Effects of vitamin A deficiency and ked resistance on the feeding of Dermacentor andersoni on sheep

Abstract

Survival of male but not of female ticks ( Dermacentor andersoni Stiles) was lower, and engorged females weighed less from sheep that had acquired resistance to keds ( Melophagus ovinus (L.)) than of ticks from ked-free sheep. Survival of male and female ticks in April and early May was higher on both ked-resistant and ked-free sheep fed a vitamin A-free ration than on sheep fed a ration supplemented with vitamin A. No significant differences in tick survival were obtained on similar sheep infested in late May and early June. Additional experiments using guinea pigs as hosts in early June, July, and August showed no differences in survival of ticks on animals fed rations free of, or supplemented with, vitamin A. When all experiments were compared, there was a progressive increase in engorgement time of females from 7 to 8 days in April to 14 to 19 days in August. It is suggested that some overriding factor or factors in late-season infestations obscured or eliminated the differences based on diet that had been significant in the early-season infestations. Two cases of tick paralysis in sheep are recorded, both of them in sheep on a ration deficient in vitamin A in an April infestation.