THE AMERICAN JOURNAL OF HYGIENE THE CONTROL OP SCHISTOSOMIASIS JAPONICA

Transcription

1 THE AMERICAN JOURNAL OF HYGIENE VOL. 45 MAY, 947 NO. 3 THE CONTROL OP SCHISTOSOMIASIS JAPONICA I. OBSERVATIONS ON THE HABITS, ECOLOGY AND LIFE CYCLE OF ONCOMELANIA QUADBASI, THE MOLLU8CAN INTERMEDIATE HOST OF SCBISTOSOMA JAPONICUM IN THE PHILIPPINE ISLANDS i INTRODUCTION One of the methods of controlling trematode infections is to eliminate the molluscan intermediate hosts. To attack the problem of exterminating such an intermediate host intelligently it is important to know something of the life cycle and habits of the species of mollusk essential to the trematode in question. In the Philippine Islands the snail Oncomelania quadrasi is the only known intermediate host of Schistosoma japonicum. While rather exhaustive studies have been made of the snail hosts for this trematode in China and Japan, relatively little had been known about the Philippine species. It was first described in 895 by von Mollendorf from Surigao Province in From the Commission on Schistosomiasis, Commission on Tropical Diseases, Army Epidemiological Board, Preventive Medicine Service, Office of The Surgeon'General, Washington 5, D. C. Department of Preventive Medicine and Public Health, The School of Medicine of the University of Oklahoma, Oklahoma City, Okla. BY DONALD B. McMULLENz (Received for publication October 7th, 946) Northeastern Mindanao (Bartsch, 936). Further details have been given by Abbott (945). Tubangui (93) proved that it was an intermediate host of S. japonicum on Leyte. Later Tubangui and Pasco (94) reported it on Mindanao, Leyte, Samar and Mindoro in the Philippine Islands. Surveys made after the liberation of the Philippines uncovered new foci on Leyte and Samar (Sullivan and Ferguson, 946; Bang et al., 946), Mindanao ("Wright et al., 945; Abbott, 946a) and Mindoro (Hunter, 945). In these surveys observations were made on the habitat but little was learned of the detailed bionomics of the snail. In connection with attempts to control 0. quadrasi the author made some observations on its habits, eggs, rate of growth, etc. He is indebted to Lt. R. T. Abbott H(S), USNR, United States Navy Medical Research Unit no. ; Capt. M. S. Ferguson Sn. C, 5th Malaria Survey Detachment; Capt. Owen H. Graham Sn. C, 5th Malaria Survey Detachment; and the members of the Commission on Schistosomiasis for assistance and many valuable suggestions. Downloaded from at Pennsylvania State University on May 0, A.J.H., 947, VOL. 45 : pp

