3. Tsetse control (including environmental side effects)

Tsetse flies (Glossina) are important agricultural and medical vectors transmitting the African trypanosomes, the agents of sleeping sickness disease in humans and various diseases in animals (nagana). While the prevalence of disease has increased to epidemic proportions, lack of a mammalian vaccine and affordable and effective drugs has hindered disease control. Trypanosomiasis management relies heavily on the control of its single insect vector, the tsetse fly. Despite the effectiveness of some of these tools, their impact on disease control has not been sustainable due to their local nature and extensive dependence on community participation. Recent advances in molecular technologies and their application to insects have revolutionized the field of vector biology, and there is hope that such new approaches may form the basis for future tsetse interventions. The success of the genetic approaches aiming to disrupt the transmission cycle of the parasite in their invertebrate host depends on full understanding of the interaction between tsetse and trypanosomes. This article reviews the biology of trypanosome development in the fly and the multiple bacterial symbionts that inhabit the same gut environment. The availability of a genetic transformation system for the midgut symbiont allows for gene products to be expressed in vivo in the tsetse gut where they can produce a hostile environment for pathogen transmission. The characterization of gene product(s) with anti-pathogenic properties and their expression in vivo is discussed. A strategy is outlined where the replacement of susceptible insect phenotypes with their engineered refractory counterparts can result in decreased disease transmission.

Tick- and tsetse-borne diseases cost Africa ~US$4-5 billion per year in livestock production-associated losses. The use of pyrethroid-treated cattle to control ticks and tsetse (Glossina sp.) promises to be an increasingly important tool to counter this loss. However, uncontrolled use of this technology might lead to environmental damage, acaricide resistance in tick populations and a possible exacerbation of tick-borne diseases. Recent research to identify, quantify and to develop strategies to avoid these effects are highlighted.

At Mkwaja and Mivumoni ranches in NE Tanzania the catches of Glossina pallidipes, G. brevipalpis, G. morsitans morsitans and G. austeni were usually about two to three times greater in traps of the Ngu, Epsilon and F3 types than in the Blue Biconical and Pyramidal traps when used with odours. Catches from moving men were improved about three times when the men carried a black screen, and increased by about another seven times for female G. pallidipes when a vehicle was the bait. The sensitivity of surveys for tsetse was estimated to be improved about 300 times for G. pallidipes and two to five times for G. brevipalpis and G. m. morsitans when surveys use traps instead of vehicles as baits. The needs for further research and field trials with survey methods are discussed.

The 19th Conference of the World Association of Veterinary Parasitology, held at New Orleans, Louisiana, USA, 10-14 August 2003, had as its theme "Old Dreams - New Visions: Veterinary Parasitology in the 21^st Century" and was intended to stimulate discussion of the application of new technologies, together with traditional disciplines, to answer questions carried over from earlier times as well as newer questions. A paper on the control of tsetse flies and trypanosomes using molecular genetic methods was included (see 12683).

A trial to evaluate the efficacy of a 1 percent cyfluthrin pour-on formulation (Cylence, Bayer) in reducing the incidence of bovine trypanosomosis was conducted in an area of ca. 2 000 km² of the Eastern Province of Zambia. The trial area was cultivated and carried a cattle population of approximately 11 animals/km². Cattle were the main host of tsetse. Following the free-of-charge treatment of the adult cattle at intervals of 7 weeks and at a dosage of 15 ml/100 kg body weight, there was an increase in the average packed cell volume in the herd although the decline in the incidence of trypanosomal infections was more prolonged. The monthly incidence of trypanosomal infections started to decline substantially 8 months after the treatments were initiated. No trypanosomal infections were detected from 10 months after the start of the trial.