SOME EPIZOOTIOLOGICAL AND CLINICAL ASPECTS OF OVINE BABESIOSIS CAUSEDBY BABESIA-OVIS - A REVIEW

The study shows a close relationship between incidence of ovine babesi osis caused by Babesia ovis and the activity period and distribution a rea of the vector tick Rhipicephalus bursa. The most important factor limiting the distribution of this tick is a decrease in humidity. In g eneral, it is absent from areas with an average annual rainfall of les s than about 300 mm. The rate of parasitaemia and the degree of anaemi a were not correlated, Decrease of the packed-cell volume ranged from 30 to 40%. Parasitized erythrocytes were not observed to block capilla ries in the brain, which explained the absence of nervous symptoms in acute babesiosis. The kidney was the most severely affected organ, exh ibiting acute glomerulonephritis. The lesions observed were suggestive of vascular alteration and vascular stasis, leading to anoxia of the tissues, A disseminated intravascular coagulation (DIG) syndrome was r ecorded in sheep infected with babesiosis. Biochemical studies reveale d possible damage to the liver and kidneys. Most of the lambs (85%) th at were infested with larvae, and all lambs infested with adult R. bur sa ticks reacted serologically to B. ovis antigen. The serological rea ctions following infestation with the larvae occurred much later than those following infestation with the adult stage, Both transovarial an d transstadial transmission of the parasite were demonstrated. A study of antibodies to B. ovis using IFAT in hoggets and ewes revealed high serological prevalence, i.e., 88.9% in ewes and 84.5% in hoggets. No marked seasonal fluctuation was observed. The serological findings, in addition to the fact that one splenectomised lamb reacted to larval i nfestation with acute ovine babesiosis, show that the preimaginal stag es of R. bursa occurring in the winter can transmit B. ovis, usually c ausing a sub-clinical disease. This might play a major role in pre-imm unizing and strengthening the premunition of the sheep against the mai n spring challenge by the adult ticks. The sub-clinical reactions are probably due to the low infection rate of the preimaginal stages of R. bursa by B. ovis parasites. (C) 1998 Elsevier Science B.V.