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Publisher Summary This chapter presents a study analyzing the structural and immunoserologic properties of Babesia and Theileria species isolated in the United States. Babesia and Theileria species used in the study were isolated from a horse in the state of Florida and a deer native to the state of Missouri, respectively. On the basis of size, mode of multiplication, and number of parasites in infected erythrocytes using the fluorescent antibody (FA) technique, it was concluded that the infectious blood sample from Florida contained both Babesia caballi and Babesia equi. It was further demonstrated by means of the FA technique that certain forms of the parasite of equine babesiasis have a tail or fibril-like appendix. The tailed forms have been observed early in infection and are believed to be associated with the invasive stage in the cycle of development of Babesia. The fine structure of the Babesia parasite consists of a cytoplasmic membrane, a double nuclear membrane, a food vacuole, an endoplasmic reticulum, a Golgi apparatus, and vacuoles of various sizes that appear devoid of internal structure. No mitochondria or pigment (hemozoin) was observed in the cytoplasm of the parasite.

The prevalence of Theileria equi and Babesia caballi infections in the north-eastern Free State Province of South Africa was determined by examination of thin and thick Giemsa-stained blood smears, IFAT and PCR. No parasites were detected by microscopy from any blood samples collected at five study sites, Qwaqwa, Kestell, Harrismith, Vrede and Warden. Of the tested serum samples, 28/29 (96.5 %), 20/21 (95.2 %) and 42/42 (100 %) were positive by IFAT for T. equi infections in Harrismith, Kestell and Qwaqwa, respectively, and 5/29 (17.2 %>), 13/21 (61.9 %>) and 30/42 (71.4 %>) were sero-positive for B. caballi infections in Harrismith, Kestell and Qwaqwa, respectively. All DNA samples from the study sites were negative for B. caballi infections by PCR, but five samples, two from each of Kestell and Warden and one from Vrede, were PCR positive for T. equi infections. The high prevalence of antibodies against T. equi and B. caballi in the sampled horses indicates that the animals had been exposed to T. equi and B. caballi infections but the absence of parasitaemia and very low number of positive PCR samples, however, imply that T. equi and B. caballi are endemically stable in the north-eastern Free State Province.

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