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

Chapter 1: Quantitation of Theileria parva sporozoites: Review of literature 27
______________________________________________________________________________________________
T. parva infected lymphoblasts (Morrison et al., 1987; Goddeeris et al., 1990; Taracha et al.,
1992; Taracha et al., 1995). These CTL are parasite strain specific and restricted to the MHC class
1 markers (Goddeeris et al., 1986).
1.1.5. Epidemiology
The disease is generally endemic in the eastern, central and southern regions of Africa within the
geographic distribution of the tick vector (Figure 4). Tick numbers, tick infection and
susceptibility of bovine hosts are the main determinants of risk of infection (Medley et al., 1993).
Climatic factors, particularly rainfall, affecting tick distribution and survival have been found to
play a major role in the occurrence and seasonality of the disease (Branagan, 1973a; Branagan,
1973b; Short et al., 1989; Berkvens et al., 1995; Mulumba et al., 2000; Mulumba et al., 2001;
Fandamu et al., 2005). Transmission and occurrence of disease may be seasonal or not depending
on the rainfall patterns that determine the tick instars and population densities on the host (Norval
et al., 1991; Mulumba et al., 2000; Mulumba et al., 2001). The main bovine host factor
complicating the epidemiology of the disease is the carrier state in recovered or immune animals
(Neitz, 1964; Young et al., 1986; Dolan, 1986a; Bishop et al., 1992; Latif et al., 2001), which
serve as infection sources for field ticks. Other bovine host factors include susceptibility to
T. parva, resistance to ticks, breed and age (Fivaz et al., 1989; Koch et al., 1990; Latif et al., 1991;
Ndungu et al., 2005). It has been shown recently that the mean volume of erythrocytes is
correlated to the fatality of cases with smaller sizes predisposing the symptomatic animal to death
(Fandamu et al., in press).
The degree of disease establishment is broadly classified as epidemic or endemic. Endemic
stability is a state in which all calves come in contact with the disease within a few months of age
and morbidity is low (Moll et al., 1986). This situation is observed in East Africa where climatic
conditions allow the all year round possibility of the tick to infect the bovine host and where
innate resistance of the local cattle to the prevailing parasite stocks is known to occur (Moll et al.,
1984).
In contrast, Billiouw et al. (2002) have shown that this is not the case for eastern Zambia where
the disease was introduced more recently, i.e. during the early 20 th century, and tick presence and
therefore transmission pressure is seasonal.