2 60 DONALD B. MCMULLEN Observations on the habitat Most of the observations given in the present paper were made in the Leyte Valley, an extensive coastal plain on the east side of the Island of Leyte. They were made from early in May into October, 945. This period approximates the "dry season." During the months of April through September there is an average rainfall of about 6 inches per month. During the months of October through March the monthly average is about inches, with a peak of about 5 inches in December. In one of the monthly reports made by the Philippine Schistosomiasis Commission, Unit no. (January, 94), the statement is made that the snail apparently disappears during the dry season. Members of the Commission on Schistosomiasis were always able to collect snails in all the colonies under observation during the dry season of 945. The snail is amphibious and is found living under variable conditions. In the Leyte Valley and in the Maranding area in Mindanao the snail lives on the coastal plain, but well removed from any tidal effects. These areas are alluvial plains, not much above sea level, with meandering streams, and a great deal of the terrain is swampy or boggy and overgrown with grass. Even in a large area like the Leyte Valley the colonies of snails are scattered, often separated from each other by extensive areas not inhabited by this snail. In the other areas on Mindanao, Samar, and Mindoro the snails are found in small, widely separated foci. In these areas rice is grown in relatively high valleys, or near lakes well removed from the coastal plain. The snails live in places kept wet in the process of rice culture, or in the seepages at the bases of the mountains that form the walls of the valley. The endemic area near Valencia (Mindanao) is apparently of this same type but it could be viewed only from the air because of the activity of Japanese troops. It is not known whether these small foci are '' last ditch stands" or represent recent establishment of the snail in the areas. The conditions under which they live indicate that they lead a precarious existence and slight changes in the environment would eliminate the colonies. Tubangui and Pasco (94) thought that the acidity of the soil was a factor in the distribution of the snail. In the observations made by the Commission on Schistosomiasis the soil was not tested, but the ph of the water lying over the soil was used in testing the conditions in which the snails were living. Tubangui reported the ph of the soil in 5 areas as ranging from 5.65 to The range in our tests ran from 6. to 7.6, with most of the areas on the acid side. Observations indicate that when the snails live where the water has a ph of 6. to 6.6 there is considerable shell erosion. In areas "where the ph is near neutrality or slightly alkaline there is little or no erosion of the shell and the snails tend to be larger. Methods used in farming in the Leyte Valley have a decided bearing on the extent of the snail colonies. In the earlier reports on the habits of the species one gets the impression that the rice fields are the usual habitats. Observations made by members of the commission indicate that although found in rice fields, the snail finds conditions more suitable in the wet, grassy areas not under cultivation. At the time these observations were made, much of the land was not under cultivation but this may have been due partly to the war. Along the creeks and larger streams in the Leyte Valley there are often ex- Downloaded from at Pennsylvania State University on May 0, 06

3 CONTROL OF SCHISTOSOMIASIS JAPONICA. I 6 tensive flood plains. During the wet season, we were told, these are usually under water. Most of these areas were covered with a dense growth of tall grass, whose old stems and leaves formed a heavy mat of organic matter in various stages of decomposition. In some places these areas were used for grazing carabao. Prom time to time parts of the tracts were brought under cultivation and made into rice fields. Sometimes plows were used, but for the most part the cultivation consisted of driving carabao over the area until the grass was tramped down and the soil was a mass of quaking mud, about foot in depth. The rice was then transplanted from the nursery bed, which in this case was in a relatively dry, well drained plot. This radical change in the habitat, the removal of the protective covering and exposure to the sun eliminates this species of snail in the area under cultivation. Remnant colonies, of course, remain in the environs of the previously well populated sections. After the rice grows enough to shade the ground the snails can again live in the field but they were never seen there in large numbers. Apparently there is no migration into the field but a few snails are washed in by flooding, either by rains or in the process of irrigation. It would appear that as long as the area is under cultivation there are few, if any, snails present and it is only when the plot is abandoned for a time that the colony once more becomes populous. Avery (946) reports that the snails stay submerged a great deal of the time. He does not state at what time of the year his observations were made but it probably was during a part of the wet season. During the dry season most of the snails are on the surface of the moist soil in these grassy, shaded areas. They are sometimes found where there are sedges but the grass habitat is more typical. They are also found in adjacent streams and pools where they are on decaying palm fronds, coconut shells, grass, water hyacinths, etc. Although many of the snails in the streams and ponds are above the water level, they are also found beneath the water and on the muddy bottom. When the grass-swamp area is flooded the snails are distributed much as in the stream and pool habitats. Those found in the swampy areas and above the water level on the vegetation in streams, move about and feed when the surfaces are moist. Otherwise the opereulum is closed and the snail is fastened in place by a small amount of dried mucus. The snails are easily dislodged by the wind or rain drops. Early in the morning when there is a heavy dew, or after a shower, they become active and resume feeding. Dew is often heavy enough to encourage activity until noon in the heavy growths of grass. Under these conditions, whether over soil or water, they are apt to move up into the mat of vegetation, or up the living stems for several inches. In the swampy areas an extended hot, dry period causes some reduction in the colony but the deaths seem to occur primarily among the very young and the older mature snails. Observations on the life cycle of the snail For a more detailed study of the habits of 0. quadrasi, 4 collecting stations were selected in the endemic area of schistosomiasis in the Leyte Valley. One of these stations is located in the sitio of Atogune, near the barrio of San Antonio. It is in the northern end of the endemic area and about 5 miles from the beach. The snails there live in the bed of a small stream. It is fed Downloaded from at Pennsylvania State University on May 0, 06

