Cattle Today

Cattle Today



by: Heather Smith Thomas

Bovine viral diarrhea virus (BVDV) is widespread in cattle, with a high number of animals testing positive to this disease. It has been estimated that 80 percent of cattle in this country have been exposed to BVD, and that 70 to 90 percent of infections go undetected, without visible symptoms. It is the most costly viral disease in cattle, inhibiting conception, causing abortion and birth defects, and hindering the immune system -- making the animals more susceptible to other diseases. In the U.S. cattle industry BVD is costing producers an estimated $2 billion per year.

The first descriptions of BVD in North America (outbreaks of diarrhea in some herds, and erosive lesions in the digestive tract -- sometimes with nasal discharge, drooling, diarrhea and abortion) were reported more than 60 years ago. It got its name from the profuse, watery diarrhea shown by weanling/yearling age cattle, but we later learned that this was only one of many forms of disease caused by this virus. The first attempts to isolate the cause of these various problems were not successful; medical technology was not advanced enough to detect and identify the virus. Eventually researchers found that all these symptoms were caused by one virus.

TYPE I AND TYPE II VIRUS, STRAINS AND CYTOPATHIC/NON-CYTOPATHIC -- The BVD virus is an elusive villian because it comes in two forms that have different effects within the body cells (cytopathic and non-cytopathic). There are also two basic genetic types of the virus (and several strains within each type), and each of these types or strains may fall into either one of these categories regarding how they affect the cells.

When scientists were trying to isolate the virus, they used certain body tissue cells to culture it, and discovered that the cultures didn't respond the same way. Some cells became infected but were not changed. In other cultures, the virus caused obvious changes in the shape of the cells and killed them. This version of BVD virus was termed cytopathic, meaning that it killed the cells. The other version of BVD virus was called non-cytopathic, meaning that it did not destroy or change the shape of the cells.

The two basic genetic types of the virus (Type I and Type II) are about 70 percent similar in genetic make-up and can both be present in the same animal. They can also change their genetic composition during the process of multiplication. This explains why there can be such a variation in disease symptoms and in how the animal's immune system deals with the virus. Each type also has both cytopathic and non-cytopathic characteristics.

Thus there are several forms of BVD, ranging from subclinical infections (in which the animal shows no obvious signs) to a severe and highly fatal form called mucosal disease. BVD can affect the digestive, respiratory, immune, nervous and reproductive systems. It can cause abortion in cows, stillborn calves, calves born with birth defects or with immune deficiencies or persistent infection.

The BVD virus can mutate or change somewhat as it multiplies, and there are several strains of the virus, which means that the infected animal may or may not be able to mount a protective immune response or be protected from the virus by vaccination. This is why Type I and Type II viruses can produce different disease signs even though they are both BVD viruses. Since they can also both be present in the same animal this complicates the disease picture.

Some scientists subdivide the two types into more strains, with further subtle changes in genetic composition of each type. Because the BVD virus can be so varied, this is the reason some infections do not cause obvious disease, while others affect the animal's ability to develop an immune response to fight off the disease, and why BVD can appear in different systems (digestive, respiratory, reproductive, etc.) in different situations.

MUCOSAL DISEASE -- The most serious form of BVD was first called mucosal disease -- a sporadically occurring, highly fatal disease of the small intestine, in which the virus has a cytopathic effect (changing and destroying the cells). Researchers eventually discovered that mucusal disease only occurs in cattle that are infected before birth with a non-cytopathic version of the virus (they seem normal, but are persistently infected and have no immunity to the virus) and then encounter a cytopathic type of BVD virus. Since these animals cannot develop immunity (even if vaccinated), they are vulnerable to the severe effects of the cell- killing version of the virus, if that virus is a strain that is closely related to the persistently infecting virus. Not every combination of non-cytopathic and cytopathic BVD virus in a PI (persistently infected) animal results in mucosal disease however.

