Supporting sources for this post on the resurgence of malaria in Sri Lanka despite DDT spraying.
From Malaria: Principles and Practice of Malariology edited by Wernsdorfer and McGregor (1988) Chapter 45 "The recent history of malaria control and eradication. by Gramiccia and Beales pages 1366-1367
In Sri Lanka, after the minimum of 17 cases in 1963, the incidence increased markedly and practically unimpeded, reaching 537 700 registered cases in 1969. There were still 400 700 cases in 1975. Malaria in Sri Lanka was known to produce epidemics at three to five year intervals on account of low rainfall and high temperature, which favour the propagation of the vector A. culicifacies. The development of epidemic foci was facilitated by an increased population movement for chena cultivation, often deep in poorly accessible forested areas, and for gem mining which created ideal mosquito breeding grounds in abandoned pits in the proximity to the shelters built by the workers. These conditions occurred again in late 1967 and in 1968. During the successful eradication period P. falciparum had been eliminated, but in 1975, after the resurgence of malaria, this species constituted 16% of all infections.
The reasons for the upsurge were many. It was certainly facilitated by the backlog of slides accumulated in the laboratories and the comparatively low numbers of blood smears taken by health institutions that permitted a gradual build up of undetected, untreated cases. Intradomiciliary residual spraying with DDT had been withdrawn in the early 1960s because of the low number of cases (in accordance with the criteria for passing from attack to consolidation). After the resurgence was recognized, administrative and financial difficulties prevented the purchase of insecticides of which there was no residual stock, and the employment of temporary squads for spraying them when insecticides were donated. In 1968, the programme reverted from consolidation to attack phase, but by that time malaria had already taken root again in all previously endemic areas. DDT residual spraying was again applied on a total coverage basis, accompanied in some areas by mass radical treatment. These measures met with limited success, but the malaria situation deteriorated once more between 1972 and 1975. Apart from operational and administrative shortcomings, the main reason for this second increase was the development of vector resistance to DDT, to such an extent that it was necessary to change to the more expensive malathion in 1977. Residual spraying with malathion was the main measure of an intensive plan of operations supported by a consortium of bilateral agencies and WHO. In addition, surveillance was intensified and complementary control measure such as larviciding, space spraying, water management and drug administration were applied as necessary. The situation levelled out in 1978 and 1979 after a dramatic reduction of cases in the first year of the intensive programme, and in 1980, 47 949 cases of malaria were reported of which 3% were due to P. falciparum.
Attempts have been made to estimate the economic losses caused by this resurgence of malaria in Sri Lanka by considering the effect on education, earning capacity, food production and the Government expenditure on treatment*. During the period 1971-75 it was estimated that a total of 4.99 million school days were lost; 6.92 million man-workdays were lost, equivalent to an earning capacity of Rupees 69.19 million; the loss of rice production was equivalent to Rupees 6797 million necessitating the importation of rice at a cost of Rupees 1664 million. No attempt has been made to measure the effect on development projects, but such projects are usually situated in malarious areas and hence the workers and their families are at high risk. The total Government expenditure for treating malaria during the period 1971-74 amounted to Rupees 1034 million for outpatients and another 2395 million for inpatients. The population of Sri Lanka at that time was approximately 13.5 million.
* Country Report on the Malaria Programme in Sri Lanka presented at the WHO Consultative Meeting on Malaria, New Delhi, India 21-24 April 1976.
From chapter 46 "The Global Distribution of malaria and the present control effort" by Haworth page 1403 on Sri Lanka
The vector A culicifacies, being resistant to DDT, the countrywide malaria control campaign depends on the residual spraying of houses four times a year with malathion (which is banned fromagriculture use to delay onset of resistance of the vector mosquito to this insecticide) together with chemotherapy.
From Mosquitoes, malaria, and man: A history of the hostilities since 1880 by Gordon Harrison (1978), Pages 246--248
Each of the nations that were to attempt eradication---fifty-twoin all---had a campaign under way by 1960. In 1966, when WHO's expert committe at the special request of the World HEalth Assembly reviewed progress, the total war had thus been waged five to ten years. There were some impressive overall results. Malaria had been eradicated in once endemic areas inhabited by more than 600 million people and reduced to an occasional and minor disease for another 334 million. Ten of the fifty-two nations had achieved eradication; eleven others had banished the disease from some part of their territory.
Among the latter, India and Sri Lanka were outstanding. India had brought the number of malaria cases down from the estimated 75 million in 1951 to about 50,000 in 1961. Sri Lanka, which launched its eradication program in 1959 after twelve years of control, reduced malaria from about three million cases after World War II to just 29 in 1964. Sri Lanka, jubilant, proclaimed that "this year (1964) marks the end of an era." WHO, reviewing the world scene, hailed "an international achievement without parallel in the provision of public health service."