4 6 DONALD B. MCMULLEN by springs and is shaded by sago (Metroxylon) palms so that even in the driest weather there is always water present. After rains the stream was filled but at no time during the period of observation did it overflow. This spot seemed to offer an ideal habitat for the snail but conditions prevented the colony from extending over a large area. Another station, near the barrio of Limbujan, is located near the center of the endemic area but is only about miles from the beach. This habitat consists of a small stream bordered on either side by an extensive flood plain that is low, wet and covered with a dense growth of grass or rice fields. The spot where the collections were actually made had been used in former years for growing rice but had not been cultivated for at least 3 years. At times the area became rather dry and the activity of the snails was retarded. At other times the area was covered with as much as inches of water. A third station, on a small stream between the barrios of Kansamada and Kapahaun, is located near the center of the endemic area but is about 9 miles away from the beach. This stream was usually confined to its banks during the period of study. The snails were collected along the bank of the stream and out in the damp, grassy areas that border it. Much of the latter was made into a rice field in the last week of June. The last station is located at the southern end of the endemic area. It is a low, swampy area adjacent to a rather large stream in the barrio of Union, near Tarragona. Occasionally the soil became dry enough to inhibit snail activity but there was always some water left in the depressions. After rains there were sometimes as much as 8 inches of water running through the collecting area. The central portion of this swamp was made into a rice field in the latter part of June. The collections at each station were made every weeks, except during parts of July and August when a survey was being made on the island of Mindanao. The collections usually consisted, of about 400 snails. During the period of observations 6,447 snails from these 4 stations were collected and measured to the closest 0.5 millimeter. The results show: () sexual dimorphism; () the relationship between the average length and the percentage of young snails present; (3) when a new generation appeared at a station; (4) the rate of growth of the snails under variable conditions; (5) the approximate time of egg laying and hatching; and (6) some indication of the length of the life of the snail. The adult snail. It was found that when population groups of 0. quadrasi were measured, and the lengths plotted, the typical distribution of the adults was a bimodal curve. Abbott (946b) has shown that the larger mature snails, averaging about 4.75 millimeters, are females, while the group of snails averaging about 4.00 millimeters is made up of males and some immature females. With one exception, this sexual dimorphism is quite evident in all of the 4 collections made in this study. Evidence of this size difference is shown in figures to 4, inclusive. Figure shows the only collection, made on May th at the Kansamada-Kapahaun Station, where sexual dimorphism is not shown. The other curve, from a collection made September th at the same station, shows the typical distribution. Apparently the first collection was from a population group that was just reaching adulthood. In the collection taken weeks later the modes of the two sexes fall at 4.00 and Downloaded from at Pennsylvania State University on May 0, 06

5 CONTROL OF SCHISTOSOMIASIS JAPONICA. I MAY II a S PT MB H LENGTH IN MM. FIGURE. A comparison of the percentage of 0. quadrasi found in each size group in the first collection and one made 4 months later at Kansamada-Kapahaun millimeters. In later collections the mode of the adult female group goes up to 5.00 millimeters. Abbott (946b) also found one area that demonstrated this same condition. It will be noted in the figures that in the typical collection the adult male and young female mode is usually found between 3.75 and 4.5 millimeters. The adult female mode is usually 4.75 or 5.00 millimeters. It will also be noted that the male-young female group usually has a greater number of snails than the group containing the mature females. This difference is due to the presence of the young females. The egg. Lt. R. T. Abbott H(S), USNR, United States Naval Medical Research Unit no. (946b), discovered the eggs of 0. quadrasi and with his assistance it was possible to find and observe them. They are laid singly and measure about 0.70 millimeter in diameter. They are fastened to wet, decaying coconut husks, leaves and similar objects. They are cemented to the surface and covered with brownish material that seems to be composed of fine sand grains and organic matter. The entire mass measures to.5 millimeters. The protective coloration is excellent and the eggs are very difficult to find. When the eggs are removed from the surface, the underside, not covered with the brownish material, Downloaded from at Pennsylvania State University on May 0, 06

