Ecology and epidemiology of rabies in humans, domestic animals and wildlife in Namibia, 2011-2017

Abstract

Rabies is a fatal zoonotic disease that causes a heavy burden on societies. Namibia, a country in southern Africa, is aiming at controlling the disease in its main reservoir, the domestic dog. To facilitate the implementation comprehensive information on the ecology and epidemiology of the disease and surveillance is of utmost importance. The study presented assesses the baseline data for both human and animal rabies surveillance in Namibia in recent times and establishes correlations with ecological and socio-economic data in order to provide an up-to-date picture on the epidemiology of rabies in Namibia. For instance, it was important to identify the main drivers in the epidemiology, and whether the control strategy by mass vaccination of dogs is undermined by cycles of rabies in wildlife. Rabies in humans was reported mainly from the Northern Communal Areas (NCAs), with a total of 113 cases from 2011 to 2017, representing an incidence of between 1.0 and 2.4 annual human rabies deaths per 100,000 inhabitants. Kavango, the region with the highest human rabies incidence was also the region with the lowest animal rabies surveillance intensity. Generally, the vast majority (77%) of dog samples originated from communal farm land, followed by urban areas (17%), while only a small fraction (3%) was submitted from freehold farm areas. In contrast, kudu and eland submissions were almost exclusively from freehold farmland (76%) and urban areas (19%), whereas the submission of cattle samples was evenly distributed among freehold farms (46%) and communal farm land (46%). The likelihood of sample submission decreased exponentially with distance to one of the two laboratories. Overall, 67% (N = 1,907) of all samples submitted tested rabies-positive, with the highest positivity rate observed in kudus (89%) and jackals (87%). The transmission cycle of rabies in dogs appears restricted to the northern communal areas of Namibia, whilst rabies in wildlife species is predominately reported from farmland in central Namibia, mostly affecting kudu (Tragelaphus strepsiceros) and livestock with a likely reservoir in wildlife canids such as jackals or bat-eared foxes. The analysis confirms the presence of two independent transmission cycles in Namibia with little geographic overlap, thus allowing for a sustainable control of rabies in dogs in the NCAs.

Fig 1

a) map depicting the location of Namibia in Africa (right) and the boundaries and names of the Namibian districts and the veterinary cordon fence (dark grey). The color scale indicates the average rabies incidence per 100,000 people and per region. The locations of the laboratories, i.e. the CVL in Windhoek (a) and in Ondangwa (b) is shown. The absolute number of cases per regions and the rabies incidence per 100,000 people is also provided in brackets. Below: temporal trend (b) and the distribution per age class and gender (c) of human rabies cases in Namibia, 2011–2017.

Fig 2

Surveillance intensity (number of animals submitted for testing per 100,000 inhabitants per year) per region (a). To the right: Absolute numbers of submissions to the laboratories in Windhoek and Ondangwa per distance (b). Because the area as a potential origin of submissions increases with radial distance, the numbers of submissions were standardized and expressed per 100sqkm (c).

Fig 3. Spatial distribution of rabies surveillance data in Namibia, 2011–2017.

Namibian land use is indicated on the map (left), and the proportion of submissions in relation to land use for kudu, dogs, cattle and other wildlife is indicated (right).

Fig 4. Rabies surveillance in dogs: Rabies cases (red dots), reported rabies suspects (orange dots) and dog samples tested negative (blue dots).

The vast majority of rabies cases in dogs were reported from the NCAs north of the veterinary fence (n = 600), while in the remaining part of Namibia only 44 cases were confirmed. The underlying shading represents the relative human population density with darker colors indicating a higher density. Below: Enlargement of the area in the NCAs with the highest numbers of rabies cases. Main roads (blue, pink) are indicated. To the right: charts showing the absolute number of rabies cases or rabies suspects per density class (left y-axis) and the density of rabies cases or rabies suspects per km² per respective density class (right y-axis).

Fig 5. Spatial display of surveillance data (2011–2017) for livestock animals (left), kudus (right).

Densities of kudus and livestock are indicated. Below: charts showing the absolute number of rabies cases or rabies suspects per density class (left y-axis) and the density of rabies cases or rabies suspects per square kilometer per respective density class (right y-axis).