Transmission thresholds for dengue in terms of Aedes aegypti pupae per person with discussion of their utility in source reduction efforts

The expense and ineffectiveness of drift-based insecticide aerosols to control dengue epidemics has led to suppression strategies based on eliminating larval breeding sites. The present work attempts to estimate transmission thresholds for dengue based on an easily-derived statistic, the standing crop of Aedes aegypti pupae per person in the environment. We have developed these thresholds for use in the assessment of risk of transmission and to provide targets for the actual degree of suppression required to prevent or eliminate transmission in source reduction programs. The notion of thresholds is based on 2 concepts: the mass action principal- the course of an epidemic is dependent on the rate of contact between susceptible hosts and infectious vectors, and threshold theory - the introduction of a few infectious individuals into a community of susceptible individuals will not give rise to an outbreak unless the density of the vectors exceeds a certain critical level. We use validated transmission models to estimate thresholds as a function of levels of pre-existing antibody levels in human populations, ambient air temperatures, and the size and frequency of viral introduction. Threshold levels were estimated to range between about 0.5 and 1.5 Ae. aegypti pupae per person for ambient air temperatures of 28 degrees C and initial seroprevalences ranging between 0 percent to 67 percent. Suprisingly, the size of the viral introduction used in these studies, ranging between 1 and 12 infectious individuals per year was not seen to significantly influence the magnitude of the threshold. From a control perspective, these results are not particularly encouraging. The ratio of Ae. aegypti pupae to human density has been observed in limited field studies to range between 0.3 and >60 in 25 sites in dengue-epidemic of dengue-susceptible areas in the Caribbean, Central America, and South East Asia. If, for purposes of illustration, we assume an initial seroprevalence of 33 percent, the degree of suppression required to essentially eliminate the possibility of summertime transmission in Puerto Rico, Honduras, and Bangkok, Thailand was estimated to range between 10 percent and 83 percent; however in Mexico and Trinidad, reductions of >90 percent would be required.(AU)

Spetial and temporal distribution of house dust mite (Astigmata: Pyroglyphidae) allergens Der p 1 and Der f 1 in Barbadian homes

Houses features contribute to house dust mite abundance and, therefore, exposure to mite allergens. Our study assessed the hypothesis that modernization of the domestic environment in a tropical setting may lead to a level of allergen from the house dust mites Dermatophagoides pteronyssinus (Trouessart) and D. farinae Hughes that previously has been defined clinically as at risk for people who suffer from allergic disease. Allergen (Der p 1 and Der f 1) levels were measured at 4 sites (mattress, bedroom floor, living room floor, and furniture) in 17 houses in Barbados during dry and rainy seasons. Der p 1 (17 of 17 homes) at all 4 sites did not vary significantly from the dry to rainy season. Allergen levels varied according to site, and were highest in living room furniture in both seasons (geometric mena 40.37 and 64.17 micrograms/g, respectively). Concentration of Der p 1 allergens were higher in concrete than in wood or mixed concrete and wood houses. Der f 1 (9 of 17 homes) levels were lower than Der p 1 by 1/1,000 (both seasons). Results indicated that season is less important in regard to levels of Der p 1 than house construction and confirm other studies that implicate D. pteronyssinus as a more abundant source of allergen than D. farinae in this tropical setting.(AU)

The role of the house dust mite and other household pests in the incidence of allergy among Barbadian asthmatics - abstract

The incidence of allergy to 14 different household pests, including 2 house dust mite species (D. farinae and D. pteronyssinus), was investigated among 156 asthmatic children in Barbados. Independent variables, including urban/rural residence, building material of the home, and presence of carpeting, were found to be significant factors related to the house dust mite allergy. The continuing trend of modernization of the domestic environment, which provides an optimal microhabitat for several household pests, is implicated as a probable contributing factor in the increasing overall prevalence of asthma reported in Barbados. Children between the ages of 5 and 18 years with a diagnosis of bronchial asthma were selected from attendees at six polyclinics distributed throughout the island, an urban private general practice, and the accident and emergency department of the only acute general hospital on the island. Skin tests were done by scratching, and by intradermal injection, if the scratch test proved negative. Allergy to the house dust mite was the most common of the reactions to the household pests tested, and was found in 81 percent of the asthmatic children (AU)