6 64 DONALD B. MCMULLEN is clear and the developing snail can be seen. The newly hatched snail measures about 0.50 millimeter in length. Evidence obtained from the sections of the population groups at the 4 stations indicate that egg laying is almost continuous, but at certain times it is greatly increased. Copulation of the snails was often seen while collecting in the field. Appearance of new broods. Some young were found in all 4 areas during the entire period of observation. At certain times, however, there was evidence that there had been one or more definite periods of greater reproductive activity. The series of collections that show this most clearly, because only one group of to SEPTEMBER 4- t> SEPTEMBER 3 o o OCTOBER. 3 o A *J \ /\//\ A\ " \ young is involved, are the snails collected at Kansamada-Kapahaun on September 4th, September 8th, and October 3th. Figure compares the results obtained in the first collection at this station, May th, and those obtained on September 4th. In the first collection there were no very young snails; the smallest measured.50 millimeters, and later was proved to be at least 8 weeks old. In this collection there were very few immature snails and the same is true in those that followed, until the one made September 4th. At this time a small, but definite, group of young appears, with a mode at.00 millimeter. Figure shows what happened to this group. i i v IIt LENGTH IN MM. FIGUEE. A comparison of the percentage of 0. quadrasi found in each size group in 3 consecutive collections at Kansamada- Kapahaun. 0 Downloaded from at Pennsylvania State University on May 0, 06

7 CONTROL OF SCHISTOSOMIASIS JAPONICA. I 65 By September 8th this mode shifts to.50 millimeters, indicating a growth of 0.50 millimeter. Two weeks later the mode has shifted to.00 millimeters, indicating that growing conditions were good. It will also be noted that the group of young snails remains small, even when the increase in size assists in the collection. This small group of snails would not be sufficient to maintain the large colony at this station. It is concluded, therefore, that during this entire 5-month period there was nothing that approached major reproductive activity. The data obtained from the first collection indicated that there had been little egg laying for about months previous to May th and most of the snails present were much older. At Limbujan and Union there were several large broods of snails. At times more than half of the collection would be made up of immature snails. At Limbujan the first definite brood appeared on July 4th. A second, and very large one, appeared on August 5th. A third one appeared in the collection made on October 3rd. At Union the first brood appeared on June 6th. On August 5th, collecting conditions were ideal and two definite groups of young were found, one with a mode at 0.75 millimeter and another at.75 millimeters. The conditions in this area at the time of the preceding collection were particularly unsuited for finding the small snails and probably accounts for not detecting the presence of the larger of the two broods earlier. The size difference in the two groups indicates that they were about 4 weeks apart in development. In the last two collections, made September 8th and nd, the results show the two broods continuing their development but no new ones appear. At the Atogune station a situation was found that was quite different from that found at Kansamada-Kapahaun. The conditions were apparently ideal for almost continuous production of young and for their rapid growth. For this reason it was often difficult to follow the different crops of young, from their first appearance until they became adults. In the collection made on June 3th, for example, 3 different modes were found, in addition to the two usually found in the adult snails. Figure 3 shows the results obtained in 3 of the least confusing collections from Atogune. The snails obtained on September 5th show two groups of young, one with a mode at.00 millimeters and another at 3.00 millimeters. In the September 0th collection a new and relatively large brood appears, with a mode at.50 millimeters. The youngest group in the previous collection has reached a mode of.50 millimeters and the older group is apparently merging with the male-young female section of the population. In the October 3rd collection the youngest group from the previous collection has grown 0.50 millimeter, as shown by the shift of the mode from.50 millimeters to.00 millimeters. The smaller group of young that first appeared in the September 5th collection seems to have lost its identity. At least 8 different broods of young appeared at Atogune during the period of investigation. Average length and percentage of young. As would be expected, when a number of small snails appeared in a population the average length decreased. This continued until the production of young decreased or until the growth of the new generation was sufficient to cause an increase in the average length. At 3 of the stations: Atogune, Limbujan and Union, young snails were present in considerable numbers at all times but Downloaded from at Pennsylvania State University on May 0, 06

