The Epidemiology of BTB in Malawi

Limited data are available for the distribution, spread, and impact of BTB in Malawi. The number of animals and individual herds infected, sources of infection, the modes of spread, and other drivers of the epidemiology of the disease are poorly investigated.

Age There are contradictory data indicating that in Malawi the prevalence of BTB and age are not linked (Bedard et al. 1993). Generally, however, very few calves react to the CCT (Berggren 1981). These contradictory findings may be related to a number of factors that were not further investigated.

Breed Although there are differences of opinion as to whether breed does, in fact, determine susceptibility, a number of studies seem to support this notion, and many of the differences in the prevalence of BTB in Malawi can be attributed to the location of a specific breed of cattle (Bedard et al. 1993). In southern Malawi 54.5% of zebu cattle carcasses were totally condemned compared to the 66.7% of the Sussex breed (Ellwood and Waddington 1972). Of cattle tested at dip tank sites (representing village zebu cattle), 4.8% reacted to the CCT, while in the bulking- group animals (dairy farms with introduced European breeds), 2.6% reacted. Taking breed into consideration, only 2.7% of the pure- and crossbred dairy cows were positive, compared to 4.7% of the zebu cattle. The data are contradictory and the reactors are often found in clusters within dip tank or bulking groups. The type of farming practice appears more likely to be the cause of the perceived breed predis­position to the disease.

Sex Only one study in Malawi assessed the relationship between the prevalence of BTB and the sex of the animal. The number of reactors was significantly higher in oxen (9.6%) and bulls (8.4%) than in females (3.3%) (Bedard et al. 1993).

Geographic Location There was a general belief that BTB was unevenly spread in Malawi and that higher prevalence rates were recorded in the northern region (Bedard et al. 1993). Other studies (Bedard et al. 1993; Ellwood and Waddington 1972) similarly indicated a clustering of BTB in certain geographic areas and herds, but methodological differences might account for these discrepancies. The reports of the Central Veterinary Laboratory for the period 2011-2015 (Table 16.2) provided more comprehensive data. The largest number of BTB outbreaks occurred in Lilongwe and Kasungu (central) and Mzuzu (north) ADDs that is likely to be due to the concentration of smallholder dairy farms that are subject to compulsory screening for BTB because of public health requirements. The ADDs with the highest number of BTB outbreaks (Shire Valley and Blantyre) are in the South with its concentration of commercial feedlots that are able to pay for BTB screening services. The existing information remains fragmented and is mostly gathered by passive surveillance and opportunistic tuberculin skin testing operations.

Spatial clustering of BTB may be associated with other unknown locality-specific determinants. While the role of wildlife in the epidemiology of BTB in Malawi is unknown, disease hotspots, as is seen in Zambia, may be associated with the presence of wildlife reservoirs (Munyeme et al.

Herd Management Livestock management practices in Malawi are linked to their geographic location, and the type of system seems to play a pivotal role in deter­mining the prevalence of the disease. Dip tank cattle have a higher BTB prevalence than cattle from bulking groups probably because of their large herd sizes and the intermingling of animals around the dipping tanks (Bedard et al. 1993). In terms of determining priorities for future BTB control programs, it would probably be more appropriate to focus on either bulking or dipping tank groups, rather than to use the data based on geographic location, breed, or sex to determine priority areas in which to initiate control programs.

Several factors are likely to contribute to the increased risk of BTB in smallholder dairy farms in Malawi. The importation of pure-bred dairy cattle, mostly from Zambia and South Africa, may further contribute to the problem because of the occurrence of BTB in cattle and wildlife populations in those countries (Michel et al. 2006; Munyeme et al. 2010).

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