Estimating the distribution and abundance of Rhipicephalus appendiculatus in Africa

The brown ear tick Rhipicephalus appendiculatus is responsible for transmitting the parasite Theileria parva in eastern, central and southern Africa, where it causes East Coast fever, Corridor disease and January disease in cattle. In an effort to assess the impact of these diseases and their control on livestock production in the region, studies are underway to model the factors controlling the distribution of the vector tick. Three recent studies have attempted to quantify and evaluate the variables influencing the distribution of R. appendiculatus, and these are reviewed and discussed in this paper.

The relationship between the distribution of an ecoclimatic index of suitability for the tick, calculated by the model CLIMEX on a 25 km² interpolated climate database for Africa, and of recorded observations of the tick itself, are evaluated. The sensitivity and specificity of the ecoclimatic index were calculated as 70.2–82.6% and 69.4–84.7%, respectively. In addition, addition, satellite-derived mean maximum normalized difference vegetation index (NDVI) for 1987 was assessed as a predictor of R. appendiculatus habitat in eastern Africa, by comparing the distribution of NDVI values in areas in which the presence of the tick has and has not been recorded. Some visual correlation was observed between mean maximum NDVI values of 0.15 or greater and known R. appendiculatus distribution in Kenya. In addition, this range of NDVI values corresponded closely to areas in Ethiopia where the tick does not occur but for which climatic suitability was predicted by CLIMEX. However, these NDVI values should not be considered universal predictors of habitat suitable for R. appendiculatus, and further study of the NDVI with ground truthing of habitat is considered necessary.     

An assessment of the economic impact of heartwater (Cowdria ruminantium infection) and its control in Zimbabwe.

Heartwater, caused by the rickettsial organism Cowdria ruminantium, is a serious constraint to livestock development in much of sub-Saharan Africa. Traditionally, the disease has been controlled by the use of chemical acaricides to control the vector tick. The University of Florida/USAID-supported heartwater research project (based in Zimbabwe) is developing a new inactivated vaccine to control the disease. In order that the vaccine is used effectively, the project has been studying the epidemiology of the disease in different livestock production systems of Zimbabwe, and evaluating the economic impact of the disease and of its future control using a vaccine such as the one under development. Initially, field studies were conducted to characterise the communal and commercial livestock-productions systems at risk from heartwater and to understand the epidemiology of the disease. The data from these studies were then applied to an infection-dynamics model of heartwater, which was used to provide estimates of disease incidence and impact under various scenarios over a period of 10 yr. Two principal outputs of the epidemiological model (cumulative annual heartwater incidence and infection-fatality proportion) were key inputs into an economics model. The estimated total annual national losses amount to Z$ 61.3 million (US$ 5.6 million) in discounted value terms over 10 yr. Annual economic losses per animal in the commercial production system (Z$ 56 discounted values) are 25 times greater than the losses in the communal system (Z$ 2.2). The greatest component of economic loss is acaricide cost (76%), followed by milk loss (18%) and treatment cost (5%). Losses in outputs other than milk (beef, traction and manure) appear to be minimal. A new vaccine has the promise of a benefit: cost ratio of about 2.4:1 in the communal and 7.6:1 in the commercial system. A control strategy based on a new vaccine would yield additional non-financial benefits to farmers and the government resulting from reductions in the use of chemical acaricides.     

The control of heartwater on large-scale commercial and smallholder farms in Zimbabwe.

