Zooprophylaxis: are we in Plato's cave?

Abstract

Zooprophylaxis, ‘the use of wild or domestic animals, which are not the reservoir host of a given disease, to divert the blood-seeking vector from human hosts of that disease’ 1xSee all References1, would seem to be a sensible and sustainable approach for malaria control in areas in which people already have domestic animals. However, the results from several studies on zooprophylaxis are ambiguous – a strong zooprophylactic effect was seen in Sao Tome 2xDogs as a favoured host choice of Anopheles gambiae sensu stricto (Diptera: Culicidae) of Sao Tome, West Africa. Sousa, C.A. et al. J. Med. Entomol. 2001; 38: 122–125Crossref | PubMedSee all References2 and Papua New Guinea 3xBlood-feeding and resting behaviour in the Anopheles punctulatus Donitz complex (Diptera: Culicidae) from coastal Papua New Guinea. Charlwood, J.D. et al. Bull. Entomol. Res. 1985; 75: 463–475Crossref | Scopus (2)See all References3, whereas in other areas the presence of animals did not have any protective effect 4xSee all References, 5xFailure of passive zooprophylaxis: cattle in Pakistan is associated with a higher prevalence of malaria. Bouma, M. and Rowland, M. Trans. R. Soc. Trop. Med. Hyg. 1995; 89: 351–353PubMed | Scopus (54)See all References, 6xHousehold risk factors for malaria among children in the Ethiopian highlands. Ghebreyesus, T.A. et al. Trans. R. Soc. Trop. Med. Hyg. 2000; 94: 17–21PubMedSee all References. A mathematical model 7xEffectiveness of zooprophylaxis in malaria control: a theoretical inquiry, with a model for mosquito populations with two bloodmeal hosts. Sota, T. and Mogi, M. Med. Vet. Entomol. 1989; 3: 337–345Crossref | PubMedSee all References7 predicts that, by providing more hosts per mosquito and therefore, reducing density-dependent feeding failure, animals might actually increase the risk of malaria transmission. Most studies have only considered the possible protection afforded when the alternative hosts, particularly cattle, are in the vicinity of humans. Thus, the herder who keeps his cattle close to his house is assumed to be more zooprophylactically sensible than the herder who keeps his cows at a distance 4xSee all References4. Supposing the cattle kept at a distance were stocked close to the only water available in the area, which happens to be the only mosquito-breeding site around. Then, the mosquitoes (being lazy or pragmatic) might be more readily diverted onto these cattle than those kept close to the houses, where other, more tasty, meals are available.A large, highly localized zoophilic population is not a problem. Indeed, there is a good argument for genetic manipulation of Anopheles funestus, the major malaria vector in East Africa, by introducing transgenes associated with zoophily. Alternatively, species replacement could be encouraged by providing suitable hosts for zoophilic mosquitoes, such as Anopheles rivululorum, which are larval competitors of A. funestus. A potential zooprophylactic scenario might comprise a herd of cattle close to a breeding site with a line of neem trees between the cattle and the village. Personal protection would then be provided by our oldest companion, one that we know anthropophilic mosquitoes like to feed on 2xDogs as a favoured host choice of Anopheles gambiae sensu stricto (Diptera: Culicidae) of Sao Tome, West Africa. Sousa, C.A. et al. J. Med. Entomol. 2001; 38: 122–125Crossref | PubMedSee all References, 3xBlood-feeding and resting behaviour in the Anopheles punctulatus Donitz complex (Diptera: Culicidae) from coastal Papua New Guinea. Charlwood, J.D. et al. Bull. Entomol. Res. 1985; 75: 463–475Crossref | Scopus (2)See all References – I have never known a dog who did not welcome the opportunity to be an alternate host for endophilic mosquitoes by undergoing the hardship of sleeping at the foot of the bed rather than outside among the ducks.