Publisher Summary The piroplasms are protozoa that are highly pathogenic to cattle, sheep, goats, and occasionally even to man. They comprise two genera—namely, Theileria and Babesia . The diseases they induce, known collectively as “theilerioses” and “babesioses,” cause fevers and lead to important economic losses in the tropics, subtropics, and southern Europe. This chapter highlights the differences between the life cycles of Babesia and Theileria species with respect to their morphology, studied by means of light and electron microscopy. The chapter describes the life cycle of piroplasms. They have a typical sporozoan life cycle comprising three phases: (1) Schizogony, an asexual reproduction phase in the vertebrate host. (2) Gumogony, the formation and fusion of gametes inside the intestinal cells of ixodid ticks. (3) Sporogony, an asexual reproduction in the salivary gland of the tick leading to the infectious, saliva-transmitted sporozoites. Comparative biological and morphological studies show that the economically important piroplasms comprise three groups: (1) Babesia species sensu strictu ; (2) Bubesia equi , B. microti ; and (3) Theileria species.

The piroplasms: life cycle and sexual stages.

The taxonomy of parasites of the genus Theileria of domestic animals appeared straightforward to most specialists in the western world about 20 years ago. Apart from the six species shown in Table 1, there were two doubtful ones in camels and reindeer, T. camelensis and T. tarandirangiferis. Species were differentiated by the host in which they were found, their pathogenicity and epidemiology, absence or presence of the carrier state after recovery and the influence of splenectomy, numbers of piroplasms and schizonts, size and percentages of different morphological types of piroplasms and size of the macroschizont. Immunological strain differences had been reported for T. annulata by Adler & Ellenbogen (1935) and subsequent authors, but it was generally believed that there was only one antigenic type of T. parva, even though Jezierski et al (1959) had demonstrated an antigenic difference between strains from Zaire and South Africa.

Theilerial Species of Domestic Livestock

Simultaneous detection of Anaplasma and Ehrlichia species in ruminants and detection of Ehrlichia ruminantium in Amblyomma variegatum ticks by reverse line blot hybridization

Parasites have evolved an extraordinary variety of mechanisms for surviving in the face of the natural and acquired immune responses of their hosts. A selection of the mechanisms serves to illustrate the challenge involved in developing strategies for immunising against parasites.

Games parasites play: how parasites evade immune surveillance

An extensive range of serological and molecular diagnostic assays exist for most of the economically important Theileira species such as T. annulata, T. equi, T. lestoquardi, T. parva, T. uilenbergi and other more benign species. Diagnostics of Theileria is considered with regard to sensitivity and specificity of current molecular and serological assays and their use in epidemiology. In the case of serological assays, cross-reactivity of genetically closely related species reduces the use of the gold standard indirect fluorescent antibody test (IFAT). Development of antigen-specific assays does not necessarily address this problem, since closely related species will potentially have similar antigens. Even so, serological assays remain an important line of enquiry in epidemiological surveys. Molecular based assays have exploded in the last decade with significant improvements in sensitivity and specificity. In this review, the current interpretation of what constitute a species in Theileria and its impact on accurate molecular diagnostics is considered. Most molecular assays based on conventional or real-time PCR technology have proven to be on standard with regard to analytical sensitivity. However, consideration of the limits of detection in regard to total blood volume of an animal indicates that most assays may only detect >400,000 parasites/L blood. Even so, natural parasitaemia distribution in carrier-state animals seems to be above this limit of detection, suggesting that most molecular assays should be able to detect the majority of infected individuals under endemic conditions. The potential for false-negative results can, however, only be assessed within the biological context of the parasite within its vertebrate host, i.e. parasitaemia range in the carrier-state that will support infection of the vector and subsequent transmission.

A review of Theileria diagnostics and epidemiology.

Infection and Transformation of Bovine Lymphoid Cells in vitro by Infective Particles of Theileria parva

Theileria mutans (Aitong) isolated from cattle exposed in the Narok District of Kenya and blood-passaged through cattle 8 times, appeared to have lost its original pathogenicity for cattle. It was demonstrated that the parasite was trans-stadially transmissible by the tick Amblyomma variegatum but not by Rhipicephalus appendiculatus. Four tick–bovine passages were made using A. variegatum, and infective parasites were also harvested from A. variegatum nymphs which had been fed for 5 days on rabbits. Blood containing piroplasms, or lymphoid cells infected with schizonts taken from cattle, at the 2nd tick–bovine passage were shown to be infective on inoculation.The course of the T. mutans infections in the cattle was studied. In tick-induced infections macroschizonts occurred transiently, persisting longest in circulating lymphoid cells. Microschizonts were rarely detected. The macroschizonts were morphologically distinct from those of other Theileria species described from East Africa.In tick-induced infections, the piroplasm parasitaemia increased rapidly and there was evidence of restored pathogenicity of the parasite since high piroplasm parasitaemias were associated with a marked anaemia.

R. C. Payne

Papers

Infection and Transformation of Bovine Lymphoid Cells in vitro by Infective Particles of Theileria parva

Studies on the transmission and course of infection of a Kenyan strain of Theileria mutans.