The American Journal of Tropical Medicine and Hygiene - Volume 6, Issue 5, September 1957

A new technique is described for studying the reactions of Simulium larvae to insecticides in the laboratory. By means of a constant stream of air bubbles directed against the inner walls of a large glass test jar, submerged larvae are induced to leave the vegetation on which they were fixed and to become attached to the inner wall of the jar at the point of maximum turbulence. In this position they can be counted and observed throughout the experiment.

The larvae receive no handling at all, and mortality in untreated controls remains low for at least 48 hours after larvae have been collected from the field. The water in the experimental jars is replaced with fresh stream water 2 or 3 times during the course of each experiment, and there is no need for a constant supply of untreated and unfiltered running water.

The technique has been worked out with particular reference to Simulium damnosum, which react well to these experimental conditions.

DDT in acetone solution produces a high mortality among Simulium damnosum larvae at concentrations much lower than normally used in field control. After a 1-hour exposure the LD50 is 0.005 parts per million on a 24-hour basis, and 0.002 ppm. on a 48-hour basis. Mortality approaches 100 per cent 48 hours after a 1-hour exposure to 0.01 ppm. A 30 per cent emulsion concentrate formulation tested was rather less effective, under laboratory conditions, than DDT in acetone.

Under laboratory conditions a 1-hour exposure to Dieldrin produces a 24-hour mortality of 100 per cent in full grown Simulium damnosum larvae at various concentrations down to 0.001 parts per million. Dieldrin (HEOD) in acetone was effective less than a 20 per cent emulsifiable concentrate tested. Although 0.001 ppm produces such a high mortality, the efficiency of the insecticide falls off rapidly below that point, and there is no appreciable effect after a 1-hour exposure to 0.0002 parts per million. In comparing the best results obtained with Dieldrin—in emulsion form—with the best results obtained with DDT—in acetone—Dieldrin appears to be effective at concentration at least 10 times lower than with DDT.

During the period from 1952 to September 1955 there occurred in Niigata, Yamagata, and Nagano Prefectures, Japan, a series of intoxications from squid and octopus, involving 758 known outbreaks and a total of 2,874 persons. From June to September 1955 there were a minimum of 108 outbreaks, involving 2,159 persons in the Niigata Prefecture, Miyaka, Mikura, and Hachijo Islands, caused by scombroid and carangid fishes. The actual number of persons involved is believed to be considerably greater than is indicated by the preceding figures. The morbidity rate for the cephalopod intoxications was 27.1 per cent, and the fish intoxications 24.6 per cent. The mortality rate was found to be 0.77 per cent in given series of fish poisonings, and 0.84 per cent for the cephalopods. All of the intoxications occurred during the months of June through September. The symptomatology was that of a gastrointestinal upset. Neurological symptoms were largely absent, and the patients generally recovered within a period of 48 hours. Bacteriological tests failed to show evidence of any of the ordinary types of bacteria known to cause food poisonings. The etiology of these outbreaks is still unknown, but epidemiological studies are continuing.

A series of tests in tubs (semi-natural conditions for test snails) was run for approximately two years to determine the relationship of temperature to molluscicidal activity of dinitro-o-cyclohexylphenol (DCPH), sodium-pentachlorophenate (NPCP) and copper sulfate (CuSO4), at 3 ppm., against the snail, Biomphalaria boissyi, in Egypt. The mean percentages of mortality among snails exposed to these toxicants was markedly increased with elevation of temperatures in the test tubs of artificial as well as by natural (seasonal) heating. Similar relationships were apparent when molluscicides were applied in test plots or sections of irrigation canals at different times of the year. In the interest of molluscicidal efficiency and economy, temperature should be considered in the practice and in the planning of snail control programs in the field.

Oncomelania nosophora maintained on moist filter paper alone for a period of one year did not lay eggs. The lack of soil for the formation of the egg capsule and/or the inadequacies of the diet may account for the absence of egg laying. The survival rate of the snails maintained on moist filter paper for one year was slightly lower than those maintained in standard rearing dishes. When transferred to standard rearing dishes, the snails produced young in the normal reproductive pattern for the species.

Oncomelania quadrasi and O. nosophora females, when mated and isolated as pairs, laid an average of 0.7 eggs per day. The frequency of egg-laying ranged from daily to a lapse of about 50 days. The maximum number of eggs laid by a female snail in a single day was 24 for O. nosophora and 23 for O. quadrasi. O. formosana laid an average of 1.3 eggs per day, and the maximum number of eggs laid by one snail in one day was 19. The frequency of egg laying ranged from daily to an interval of 12 days. The average incubation periods for eggs for O. quadrasi, O. nosophora, and O. formosana were 18.4, 21.4, and 19.5 days respectively. The average sizes of the eggs with capsules of the three species in the same order were 0.6, 1.1, and 1.0 mm., while the average sizes of the newly hatched snails were 0.5, 0.6, and 0.55 mm.

The growth rates for O. quadrasi, O. nosophora, and O. formosana were 0.2 mm., 0.4 mm., and 0.3 mm. per week respectively. In general, the males and females grow at approximately the same rate for the first 11 weeks for O. quadrasi and 16 weeks for O. nosophora. At this time the rate of the males was reduced and size dimorphism occurred.

Female snails were raised from isolated eggs and were followed through to maturity. They were mated only once to male snails reared under the same conditions. After the mating and isolation, O. quadrasi, O. nosophora, and O. formosana produced young snails for average observation periods of 285, 180, and 135 days respectively. The maximum periods of reproduction per species were 385, 213, and 200 days respectively. Snails were found to be able to produce several hundred young from one mating.

The duration of copulation of O. quadrasi is from about one hour to four and one-half hours and for O. nosophora about one hour to five and one-half hours. O. nosophora does not copulate as readily as O. quadrasi.

Introduction to Statistical Analysis, by Wilfrid J. Dixon and Frank J. Massey, Jr. 2nd edition. 488 pages illustrated. New York, McGraw-Hill Book Company, Inc., 1957. $6.00

Acta Leidensia, Edita cura et sumptibus, Scholae Medicinae Tropicae (Mededelingen uit het Instituut voor Tropische Geneeskunde Leiden, Rapenburg 33) Volumen XXVI–XXVII. 346 pages illustrated. Leiden, Universitaire Pers Leiden, 1957.

Public Health in Indonesia, Problems and Planning, by J. Leimena. 170 pages. Den Haag-Djakarta, N.V. v/h G.C.T. van Dorp & Co., 1956.

Fundamentals of Microbiology, by Martin Frobisher. 6th edition. Philadelphia, W. B. Saunders Company, 1957. $6.50.

Biochemical Disorders in Human Disease, ed. by R. H. S. Thompson and E. J. King. 843 pages, illustrated. New York, Academic Press, Inc., 1957. $12.50

Annual Epidemiological and Vital Statistics 1954. World Health Organization. 617 pages. Geneva, World Health Organization, 1957. $10.00 (Bilingual edition, French and English).