Background
In cattle, TB is generally a chronic wasting disease, and infected cattle survive for several months or years, often without manifesting clinical signs (Morris et al. 1994). The chronic, insidious nature of bovine tuberculosis (BTB) poses a significant challenge when attempting to contain the disease once it is introduced into a susceptible population.
If, as is often the case, infection in a herd remains undetected, the disease spreads silently from individual to individual and ultimately also to in-contact, non-infected herds, particularly when infected animals are allowed to roam freely.Chronic diseases such as BTB are difficult to investigate if one is dependent on the presence of clinical signs alone because diseased cattle often only manifest ill-thrift with depressed fecundity and productivity without regular mortalities (Jolles et al. 2005). Further challenges posed by BTB include its wide host range, the variety of interspecies transmission patterns (Skuce et al. 2012), and the large variation of different host species in their susceptibility to infection with Mycobacterium bovis (Drewe et al. 2014).
Africa is a vast continent, and the management practices vary substantially from the traditional nomadic and pastoral systems found in most of Africa to increasing small-scale urban farming and the sophisticated commercial farming systems found in countries such as South Africa, Namibia, and Botswana giving them access to international markets. Given the differences in disease dynamics in each of these systems, it should be expected that the epidemiology of BTB would accordingly
A. B. Dibaba (*)
Department of Pathobiology, College of Veterinary Medicine, Tuskegee University, Tuskegee, AL, USA
e-mail: adibaba@tuskegee.edu
C. J. Daborn
Tigoni Veterinary Services (TVS), Nairobi, Kenya
e-mail: tvs@habari.co.tz
© Springer Nature Switzerland AG 2019 89
A.
B. Dibaba et al. (eds.), Tuberculosis in Animals: An African Perspective, https://doi.org/10.1007/978-3-030-18690-6_7 differ substantially, and it would be dangerous to extrapolate between the different systems.There is an ample amount of epidemiological information available about BTB in cattle, and the other domestic and wild animals in Africa, but the information is often incomplete, fragmented, historical, contradictory, and difficult to access. The purpose of this chapter is to collate the available information, highlighting the complexities and contradictions, signposting the direction for further studies, and making the existing knowledge more readily available to those with an interest in the epidemiology and control of BTB in Africa.
In the developed countries, the interaction between the agent and hosts and prevailing farming systems are well documented, and the information is sufficient to predict the likelihood of BTB being contracted. Under these circumstances, risk factors determining the probability of infection and transmission of M. bovis are intimately linked to factors that affect the susceptibility of exposed animals (Skuce et al. 2012). Most livestock in Africa are farmed within regions where no formal control measures for BTB in the form of the conventional test-and-slaughter policy are applied (Cosivi et al. 1998). There are many gaps in the continent’s datasets containing information about the distribution and prevalence of BTB. Likewise, there are only a few reports describing host and environmental factors that influence the transmission of M. bovis between various susceptible animal species and humans (Drewe et al. 2014).
Studies conducted in Africa elucidated some of the predominant risk factors, including a variety of farm management practices, the presence of wildlife reservoirs, and environmental factors that play a role in the transmission of the disease. Specific factors relevant to disease management, and those that are generally accepted to promote the introduction and spread of BTB, include the uncontrolled movement of animals (Oloya et al.
2007), intensification of husbandry systems (Elias et al. 2008), and contact with wildlife reservoirs (Michel 2008). Other locality-specific risk factors on the continent that will influence the application of practical preventative measures have been identified.A recent analysis of wildlife-related risk factors dealt with the presence and spread of M. bovis in different wildlife populations (Gallagher et al. 1972; Bengis et al. 1996; Michel 2008) and the role that they may play in sustaining and disseminating the infection to other wildlife species and livestock. In Africa, this information is vital to consider when devising effective ways of controlling the disease and reducing interspecies transmission, should the approach be to eventually attempt to eradicate the disease from the continent.
It is clear that the risk of contracting BTB is multifactorial, and its investigation poses several, and often complex, challenges. To explore this complexity, the methodology of the Epidemiologic Problem Oriented Approach (EPOA) is applied to present the epidemiology of BTB in Africa.
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