BVD/MD

Pathogenic Agent

Which one is persistently infected?

Bovine viral diarrhea virus (BVDV)

Etiology

BVD/MD virus infection is a common disease in most countries. There are different forms of manifestation, depending on the time of transmission, the strain of infecting virus, and the individual immune status of the host. Several respiratory, reproductive, alimentary and skin problems can be traced to infection by BVDV. Cattle of all breeds, ages and sexes are susceptible to infection. Main sources of transmission are persistently infected cattle. By shedding the virus in secretions and excretions, they put susceptible animals at risk. Detection of virus carriers is the main problem in controling BVDV infection.

BVDV is able to cross the placental barrier and infect the fetus. Persistently viremic cattle have been infected in the dam by noncytopathic strains of BVDV, prior to achieving immunocompetence (i.e. before the 125th day of gestation). At that age, the fetus is not able to recognize the virus as a foreign body and does not produce antibodies to fight it.Infection by cytopathic strains of virus in early stages of pregnancy results in embryonic death, resorption of the fetus, or mummification. Infection of unprotected dams in midpregnancy by any strain of virus is followed by abortion, stillbirth, or birth of undersized, weak calves that die within a few days. Cerebellar hypoplasia and eye defects are often observed.

If infection occurs at advanced stages of pregnancy, the risk of abortion, stillbirth, and birth of undersized, weak calves diminishes because the fetus is able to produce antibodies that eliminate the virus. Infection of seronegative calves and non-pregnant cattle by either strain of virus results in an outbreak of BVD or is followed by subclinical infection, depending on individual immune capacity. BVDV is replicated in the cells of the alimentary tract, but also in leukocytes, reducing their number drastically for a period of one to two weeks. Deprived of important defense mechanisms, the animal is redisposed to various infections by other agents. The susceptibility to BRD and other diseases is increased.

Clinical Signs

Hydrocephalus

The signs vary depending on the time of infection, immune status of the host, and infecting strain of BVD virus.

Subclinical infection

This is the most frequent form of BVDV infection. Affected cattle have slightly reduced milk yields and sometimes increased body temperatures. Production of virus neutralizing antibodies in serum is followed by rapid recovery in a few days. The presence of antibodies provides protection against clinical disease for years.

Acute BVD

This form occurs in cattle that are infected after they achieve immunocompetence. Following an incubation period of 5 to 7 days body temperature rises to 40° to 41°C (104-106°F), and returns to normal after about two days. About this time a profound watery diarrhea occurs.The signs include anorexia, depression and fatigue. Some cattle die of dehydration but mortality is low. Antibodies can be found in serum of recovered animals.

Respiratory disease

Immunosuppression caused by BVDV infection is followed by increased susceptibility to infection and outbreak of diseases caused by other agents, particularly those that cause bovine respiratory diseases. Respiratory infections are more severe, courses take longer, and treatment often fails if animals have a history of prior BVDV infection. Calves aged 2 months and older are most susceptible, because of decreased protection by maternal antibodies.

Acute mucosal disease- acute MD

This form of disease can only be observed in cattle that have been infected by noncytopathic strains of BVDV prior to achievement of immunocompetence (before the 125th day of gestation). At that age, the fetus is not able to recognize the virus as a foreign body, and does not produce antibodies. Infected animals remain virus carriers for life. Some infected calves are developmentally delayed, have reduced daily gains and rough hair coats. Unfortunately, most viremic cattle can not be distinguished from herdmates. They survive for varying periods up to adulthood until they develop fatal mucosal disease. According to latest research, the outbreak of acute MD occurs subsequent to mutation of noncytopathic to cytopathic strain of virus in persistently viremic cattle or after superinfection by cytopathic strains. Clinical signs include profound depression, anorexia, rumenstatis, profuse and watery diarrhea. Feces have a foul odor and often contain mucus and blood. Erosions in the entire oral cavity are significant. It becomes ulcerated, leaving denuded, painful areas of mucosa. Erosions may also appear on the muzzle and interdigital space. Body temperatures rise up to 104 - 106°F (40° to 41°C). Acute mucosal disease is always fatal. Death due to dehydration occurs 5 to 7 days after the onset of clinical disease.

Chronic mucosal disease

Chronic MD is also only developed by persistently viremic cattle and signs are similar to acute MD. There is research required to reveal why some viremic cattle develop chronic courses and other acute MD. The course may take months. Affected animals finally die due to starvation.

Hemorrhagic syndrome

This is a peracute, not very frequent form of infection by noncytopathic strains of BVDV. It was only lately discovered to be part of the BVD/MD complex and research is still ongoing. Affected animals die within hours from blood loss due to hemorrhagic diathesis.

