In vitro quantitation of Theileria parva sporozoites for use - TropMed ...

Chapter 1: Quantitation of Theileria parva sporozoites: Review of literature 21
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1.1.2.2. Hosts
T. parva is totally dependent on its hosts and cannot survive as a free living organism. Buffalo and
cattle are the intermediate hosts while the sexual stages occur in three-host ixodid ticks
Rhipicephalus appendiculatus and Rhipicephalus zambeziensis, which are principal field vectors
of the parasite (Theiler, 1904; Lawrence et al., 1983). The tick is considered the definitive host
because the parasite undergoes a sexual propagation in this host. R. appendiculatus and
R. zambeziensis are three-host ticks meaning that each instar feeds on a separate host and in
between drops to the ground where moulting takes place (Figure 1). It does not infect goats or
sheep under natural conditions though experimentally, ovine lymphocytes pre-treated with
proteases are susceptible in vitro but the internalised sporozoite does not differentiate into the
schizont stage (Syfrig et al., 1998). T. parva infected bovine lymphoblasts can also be established
experimentally in severe combined immunodeficiency (SCID) mice where they develop into
tumours (Fell and Preston, 1993).
The parasite is transmitted transstadially, meaning infection picked up in one instar is transmitted
at the next instar (larva to nymph or nymph to adult) (Lawrence et al., 2004). Transovarian
transmission of Theileria spp does not occur in ticks (Theiler, 1904) and neither will infected larva
transmit infection as adults if the intervening nymphal stage feeds on uninfected hosts (Lawrence
et al., 2004).
Figure 1: Life cycle of Rhipicephalus appendiculatus