THE EPIDEMIOLOGY OF BOVINE TROPICAL THEILERIOSIS IN TUNISIA AND THE
DEVELOPMENT OF A LIVE ATTENUATED VACCINE
Tropical theileriosis, caused by the tickborne protozoan
Theileria annulata is a most important disease of improved, and the
most productive, dairy cattle in Tunisia. The scientific work
undertaken during the project consisted of epidemiological
investigations on tropical theileriosis and attenuation of Tunisian
stocks of Theileria annulata. The project (STD2A-260) was funded by
the European Union, and was coordinated by Professor C.G.D. Brown,
CTVM, University of Edinburgh.
Sero-epidemiological investigations on tropical
theileriosis showed that the infection is widespread in the
sub-humid and semi-arid zones of Tunisia, and also present in the
cork oak (Quercus suber) level of the humid zone. Both endemically
stable and endemically unstable situations were identified.
Endemic instability, with the highest clinical disease incidence,
was mainly observed in purebred exotic cattle, while endemic
stability was recorded in indigenous cattle under the traditional
management system. Tick surveys confirmed the role of Hyalomma
detritum as the major vector of T.annulata in Tunisia. In
endemically stable situations high tick infestation rates were
observed on cattle, ensuring an early primary infection of calves
during the first summer of life. In endemically unstable
situations lower infestation rates were recorded, resulting in
varying proportions of uninfected, naive cattle up to and following
the third theileriosis seasons.
A preliminary evaluation confirmed the economic importance
of tropical theileriosis. In the sub-humid zone where a high
incidence of clinical disease was recorded in exotic dairy cattle,
the impact of the disease led to preliminary loss evaluation of 18
Tunisian Dinars per female in the survey area.
Nationwide studies on the distribution of T.annulata and
its vector ticks were pursued in this project; these studies
confirmed the presence of both H.detritum and T.annulata in the
cork oak bioclimatic stage in the humid zone.
Six stocks of T.annulata were isolated from clinical cases
of theileriosis in the field and established in vitro as cell
cultures. Five of these stocks were also isolated as infected
ground up tick supernates (GUTS) produced from the laboratory tick
colony of H.anatolicum excavatum and, in one case, from naturally
infected H.detritum ticks. These were cryopreserved in liquid
nitrogen as definitive, representative stabilates of Tunisian
stocks of T.annulata.
A high degree of phenotypic and genotypic polymorphism was
observed both between and within the T.annulata stocks. Four of
these stocks were maintained in continuous cell culture until
passage 200. In vivo testing of three stocks confirmed their
attenuation at passage 100, or at passage 200, depending on the
stocks used. Experiments to evaluate immunogenecity confirmed that
a decrease in the degree of immunity to heterologous challenge was
observed while the homologous immunity was still strong.
Transmission to ticks followed by reversion to virulence
was observed with one of the three attenuated stocks.
Safety experiments performed on milking Friesian Holstein
cows, showed that vaccination with passage 100 of the Batan 2 stock
produced a decrease of 3% in milk production. Vaccination with
Batan 2 at passage 200 was better tolerated by lactating cows and
no milk loss was detected when compared with unvaccinated control
animals over a period of 3 weeks following vaccination. This cell
line, at passage 200, was selected for further trials and field
testing as a candidate vaccine.
For further information contact:
Dr M.E.A. Darghouth
ENMV, Sidi Thabet
Professor C.G.D. Brown
University of Edinburgh
Easter Bush, Roslin
Midlothian EH25 9RG