Epidemiological analysis of tick-borne diseases in Zambia
Introduction
Tick-borne diseases (TBDs) are one of the most important constraints to livestock production in developing countries, including Zambia (Makala et al., 2003, Minjauw and McLeod, 2003). They are responsible for high morbidity and mortality resulting in decreased production of meat, milk and other livestock by-products together with the loss of draught power. TBDs are also a significant restraint on the genetic improvement of indigenous breeds of cattle, sheep and goats, since they prevent the introduction of more productive exotic breeds. The most important TBDs of cattle in Sub-Saharan Africa are East Coast fever (ECF) caused by Theileria parva, babesiosis caused by B. bovis and Babesia bigemina, anaplasmosis caused by Anaplasma marginale and heartwater caused by Ehrlichia ruminantium (Makala et al., 2003). Other species of Theileria known to infect cattle in Zambia include T. mutans, T. taurotragi and T. velifera (Jongejan et al., 1986b, Musisi et al., 1984).
Important ticks of cattle in Zambia include Rhipicephalus appendiculatus, R. microplus, R. decoloratus and Amblyomma variegatum (Pegram et al., 1986). R. appendiculatus and A. variagatum are found throughout the country with a single generation occurring per year (Pegram et al., 1986). Previously R. microplus was only found in the Eastern and Northern Provinces, while R. decoloratus was found throughout the country (Pegram et al., 1986, Berkvens et al., 1998). However evidence presented by Jongejan et al. (1988) suggested that R. microplus is spreading westwards into Central Zambia.
Knowledge of the epidemiology of these diseases is important for the design and implementation of control strategies. However, despite the considerable economic loss associated with these diseases, information on their basic epidemiology in Zambia is inadequate. Available data is based upon serological tests or microscopic examination of blood smears, diagnostic techniques with shortcomings that are well-documented (Bose et al., 1995). Moreover, it has been more than twenty years since any comprehensive studies on the epidemiology of babesiosis and anaplasmosis in Zambia have been conducted (Jongejan et al., 1988) and there is no available data on heartwater in cattle. Although there have been a number of studies on ECF (Berkvens, 1991, Billiouw et al., 1999, Fandamu et al., 2005), there is no information on the prevalence of T. parva in most parts of the country. This paucity of epidemiological information is a significant impediment to the development of cost-effective control strategies. In addition, with extensive movement of cattle and frequent changes in Governmental disease control policies (Chilonda et al., 1999) continuous monitoring of disease epidemiology in Zambia is important.
The Zambian environment represents a multi-tick species vector ecology (Pegram et al., 1986), and mixed infections of different tick-borne pathogens in host populations is probably commonplace. Interaction between these infections is important because it could potentially affect the outcome of therapeutic intervention, and may influence the severity of disease. However, the relative importance of single and mixed species forms of TBD infections in relation to productivity losses in cattle is largely unknown.
There is no information concerning the potential risk factors associated with the occurrence of TBD in cattle in Zambia. Risk factors that have been identified to influence the occurrence of TBDs include the distribution and abundance of tick vectors, the abundance and movement of cattle populations and the host resistance to both the tick and the responsible pathogen (Bakheit and Latif, 2002, Norval et al., 1988, Salih et al., 2007). Quantifying the influence of these determinants and characterising local variations in disease epidemiology are essential for assessing the range of TBDs and their potential to spread. In addition, identifying which of these factors are the most important would assist in designing cost-effective control strategies. This study aimed to provide information that could be used for estimating and controlling the risk of TBD in Zambia. Therefore, the epidemiology of tick-borne parasites of cattle in Lusaka, Central and Eastern Provinces of Zambia was assessed in both dry and wet seasons and the data used to answer the following questions: (1) what is the prevalence of tick-borne pathogens of cattle in Zambia? (2) can interactions be identified between the various tick-borne pathogens in individual animals? (3) what are the risk factors associated with the occurrence of these pathogens in cattle? (4) what are the effects of single or multiple species infections together with other confounding variables on a major clinical parameter (PCV)? and (5) what factors influence tick burden on the cattle population?
Section snippets
Study area
The study was conducted in three Provinces of Zambia; Eastern, Lusaka and Central Province (Fig. 1). The Eastern Province covers 69,000 km2, about 9% of Zambia's total area, and is divided into eight districts. Sampling was undertaken in two districts (Katete and Petauke). The plateau of the Eastern Province has a flat to gently rolling landscape with altitudes ranging from 900 to 1200 m. The vegetation is miombo woodland dominated by tree species such as Braychystegia and Julbernadia (Van Den
Prevalence of tick-borne pathogens in the dry and wet seasons in Central, Lusaka and Eastern Provinces
The detected prevalence of each of the seven tick-borne pathogens prevalent in Zambia ranged from 0% to over 83% in the three surveyed provinces and variation in prevalence between the seasons and the provinces was detected (Table 1). In Eastern Province, the prevalences of all the pathogens were significantly higher in the wet season than in the dry and for both seasons, T. mutans was the most prevalent, followed by Anaplasma spp while B. bigemina was the least prevalent. In Lusaka Province (
Discussion
Tick-borne diseases are an important constraint to livestock production in tropical and sub-tropical regions (Jongejan and Uilenberg, 2004). However, information on their distribution, abundance and the factors that affect their occurrence is inadequate or entirely lacking in many of these countries including Zambia. Epidemiological surveys using specific, sensitive and reproducible laboratory tests are required and this provided the rational for the analysis reported in this paper.
All the TBDs
Acknowledgements
Martin Simuunza was on a Commonwealth PhD Scholarship. The assistance rendered by Mr. Innocent Nyambe during sample collection exercise is greatly appreciated. We are also thankful to Margaret Baird, Drs. Jane Kinnaird and Huseyin Bilgic for the assistance rendered during the laboratory analysis of the samples. We are grateful to the Centre for Tropical Veterinary Medicine (CTVM) and Katherine M. Kocan of Oklahoma State University for the provision of the DNA stocks that were used in the
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