It was that. Yet it was a significantly smaller achievement than had been hoped. The grand statistics suggested a broadly sweeping victory which closer examination showed had not been won. Of the ten countries where eradication had been achieved, four were in Europe, the other six in the Americas---Chile and five Caribbean islands. As the experts noted, 638 million people still lived where malaria was actively transmitted and still constituted a major cause of sickness and death; 360 million were in Africa and half of these lived where no efforts were yet being made to fight the disease. Despite some successes in experimental programs, victories in Mauritius and Reunion and progress in the south, nowhere in equatorial Africa were there indications that transmission might be stopped. In Asia, counterbalancing success in India, Pakistan had yet to launch any campaign at all in half the country.
But more damaging than any of the specific shortcomings was the fact that an all-out war of extermination had been in progress everywhere for at least five years, and in some places for ten, without yielding one sure victory in any major tropical area. The essence of eradication strategy had been speed in execution. Three to four years in attack ought to have been enough, and if resistance in the mosquitoes had developed at the threatened rate that might have been all the time there was. Now after ten years the strategists were talking not about final victory but about progress toward it taking place in some still indefinite future.
The original idea had been by massive and intensive spraying to end transmission simultaneously throughout areas large enough to hold thereafter, without DDT, against both lingering small foci within the region and incursions from outside. But in practice the massive nationwide campaign was but a statistical generality of many small battles, fought with uneven skills in conditions of disparate difficulty. As victory was not to be had all at once, and as everyone was in a hurry to cut off the spray and show results, the battles began to be called off one at a time---often in relatively small districts wholly surrounded by others where the fight went on. Many of these could not be held and the attack had to be resumed. Both India and Sri Lanka, models for the feasibility of eradication, slipped back a little between 1960 and 1965. It was not much. But in the circumstances even the slightest regression was ominous. The goal of eradication had to be postponed and, by definition, time was on the other side.
WHO was of course concerned but, it would appear, not gravely so. As in the 1950s the campaign leaders had been rushed into a program of eradication by fears of insect resistance, now ten years later they were beguiled into accepting more distant goals by seeing that their fears had been somewhat exaggerated. To be sure, resistance had spread rapidly: From the five species of anophelines proved resistant in 1956, the number had risen to thirty-eight in 1968. But in many places the declining vulnerability was not yet sufficient to interfere with control. In India, for instance, Anopheles culicifacies, a major vector widespread in the country as a whole, was shown to be resistant but only in a few isolated localities---not enough to make any real difference. From Sri Lanka when spraying stopped in 1964, reports that culicifacies there remained wholly susceptible were encouraging. Most of the thirty-eight resistant species tolerated DDT or dieldrin but not both and so could be controlled by switching insecticides. WHO's pesticide experts concluded that around the Persian Gulf and in several countries in Central America "resistance challenges the outcome of the campaign," but that elsewhere it was still more of an inconvenience than a major obstacle. So there was still time---time, the experts thought, "for a more thorough study and analysis" of the program the better to adapt it to the capacities of those countries still struggling.
In India the 50,000 malaria infections proved in 1961 had become 100,000 by 1965. That was not a great increase and might indeed be factitious since in the interim the detection system had become much more thorough. Similarly in Sri Lanka a few more cases turned up in 1965 than in 1964 but the reason was clearly that a few local foci of infection persisted, mostly in highly inaccessible parts of the island. There seemed no reason for alarm. To all intents and purposes, the major gains were being, held and other countries were still moving ahead even if not on schedule.
The ideal of continuing attack until extensive areas enclosed by natural barriers to transmission were cleared was still more difficult to translate into politically practicable criteria. No error was more egregious in practice than the premature establishment of islands of surveillance surrounded by areas under attack, or conversely the tolerance within large consolidation areas of considerable enclaves without natural boundaries where transmission persisted. The error came in part from the genuine difficulty of deciding just how large a defensible consolidation zone had to be, but in greater part from the manifold political and economic pressures to get off the DDT wherever it seemed even marginally possible. The result was a gerrymandered patchwork of defense zones whose frontiers were certain to be regularly and even massively reinvaded.
Endemic malaria returned to India rather like the turnaround of a tide, slowly at first and then with a broad sweep. From 1961 through 1963 there were less than 100,000 cases in the entire country. In the next three years the number moved from 100,000 to 150,000. In 1967 and 1968 it reached 275,000 and in 1969, 350,000. Then the barriers gave way. Cases doubled in 1970 and doubled again in 1971. At that point about a quarter of the units resumed attack and for the next three years the spread was checked but not rolled back. In 1975 the cost of DDT, a petroleum product, soared in response to the steep increase in oil prices. Malathion, which had to be substituted for DDT where Anopheles culicifacies had developed resistance, was still more expensive. India ran short. Almost two and a half million cases were recorded in 1974, and the next year that number once more than doubled.* In 1977 according to some estimates the number of cases reached at least 30 million and perhaps 50 million. The proportion of potentially lethal falciparum infections inexorably mounts as Plasmodium's reconquest of India tragically goes on.
