Eva M. De Clercq

Global risk mapping for major diseases transmitted by Aedes aegypti and Aedes albopictus

OBJECTIVES The objective of this study was to map the global risk of the major arboviral diseases transmitted by Aedes aegypti and Aedes albopictus by identifying areas where the diseases are reported, either through active transmission or travel-related outbreaks, as well as areas where the diseases are not currently reported but are nonetheless suitable for the vector. METHODS Data relating to five arboviral diseases (Zika, dengue fever, chikungunya, yellow fever, and Rift Valley fever (RVF)) were extracted from some of the largest contemporary databases and paired with data on the known distribution of their vectors, A. aegypti and A. albopictus. The disease occurrence data for the selected diseases were compiled from literature dating as far back as 1952 to as recent as 2017. The resulting datasets were aggregated at the country level, except in the case of the USA, where state-level data were used. Spatial analysis was used to process the data and to develop risk maps. RESULTS Out of the 250 countries/territories considered, 215 (86%) are potentially suitable for the survival and establishment of A. aegypti and/or A. albopictus. A. albopictus has suitability foci in 197 countries/territories, while there are 188 that are suitable for A. aegypti. There is considerable variation in the suitability range among countries/territories, but many of the tropical regions of the world provide high suitability over extensive areas. Globally, 146 (58.4%) countries/territories reported at least one arboviral disease, while 123 (49.2%) reported more than one of the above diseases. The overall numbers of countries/territories reporting autochthonous vector-borne occurrences of Zika, dengue, chikungunya, yellow fever, and RVF, were 85, 111, 106, 43, and 39, respectively. CONCLUSIONS With 215 countries/territories potentially suitable for the most important arboviral disease vectors and more than half of these reporting cases, arboviral diseases are indeed a global public health threat. The increasing proportion of reports that include multiple arboviral diseases highlights the expanding range of their common transmission vectors. The shared features of these arboviral diseases should motivate efforts to combine interventions against these diseases.

High-resolution predictive mapping for Rhipicephalus appendiculatus (Acari: Ixodidae) in the Horn of Africa

The brown ear tick Rhipicephalus appendiculatus, vector of East Coast fever (ECF) and related cattle diseases caused by Theileria parva has never been reported from the Horn of Africa. Habitat suitability for this tick species was predicted using Maxent modelling technique based on R. appendiculatus records in Sub-Saharan Africa. Two models were developed: the first is based on the tropical R. appendiculatus distribution and the one is based on the distribution records in the temperate region of Sub-Saharan Africa. The tropical model shows favourable habitat in much of the Ethiopian highlands. The whole Djibouti, the south eastern Ethiopian lowlands, majority of Somalia and Eritrea were found to be not suitable for the survival and development of this tick species. Highly suitable areas occur in areas which have moderate temperature and high precipitation. Introductions of R. appendiculatus into the Horn of Africa probably have been prevented by the natural barrier between the known R. appendiculatus distribution range in East Africa and the Horn of Africa. The effect of an introduction of R. appendiculatus and thereby ECF into the Horn of Africa could be catastrophic since the cattle in this area have no immunity against ECF, and mortality might be considerable in all age groups of cattle.

West African Cattle Farmers’ Perception of Tick-Borne Diseases

Worldwide, cattle production is struggling to face the negative impacts caused by ticks and Rhipicephalus (Boophilus) microplus is one of the most harmful ticks for livestock. Most of the people in West Africa depend on cattle farming and subsistence agriculture. The presence of ticks on cattle is a major problem faced by smallholder farmers who fight for their livelihood. National and regional tick control programs could assist these rural communities in protecting their livelihoods against ticks and tick-borne diseases, but only if they take into account the targeted herders and their perception on cattle management and tick control. This paper aims to provide a better insight in the socio-economic characteristics of Beninese cattle farmers, and their perception on tick burden, as well as to document common tick control strategies. Different tick species and their seasonality are well understood by cattle herders. For tick control, many still use manual tick removal, especially in the north of the country. The high cost of acaricides, the lack of financial means of African farmers, and of the local stockbreeders in particular, limits the use of acaricides in livestock breeding in Benin. While aiming to increase the meat or milk production of their animals, stockbreeders who can afford it sometimes turn to an abusive use of acaricides, which might in time lead to an increase in tick resistance. This study remains one of the rare studies to report extensively on the perceptions of West African cattle herders.

Classification of anthropogenic landscapes

Several vector-borne diseases thrive in and emerge from transition areas between urban, rural and natural areas. In West Africa, these are generally difficult to map because they are composed of complex mixtures of land cover types. In this paper, we use weekly NDVI time series to map transition environments. The high temporal resolution increases the chance of obtaining cloud-free images and allow to study landscape phenology. While NDVI time series data are mostly used for the study of natural vegetation, this paper focusses on human modification of the landscape. The resulting maps of human landscape utilization are useful for assessing the exposure to disease vectors.

Heterogeneity in a communal cattle-farming system in a zone endemic for foot and mouth disease in South Africa

In South Africa, communal livestock farming is predominant in the foot and mouth disease control zone adjacent to the Greater Kruger National Park (KNP), where infected African buffaloes are common. During routine veterinary inspections of cattle in this area, a large amount of production and demographic parameters were being recorded. These data were collated for a five-year period (2003-2007) in three study sites to better understand the temporal dynamics and spatial heterogeneity in this system. A decreasing gradient from South to North with respect to both human and cattle population densities was observed. Rainfall and human population density alone could explain 71% of the variation in cattle density. Northern and central sites showed an overall decrease in total cattle numbers (15.1 and 2.9%, respectively), whereas a 28.6% increase was recorded in the South. The number of cattle owners in relation to cattle numbers remained stable during the study period. Only 4.0% of households in the South own cattle, compared to 13.7 and 12.7% in the North and Centre. The overall annual calving rate was 23.8%. Annual mortality rates ranged from 2.4 to 3.2%. Low calf mortality (2.1%) was recorded in the North compared to the South (11.6%). Annual off-take in the form of slaughter averaged 0.2, 11.7, and 11.0% in the North, Central and South sites, respectively. These figures provide valuable baseline data and demonstrate considerable spatial heterogeneity in cattle demography and production at this wildlife-livestock interface, which should be taken into consideration when performing disease risk assessments or designing disease control systems.