8 66 DONALD B. MCMULLEN SCPTCMBEH S a- -o SCPTCMBCR. 0 o o OCTOBER LENGTH IN MM. FIGURE 3. A comparison of the percentage of 0. quadrasi found in each size group in 3 consecutive groups at Atogune. began to increase soon after the study began. It is possible that a part of this apparent increase was due to greater proficiency, with practice, in the art of collecting these small snails. The average length of two collections, made just weeks apart but from the same place, often differed a great deal. In some instances this was due to the appearance of large numbers of young snails. At other times it was due to poor collecting conditions, because when the area was relatively dry it was difficult to find the smaller snails. During prolonged adverse periods the death of the young influenced the averages. On September 8th a collection from Union shows that 66.8 per cent of the snails were young. In the next two collections the percentage of young snails decreased progressively to 55.6 per cent, then to.9 per cent. This reduction in the number of young is due primarily to the high mortality rather than their growth to maturity. In spite of these variables the average length of the snails from these 3 stations tends to decrease until near the end of the period of observation. At this time the average length remains about the same or increases slightly. At the fourth station, Kansamada- Kapahaun, some young snails were always found but there was never a major reproductive period. As a result the average lengths stayed the same. The brood that appeared near the end was Downloaded from at Pennsylvania State University on May 0, 06

9 CONTROL OF SCHISTOSOMIASIS JAPONICA. I 67 so small it had little or no effect on the average length. Bate of growth. The rate of growth of some of the small snails is shown in figures and 3. The increase in size of the brood from the Kansamada-Kapahaun station can be followed easily. During each -week period the snails grew 0.50 millimeter. The series from Atogune was complicated by the presence of several broods, and can be shown more clearly in a simplified form as given in figure 5. In the August 8th collection there was a group of young snails with a mode at.00 millimeters. In the next 3 collections growth is shown by the shift of A-. A I o * / " / ' / ' r / ' / * I! I! I '! 6 \ \ \ \ \ \ \\ \ \\ \ \ i \\ \ \\ k.. the mode, 0.50 millimeter for each - week period. By September 0th this group was approaching maturity and lost its identity among the adults. By projection of this growth curve it appears that the brood hatched about June 7th. In the closely related species, Oncomelania (Katayama) nosophora (Robson), the eggs hatch about weeks after laying (Sugiura, 933). In the laboratory Abbott (946b) observed that the period from laying to hatching in O. quadrasi is about the same. In that case, the eggs at the Atogune station were probably laid about June 3th. At times when the habitat became dry growth of the AUGUST 5 a ajk/gust 9 a l\ l\ / '' «-h ; ' / J* ' / V / / / V r P t ' i ii i i "A, V, Downloaded from at Pennsylvania State University on May 0, O LCNGTH IN MM. FIOUEE 4. A comparison of the percentage of 0. quadrasi found in each size group in two collections made at Limbujan over a -week period when conditions were adverse.