Mucosal disease has several forms. There is an acute form with a very high mortality rate and a chronic form that also has high mortality rate but a much longer course. Cattle most likely to develop mucosal disease are usually six months to two years old, and though only a small percentage of the herd may be affected, nearly all of the affected animals die. Often these young animals were persistently infected with the same strain of non-cytopathic virus and were then exposed to an animal with a cytopathic virus.

Mucosal disease is often characterized by profuse, watery diarrhea (which may contain blood and gut lining), which generally develops two to three days after the animal exhibits fever, weakness, depression and lack of appetite. There are usually lesions in the mouth that may involve the lips and tongue, or even the nasal cavity. The animal becomes emaciated and dehydrated. Acute cases usually die within a few days or weeks. A few become chronic and may survive for up to 18 months, becoming progressively emaciated. Diarrhea in these cases may be continuous or intermittent and some animals may develop chronic bloat. Lesions in the mouth and skin are slow to heal and the animal may become lame. Treatment is not recommended because even if there is a slight chance for recovery, the animal will be infected for life and should be culled.

INFECTIONS SPREAD VIA BREEDING -- Semen from bulls that are infected may contain the virus. Reduced conception rates have been reported in otherwise healthy cows that were bred to (or inseminated with semen from) persistently infected bulls. Bulls affected with acute BVD may shed the virus in their semen for awhile after being infected. BVD infection at the time of breeding can result in reduced pregnancy rates due to significant embryo or fetal loss.

SIDEBAR: BIRTH DEFECTS CAUSED BY BVD -- Infection of the fetus any time between 100 to 150 days of gestation can result in a variety of congenital defects, since this stage is when the nervous system is in its final stages of development and the fetal immune system is developing. BVD infection at any stage of gestation may retard fetal growth, resulting in lower birth weight and poor bone growth. Lung development may be incomplete. Skeletal defects may include a jaw too short, or fused joints. Another defect sometimes seen is less than normal amount of hair, curly hair, or hairlessness.

Defects involving the nervous system include inadequate brain development, incomplete development of the cerebellum (the portion of the brain involved with coordination of movements —affected calves have trouble standing up), water on the brain and other brain problems. Congenital defects involving the eyes may include cataracts, opaque cornea, inflammation of the optic nerve, atrophy or abnormality of the retina, and varying degrees of blindness.

SIDEBAR: BVD CAN BE THE UNDERLYING CAUSE OF OTHER DISEASE OUTBREAKS Since BVD infection suppresses the immune system, infected cattle (especially young calves) may have a higher incidence of other diseases, including pnuemonia, scours, pinkeye, footrot, diptheria, etc. Pregnant cattle may abort--with outbreaks of lepto, IBR and other diseases -- simply because they were not able to develop immunity to those diseases. BVD infection can be difficult to diagnose because it shows up in so many different ways. If a rancher has problems with several types of calf scours and a high incidence of respiratory diseases, this may be a clue that BVD is part of the problem. And besides affecting the immune system, BVD by itself can cause illness in calves.

A herd health program may be ineffective because BVD infected cattle don't mount a very good response to vaccinations. So even if a rancher diligently vaccinates against lepto, IBR, pinkeye and other common diseases, some of the vaccinated cattle may develop those diseases. The rancher may think the vaccine didn't work, when in reality the animal was unable to develop a good response.

In one study, BVD virus was the virus most often found in the lungs of feedlot cattle with pneumonia, and was usually found in conjunction with Pasteurella. Infection with the BVD virus has been associated with In outbreaks of respiratory disease complexes in feedlots, and on the ranch. In young calves with multiple viral infections, BVD virus is the most frequently found pathogen. Infection with BVD virus has been shown to impair the ability of calves to fight lung infections caused by bovine herpesvirus 1 (infectious bovine rhinotracheitis or IBR). Many vaccines combine IBR and BVD, to give protection against both of these viruses.


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