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Malaria in Oases of Eastern Saudi Arabia¹

1. Malaria has been an important health problem in the Qatif and al-Hasa oasis for centuries. The disease is sharply delimited to cultivated areas in the Eastern Province and affects the settled oasis population (310,000). The nomadic Bedouins are normally free of the disease.

2. Oasis malaria is man-made malaria and is directly connected with the local practice of agriculture. Local irrigation methods and drainage schemes furnish vector breeding places; hence malariousness of villages is directly dependent on their nearness to, and extent of, adjacent irrigated acreage.

3. Uncontrolled malaria may reach hyperendemic proportions. As such, it represents a major potential cause of morbidity and a ranking cause of mortality in the oases. Both industry and agriculture suffer when the disease is uncontrolled; and if these activities are to continue to prosper, adequate provision for malaria control must be made.

4. Precontrol morbidity (1941 through 1947) in adults was measured by cases reported in Saudi Arab employees of the Arabian American Oil Company. Annual attack rates varied from slightly over 1,000/10,000 in less malarious years to 2,300/10,000 in more malarious years. The mean annual morbidity rate in this group of adults was 1,631/10,000 during precontrol years.

5. Precontrol mortality (1941 through 1947) averaged 22/100,000 in the above employee group (only eight employee deaths). With 43 malaria deaths of patients from the general population occurring in Aramco hospitals during this same period, a proportionate mortality rate of 11.6 per cent was recorded. This rate was calculated from total deaths reported on death certificates in Saudi Arab employees, their dependents, and other non-Aramco members of the general public.

6. Precontrol malaria surveys (1947) in typical oasis villages in the Qatif area showed an average parasite rate (2–14-year age group) of 85 per cent. The infant parasite rate was 100 per cent. The gametocyte rate in the 0–11-month age group was 27 per cent; in the 12–23-month group, 24%; and for the 2–14-year composite, 13%. A comparable spleen rate in the 2–14-year age group was 94 per cent. Spleen rates increased with age; the reverse was true of parasite rates. The average spleen size was 2.2, average enlarged spleen was 2.3, and splenometric index was 217.1 in the 2–14-year age group. These data emphasize the public health importance of malaria in these oases.

7. Falciparum malaria was predominant over vivax and malariae, both in available adult morbidity data and in village parasite surveys of children. From original Qatif surveys in typical oasis villages, the 85 per cent parasite rate consisted of falciparum, 35.5 per cent; vivax, 27.4 per cent; malariae, 14.2 per cent; species undetermined, 1.2 per cent; and mixed infections, 6.6 per cent. The same general proportionate species distribution occurred among employee cases when species determinations were available. In earlier years, a majority of cases reporting to hospitals was determined as clinical malaria without reference to species of parasite.

8. The following anophelines occur in the area: A. stephensi, fluviatilis, pulcherrimus, sergenti, multicolor, and coustani var. tenebrosus. Of these stephensi is the only important vector; fluviatilis may be of secondary importance, but its role in local malaria transmission is unknown.

9. The local vector, Anopheles stephensi, is present in enormous numbers especially during spring and fall transmission seasons. It rests in man-made shelters during the day and is active in feeding only after sunset. Blood meal sources include a variety of warmblooded animals; but on the basis of limited precipitin test data, man is not the favored host. Stephensi accounts for 90.6 per cent of anophelines found in houses and for 79.3 per cent of anophelines recoverd from light-trap collections. The New Jersey electric mosquito trap has proved useful in local anopheline surveys.

10. Stephensi utilizes a wide variety of surface waters for breeding places. Among the more important are drainage ditches, seepage areas, ponds, shallow wells, borrow pits, and shallow puddles. Rice fields do not add appreciably to the local malaria problem. Artificial containers are of no local importance as stephensi breeding sites. High degrees of salinity and even some degree of pollution are not deterrents to breeding.