10 68 DONALD B. MCMULLEN o / / / u % / V JUNE JUNC JULY JULY AUG. AUG. SCPT. StPT 3 7 II* 5* 8 3 S 0 NO COLLECTIONS MADE. FIGURE 5. The changes in the mode of a group of young 0. quadrasi collected at Atogune at -week intervals. snails was retarded. This was shown at the Limbujan and Union stations. At times the growth would be only 0.5 millimeter between collections and at Limbujan a large group of young failed to show growth during one -week period. The results of these two collections are shown in figure 4. During the period from August 5th to August 9th the area was especially dry. It will be noted that during this time the mode representing the young remains the same. The average size of the young snails is slightly greater in the second collection, as is shown by the small shift to the right. This apparent small increase in size may be due partly to the poor collecting conditions, however. The evidence gathered from all the data indicates that under optimum conditions the young snail grows at the rate of about 0.5 millimeter per week. The period from egg-laying to the mature snail apparently is at least 4 to 5 months. Life span of the snail. The collections from the Kansamada-Kapahaun station give the best indication of the length of life of the adult snail. During the period from May th to October 3th the same adult group of snails made up nearly the entire population, and they had not produced what could be termed a new generation. From this it can be concluded that the adult snail certainly lives at least 5 months, but there are no data to indicate just how long they do live. As has been shown in a previous section, the snails grow at the rate of about 0.5 millimeter per week under optimum conditions. It requires 4 to 5 months for an adult snail to develop from a newly laid egg. This would indicate that the life span of 0. quadrasi was at least year. Sugiura (933) reports that 0. nosophora lives for 5 years or more. The present author does not believe that the Philippine species lives that long but does not have concrete evidence to prove the point. DISCUSSION The full importance of the relationship of the life cycle of the snail, together with its control, and the epidemiology of schistosomiasis japonica, is still to be determined. In other attempts to control snails it has been found that the eggs can withstand chemicals that kill the young and adult snails. With the snails that have a definite annual period for egg laying, certain times of the year are better than others for instituting control measures. In the case of O. quadrasi, where egg laying may occur Downloaded from at Pennsylvania State University on May 0, 06

11 CONTROL OF SCHISTOSOMIASIS JAPONICA. I 69 at almost any time and varies from colony to colony, the problem of finding a chemical that will kill the eggs is introduced. From the study of the development of the snail it is possible to estimate approximately when the earliest mature infections will be found in the young snail. In laboratory-infected snails cercariae of 8. japonicum have appeared in as short a time as 8 weeks, but development in 0. quadrasi was found by Hunter et al. (946) to take weeks under laboratory conditions. With both the rate of growth and time required for development of the infection known, it might be possible for cercariae to emerge from snails that measure about.50 millimeters in length, if infection took place soon after hatching and development took place in 8 weeks. In crushing snails collected in the field and in placing snails in water for cercarial emergence, the smallest snails found with mature cercariae measured 3.00 millimeters. These snails could not have been less than 0 weeks old, and if their growth had been retarded by environmental conditions they could have been to 4 weeks of age. Data from the crushed snails indicate that penetration by the miracidium is not confined to the very young. Four adult females, measuring 4.75 to 5.00 millimeters in length, were found with young sporocysts. These snails were at least 7 weeks old and, since they had reached maturity, may have been much older. The infections in these snails probably were not more than 6 weeks old. At the beginning of the study of the snail population at the 4 stations, the snails were crushed and examined for trematode infections. Need for cercariae in other experiments, lack of time and dissecting microscopes made it necessary to discontinue this as a routine procedure in most of the collections. If it had been continued it might have given some indication when most of the infections were acquired by the snails. In the collections where the snails were crushed all of the 8. japonicum infections were divided into the following categories: () young daughter sporocysts; () immature cercariae; (3) mature cercariae. The first group included any infection that consisted of young sporocysts that did not have cercariae developed to a point where they could be recognized. In this group it is impossible to say more than that these sporocysts were schistosome-like. On the other hand, there were very few trematode infections, other than 8. japonicum, in these snails. During the period of observation the investigator crushed,3 snails and other members of the commission crushed many more and no strigeids were found. The trematode infections found, other than 8. japonicum, could be easily differentiated in any stage of development. There were two snails that had infections with monostomes and 4 had xiphidiocercariae. It appears safe to assume that all the immature schistosome-like infections were actually 8. japonicum. The second category was made up of the infections which had cercariae that could be differentiated but were not mature enough to emerge, i.e., not active enough to be able to swim to the surface film in the normal fashion. The third category included all those in which mature cercariae were present. One could not be certain that the snail with the last type of infection had actually given off cereariae but for the purpose of discussion this classification worked very well. During May, 568 snails from the Kansamada-Kapahaun station were Downloaded from at Pennsylvania State University on May 0, 06

