Disease prevention and anti-vector campaigns: insects.

Control of insect vector populations is an integral part of disease management but has many challenges. Area-wide campaigns, mainly based on insecticide administration, are most effective for control of insect populations, whereas disease prevention is more localised and protects a smaller number of animals against insect vector contact. Various control and prevention techniques are available for use against most insectvectors and are illustrated here by focusing on two important insect groups: biting midges and tsetse flies. Biting midges (Culicoides) present a major threat and challenge to disease and vector control because of limited large-scale control options and the huge population sizes and wide distribution of these insects. Localised disease prevention forms the basis for control, and there is a need for better understanding of the ecology and biology of these insects in order to develop large-scale control techniques. The necessary techniques to effectively control tsetse flies (Glossina) and trypanosomosis exist for both localised and area-wide control. The development of a new, cost-efficient device has had a significant impact in the control of both human and animal trypanosomosis. This is especially relevant in Uganda, where the movement of livestock for trading purposes is implicated in disease distribution and poses an immediate health threat where the two forms of the disease overlap. Although many successes have been achieved, continued research and development is needed to keep abreast of the multitude of challenges in insect vector control.

Prospects for the development of odour baits to control the tsetse flies Glossina tachinoides and G. palpalis s.l.

Field studies were done of the responses of Glossina palpalis palpalis in Côte d'Ivoire, and G. p. gambiensis and G. tachinoides in Burkina Faso, to odours from humans, cattle and pigs. Responses were measured either by baiting (1.) biconical traps or (2.) electrocuting black targets with natural host odours. The catch of G. tachinoides from traps was significantly enhanced ( approximately 5x) by odour from cattle but not humans. In contrast, catches from electric targets showed inconsistent results. For G. p. gambiensis both human and cattle odour increased (>2x) the trap catch significantly but not the catch from electric targets. For G. p. palpalis, odours from pigs and humans increased (approximately 5x) the numbers of tsetse attracted to the vicinity of the odour source but had little effect on landing or trap-entry. For G. tachinoides a blend of POCA (P = 3-n-propylphenol; O = 1-octen-3-ol; C = 4-methylphenol; A = acetone) alone or synthetic cattle odour (acetone, 1-octen-3-ol, 4-methylphenol and 3-n-propylphenol with carbon dioxide) consistently caught more tsetse than natural cattle odour. For G. p. gambiensis, POCA consistently increased catches from both traps and targets. For G. p. palpalis, doses of carbon dioxide similar to those produced by a host resulted in similar increases in attraction. Baiting traps with super-normal (approximately 500 mg/h) doses of acetone also consistently produced significant but slight (approximately 1.6x) increases in catches of male flies. The results suggest that odour-baited traps and insecticide-treated targets could assist the AU-Pan African Tsetse and Trypanosomiasis Eradication Campaign (PATTEC) in its current efforts to monitor and control Palpalis group tsetse in West Africa. For all three species, only approximately 50% of the flies attracted to the vicinity of the trap were actually caught by it, suggesting that better traps might be developed by an analysis of the visual responses and identification of any semiochemicals involved in short-range interaction.

An update of the bovine trypanosomosis situation at the edge of Hluhiuwe-Imfolozi Park, Kwazulu-Natal Province, South Africa.

To obtain updated data on and assess the contribution of trypanosomosis to the disease burden of cattle kept at the edge of the Hluhluwe-iMfolozi Park, a survey was conducted at Mvutshini Dip. Use was made of a purposeful sampling strategy by restricting sampling to animals that the livestock owner considered to be in poor condition. Of a total of 76 blood samples collected, 26 were parasitologically positive and 46 were positive on PCR/RFLP. Almost all infections were due to Trypanosoma congolense savannah subgroup. A total of 63 animals had a PCV < or = 24% and were considered to be anaemic. Results from the survey show that trypanosome infections contribute significantly to the overall burden of disease in the area. Further research is required to develop appropriate control methods.

Selective use of odour-baited, insecticide-treated targets to control tsetse flies Glossina austeni and G. brevipalpis in South Africa.

The effectiveness of odour-baited targets treated with 0.8% deltamethrin in controlling Glossina austeni Newstead and G. brevipalpis Newstead (Diptera: Glossinidae) was evaluated in Zululand, South Africa. Targets were initially deployed in the three habitat types (grassland, woodland and forest) of two adjacent areas at a density of four targets per km(2). One area functioned as the treatment block (c. 35 km(2)) and included the focus of the target deployment, and the second area functioned as a barrier block (c. 40 km(2)) against tsetse fly re-invasion from the untreated area to the south. After 8 months, targets were removed from open grassland in both areas and target density in wooded habitats and sand forest was increased to eight per km(2). Twelve months later, all targets were removed from the barrier block and used to increase target density in the wooded and sand forest habitats of the treatment block to 12 per km(2). This target density was maintained for 14 months. In the treatment area, a 99% reduction in G. austeni females occurred after 13 months at a target density of eight per km(2) in wooded habitat; this was maintained for 22 months. Reduction in G. brevipalpis was less marked. The relatively poor reduction in G. brevipalpis is attributed to the high mobility of this species and its distribution throughout less wooded and more open habitats.

Protective netting, an additional method for the integrated control of livestock trypanosomosis in KwaZulu-Natal Province, South Africa.

Studies were conducted in KwaZulu-Natal, South Africa, to evaluate the effectiveness of netting in preventing Glossina austeni and Glossina brevipalpis from entering H-traps. Results indicated that a net of 1.5 m in height was effective in reducing catches of G. austeni by 59.6% and catches of G. brevipalpis by 80.9%. Increasing the net height to 2.5 m, reduced catches by 96.6% and 100% for G. brevipalpis and G. austeni, respectively. Nets of this height also reduced catches of horse flies by 55%. Although the potential use of protective netting has limitations in tsetse-infested areas of rural northern KwaZulu-Natal, it is a low-technology method that can be used as part of integrated disease management strategies.

Abundance and distribution of the tsetse flies, Glossina austeni and G. brevipalpis, in different habitats in South Africa.

The distribution and abundance of Glossina austeni Newstead and Glossina brevipalpis Newstead (Diptera: Glossinidae) were studied in the three main vegetation types in Zululand, KwaZulu-Natal, South Africa. During a period of 12 months, a trap transect consisting of 38 H-traps traversing the three vegetation types was monitored. The Index of Apparent Abundance (IAA) for G. brevipalpis was high in indigenous forest and open grassland but lower in exotic plantations. Glossina austeni, on the other hand, was captured mainly in or adjacent to indigenous forest. The seasonal trend in the IAA did not differ between vegetation types. The findings on the distribution of G. brevipalpis are in contrast with the historic records. Historically, this species was considered to be restricted to areas with a dense overhead canopy and high relative humidity. The repercussions of these findings for the epidemiology of livestock trypanosomiasis and the control of tsetse in Zululand are discussed.