Nearly 4 million cattle are fed in Japan as dairy and beef cattle. More than 50% of dairy cattle (800,000 cows) and 20% of beef cattle (500,000) are fed in Hokkaido, Japan. Young and adult cows in addition to dairy products are delivered throughout Japan from Hokkaido. Moreover, many dairy cows are brought to Hokkaido as calves, and return to their home farm when pregnant. Vaccinations for some diseases are compulsory to prevent infectious diseases when cows move feeding places and particularly when they are introduced to common grazing farms.
Twenty-seven ruminant, equine, swine, avian, and bee diseases are designated as regulated domestic animal infectious diseases (1) as is paratuberculosis (Johne's disease; JD). Another 71 diseases in domestic animals have been monitored as non-regulated diseases since 1998 (2). Bovine viral diarrhea (BVD) and enzootic bovine leukemia (EBL) are non-regulated diseases. The prevalence of JD has been periodically monitored for a long time, and eradication programs are ongoing and also being developed.
Typical clinical symptoms are exhibited by JD-affected animals, and its spread in farms is a serious issue. In Hokkaido, a compulsory examination for the antibody against Mycobacterium avium spp. paratuberculosis, the pathogen of JD, is performed once every 5 years by all farms including dairy and beef cattle. Surveillance for a few decades cannot eradicate JD because the pathogen may remain latent in herds for long periods of time and difficulties are associated with its detection in the early stage of the disease. Although bovine leukemia virus (BLV), the pathogen of EBL, is latent in herds, preventive management is possible.
Bovine viral diarrhea virus (BVDV) is associated with various subclinical to fatal diseases. Persistent infection (PI) has been recognized as a serious threat to the cattle industry. PI animals can then subsequently develop mucosal disease which is often fatal. The immune dysfunction associated with BVDV infection has been associated with bovine respiratory disease complex and hemorrhagic syndrome. These disease syndromes have a serious economic impact on cattle producers.
Vaccines are useful tools for the protection of infectious diseases. BVDV is the only one of the three endemic diseases (BVD, EBL, and JD) that cattle are vaccinated for in Japan. The control of PI by BVDV is a key point for its eradication; however, difficulties are associated with detecting PI animals because not all infected animals exhibit the typical clinical symptoms and fetal infection cannot be estimated. Moreover, it is difficult to discriminate whether the immunization status was achieved by vaccination or infection. An update on BVD in Japan is presented in this review.