INDOOR RESIDUAL SPRAYING OF DDT FOR MALARIA CONTROL

We wish to comment on O'Shaughnessy's article[1] on the use of DDT to control malaria from our years of malaria eradication experience.[2]

From 1948 to 1958, H.H.C. participated in Taiwan's malaria program. In 1950, there were 1.2 million cases of malaria out of a population of 7.5 million.[3],[4] After a period of DDT indoor residual spraying from 1953 to 1957,[3],[5-7] the number of cases dropped to 533. Subsequent surveillance, case detection, treatment, and focal indoor residual spraying resulted in only 19 cases in 1965 (out of a population of 12.4 million) and the World Health Organization (WHO) declared that malaria has been eradicated on the island. Remarkably, the total program cost per capita from 1953 to 1957 was NT $3 (US $0.10 at the current exchange rate).[6]

The story of parachuting cats into malaria-infested areas to counteract the increasing rodent populations after the DDT spraying program killed the native cats is cited as a parable of ecosystem meddling and has become an urban legend. In Taiwan, we knew indoor residual spraying killed cats and other animals, so we asked villagers to keep them out of the house for several days after the spraying. Surveillance of 682 houses sprayed (264 with cats) found that 43 cats died, along with many mice and chickens.[3] Some animals died in a day, suggesting that direct toxicity was the issue, not biomagnification.

We are skeptical about the validity of the reports of cats being parachuted into Sarawak in 1960. An airdrop of DDT supplies in 1954 during the pilot project is mentioned in another source,[8] but supplies were always transported via speedboats during H.H.C.'s tenure as the malaria advisory team leader in Sarawak from 1962 to 1968. Colbourne et al. never mentioned parachuting cats to us or in their article, which focused on the Sarawak malaria epidemic from 1955 to 1958.[9] Similarly, we heard no mention of such an incident from the WHO Senior Malaria Advisor from 1959 to 1961.

In Sarawak, Malaysia, we heard complaints about spoiled thatch roofs of farm huts[10] in the neighboring area of Sabah (North Borneo), but never Sarawak. Longhouses — the typical indigenous abode in Sarawak at the time — have roofs of billian (extremely hard wood impervious to moth larvae)[8]; thatching was not used so longhouses were unaffected.

Despite claims to the contrary, we do not believe that there were any major typhus epidemics or outbreaks of sylvatic plague in Sarawak due to increased vector populations. In fact, in Taiwan, we experienced a positive consequence of the spraying program: head lice disappeared. Today's reader might cringe, but villagers often asked to have their heads sprayed with DDT!

Indoor residual spraying is controversial, but it is appealing for a variety of reasons. It contaminates the environment less than larviciding or aerial spraying. It is economical. Spraying can be done once or twice a year for 3 to 5 years followed by focal spraying.

Malaria eradication has failed for multiple reasons: drug resistance, continuous mosquito exposure, geographical challenges, and political instability. Although restarting indoor residual spraying alone is insufficient to control malaria, fears of environmental destruction are overblown. It is time to retire the cat story and reconsider DDT's role in the fight against malaria.

I would like to extend my thanks to Chen et al. for adding helpful insights to the points made in my article from the elder Chen's long career in the World Health Organization's Taiwanese malaria program. I was especially heartened to read that records were kept detailing the number of cats killed as a result of indoor residual spraying in Taiwan and Chen et al.'s conclusion, which supports my own, that the deaths were a result of direct ingestion rather than biomagnification. It was also interesting to read (and be corrected) that thatched roofs were only applied to farm huts and not the longhouses in the area. Another source[1] separately describes the "cat drop" and the thatched-roof problem, but the two events must have been connected as a result.

I agree with Chen et al. that air drops were rare in the area but I am convinced that a "cat drop" did occur. Interestingly, Tom Harrison, who lived in the area at the time, was the first to describe the "cat drop," but did not provide many details to substantiate its occurence.[2] Interestingly, his biographer reports that an air drop was only made because a transport plane could not land on the short strip at Bario, Sarawak.[3] Furthermore, the account of the drop made by Morrison[4] includes many details that I did not provide in my article (such as the three days it took to make the drop owing to bad weather), but which further substantiate the existence of the event. Finally, none of the accounts other than the "parable" suggest the World Health Organization was involved, which explains why there was no communication of any type from that agency regarding the event.