During the mid-1990s, reports indicated that cattle herds in California, Kentucky, Michigan, New York, Ohio, Pennsylvania and Wisconsin were being affected by an atypical Bovine Viral Diarrheal (BVD) virus.
It caused disease in cows, calves and heifers and higher-than-expected morbidity (illness) and mortality (death).
Dairy herds showed high mortality and clinical signs including high fever (107 degrees or higher), reduced feed intake, decreased milk production, occasional diarrhea, respiratory signs and death within 48 hours of onset.
BVD virus was isolated in many of these herds.
Two kinds. According to the USDA Animal Plant Health Inspection Service, two distinct biotypes of BVD virus have been identified: cytopathic and noncytopathic, and both biotypes occur in each of the genomic (genetic) types (type I and type II).
The two viral genotypes share some antigenic determinants, in other words, antibody against type I BVD virus will cause immunity to type II BVD, but less completely than to type I BVD virus.
Immunity. According to information from Auburn University’s Kenny Brock, formerly a researcher at the Ohio Agricultural Research and Development Center, there is considerable antigenic diversity among BVD virus strains, and vaccines should provide immunity against these diverse strains.
Due to the wide ranging genetic diversity of BVD virus, a multivalent vaccine containing at least 2 antigenically different strains is recommended.
Most vaccine manufacturers have developed BVD vaccines containing antigens that confer type I and type II BVD immunity.
Vaccination is essential to the prevention and control of BVD.
Select vaccine. Managers should consider several factors in selection and use of vaccines:
* Purpose of vaccination;
* Management factors and timing of vaccination;
* Risk of disease; and
* Type and age of animal.
Different vaccination practices may be recommended for different groups of animals in a dairy herd.
Vaccination of weaned calves and growing animals is primarily directed toward preventing acute infections and associated respiratory tract infections.
Vaccination of breeding animals such as cows and replacement heifers is directed toward preventing acute infections plus providing protection of the fetus during gestation.
Breaking cycle. In order to break the vicious cycle of persistent infections, vaccination of cows and heifers must provide protection against fetal infection.
However, vaccination of pregnant animals will not provide complete protection of the fetus.
Therefore, it is necessary to prevent exposure of pregnant animals during the first trimester of gestation.
Since there are pregnant animals on most dairy farms at all times, it is important to prevent exposure of the herd to infected animals at all times.
Prevention. According to Brock, here are the Top 10 ways to prevent BVD in your herd:
* Maintain a strict level of herd biosecurity.
* Purchase only open animals that are known to be BVD virus negative prior to purchase.
* Isolate any new additions or animals re-entering the herd for a minimum of 30 days.
* Test all new additions for BVD virus and vaccinate all new additions during the quarantine period.
* Maintain good sanitation and routinely disinfect contaminated areas. Prevent contamination from outside sources.
* Prevent contact with neighboring animals, regardless of disease status.
* Protect pregnant animals from potential sources of exposure, especially during the first three months of gestation.
* Prevent mixing of animal groups immediately prior to breeding and during the first three months of gestation.
* Conduct surveillance for BVD virus by performing necropsy on animals that die of respiratory diseases and collect blood samples on any calves that are poor-doers and calves that have respiratory disease.
* Vaccinate the cow herd on an annual basis. Make sure heifers are vaccinated at 6 months of age, boostered 30 days later, and re-vaccinated prior to breeding.
Diagnosis. Diagnosis of BVD infections is difficult and requires persistence.
Although BVD infections are widespread, most infections are subclinical and go undetected.
Acute (severe) infections are characterized by the presence of virus in the blood for only 2 to 3 days, approximately 5 days after infection.
To verify infection, blood samples must be taken during this narrow window. Persistent infections can be diagnosed easily from a blood sample or tissue sample by virus isolation.
However, colostrum antibodies may interfere with diagnosis. Diagnostic tests include virus isolation (the most dependable method), ELISA from blood samples, immunohistochemistry from tissue samples such as an ear notch, and polymerase chain reaction (PCR) assay from blood or tissue samples.
Blood testing for BVD antibody (serology) is a screening method, but is difficult to interpret for diagnostic purposes.
PCR of bulk milk samples can be used for herd screening.
Type of vaccine to use. Two types of BVD vaccines are currently available: modified live virus or attenuated, and killed virus or inactivated.
MLV vaccines contain viruses that have been altered so that they have reduced capacity to cause disease, but still induce protective immunity.
MLV vaccines must replicate inside the vaccinated animal’s body in order to produce an immune response.
Killed virus vaccines contain viruses that have been treated by a chemical or physical means to prevent them from replicating in the vaccinated animal.
With all vaccines and medications, read and follow all label directions. Proper administration of vaccines is absolutely critical to success.
Failures. Common causes of failures in vaccination programs include improper mixing; delays in use following rehydration of MLV vaccines; improper use of disinfectants in syringes; and not following label vaccination schedules.
For more information on BVD, proper use of vaccines, and diagnostic tools, visit the Auburn University BVD Web site at: www.vetmed.auburn.edu/bvd.
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