Analysis of the effect of inapparent bovine paratuberculosis.

Inapparent paratuberculosis was studied in a Guernsey herd with a history of paratuberculosis (Johne's disease); the herd was maintained at between 900 and 950 cattle. Fecal specimens and intestinal tissue specimens from any of these that were slaughtered were examined culturally for the presence of Mycobacterium paratuberculosis. The reasons given for culling and slaughtering of cows from this herd were compared with infection status determined by culturing. Less than one-third of the culturally confirmed paratuberculous cattle were culled because of clinically apparent paratuberculosis. Mastitis was the reason for culling 3.6% of the non-infected cattle and 22.6% of the cattle with inapparent paratuberculosis. Infertility was also significantly higher in cows with inapparent paratuberculosis than in noninfected cows in the same herd. Separation of parturient dams and calves from the rest of the herd was shown to materially reduce the level of infection and incidence of clinical paratuberculosis.