12 70 DONALD B. MCMULLEN crushed and none of them was infected with trematodes. During this same period 57 snails were examined from the Limbujan station and 5 contained 8. japonicum, giving an infection rate of 0.95 per cent. At the Union station only one snail of 99 examined had an infection with 8. japonicum, or a rate of 0.33 per cent. The first collection of 53 snails taken at the Atogune station was found to have 44 snails (7.4 per cent) infected with 8. japonicum. This was the highest rate of infection found, and since it was difficult to obtain cercariae for other experiments, later collections from this area were turned over to other members of the commission. The last collection at this station was made October 3rd TABLE and the snails were crushed for comparison with those in the first collection. The results are found in tables and. On May 6th, 7.4 per cent of the snails collected contained 8. japonicum. In the October 3rd collection only 33 of 459 snails (7.7 per cent) were infected with this trematode. This difference may be due partly to the great increase in the number of young. At this station reproduction had been going on almost continuously and at a high rate. The last collection contained 45.8 per cent young, compared with 9.8 per cent in the first collection. As will be seen in the table, the smallest snail found to contain a recognizable infection was.50 millimeters in length. If all the snails in both collections less than.50 milli- Results obtained in crushing a collection of O. quadrasi and examining them for infections, Atogune, May 6th Length (millimeters) Totals Measurements ^j A onaila IN Oft Oillkllo negative Young sporocysts Immature cercariae 3 Data from crushing No. snails positive Mature cercariae * 6 7 Other infections.. _ 0 Downloaded from at Pennsylvania State University on May 0, 06 * One of these appeared to be very old; i.e., walls of the sporocyst were dark and pigmented.

13 CONTROL OF SCHISTOSOMIASIS JAPONICA. I 7 TABLE Results obtained in crushing a collection of O. quadrasi and examining them for infections, Atogune, October Srd Measurements Data from crushing Length (millimeters) Totals negative Young sporocyste * Immature cercariae 5 No. snails positive Mature cercariae Other infections * Mother sporocyst. t Xiphidiocercariae. meters are ignored, the difference in the rate of infection is still great. In the May 6th collection this procedure gives 0. per cent of the older snails infected, while in the October 3rd collection the rate is.3 per cent. The ratio of the immature infections gives another indication that there is actually a decrease in the incidence. The rate of immature to mature infections in the October 3rd collection is about half that found in the May 6th collection; i.e., 8 to 5 and 7 to 7. The numbers examined are small but there seems to be some indication that there is a decrease in the rate of infection during It the dry season. This would seem to substantiate the statement made in the Philippine Schistosomiasis Survey Unit no., Progress Eeport for January, 94. They had observed that the snails are more apt to have infections after the rainy season. Of course, the observations presented here are from one small area and it is possible that the rates of infection in different areas can be as variable as the production of young. If most of the infections are acquired by the snails in the early part of the rainy season in October and November, when the miracidium concentration might conceivably be greatest, there Downloaded from at Pennsylvania State University on May 0, 06