As part of a series of studies associated with the development of improved vaccines for heartwater (a tick-borne disease of ruminant livestock caused by Cowdria ruminantium), field surveys were carried out to assess losses associated with the disease and the costs associated with controlling it in the two main agro-ecological zones of Zimbabwe (lowveld and highveld) where heartwater is believed to be endemic and epidemic, respectively. In each zone, a cross-sectional study was performed in the main farming systems (smallholder (SH) and large-scale commercial (LSC) beef and dairy), followed by longitudinal studies in the same sectors to improve data accuracy for some parameters. Suspected heartwater-specific mortality in cattle was similar in all LSC sectors (p = 0.72) accounting for a median 1% mortality risk. Heartwater-specific mortality in SH areas was not assessed due to poor diagnostic ability of the farmers. Few LSC farms and SH households kept sheep; suspected heartwater-specific mortality in LSC sheep was 0.8% in the lowveld and 2.4% in the highveld. Goats were a major enterprise in SH areas but not on LSC farms. Suspected heartwater mortality in LSC goats was 0.8% at one site in the highveld and 17.5% on a farm in the lowveld. Application of acaricides was the major control method for heartwater and other tick-borne diseases on both SH and LSC farms. On LSC farms, plunge dipping was used most frequently and the number of acaricide applications ranged widely between 3 and 52 per year. The total cost of acaricides per head per annum was higher in highveld dairies than in highveld and lowveld beef enterprises (p = 0.03). In SH areas, cattle plunge dipping was conducted by the government with an average frequency of 8 +/- 2 (sd) immersions per annum in both the lowveld and highveld. The type of tick control on sheep and goats in all production systems was highly variable (ranging from none to hand removal or intensive acaricide treatment). Suspected heartwater cases on LSC farms were treated with tetracyclines; treatment was not reported in SH areas. Reported treatment costs were high (median Z$ 120) and highly variable (range Z$-833). Vaccination against heartwater with the live, blood-based vaccine was reported on only one LSC farm. LSC farms applying acaricide 30 or more times per year reported higher morbidity (p < 0.0001) and mortality (p < 0.0001) than farms applying acaricides less than 30 times a year. This finding supports the use of reduced tick control in the management of heartwater in Zimbabwe.     

Reducing a spatial database to its effective dimensionality for logistic-regression analysis of incidence of livestock disease

Large databases with multiple variables, selected because they are available and might provide an insight into establishing causal relationships, are often difficult to analyse and interpret because of multicollinearity. The objective of this study was to reduce the dimensionality of a multivariable spatial database of Zimbabwe, containing many environmental variables that were collected to predict the distribution of outbreaks of theileriosis (the tick-borne infection of cattle caused by Theileria parva and transmitted by the brown ear tick). Principal-component analysis and varimax rotation of the principal components were first used to select a reduced number of variables. The logistic-regression model was evaluated by appropriate goodness-of-fit tests.     

Land-use and land-cover dynamics in response to changes in climatic,biological and socio-political forces: the case of southwestern Ethiopia

Few studies of land-use/land-cover change provide an integrated assessment of the driving forces and consequences of that change, particularly in Africa. Our objectives were to determine how driving forces at different scales change over time, how these forces affect the dynamics and patterns of land use/land cover, and how land-use/land-cover change affects ecological properties at the landscape scale. To accomplish these objectives, we first developed a way to identify the causes and consequences of change at a landscape scale by integrating tools from ecology and the social sciences and then applied these methods to a case study in Ghibe Valley, southwestern Ethiopia. Maps of land-use/land-cover change were created from aerial photography and Landsat TM imagery for the period, 1957–1993. A method called `ecological time lines' was developed to elicit landscape-scale explanations for changes from long-term residents. Cropland expanded at twice the speed recently (1987–1993) than two decades ago (1957–1973), but also contracted rapidly between 1973–1987. Rapid land-use/land cover change was caused by the combined effects of drought and migration, changes in settlement and land tenure policy, and changes in the severity of the livestock disease, trypanosomosis, which is transmitted by the tsetse fly. The scale of the causes and consequences of land-use/land-cover change varied from local to sub-national (regional) to international and the links between causes and consequences crossed scales. At the landscape scale, each cause affected the location and pattern of land use/land cover differently. The contraction of cropland increased grass biomass and cover, woody plant cover, the frequency and extent of savanna burning, and the abundance of wildlife. With recent control of the tsetse fly, these ecological changes are being reversed. These complex patterns are discussed in the context of scaling issues and current conceptual models of land-use/land-cover change.