11. Local climate is characterized by extremely hot summers and temperate winters, with oppressively high humidity in late summer and early fall. Weather data from Aramco's Ras Tanura weather station were used in studying effects of climate on seasonal distribution of malaria and on vector densities. Mean weekly and monthly temperatures (1945–1956), mean weekly and monthly relative humidities (RH) based on 0900 readings (1945–1949), and on daily means (1949–1956) were calculated. Monthly rainfall data are available from 1945–1956. The average annual rainfall is 3.23 inches, occurring sporadically from November through May, with December having the highest average monthly rainfall (1.14 inches). Mean monthly temperatures for the 12-year period (1945–1956) ranged from a low of 60.7° F in January to highs of 92.5° F in July and August. Mean monthly RH based on 0900 readings (1945–1949) ranged from a low of 50.1 per cent in August to a high of 78.4 per cent in December. When calculated from daily means (1949–1956), mean RH ranged from a low of 53.8 per cent in June to a high of 73.3 per cent in February. With wide daily fluctuations in humidity, 0900 readings and subsequent calculations from them gave a more orderly seasonal progression and recession in RH. Hence the latter are recommended for any future analysis of RH.

12. Winter temperatures are low enough to inhibit, but not severe enough to stop, stephensi breeding. Spring and fall provide ideal conditions for development of maximal seasonal population densities. April and October are peak months. The hot, dry summer is the most critical period in the seasonal life cycle of the vector, and lowest populations of the year are thus found from June through September. More favorable microclimates play an important role in protection from unfavorable extremes of temperature and humidity during this critical summer period.

13. Malaria morbidity occurs in a characteristic seasonal cycle with peak rates in spring (May) and fall (November). Hot summer months usually show lower malaria morbidity rates than do winter months. This seasonal distribution is controlled largely by temperature variations, with RH playing an important secondary role in summer and early fall. These combinations of temperature and humidity affect total vector population, individual vector longevity, infectivity of vector, development of parasite in the mosquito host, and seasonal behavior patterns in the human host. Rainfall is of little or no significance in seasonal distribution of local malaria.

14. Optimum malaria transmission occurs in October when mean monthly temperatures of 82° F and mean RH of 65 per cent or above occur. This represents an optimum temperature for the extrinsic incubation period of the parasite, and this temperature-humidity combination also favors vector longevity. Preceding optimal vector temperatures (90° F) have allowed maximum vector densities to be attained earlier. This results in a maximum transmission potential in October with maximal morbidity following in November. Cooler temperatures inhibit transmission, and reduced morbidity occurs during the winter months.
In spring, optimum temperatures are reached in late April and early May with favorable humidity declining to unfavorable levels before optimal vector developmental temperatures are reached. Hence, spring vector densities do not reach full potential as they do in the fall. As a result of this onset of hot, dry conditions, the spring malaria season is shorter and morbidity is less than in the fall. Unfavorable humidity (below 60% RH) along with increasing unfavorable temperatures decreases vector longevity, and transmission is cut sharply in June. Low midsummer morbidity follows, until a combination of optimum temperatures and favorable humidities reappear causing a sudden rise in transmission in October and peak morbidity in November to complete the seasonal cycle.

15. A standard seasonal pattern of malaria morbidity based on average temperature-humidity conditions is presented. Variations from this standard morbidity pattern can be predicted on the basis of preceding months' variations from average temperatures and RH. This is of potential practical importance in advance planning for treatment of cases if control measures are discontinued and seasonal epidemics reoccur. Thus, November will be more malarious than usual if October mean humidities are above the normal 65 per cent and approach 70 per cent or higher. If October temperatures remain abnormally high, the fall peak in morbidity may be delayed until December. In spring, if May RH remain favorable (60% RH or above) instead of beginning their normal decline toward early summer lows, a higher-than-normal June morbidity rate may be expected.

16. Residual sprays alone can control local malaria effectively. Annual applications of 200 to 250 mg/sq ft of DDT beginning in 1948 gave adequate malaria control until late 1953 when development of DDT resistance became apparent. This resistance prevented adequate malaria control in 1954 and 1955. Dieldrin replaced DDT as the insecticide of choice in 1955. Annual applications have since proved dieldrin more effective than DDT had ever been in reducing malaria morbidity. For best results, residual applications should be completed in September so that the insecticide deposit will be at maximum strength and effectiveness during the peak October transmission season. Any delays in application after this date will be reflected in higher morbidity rates in November and December. Such precise timing may be less important with longer-lasting dieldrin (over 12 months) but should be continued wherever possible. September applications will insure treatment of all new dwellings or those missed the previous year before October transmission season begins.

