Chagas disease vector control through different intervention modalities in endemic localities of Paraguay.

In a field study carried out in three rural communities in Paraguay in a zone endemic for Chagas disease, we implemented three different vector control interventions--spraying, housing improvement, and a combination of spraying plus housing improvement--which effectively reduced the triatomine infestation. The reduction of triatomine infestation was 100% (47/47) in the combined intervention community, whereas in the community where housing improvement was carried out it was 96.4% (53/55). In the community where fumigation alone was used, the impact was 97.6% (40/41) in terms of domiciliary infestation. In all the houses where an intervention was made, an 18-month follow-up showed reinfestation rates of less than 10%. A serological survey of the population in the pre- and post-intervention periods revealed a shift in positive cases towards older age groups, but no significant differences were observed. The rate of seroconversion was 1.3% (three new cases) in the community with housing improvement only, but none of these cases could have resulted from vector transmission. The most cost-effective intervention was insecticide spraying, which during a 21-month follow-up period had a high impact on triatomine infestation and cost US$ 29 per house as opposed to US$ 700 per house for housing improvement.

PIP: Chagas disease is most commonly spread by vectors in areas where inadequate housing, crowding, and poor hygienic conditions facilitate the proliferation of triatomines and help to maintain their domestic cycle. Chagas disease is widely distributed in America, from the southern US to southern Argentina, with a prevalence of 5-60%. In 3 rural communities in Paraguay in a zone endemic for Chagas disease, insecticide spraying, housing improvement, and a combination of spraying plus home improvement were implemented in a field study to control the vectors of Chagas disease. The insecticide applied was lambdacyhalothrin in a wettable powder formulation, while houses were modified to have smooth, flat, crack-free walls and ceiling surfaces, as well as better ventilation and illumination. Each intervention effectively reduced the vector of Chagas disease, with triatomine infestation reduced by 100% in the combined intervention community, 96.4% in the community in which only houses were improved, and 97.6% in terms of domiciliary infestation where only fumigation was used. In all houses in which an intervention was made, an 18-month follow-up showed reinfestation rates of less than 10%. A serological survey of the population during the pre- and post-intervention periods showed a shift in positive cases toward older age groups, but no significant differences were observed. The rate of seroconversion was 1.3% (3 new cases) in the community with housing improvement only, but none of those cases could have resulted from vector transmission. Insecticide spraying was the most cost-effective intervention: US$29 per house compared to US$700 per house for housing improvement.

Residual effect of lambdacyhalothrin on Triatoma infestans.

Insecticidal residual effect and triatomine infestation rates in houses of a community fumigated with lambdcyhalothrin (Icon) are reported. No mortality was observed in 5th-instar Triatoma infestans nymphs in 72-hr exposure test on three different surfaces, one month after fumigation for a dose of 31.5 mg am/m2. However, during post-exposure observation a mortality of 60% was recorded for those insect exposed on sprayed woodboard. The results observed with mud-containing treated walls, were markedly poorer (0% of mortality). Twelve month after spraying 40% of mortality was observed on first-instar T. infestans nymphs in 72-hr exposure test on woodboard, but lower mortality rates were observed in mud-containing materials. When the effect of deltamethrin (109 mg ai/m2) and lambdcyhalothrin (94 mg ai/m2) was compared, the former did not appear to be superior at similar loads. Both have showed a mortality rate of 30% on 5th-instar T. infestans nymphs three months post-fumigation. The dose utilized in the field fumigation was enough to get a significant (p < 0.0001) control of triatomine domestic infestation, since it was sufficient to keep 95% of the houses uninfested throughout 21 months following treatment, when compared with baseline situation. A remarkable knock-down effect on adult and nymphs forms of the insect and a high in situ mortality were observed as a result of its application, even at very low doses.