Diagnosis

Cerebellar hypoplasia

Diagnosis on individual animals can be confirmed by virus isolation from blood samples (EDTA) or nasal swabs, increased antibody titers in paired serum samples or at necropsy by finding shallow erosions confined to the alimentary tract. It is of enormous importance to survey the livestock for virus carriers whenever you suspect BVDV infection in a single animal. Serum samples of at least 10 cattle aged 6 to 24 months should be analyzed for antibodies at the laboratory. If antibodies are found, it is essential to find and eliminate the virus carriers among the livestock. Send blood samples (EDTA) of all cattle up to the age of 3 years to your lab for virus detection.

Differential Diagnosis

IBR, BRSV infection, infectious bovine enzootic bronchopneumonia, bovine malignant catarrh, FMD, vesicular stomatitis, salmonellosis, Johne's disease, rinderpest.

Prevention

Prevalence and course of most viral infections is influenced by livestock living conditions.

It is essential to evaluate the present immune status and thereby disease prevalence of the livestock prior to any program. Collect blood samples of at least 10 cattle aged 6 to 24 months and send them to your laboratory for detection of antibodies. If antibodies are discovered, it is essential to find and eliminate virus carriers in the herd. Send blood samples (EDTA) of all cattle up to the age of 3 years to your lab for virus detection. Without immediate elimination of virus carriers, vaccination will not be successful. The presence of even one infected animal will put all livestock at risk. Closed herds require estimating economic gains of implementing a vaccination program.

Vaccination program/ metaphylaxis

• Vaccinate the livestock with an inactivated vaccine
• 
  
• revaccinate after 4 weeks
• vaccinate all cattle annually 6 weeks prior to breeding season with modified live vaccine
• make sure that new animals in the herd have been vaccinated twice with live vaccine at monthly intervals before introduction to the herd.

The livestock are also at risk, if there are no antibodies discovered. If these cattle are exposed to BVDV (e.g. by virus carriers among new arrivals) they are unprotected. New arrivals should be isolated and screened with a virus isolation test (EDTA samples) prior to introduction into the herd. Virological testing and culling of positive newborn calves is essential, as is a regular herd screening test. Because of the high prevalence of BVDV in many areas of the world the risk of contact with BVDV may be great in herds that are proven free of the pathogenic agent and therefore susceptible to infection. Subsequent economical losses require consideration of a Vaccination program/ prophylactics

• vaccinate calves at the age of 3 months with modified live vaccine
• revaccinate with live vaccine at the age of 5 months
• use live vaccine again 6 weeks prior to breeding
• annually revaccinate with inactivated vaccine to maintain vaccination status

...or according to recommendations by your vaccine manufacturer Initial administration of inactive vaccine reduces temporary shedding of virus subsequent to modified live vaccination.

some tips

• only live vaccines provide a reliable and lasting protection
• f systemic infection and thus fetal infection
• pregnant cows should not be vaccinated with live vaccine
• pregnant cows should not contact recently live vaccinated cattle, because of their transient shedding of vaccine virus
• live vaccine may cause an outbreak of mucosal disease in persistently infected calves
  ...or according to recommendations by your vaccine manufacturer
  • avoid mechanical transmission
  • fly control
  • disinfection

Treatment

There is no specific treatment. Supportive therapy is indicated, but prognosis for severe cases with profuse watery diarrhea and marked oral lesions or chronic MD is unfavorable. Slaughter or euthanasia should be considered.

Literature

• Bezek, D.M., Bovine Virus Diarrhea Virus Infection: Individual and Herd Diagnosis, Compendium on Continuing Education, Vol. 17, No 8, August 1995
• Blood, D.C., Radostis, O.M., Veterinary Medicine, 1989
• Franck, R., Which One Has BVD ? Dairy Herd Management, October 1994
• Fraser, C.M. et al., The Merck Veterinary Manual, 1991
• Frey, H.-R. et al, Praevalenz und klinische Symptomatik persistenter BVD-Virusinfektionen in Rinderbestaenden Niedersachsens, Der praktische Tierarzt, Januar 1996
• Gardner, C., BVD - an "inside job" Dairy Herd Management, September 1993
• Mantey, S., Heifer Enemy No.1, Dairy Herd Management, December 1993
• Quaife, T., Don't Let The Wrong Animals In Dairy Herd Management, August 1995
• Smith, B.P., Large Animal Internal Medicine, 1990
• Wolf, G. et al, Impfindikation und Impfstrategie bei BVD, Der praktische Tierarzt, Juni 1996