Trapping of free-living, unfed adult and nymphal Amblyomma hebraeum in heartwater endemic areas of South Africa, and the prevalence of Cowdria ruminantium in a sample of adult ticks

Abstract

The main objective of this study was to use the Attraction-aggregation-attachment¬pheromone/carbon dioxide (AAAP/C02) trap on a sustainable basis at six different field sites in South Africa. This trap was developed in Zimbabwe, but had not been used successfully in the field for the collection of free-living adult and nymphal A. hebraeum. A two-year collection survey was carried out at one of the sites, the Rietgat communal grazing area (CGA) where 1 196 adult and 292 nymphal A. hebraeum were trapped with the AAAP/CO2. Only free-living, unfed adult and nymphal A. hebraeum were collected, as these ticks were considered to be epidemiologically more credible than ticks collected off hosts. A distinct seasonal appearance of adult ticks was noted in both 1996 and 1997, and this could explain the difficulty experienced in collecting these ticks in the field in the past. Peak numbers of adult ticks were collected from late spring (September/October) to midsummer (November - January). This was followed by a sharp decline to very low counts for the remainder of the year (February - August). Field work was also conducted at five other sites in South Africa. At three of these sites, the AAAP/CO2 trap was used successfully, these included a farm near East London (n = 187 adults, 17 nymphs) Kruger National Park (KNP) (n = 447 adults) and the Songimvelo Game Reserve (SGR) (n = 48 adults). At the two other sites, namely the MEDUNSA campus (n = 31 adults) and at a farm near Warmbaths (n = 25 adults), the AAAP/CO2 trap was not really successful. A total of 1 934 adult and 309 nymphal A. hebraeum were collected with the AAAP/CO2 trap. A sample (n = 570) of the adult ticks collected from the Rietgat CGA (n = 434), the KNP (n = 88) and the SGR (n = 48) was tested for C. ruminantium with a specific PCR assay developed at the UFIUS AID/SADC Healtwater Research Project in Harare, Zimbabwe. Nearly nine per cent (8.9%) of the ticks from the Rietgat CGA, 5.7% from the KNP and 25% from the SGR were positive for C. ruminantium. The overall infection rate of 9.8% for the total sample (n = 570) is similar to others recorded in southern Africa. This was the first time that a large, statistically-relevant sample of free-living, unfed adult A. hebraeum collected with a AAAP/C02 trap, from a variety of different ecological areas has been processed with a C. ruminantium-specific PCR. The epidemiological data from this project should be more credible than those from many of the previous surveys, where feeding ticks were collected off hosts, and indirect methods used to determine C. ruminantium prevalence.