Материал: Bovine Viral Diarrhea Virus Diagnosis, Management, and Control

tion been used in addition to the serological testing. In the other herd with a false negative result, a single persistently infected calf of 3 months of age was missed. This animal was reported not to have had contact with the target population. The authors noted that the overall sensitivity of the serologic evaluation of five unvaccinated heifers for determining herds with persistently infected cattle could be improved by periodic retesting of herds that had negative results. The use and testing of sentinel animals in vaccinated herds has been recommended as an important element of a BVDV control program (Dubovi, 2002).

CONTROL BY VACCINATION

In recent years there have been two major trends in vaccine research and development: the inclusion of BVDV 2 strains in commercial vaccines and the evaluation of the efficacy of BVDV vaccines for protection against fetal infection. The latter, an assessment of efficacy for fetal protection, has not been a requirement for the licensing of vaccines in the U.S. and Canada. For licensing, the vaccines are required only to show protection against acute infections. However, in recent years, efforts by both individual researchers and vaccine companies have been intensified to evaluate BVDV vaccines for their efficacy in providing fetal protection. The ability to demonstrate and claim vaccine efficacy for fetal protection should give those vaccine companies a competitive advantage over others that cannot make such a claim for their products. This will be the driving force for new and better products in the absence of new regulations.

Recently, experiments involving immunization with BVDV-specific nucleic acids (DNA or RNA) have been performed. The first of these utilized a recombinant plasmid containing the BVDV E2 gene with a human cytomegalovirus promotor for DNA immunization of mice (Harpin et al., 1997). Neutralizing responses against BVDV 1 strains were generated, which persisted for 6 months after the last injection. In cattle, immunization with the plasmid produced both a neutralizing antibody response and a cellular immune response, but immunization was only partially protective when the cattle were inoculated with a challenge virus (Harpin et al., 1999).

Vassilev et al. (2001) used full-length, infectious BVDV RNA derived from a recombinant, infectious cDNA clone (NADL strain) to immunize cattle and sheep against BVDV. The RNA was coated onto gold microcarrier particles and delivered to the inoc-

ulation sites using a gene gun delivery system. Serumneutralizing antibody titers of >212 and >27 were generated in calves to the NADL strain of BVDV 1 and to a BVDV 2 strain, respectively.

Muñoz-Zanzi et al. (2002) developed models to predict the age of dairy calves when colostrumderived BVDV antibodies would decline to a level that would no longer offer disease protection or interfere with vaccination. The ages at which 50% of the calves were predicted to have low antibody titers (<1:16) were, for two herds, 107–111 days for BVDV 1 and 75–81 days for BVDV 2. The ages at which 50% of the calves were predicted to be seronegative (titer <1:8) were, for the same herds, 139–143 days for BVDV 1 and 110–118 days for BVDV 2. These ages were considerably shorter than earlier estimates for the persistence of colostral antibodies (6–10 months). The differences in these estimates may be due, in part, to the large sample size (466 calves) used in this study or due to differences in protocols for administering colostrum.

ERADICATION

The eradication programs without vaccination that began in the 1990s in the Scandinavian countries have proven to be very successful in reducing the number of herds with BVDV infections. For instance, 52% of herds were estimated to be infected in Sweden in 1993, whereas in 2002, 93% of dairy and 88% of beef herds were considered BVDV-free (Lindberg, 2002). Eradication of BVDV appears to have been even more successful in Denmark where, in 2002, approximately 99% of dairy and 98% of beef herds were free of BVDV (Bitsch et al., 2002).

In Germany, an ongoing control program for eventual BVDV eradication is voluntary and partially subsidized by the state (Greiser-Wilke, 2002). Vaccination is practiced in Germany to reduce virus circulation and to minimize economic losses. A killed vaccine is used for basic immunization and an attenuated live virus vaccine is used to booster the immune response. Screening for removal of persistently infected animals is performed using an antigen capture ELISA. If all animals up to 3 years of age in a herd are negative, the herd is classified as “BVDV unsuspicious.”

In 2002, the Academy of Veterinary Consultants (AVC), presented a position statement on BVDV for eventual eradication of the virus in North America at a USDA-sponsored meeting (Grotelueschen, 2002). The AVC, established in 1970, is an association of veterinarians involved in beef cattle medicine, herd health programs, and consultation. Its members con-

sist of veterinarians from both the U.S. and Canada. The BVDV position statement reads:

The beef and dairy industries suffer enormous loss due to effects of bovine viral diarrhea virus (BVDV) infection. The highly mutable nature of BVDV and the emergence of highly virulent strains of BVDV contribute to limited success of present control programs. Also persistently infected cattle are the primary source of infection and effective testing procedures are available to identify those infected carriers. Therefore, it is the resolve of the Academy of Veterinary Consultants that the beef and dairy industries adopt measures to control and target eventual eradication of BVDV from North America.

The AVC position statement is likely only a first step toward a more comprehensive program against BVDV in North America, and the support of producer groups will be critical before such a program becomes a reality. The consensus of those present was that any eradication program in North America will likely include vaccination, and that vaccines that provide maximum fetal protection will be crucial to the success of this type of program.

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