The Theileria parva carrier-state in cattle on commercial farms on
Zimbabwe was investigated using parasitological and serological methods.
The proportion of cattle showing Theileria piroplasms on two farms,
which had recent histories of disease outbreaks, were 64% ( n=106, total
of heifers and weaned calves examined) and 71.5% ( n=60) while the
proportion of T. parva antibodies for the same animals were 59% and
98.5%, respectively. On four farms where no cases of the disease
occurred for over 10 years, the average proportion of animals showing
piroplasms and antibodies were 55.4% (range 32-82, n=223) and 73%
(range 47-91, n=223), respectively. However, on another three farms
which had no history of theileriosis outbreaks these proportions were
very low, being 11.4% (0-24, n=157) for piroplasms and 12.2% (5-23,
n=157) for antibodies. The mean infection rate in unfed Rhipicephalus
appendiculatus adults collected from farms with a high prevalence of
cattle which were carriers of Theileria piroplasms during the tick
activity season was 29% (range 12-60%) with 9.3 (range 2-18.7) mean
infected acini per infected tick. The infectivity of different tick
batches to susceptible cattle produced a wide spectrum of theileriosis
reactions. Laboratory controlled experiments were carried out to study
the persistence of T. parva (Boleni) piroplasms in cattle immunized
with this strain as well as its infectivity for ticks and its subsequent
transmissibility to cattle. Examination of the salivary glands of 15
batches of ticks collected from six immunized cattle on three different
occasions over 18 months showed that none were infected with Theileria
parasites. However, the infectivity of other ticks in the same batches
to susceptible animals was demonstrated 6, 10 and 18 months after cattle
had been immunized with Boleni stabilate.
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