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Foot-and-mouth disease

Retroviridae

Bovine leucosis

a Also affects goats, as do caprine arthritis-encephalitis and peste de petits ruminants.

Source: Adapted from Coetzer et al., 1994.

a Also affects goats, as do caprine arthritis-encephalitis and peste de petits ruminants.

Source: Adapted from Coetzer et al., 1994.

The lack of response of viruses to antibiotics makes treatment of viral diseases particularly problematic, although progress is being made toward the development of new vaccines (e.g., for Rift Valley fever [Morril et al., 1997]) and new antiviral compounds (e.g., polyoxometalates effective against respiratory syncytial virus [Barnard et al., 1997]). Regardless of these efforts, dairy producers should continue to maintain both animal hygiene and good management techniques to ward off viral infections.

Viral infections have variable effects on milk production. Bovine diarrhea virus has been reported to have severe economic impact in dairy herds both through lower milk yield and more severe disease in calves (Moerman et al., 1994). Bovine respiratory syncytial virus has no significant effect on milk production (Van der Poel et al., 1993). Bovine leukemia virus has been reported in one case to decrease milk yield and in another to increase yield (Rulka et al., 1993). Dairy cattle having a genetic potential for high milk production have a greater tendency toward infection with bovine leukemia virus, which probably explains why cows having subclinical infections with this virus sometimes produce more milk (albeit with lower milkfat content) than do uninfected animals in the same herd (Wu et al., 1989).

G. Bovine Spongiform Encephalopathy

Bovine spongiform encephalopathy (BSE), commonly known as ''mad cow disease,'' is a transmissible slow-acting fatal neurodegenerative disease whose symptoms include abnormal gait, nervousness, and ataxia. The disease was first identified in Britain in 1987 (Wells et al., 1987), and by mid-1998 the number of confirmed cases in that country had reached 173,915 (Patterson and Painter, 1999). Epidemiological studies suggest that approximately 903,000 cattle were infected between 1974 and 1995; apparently from consuming offal mixed into the feed following a change in processing methods by renderers. Most infected animals were beef cattle that had been slaughtered before demonstration of symptoms, and it is suspected that approximately 446,000 infected animals entered the human food chain. BSE was also widely distributed in dairy cows, and it is thought to have infected 59% of British dairy herds. The epidemic has dissipated after changes in feeding practices and the forced destruction of hundreds of thousands of infected animals; however, infected cattle have recently been identified in several other European countries.

Evidence has accumulated that BSE and other transmissible spongiform encephalopathies (TSEs), which have been identified in many domestic and wild mammalian species, are caused by prions, an abnormal form of PrP, a cell surface glycoprotein (Prusiner, 1997). The abnormal form, designated PrPSc, can convert PrP to additional PrPSc. Because PrPSc is resistant to proteases, it accumulates to concentrations that cause degeneration of the brain and reticuloendothelial tissues by a yet unknown mechanism. There is evidence that (a) BSE may have arisen from scrapie, a TSE of sheep and goats and (b) BSE may have been transferred in several cases to humans, resulting in a variant Creutzfeldt-Jakob disease (reviewed in Patterson and Painter, 1999).

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