Repellents may help prevent transmission of malaria when insecticides and prophylactic drugs cannot be used, but the repellents presently available must be applied frequently and are not entirely acceptable cosmetically. A repellent that would be effective for several days would be extremely useful in malaria prevention. A feeding deterrent might be more persistent on the skin than a true repellent though less acceptable in other ways, and might be used in combination with a repellent to provide a general-purpose treatment.
The amount of research having to do with the development of repellents is small compared with that devoted to insecticides. Still, several thousand compounds have been screened, and the most promising leads developed in the screening program have been exploited in systematic synthesis programs. Human hosts have been used for most testing, and the selections of promising leads have been based on the persistence of a minimum effective concentration on the skin or clothing for a minimum acceptable period. The final evaluation has usually been based on the relative effectiveness of new and standard repellents tested concurrently against a wide range of species.
Studies with 3 standard repellents—deet, dimethyl phthalate, and ethyl hexanediol—indicate that the differences in the length of protection provided when they are applied to the skin result more from the differences in the minimum effective concentrations and less from differences in persistence on the skin. Differences in the length of protection provided by the same repellent to different individuals result more from the relative rates of loss than to differences in the attractiveness of the hosts. The 3 standard repellents are lost by evaporation, absorption, and rubbing; they are not decomposed on the skin and do not lose effectiveness by admixture with skin secretions, at least to a degree that affects the protection provided.
The relative effectiveness of repellents on human skin may vary with the species of insect, e.g., dimethyl phthalate is about as effective as deet and more effective than ethyl hexanediol against Anopheles quadrimaculatus Say, but less effective than deet against A. albimanus Wiedemann, less effective than ethyl hexanediol against A. farauti Laveran, and less effective than either against some species of Aedes.
Repellents have not been tested as widely against anopheline mosquitoes as against some pest species, but the tests that have been made indicate that the standard repellents would remain effective for several hours on unrubbed skin; all are susceptible to removal by rubbing or immersion in water. In some situations, it may be necessary to treat the clothing as well as the exposed skin. Wide-meshed cotton netting treated with repellents is sometimes preferable to standard bed nets or head nets.
Attempts to increase the usefulness of repellents by incorporating them into formulations intended to improve their cosmetic acceptability or retention on the skin or both have met with little success.
New and improved repellents might be produced in synthesis programs as a result of following leads developed in screening programs. In the past this method has produced such improved compounds as deet, but it seems improbable that leads from presently known repellents will produce compounds that will remain on the skin for several days. Studies of the physiology of repellent action might lead to radically new compounds, and far-ranging screening programs might open up radically new leads. A study now underway is designed to produce compounds with long-lasting repellency by the gradual release of small quantities of a highly active repellent from a precursor-type molecule anchored to the epidermal surface.
The prospect of developing a systemic repellent seems remote.
The methods used to screen and evaluate repellents include, in order of preference, tests with human hosts, animal hosts, inanimate attractants, and no attractants whatever.
Investigations of the attraction of anophelines have been concerned with differences in attractiveness between various species of hosts (including man), differences between individuals, and the factors causing differences in the degree of attraction, particularly carbon dioxide (CO2), moisture, heat, and odors. The results obtained by different investigators have not always been in agreement, especially in regard to the action of CO2. However, some of the inconsistencies may result from differences in test techniques or, when olfactometers are used, from the failure to discriminate between true attractants, arrestants, and probing stimulants.
In general, most anophelines prefer dark resting places whether unfed, recently blood-fed, or gravid.
