animals with mucosal disease is not recommended. 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 veterinarian for proper diagnosis, to know whether it’s worth trying to treat the sick animal or not.