Average Household Size and the Eradication of Malaria

Efforts to eradicate malaria during the 20th century succeeded in some parts of the world but failed in others. Malaria also disappeared spontaneously in several countries for reasons that remain an enigma. The connection between malaria and poverty has long been noted. Here we focus on a specific aspect: household size, which has hitherto received little attention. We find strong evidence that when average household size drops below four persons, the probability of malaria eradication jumps dramatically and its incidence in the population drops significantly. This effect is independent of all commonly-studied explanatory variables and was globally valid across all climate zones irrespective of counter measures, vector species, or Plasmodium species. We propose an explanation based on the dispersal mechanism of the parasite. Malaria is transmitted at night by mosquito bite. The mosquito typically spreads the Plasmodium only locally over short distances to new human victims. To survive, the Plasmodium depends on infected humans making social contacts over longer distances. When household size decreases sufficiently, these contacts cross a threshold value that changes the balance between extinctions and replacements and the Plasmodium disappears on its own. We test this interpretation by contrasting our malaria model with dengue fever, which is also poverty-related and mosquito-borne but transmitted differently, namely through daytime exposure. Household size is uncorrelated with dengue incidence, whereas an indicator of outdoor work that is insignificant in the malaria model is highly significant for dengue. We conclude that poverty-induced malaria infection risks are likely to persist, but a focus on reducing effective household size can be a feasible and promising means of its eradication.