Competitive displacement and acaricide resistance of two Rhipicephalus (Boophilus) species collected on commercial farms in South Africa.

Rhipicephalus (Boophilus) microplus, an invasive species to Africa, and the endemic R. (B.) decoloratus are of high economic importance in the cattle industry. Invasion of the alien species in South Africa has mostly been reported for traditional communal grazing areas where it seemed to be rapid and, in some cases, even replaced the native species. The alien species is also assumed to already be resistant to acaricides upon invasion. The presence of R. (B.) microplus on commercial farms was therefore investigated and resistance screening of both species to field concentrations of cypermethrin, amitraz, and chlorfenvinphos was determined by means of the larval immersion test. Results showed that only 3.7% (of 383) tick collections submitted were R. (B.) microplus populations. A further 1.6% (of 383) showed co-existence of the two species. Comparing the level of resistance to the acaricides between the two species indicated a mean phenotypic resistance of 66.2 and 26.5% of R. (B.) decoloratus populations to cypermethrin and amitraz, respectively. This was significantly lower for R. (B.) microplus, with 23.0 and 4.1% of its populations resistant to cypermethrin and amitraz, respectively. Closed commercial farming areas seemed to have a preventative advantage for the invasion of R. (B.) microplus and displacement of R. (B.) decoloratus by R. (B.) microplus. Regular monitoring of these two species may be of high importance to prevent unnecessary financial losses due to insufficient control and increased awareness of the threat of Asiatic babesiosis vectored by R. (B.) microplus.

Evolution of acaricide resistance of Rhipicephalus decoloratus on commercial farms in South Africa.

The development of tick resistance to chemical control plays a major role in the increasing global economic impact of ticks on cattle farming. Reports on acaricide resistance of Rhipicephalus decoloratus, endemic to Africa and South Africa, are relatively few compared to the closely related and globally distributed Rhipicephalus microplus. In South Africa, ectoparasite control became the sole responsibility of each commercial producer when compulsory dipping was phased out from 1984. Different acaricidal management strategies resulted in the simultaneous development of resistance to various acaricide groups. The establishment of a Pesticide Resistance Testing Facility provided the opportunity to test Rhipicephalus (Boophilus) populations, submitted from all over South Africa, for resistance where failure of chemical control was experienced. The number of populations resistant to cypermethrin (CM) was significantly higher than those tested as resistant to amitraz (AM), or chlorfenvinphos (CFVP). No significant difference was found between the number of populations resistant to AM and CFVP. The evolution of R. decoloratus resistance at the end of a 12 year period indicated a stable but high prevalence of 90% overall resistance to CM. The same trend was seen for AM-resistant R. decoloratus populations but at a lower level of just over 40%. In contrast, CFVP resistant R. decoloratus populations showed a decreasing trend with near-total reversion to susceptibility. Multi-resistance was present in more than 50% of populations tested with the highest incidence in the Eastern Cape, KwaZulu-Natal, and Western Cape provinces.

Acaricide resistance of Rhipicephalus (Boophilus) decoloratus (Acari: Ixodidae) on commercial farms in South Africa: filling a gap in historical data.

In South Africa, acaricides are widely used for tick control but very few reports are available on resistance development of Rhipicephalus (Boophilus) decoloratus Koch to chemical control on commercial farming systems in Africa, south of the Sahara. Resistance to different acaricide classes reported over the years was mostly from localised communal farming systems. This report addresses the lack of available information on resistance development by reviewing results found during a National Tick Resistance Survey carried out from 1998 to 2001, laying the foundation for more recent research on resistance development, and the evolution of resistance over the years. One hundred and eighty R. decoloratus populations were randomly collected from commercial farming systems, covering most of the provinces of South Africa. Larval immersion tests were used to determine phenotypic resistance for each tick population and 6.6% of the populations tested were found to be resistant to amitraz, 35.5% to cypermethrin, and 36.1% to chlorfenvinphos. Multi-resistance to all three acaricides was found in 1.2% of populations and a further 25.8% of the populations were resistant to two acaricides. The detection of resistance of Rhipicephalus (Boophilus) species to currently used or new acaricides is an essential tool in resistance management. The acaricides tested for the resistance of R. decoloratus during the survey are currently still in use in South Africa and these historical results, never published before, can be valuable and can act as reference data to determine the evolution of resistance development to acaricides in more recent studies.

Biogeography of the theileriosis vector, Rhipicephalus appendiculatus under current and future climate scenarios of Zimbabwe.

Climate directly influences the epidemiology of vector-borne diseases at various spatial and temporal scales. Following the recent increased incidences of theileriosis in Zimbabwe, a disease mainly transmitted by Rhipicephalus appendiculatus, we determined lethal temperatures for the species and current and possible future distribution using the machine learning algorithm 'Maxent'. Rhipicephalus appendiculatus larvae had an upper lethal temperature (ULT50) of about 44 ± 0.5 °C and this was marginally higher for nymphs and adults at 46 ± 0.5 °C. Environmental temperatures recorded in selected zonal tick microhabitats were below the determined lethal limits, indicating the ability of the tick to survive these regions. The resultant model under current climatic conditions showed areas with high suitability indices to the eastern, northeastern and southeastern parts of the country, mainly in Masvingo, Manicaland and Mashonaland Central provinces. Future predictions as determined by 2050 climatic conditions indicate a reduction in suitable habitats with the tick receding to presently cooler high elevation areas such as the eastern Highlands of Zimbabwe and a few isolated pockets in the interior of the country. Lowveld areas show low suitability under current climatic conditions and are expected to remain unsuitable in future. Overall, the study shows that R. appendiculatus distribution is constrained by climatic factors and helps identify areas of where occurrence of the species and the disease it transmits is highly likely. This will assist in optimizing disease surveillance and vector management strategies targeted at the species.