Vector competence of Glossina austeni and G. brevipalpis and characterization of Trypanosoma congolense strains from northern KwaZulu-Natal, South Africa

Abstract

Tsetse-transmitted trypanosomosis (Nagana) has been the cause of cattle losses in the past and still presents a major problem to livestock owners in northern KwaZulu-Natal (KZN), South Africa. Although information on the distribution and abundance of the tsetse flies Glossina brevipalpis and G. austeni in northern KZN exists, data on related vector competence and virulence of Trypanosoma congolense parasites, the main cause of outbreaks, are lacking. The aim of the first part in this study was to determine the rate of natural T. congolense infections in wild and colony-reared tsetse flies of the two species and their infectivity in susceptible cattle. Flies were collected from cattle dip tanks and game parks using H-traps. A total of 637 G. brevipalpis and 40 G. austeni were dissected immediately after collection to determine their infection rates. A total of 699 G. brevipalpis and 49 G. austeni flies collected at different periods from the same locations were used in xenodiagnosis experiments. Teneral colony flies were fed on infected animals under controlled conditions and dissected 21 days post infection to assess their infection rates. Of the field collected flies G. austeni harboured 7.5% parasites in the midgut (immature infections) and 7.5% in the proboscis (mature infections). In G. brevipalpis, the infection with the immature stages was lower (0.7%) and no mature infections were observed. All four batches of G. austeni collected from the field transmitted T. congolense to 3 susceptible bovines and one goat but the 10 batches of G. brevipalpis did not transmit any trypanosome parasites to any of the 10 susceptible bovines. Both G. austeni and G. brevipalpis colony flies acquired trypanosome infection from four infected bovines, with immature infections range of 19.6% - 33.3% (n = 534) and 0.4% - 4.2% (n = 882), respectively. However, mature infections were only found in G. austeni (average 3.9%). The second part of the study aimed at characterization of T. congolense stocks circulating in cattle and buffalo in northern KZN, by molecular tools and using the mouse model. A total of 30 isolates were successfully recovered from the mice inoculated with parasitologically positive field blood samples, 2 and confirmed to belong to the genus Trypanosoma by PCR of the 18S ribosomal RNA gene. The identity of 21 samples was confirmed to belong to T. congolense Savannah by TCN-1 and TCN-2 primer sets specific for the amplification of the Savannah-type T. congolense and 9 isolates amplified with the T. congolense Kilifi-specific primers. Virulence testing in mice (6 mice per group) of 21 cattle-derived and 5 buffalo-derived T. congolense isolates had revealed three categories: high, moderate and low profiles based on the pre-patent period (PPP), the patent period (PP) and the median survival time (MST). In addition, the progression of parasitaemia and the development of anaemia as judged by the packed cell volume (PCV) percentage were recorded. The number of days to the percentage drop in the PCV values, PP and the MST between mice injected with the HVS, MVS and LVS was statistically highly significant (P < 0.0001). Overall, 46.2, 30.8 and 23.0% of the 26 strains were characterized as of high, moderate and low virulence. All of the LVS except one (6/7; 85.7%) originated from locations which are not close to game parks. The third part of the study aimed at testing the susceptibility/resistance of Nguni cattle to virulent T. congolense strains selected from the “mouse model” study. The high and the low virulent strains were inoculated into 30 Nguni and 6 Friesians used as susceptible controls. A rapid development of parasitaemia was observed with both LVS and the HVS a week post infection with subsequent drop in PCV values in both cattle breeds. None of the Nguni cattle infected with the LVS required treatment while all of the Friesians were treated (PCV ≤ 19% for three consecutive days). By day 40 - 54 postinfections with HVS, 100% of Friesian cattle and only 41.7% of Nguni had received treatment. The study showed that Nguni cattle had a considerable degree of innate resistance to infection with a high virulent strain and 58.3% were able to control the progression of severe anaemia and parasitaemia similar to the well characterized trypanotolerant cattle in other parts of Africa. Eradication of trypanosomosis from affected African countries is an unrealistic goal. Efforts made in the past to control the disease through the use of trypanocidal drugs and the management of the vector have been impended by decreasing efficacy of available drugs and the difficulties of sustaining tsetse control. Therefore, new opportunities for improved productivity are necessary and can be attempted through selection for trypanotolerance (D’leteren et al., 1998).