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Abstract
The study shows a close relationship between incidence of ovine babesiosis caused
by Babesia ovis and the activity period and distribution area of the vector tick Rhipicephalus
bursa. The most important factor limiting the distribution of this tick is a decrease
in humidity. In general, it is absent from areas with an average annual rainfall of
less than about 300 mm. The rate of parasitaemia and the degree of anaemia were not
correlated. Decrease of the packed-cell volume ranged from 30 to 40%. Parasitized
erythrocytes were not observed to block capillaries in the brain, which explained
the absence of nervous symptoms in acute babesiosis. The kidney was the most severely
affected organ, exhibiting acute glomerulonephritis. The lesions observed were suggestive
of vascular alteration and vascular stasis, leading to anoxia of the tissues. A disseminated
intravascular coagulation (DIC) syndrome was recorded in sheep infected with babesiosis.
Biochemical studies revealed possible damage to the liver and kidneys. Most of the
lambs (85%) that were infested with larvae, and all lambs infested with adult R. bursa
ticks reacted serologically to B. ovis antigen. The serological reactions following
infestation with the larvae occurred much later than those following infestation with
the adult stage. Both transovarial and 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 infestation with acute ovine babesiosis, show
that the preimaginal stages of R. bursa occurring in the winter can transmit B. ovis,
usually causing a sub-clinical disease. This might play a major role in pre-immunizing
and strengthening the premunition of the sheep against the main 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.