The history of malaria eradication in Mexico and the world is the story of dichloro-diphenyl-trichloroethane (DDT), a compound first synthesized in 1874. It had no known application but was resynthesized in 1939, at which point it was recognized as an effective insecticide. DDT was used during the Second World War to control lice, fleas, and mosquito-transmitted diseases, and it was soon established as the most useful tool for controlling insect vectors of public health importance. DDT campaigns successfully controlled malaria in Europe. Its effectiveness encouraged the notion that malaria could be entirely eliminated across the globe. When the war ended, DDT was commercialized and made part of malaria control programs in various tropical countries, including Mexico.
The WHO Eighth World Health Assembly, held in Mexico City in 1955, approved the global malaria eradication campaign. This program was to be carried out in Europe, North Africa, the Middle East, some Asian countries, and Latin America. Sub-Saharan Africa was deemed "not ready"  and excluded from the supposedly "global" initiative, which therefore ignored the territories that produced 90% of the malaria cases and deaths in the world at that time. This statistic bears repetition: the countries that produced just 10% of earth's malaria infections were the only ones with which the global malaria eradication program concerned itself. The program succeeded in driving malaria out of southern Europe, North Africa, the Middle East, and some islands in the Atlantic Ocean. India and Sri Lanka almost eliminated malaria, but then it returned dramatically.
At the World Health Assembly in 1955, Latin American governments agreed to finance the campaign for five years, the amount of time WHO had deemed sufficient for eradication. Altruistic US funds (administered through UNICEF and what was then called the Pan American Sanitary Bureau, today the Pan American Health Organization) were the fuel that got the campaign going in Latin America.
But in 1960 not one of the countries of continental Latin America was free of malaria. The campaign was extended. By 1966, it looked as if Mexico might achieve eradication, but by 1970 that promise was shown to be false. Even after WHO dissolved its global efforts, Mexico's campaign went on until 1986, by which time malaria had once more spread throughout its original area of incidence and malaria morbidity had reached an alarming 140,000 cases. But eventually all countries gave up on eradication and implemented programs to control malaria, using the instruments and techniques left over from the era of eradication. Today, Mexico officially reports from one to three thousand cases annually.
DDT's efficacy during and after the Second World War made global malaria eradication seem feasible; this conviction was further supported by the following experience-based assumptions:
1. Malaria transmission occurs at night inside human dwellings, where anopheles mosquitoes bite people to feast on their blood.
2. Anopheles mosquitoes remain within human dwellings after sucking blood and rest on their interior walls.
3. Malaria parasites have a finite lifespan in human hosts, if treatment is not administered and infection is left to run its natural course. Plasmodium falciparum die in six months to one year; Plasmodium vivax survive inside human beings for one to one and a half years (and sometimes more); and humans can host Plasmodium malariae for 10 to 40 years or until they themselves die. Finally there is Plasmodium ovale, which is no concern of ours because this species has not been demonstrated to exist in the Americas.
4. The application of persistently-acting insecticides to a house's interior walls is sufficient to interrupt malaria transmission.
5. If transmission were interrupted for just three years (later upped to five), malaria would be eradicated.
6. The spraying must be done as quickly as possible, before mosquitoes develop resistance to insecticides.
Theoretically, what could be expected if these assumptions prove to be true?
When insecticide spraying began in these communities, they were home to adult mosquitoes (in the air) and to larval mosquitoes (in nearby bodies of water). The plan was to spray the walls of every single house. Mosquitoes in search of a place to rest after sucking human blood tend to land on the walls of dwellings, and all mosquitoes that landed on sprayed walls would die. Female mosquitoes – the ones that suck blood, and therefore the ones that transmit malaria – do not feed every day; they take a meal every third or fourth day in order to lay eggs. They do not, of course, all choose to feed on the same night, but it could be reasonably supposed that all adult female mosquitoes would feed on blood at least once over the course of a week. Thus, within a week of commencement of spraying, all mosquitoes that were adults on day one would have fed and then died after coming into contact with a sprayed surface.
Once the adult mosquitoes were dead, however, the biting would not have ended for good, since immature mosquitoes, unaffected by the spraying, would eventually reach the adult stage and begin their own quest for blood. Each egg present on water bodies at commencement of spraying would develop through five larval stages and one pupa, from which the adult hatches. This process takes about 20 to 30 days at tropical temperatures, so within 30 days a new anopheles mosquito population would emerge. (There should be no eggs laid after commencement of spraying, since spraying would terminate extant adult mosquitoes before they could lay eggs.) This new population would die as its forbears did, on the sprayed walls of houses they had entered in search of a meal of human blood.
If the assumption was correct, then about 30 days after the first DDT spraying the mosquito population would be eliminated and transmission would be interrupted, with no females at any stage of development left to transmit malaria. Sprayed insecticide would remain potent for five months even without further spraying, which would take care of any mosquitoes that had escaped death. Once female mosquitoes were eliminated, the only source of malaria parasites would be the blood of people who were already infected when the eradication campaign began.
Insecticide spraying coupled with antimalarial treatment should have eradicated malaria in one or two months. The WHO campaign manual for the eradication of malaria did not call for treatment, however; instead it recommended five full years of spraying, during which time all parasites would die naturally inside their human hosts.
Why wasn't treatment administered to people with malaria as their houses were sprayed, when this combination would, theoretically, have eliminated a community's parasite population in just thirty days, setting the stage for eradication maintenance? According to Dr. Cueto, Mexico and other countries were familiar with DDT since 1945 on, first sprayed experimentally and later routinely used in malaria control campaigns and, that is why there was a big push within WHO to start the campaign before the mosquitoes developed resistance to the insecticides: once they developed resistance, it was thought, eradication would be impossible. Why wasn't treatment administered along with the spraying? Because WHO thought spraying was enough, and more efficient than treatment? Because spraying without treatment would allow malaria to linger and the insecticide manufacturers to do more business? We cannot say.
In any event, many of the eradication campaign's initial assumptions turned out to be wholly or partially incorrect, and Dr. Cueto describes a host of other unforeseen troubles that plagued its efforts. What is clear today is that the failure to provide treatment to people suffering from malaria was inhumane.