Archive for the ‘HIV’ Category

THE NEW GERM THEORY OF DISEASE: SCREENING FOR MOSQUITOES

Monday, December 13th, 2010

The general solution for acute infectious diseases is to invest in interventions that reduce transmission from the sickest people. This investment should make pathogens evolve toward benignity because the sickest people tend to house the nastiest pathogens. The particular solution depends on the particular category of disease.
For diseases transmitted by vectors such as mosquitoes, mosquito-proof housing should push the host-parasite relationship toward benignity. When houses and hospitals do not restrict entry of mosquitoes, very sick individuals are particularly vulnerable to mosquito bites. When houses and hospitals are mosquito-proof, mosquitoes do not have access to a person sick in bed. The parasites in that person are therefore taken out of the competition. Transmission occurs instead from people who are sufficiently healthy to walk around outside. Because these people tend to be infected with more benign parasites than are severely ill people, mosquito-proof housing should tip the competitive balance in favor of the milder strains.
Mosquito-proof netting might seem like a reasonable and cheaper way to accomplish the same thing. But it is bound to be less effective because it demands more of the sick person and those who care for the sick. Can one count on a sick person to remember to tidy up the mosquito netting and to be motivated to do so? People generally use mosquito netting to avoid being infected rather than to avoid infecting others. Caregivers too tend to be focused on the patient rather than the long-term well-being of the society. They may be less inclined to be careful about mosquito netting if the person already has malaria.
Mosquito-proof housing obviates these problems because a sick person needs to do only what that person feels like doing: lying down and taking it easy. Similarly, the caregivers need focus only on their patients. Nor do the people involved have to put up with short-term inconveniences (shots or periodic spraying of houses) for a long-term gain. They simply have to be willing to accept something that they would like anyhow: higher quality housing.
Skeptics may argue that it cannot be done for a reasonable price. It already has. Just before the discovery of DDT and the synthetic quinine derivatives, the Tennessee Valley Authority was confronted with a malaria problem. The dams it had constructed during the 1920s along the border between Tennessee and Alabama had created mosquito-breeding areas; by the mid-1930s about half the Alabaman residents along the border had malaria infections. Faced with limited options, the TVA decided to embark on a major mosquito-proofing campaign. They divided the area into eleven zones and, between 1939 and 1944, mosquito-proofed every house at a cost of about one hundred dollars per house. Mosquito-proofing was maintained at a cost of about ten dollars per year per house (all 1940 dollars).
Skeptics may argue that screening of houses may do little to influence malaria transmission because much transmission may be occurring outside houses. The outcome of the TVA program provides some insight into this possibility. The mosquito-proofing was staggered so that some zones were completed sooner than others. By the time the mosquito-proofing was completed in a zone, the prevalence of infection had generally fallen to less than 10 percent. Within a couple of years after that, the prevalence was less than 1 percent in virtually every zone.
Skeptics may also argue that people will not tolerate the reduced airflow associated with screens. The TVA personnel were worried about this problem too in the stifling Alabama summers. They reported a few libertarians who threw furniture through the screens in protest, but overall they had virtually 100 percent compliance. In general the people were pleased to get the screens so that they could now escape the buzzing and biting of mosquitoes.
Skeptics may argue further that Alabama is not Nigeria. Differences between countries will undoubtedly influence the outcome of such interventions. In some countries the intervention will be more economically and politically feasible than in others, especially with regard to the initial intervention and long-term maintenance. The logical starting point would be to try the intervention in those countries that seem most appropriate and then to move on to assess the limits of the strategy.
Skeptics may still argue that we do not even know whether, after all the trouble, screening will have the desired evolutionary effect. Yes, we do need to try the intervention to see whether it will work. But this prospect is as close to a win-win situation as one could realistically ask for. If the evolutionary argument is wrong, and no evolution toward benignity occurs, then a large number of people will at least have obtained better housing and will have fewer mosquito bites and a lower rate of infection. If the malaria responds as the malaria in Alabama responded, it will be virtually eradicated. If the local malaria is not eradicated, the intervention will serve as a test of the evolutionary hypothesis.
If such tests prove the evolutionary argument correct, the death and suffering associated with malaria and other vector-borne diseases in the area may be dramatically reduced because each infection will be 1 less damaging—one cannot say by how much without running the intervention, but the geographic variation in the harmfulness of malaria provides some indication. In areas with very restricted seasonal abundance, such as along the northern edge of sub-Saharan endemism for falciparum malaria, and in northern latitudes for vivax malaria, infections are relatively benign. Even where malaria transmission is intense, such as in Gabon, mild strains of the falciparum organism coexist with harmful strains, indicating two things. First, the raw material for evolution to benignity is there. Second, the mild strains, being already able to persist, might need only a slightly increased competitive advantage to dominate or even displace the more harmful strains.
Obviously I have been talking with some skeptics over the years. Perhaps the most frustrating response, though, is dismissal on the grounds that the intervention is primitive. This kind of dismissal is sometimes made by those who are convinced that the answer must lie in some technological breakthrough, even though mosquito-proofing of houses is supported by solid evidence, and the high-tech applications provide no evidence of an imminent breakthrough. The mosquito-proofing approach will undoubtedly incorporate high-tech advances in materials science and molecular methods in many aspects of the intervention, surveillance, and testing. But even if it did not, why care? What matters is what works, not whether it involves mesh, boards, and nails.
Finally, it is worth noting that the same intervention should simultaneously cause evolutionary reductions in other vector-borne diseases in the region. The experience with dengue along the Mexican border suggests that a mosquito-proofing campaign would reduce the damage from this viral disease as well. The cost-benefit ratio could be tremendously favorable.
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