Perhaps even more bitterly disappointing was the reverse that took place in Sri Lanka. Statistically at least, Sri Lanka had appeared on the point of abolishing malaria as early as 1954. But instead of continuing the attack past the point of apparent victory to make it sure, the government yielded to the temptation to save money and progressively stopped spraying in district after district as soon as the criteria were minimally met. When malaria resurged in 1956 following an abnormal drought, Sri Lanka resumed full attack. In 1957 the endemic dry zone was completely resprayed. The surveillance organization, the oldest and probably the best in Asia, was overhauled. A supplementary budget was approved and the United States offered vehicles and sprayers.
In 1959 the new attack was formally reorganized to set eradication as its target. Within three years thereafter, as noted, Sri Lanka stood once more on the brink of total victory. Or so it seemed. The remaining few cases were in remote forest areas, often among people who lived in temporary camps or houses without walls. Intractable as these holdouts were, they did not appear to threaten the country.
Nevertheless on close examination ominous facts turned up. Among the handful of cases an extraordinarily high proportion were infections of Plasmodium falciparum. As falciparum infections do not relapse, all of these had to be newly contracted. The Sri Lankan malaria service had outdone itself in collecting blood slides annually---from 16 percent of the population instead of the recommended 10 percent. But a disproportionate number came from the least dangerous areas, where malaria had been moderate, as compared to those where it had been hyperendemic. Moreover WHO consultants found large backlogs of unexamined slides. In one focus of infection to which several falciparum cases were traced, no blood slides at all had been taken by the two apothecaries responsible. A small matter perhaps but the man from WHO noted this "with a sense of uneasi- ness." The slow, steady rise in cases throughout the country by then (1965), he thought, "suggested that the situation had been smouldering for a longer time than originally realized." Perhaps the most worrying fact in Sri Lanka was that it was proving impossible to seal off the remote foci of infection. After a 1966 outbreak of malaria in a village of 258 people, investigators traced the source to a nearby contractor's camp. They then discovered that workers from that camp had dispersed to 58 other villages and among them eight had carried parasites.
Despite these rumblings of trouble the epidemic that hit the island in 1968-69 was shocking, unexpected and deeply discouraging The few score cases suddenly multiplied into more than half a million. In a single season parasites reestablished themselves almost throughout the areas from which they had been so expensively driven in the course of twenty years. Sri Lanka went back to the spray guns, reducing malaria once more to 150,000 cases in 1972; but there the attack stalled. Anopheles culicifacies, completely susceptible to DDT when the spray stopped in 1964, was now found resistant presumably because of the use of DDT for crop protection in the interim. Within a couple of years, so many culicifacies survived that despite the spraying malaria spread in 1975 to more than 400,000 people.
From Mosquito: A Natural History of Our Most Persistent and Deadly Foe (2001) by Spielman pages 176-177
By all accounts, the DDT that had been sprayed in Sri Lanka had been so effective that the defeat of the mosquito should have been complete.
The mystery would have remained but for my review of the government's records from the malaria campaign. The files were kept in a small office down a long corridor in the dilapidated Ministry of Health building in Colombo. Thick manila folders contained the plan of attack, the records of fieldwork performed, and surveillance results. Everything was there, right down to the listing of supplies and their use.
Amid the arcane and mundane lay a buried document that revealed the hand of humankind in the return of malaria to the island. It was a memo that decreed the the eradication team change its criteria for reporting the rate of maoquitoes' resistance to the insecticide. In the beginning, the effectiveness of DDT was confirmed when 100 percent of the test mosquitoes were killed within an hour. As long as this kill rate was observed, the pesticide could be deemed effective and spraying could continue.
But halfway through the program, the time limit was suddenly doubled, to two hours. Though the reason was not recorded, it was obvious that some mosquitoes were developing resistance and the change was made to justify continued spraying. It also allowed the team to reassure political leaders and the public that their efforts were succeeding. In fact, they had already failed and the mosquitoes held the upper hand.
One question keeps bothering me: why haven't mosquitoes develpoed resistance to malathion?
Perhaps malathion is a more expensive insecticide and studies related to resistance to a cheaper product as DDT could have been tainted with "unethical interests" (that is "ordered", or made by "paid liars"?
Mosquitoes have developed resistance to malathion.