Some implications of a sexual cycle in Theileria

Abstract

SummaryRecent microscopic studies of Theilerial parasites in. ticks appear to provide convincing evidence for a sexual cycle for these parasites. This evidence would remove most of the objections to placing piroplasms in the same class as malarial and coccidial parasites. It is suggested, therefore, that the class Piroplasmasida (Piroplasmea) be abolished and that piroplasms be located in a sub-order Piroplasmorina alongside Haemosporina within the class Sporozoa (Teleospora).If the close relationship of Theilerial and malarial parasites is correct then by analogy meiotic or reduction division of Theileria probably occurs during sporogony in the tick salivary gland acinar cells. This means that all bovine stages are haploid. If these can be cloned and characterized by isoenzyme or drug resistance markers, then genetic recombinant studies, similar to those performed with malarial and coccidial parasites, could be conducted.The potential of genetic recombination greatly increases the scope of the parasite for developing new variants. This potential is greatest in areas where there is a variety of tick and host species which apply different types of selection pressure to parasites. Atypical strains of Theileria are usually isolated from such areas, and although the characteristics of these strains may not be expressed in the wild, unnatural selection by laboratory passage may reveal their presence. The apparent lability of such strains may simply be cloning of different genetic recombinants.If different strains can be cloned and characterized it may be possible by genetic recombination techniques to develop new clones which retain desired characters for use as avirulent protective vaccines, but from which undesirable characters have been eliminated.

Vaccination against Tropical Theileriosis

“…Most field isolates conferred a high degree of reciprocal immunity, 8,38‐40 and it has been suggested that a constant mixing and crossing of genetically different parasite populations occurs in nature. 41 Most isolates are, therefore, likely to consist of a mixture of “strains” and will, consequently, confer a relatively wide range of protection. On the other hand, several experiments detected T. annulata isolates with a distinctly low capacity for reciprocal immunity.…”

Theilerial parasites isolated from carrier cattle after immunization withTheileria parvaby the infection and treatment method

“…(b) A second explanation would be that during the carrier state by process of antigenic variation, the carrier parasite had changed completely. This could be assisted by the possible hybridization of the parasite after the proposed and likely recombination during the sexual cycle in the tick (Irvin & Boarer, 1980;Mehlhorn & Schein, 1985). It was hoped that the monoclonal antibody profile might provide the answers to these possibilities.…”

Restriction mapping of the genome of the protozoan parasite Theileria parva.

“…It is transmitted by the tick Rhipicephalus appendiculatus, in which, according to morphological evidence, a sexual cycle may occur (1). The developmental stages of the parasite in cattle occur within lymphocytes and erythrocytes and are considered haploid (2).…”