Epidemiology and Economic Importance of African Animal Trypanosomiasis

Introduction

African animal trypanosomiasis (AAT), also known as Nagana, is a vector-borne parasitic disease caused by an extracellular protozoan belonging to the genus, Trypanosoma (Cox, 1979). According to the Food and Agriculture Organization (FAO), it is probably the only disease that has affected the settlement and economic development of a major part of African continent. Nagana remains endemic in 37 of the 54 countries in Africa, affecting an area of approximately 10 million square kilometers of arable land, and reduces the efficiency of productivity of over 150 million cattle and 260 million sheep and goats (Leigh et al., 2015; Nyimba et al., 2015). AAT is a very significant economic and animal health issue for this sub-Saharan region (Habila et al., 2012). Furthermore, the disease has an extended impact on the crop, human settlement, and welfare, because seven million square kilometers of the six region’s land is rendered unsuitable for mixed crop-livestock ecosystems (Nigatu et al., 2015; Peregrine, 1994). AAT is estimated to cause annual losses of more than US$ 4.5 8 billion dollars through direct and indirect agricultural production costs (Bezie, 2015). It is not surprising, that the 21 countries where Trypanosomiasis is endemic are included in the world’s 25 poorest countries (Shaw, 2009), and 32 were considered highly indebted according to IAEA, (2002).

The wide distribution of AAT is attributed to the abundance of its biological and mechanical transmitting vectors known as tsetse flies and other biting flies respectively (Diall, et al., 2017). All warm-blooded animals including wildlife species have been implicated in the transmission cycle for Nagana(Reichard, 2002). Trypanosomiasis is a parasitic disorder that multiplies in the host bloodstream, lymphatic vessels and tissues including the cardiac muscles and the central nervous system. The parasite trypanosomes invade the immune system of the host because it possesses a variable surface antigen (VSG) which prevents lying by complement alternative pathways, (Vincendeau & Bouteille, 2006).

Tsetse flies, the main vectors for trypanosomes occur exclusively in Africa over an area of approximately 10 million km ², extending on both sides of the equator from 15º N to 30º S(Cattand, 2001). Tsetse flies are of primary importance in the spread and epidemiology of Nagana (Houre, 1976; ILARD, 1990) and thus AAT continues to thrive, and losses incurred are still existing in the affected countries (Meyer, Holt, Selby, & Guitian, 2016; Muhanguzi, et al., 2017). Extensive trypanosomiasis and tsetse (T&T) control operations have been running since the colonial era, however, tsetse infestation in sub-Saharan Africa has scarcely disappeared completely. Furthermore, there is no new veterinary drugs for the treatment of AAT that have been released since 1985(Anene, Chime, Jibike, & Anika, 1991) and at the same time, there is increasing resistance to the existing trypanocides. Following this, there has been a renewed interest in funding committed to AAT control and research into drug discovery and novel control methods since 2000. In addition, the Pan-African Tsetse and Trypanosomiasis Eradication Campaign (PATTEC), whose coordinating office is supported by the African Union, has set a goal for tsetse and AAT elimination. In this case elimination of AAT has been defined as “the reduction to zero in the incidence of AAT in a defined geographical area” (Molyneux, Hopkins, & Zagaria, 2004). This chapter will address the epidemiology of AAT and its possible impacts on livestock productivity. The scientific information given in this chapter will help to inform animal health practitioners, veterinary students, researchers and all interested parties on epidemiology of African animal trypanosomiasis and its economic impacts.