EVALUATION OF HUMORAL IMMUNE-RESPONSES IN CATTLE GRAZING ENDOPHYTE-INFECTED OR ENDOPHYTE-FREE FESCUE

Anecdotal reports suggest cattle with fescue toxicosis may not respond to vaccination and thus, experience increased incidence of Bovine Res piratory Disease Complex (BRDC) when shipped to feedlots. Fescue toxic osis causes hypoprolactemia in cattle. Hypoprolactemia decreases humor al immune responses in mice. Therefore, a study was conducted to compa re the magnitude of primary and secondary humoral immune responses aga inst specific antigens in cattle grazing endophyte-infected or endophy te-free fescue. Angus steers were blocked by weight and allocated into four groups. Two groups grazed endophyte-infected (EI) fescue and the other two groups grazed endophyte-free (EF) fescue. All steers were i njected LM on d 0 and 21 with lysozyme without adjuvant and concanaval in A (Con A) with sheep red blood cells (SRBC) in incomplete adjuvant of Freund. Steers were bled on days 0, 21 and 35 post-vaccination. Ave rage daily gains (ADG), alkaline phosphatase (ALP) activity, cholester ol concentrations, rectal temperatures, and serum prolactin concentrat ions were measured to confirm fescue toxicosis in steers grazing EI fe scue. Antibodies to Con A and SRBC were determined by ELISA and hemagg lutination assay, respectively. The ADG were decreased for the EI grou p during the first month. Rectal temperatures were elevated and serum prolactin concentrations were decreased in the EI group. ChoIesterol a nd ALP concentrations also were decreased in the EI group. Primary and secondary immune responses against Con A tended to be Increased and w ere increased against SRBC in the EI group. Antibodies against lysozym e were not induced in either group. in conclusion, cattle grazing EI f escue mounted similar humoral immune responses to vaccination, despite hypoprolactemia, as cattle grazing EF fescue. Increases in bovine res piratory disease in cattle maintained on EI fesfue probably is not ass ociated with lack of humoral immune response to vaccination protocols as a result of fescue toxicosis. (C) 1997 Elsevier Science B.V.