14 7 DONALD B. MCMULLEN would be time for the development of these infections before the end of the rainy season. In January, February, and March these infections would be reaching maturity. Since the snails must be in water to shed eercariae, it is possible that the greatest danger of human infection would be at this time. During the dry season a large majority of the snails live in an environment that often does not offer an opportunity for cercarial emergence. However, during the small floods that follow occasional rains in this period of the year, there must be large numbers of cercariae present. The output of the infected snails at such times would be greater than when they were submerged most of the time. The relatively smaller amount of water in the streams during the dry season may tend to concentrate the cereariae. It would appear that contact with water in infested streams during these floods would be dangerous. SUMMARY. Eggs of the snail, Oncomelania quadrasi, are described.. The egg probably hatches about weeks after being laid. The newly hatched snails measure about 0.50 millimeter in length. 3. Under ideal conditions the snail grows at the rate of about 0.5 millimeter per week. The shortest time required to reach maturity is 4 to 5 months. In areas where conditions are sometimes adverse the time required would be at least a month longer. 4. Some young snails appeared during the entire period of observation at all of the collecting stations. At three of the stations large broods of young were found. 5. At one of the stations it has been shown that the adult snails lived for at least 5 months. At the end of the period of observations there was no evidence that this group was dying out, and it had not had a major reproductive period. It would appear, therefore, that the snail probably has a life span covering at least year. 6. The smallest snails with mature cercariae, obtained in the field, measured 3.00 millimeters in length. These snails were at least 0 weeks old and must have been infected shortly after hatching, since the development of S. japonicum in O. quadrasi requires weeks under laboratory conditions. 7. Data from the crushed snails indicate that penetration by the miracidium is not confined to the very young. 8. At one station where the incidence was relatively high, there was some evidence that the rate of infection decreased during the dry season. REFERENCES Abbott, E. T. 945 The Philippine intermediate snail host (Schistosomophora quadrasi) of schistosomiasis. Occas. Papers on Mollusks, Harvard University, : a Handbook of Medically Important Mollusks of the Orient and the Western Pacific. Bull. Mus. Comp. Zool., Cambridge, Mass. (In press.) 946b The egg and breeding habits of Oncomelania quadrasi Meldff., the schistosomiasis snail of the Philippines. Occas. Papers on Mollusks, Harvard University, : Alomia, A. 94 Monthly progressive report of Philippine Schistosomiasis Survey Unit no., for the month of January. (Unpublished report.) Avery, J. L. 946 The habitat of the snail host of Schistosoma japonic-urn in the Philippines. Science, 04: 5. Downloaded from at Pennsylvania State University on May 0, 06

15 CONTROL OP SCHISTOSOMIASIS JAPONICA. I 73 Bang, F. B., Hairston, N. G., Graham, O. H., and Ferguson, M. S. 946 Studies on schistosomiasis japonica. II. Methods of surveying for schistosomiasis japonica. Amer. Jour. Hyg., 44: Bartsch, P. 936 Molluscan intermediate hosts of the Asiatic blood fluke, Schistosoma japonicum, and species confused with them. Smithsonian Misc. Coll., 95: 60. Ferguson, M. S. 945 Freshwater snails collected on Leyte, P. I., with reference to those of medical importance. Beport no. 3 to the Surgeon General, Fifth Malaria Survey Detachment and Medical Research Unit. (Unpublished report.) Hunter, G. W., III. 945 Surveys for Schistosoma japonicum in Mindoro, Philippine Islands. Eeport for July to Surgeon General, Commission on Schistosomiasis. (Unpublished report.) Hunter, G. W., Ill, Bennett, Harry J., Ingalls, James W., and Greene, Eugene. 946 The molluscan intermediate host and sehistosomiasis japoniea. IV. Experimental infection of Oneomelania quadrasi, the molluscan intermediate host of Schistosoma japonicum. (In manuscript.) Sugiura, S. 933 Studies on the biology of Oneomelania nosophora (Robson), an intermediate host of Schistosomum japonicum. Mit. Path. Inst. Med. Fakul. Niigata, 3: -8. Sullivan, E. E., and Ferguson, M. S. 946 Studies on schistosomiasis japonica. III. An epidemiological study of schistosomiasis japonica. Amer. Jour. Hyg., 44: Tubangui, M. A. 93 The molluscan intermediate host in the Philippines of the oriental blood fluke, Schistosoma japonicum Katsurada. Phil. Jour. Sci., 49: Tubangui, M. A., and Pasco, A. M. 94 Studies on the geographical distribution, incidence, and control of schistosomiasis japonica in the Philippines. Phil. Jour. Sci., 94, 74: Wright, W. H., McMullen, D. B., and Faust, E. C Survey for schistosomiasis on the Island of Mindanao. Eeport to Surgeon of Eighth Army, Commission on Schistosomiasis. (Unpublished report.) Downloaded from at Pennsylvania State University on May 0, 06