17. DDT resistance became apparent in late 1953, five years after the first DDT application. Malariometric evidence, field vector observations, and laboratory confirmation are all offered in support of this resistance development. November morbidity rates rose to over 310/10,000 in the fall of 1953 and 1954; while in earlier years of successful DDT control, November rates ranged from 19 to 62/10,000. Infant parasite rates in experimental villages (al-Ajam and Safwa) during this same period rose from 0 per cent to 31.3 per cent in 1954. Parasite rates in these villages, varying from 1.0 per cent to 14.4 per cent in control years, increased to 44.9 per cent in 1954.

18. Entomological evidence from the field showed progressively shorter intervals in return of stephensi to DDT-treated shelters. After the initial 1948 application, about 12 months elapsed before stephensi were first collected on sprayed surfaces; and then in much reduced numbers compared to untreated checks. After a second application, a 5-month interval was noted; and by 1954, only 2 months elapsed. Unfortunately, 24-hour mortalities were not observed in these checks.

19. Laboratory determination of median lethal concentrations for stephensi from areas where DDT had been first used 5 and 6 years previously showed a 10-fold resistance to DDT had developed. This was taken as final confirmation that DDT resistance had occurred to an extent that DDT was failing to give adequate malaria control.

20. Dieldrin applied at a dosage of 50 to 70 mg/sq ft has successfully replaced DDT in local malaria-control campaigns. It is a more effective residual insecticide as shown by reduction of morbidity and parasite rates to the lowest levels since control programs were instituted.

21. Success of the residual-spray technique alone in controlling malaria is shown by comparison of the precontrol morbidity mean of 1,631/10,000 in Aramco Saudi Arab employees (1941–1947) with the 1958 annual rate of 28.5/10,000. A maximum precontrol rate of 2,317/10,000 was recorded in 1943. In village surveys, infant parasite rates in both oases dropped from 100 per cent in 1947–48 to 0 per cent in 1957. Overall parasite rates (2–14-year age group) in the Qatif oasis dropped from 85 per cent to 6.4 per cent. Most of the recent clinical and parasite relapses are vivax (Aramco employees) or malariae (village children) infections. This indicates the possibility that relapses rather than new infections are responsible for most of those cases currently being seen. Overall spleen rates in the Qatif oasis (2–14-year) age group declined from 94 per cent to 3.8 per cent; average spleen size from 2.2 to 0.05; average enlarged spleen from 2.3 to 1.4 and splenometric index from 217.1 to 5.3. Thus, the residual-spray technique alone (with dieldrin as the insecticide) can adequately control malaria; and may be able to eradicate the disease in these oases if dieldrin resistance does not develop.

22. Potential dieldrin resistance makes it imperative to review application of classical malaria-control techniques to our local malaria problem. Larviciding presents problems of labor, administrative control, and increased costs. Improvements in water-management techniques offer the greatest field for mosquito-malaria-control efforts. More efficient construction, maintenance, and repair are needed for irrigation channels and drains to prevent surface water accumulations. Proper construction and valve control of artesian wells should be enforced. Drains should be kept free of vegetation and water free-flowing.

23. A general scheme for agricultural drainage already proposed should be expedited for malaria control as well as for increased agricultural productivity. Ponds and swamps should be drained and general ground-water levels lowered. Smaller breeding areas such as abandoned ditches, shallow wells, and borrow pits should be filled. Larvivorous fish should be introduced wherever practicable.

24. Individual understanding and participation in control efforts through health education should be expanded. A locally-produced film and filmstrip, posters, brochures, and school lesson plans have been made available for such programs. A recently inaugurated, local, television program is already being used to help meet these health education objectives.

25. Malaria eradication is a practical and economically desirable goal if the above control measures are fully implemented. Our most recent morbidity data and parasite surveys (1958) encourage this attempt. Special care in surveillance, reporting and investigating of individual cases will become of increased importance with continued influx of industrial immigrants. Pilgrims will also make increasing use of newly developing seaports, airports, and cross-country railways of the area for the Mecca pilgrimage. Movements of Bedouins from uncontrolled areas into the eradication zone will also present problems. In spite of obvious difficulties and need for careful, long-term planning and financing, malaria eradication is recommended as an ultimate achievable goal.

¹ Contribution from the Department of Tropical Public Health, Harvard School of Public Health. This paper represents part of a thesis submitted as a requirement for the degree of Doctor of Public Health, Harvard University.