EQUINE | Disease
hemolytic anemia. They also alter coagulation,
predisposing
to
disseminated
intravascular
coagulation, by unknown mechanisms.
Transplacental infection of fetuses from infected carrier
mares has been documented and is most common
with Theileria equi infections.
Figure 1: Life cycle of Theileria equi demonstrating the development
phase within lymphocytes prior to the infection of erythrocytes. (Courtesy
of Rothschild C.M & Knowles D.P. Equine Medicine 2016)
where the division process is repeated (figure 2).
Both parasites cause lysis of red blood cells resulting
in varying degrees of hemolytic anemia. In addition,
infected erythrocytes are removed from the circulation
by splenic macrophages further compounding the
Equids infected with T.equi appear to remain
permanently infected (lifelong), with recovered
animals
becoming
asymptomatic
carriers.
Parasitaemia is often absent in carriers but can reoccur
after immunosuppression, stress, strenuous exercise
or steroid administration. Treatment can suppress
clinical signs but is ineffective in clearing T.equi from
carriers. The incubation period is 12-19 days.
Animals infected with B.caballi can remain carriers for
up to 4 years but might be able to clear the organisms
eventually. Transplacental infection with B.caballi is
rarely reported. Incubation period is 10-30 days.
Sources of infection.
Infected ticks, mechanical vectors (contaminated
needles and syringes) and infected carrier horses serve
as the major sources of infection. Infected blood used
for blood transfusions is also a potential source of
infection.
Clinical Disease.
Figure 2: Life cycle of Babesia caballi demonstrating the direct infection
of equine erythrocytes. (Courtesy of Massero Ueti)
Theileria equi tends to cause more severe disease than
Babesia caballi and is responsible for the vast majority
of clinical cases of equine piroplasmosis cases in South
Africa. The risk of clinical infection increases with the
presence of factors such as immunological naivety and
increased density of infected ticks and horses. Clinical
infection can occur in peracute, acute, subacute,
chronic and atypical forms with in-utero infection of
pregnant mares resulting in various outcomes in the
offspring. Documented case fatality rates vary from 10
• Volume 20 Issue 1 | April 2018 •
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