or that, if present, occur in smaller quantities. It is barely possible that some means of artificially producing in lambs an immunity against the evil effects of stomach worms may be devised, but at the present time this is only a matter for speculation and experimental research. Our present knowledge of the stomach worms leads us to direct our efforts toward bringing about freedom from infection, or, as the next best thing, reducing the amount of infection to a minimum and keeping it there. LIFE HISTORY OF THE STOMACH WORM. In the adult sexual stage stomach worms are able to live and carry out their reproductive functions only in the alimentary canal of sheep or other ruminants, occurring in the largest numbers in the fourth stomach. Each female produces thousands of eggs of microscopic size which do not develop into adult worms in the body of the host in which they are deposited, but, without hatching, pass out of the intestine in the feces. In a few hours, days, or weeks, according as the temperature is high or low, these eggs, if they are not. killed by dryness or freezing-either of which is fatal to them-hatch out, and the tiny embryonic stomach worms then develop to what may be termed the final larval stage, or infectious stage. This later development likewise requires days or weeks, according to the temperature, and until the embryos have reached the infectious stage they appear to be fully as susceptible to freezing and drying as the eggs. Having reached the infectious stage, however, the worms are able to withstand long periods of dryness and severe cold, though some of them succumb comparatively early. In the infectious stage the young worms are very active in the presence of moisture, and rapidly crawl up blades of grass and other objects whenever the relative humidity of the air is at a maximum, provided the temperature is above 40° F. or thereabout; below this temperature they are inactive. A decrease in the relative humidity, with the consequent evaporation of the moisture from the surface of grass blades and other objects, stops the migrations of the worms, and they become quiescent and remain in a condition of suspended animation wherever they happen to be at the time. During the next period of wet weather, dew, rain, or fog, they again become active and climb still higher on the grass. This crawling up grass blades is evidently most advantageous to the worm, as it thereby gets into a position from which it is much more likely to attain its final abode within the stomach of a sheep or cow than if it stayed down on the ground. When swallowed by a sheep or other ruminant, the embryonic stomach worm, if it has reached its final larval stage, whether active at the time or in a state of suspended animation, continues its development and in the course of two or three weeks has reached maturity. The length of life of individual worms in the stomach has not been determined. We have kept infested sheep in pens with board floors which were kept clean by sweeping and frequent scrubbing, feeding the sheep from raised racks and supplying water in a trough which was frequently cleaned, for varying periods up to a maximum of nineteen months, and found them still infested. As the possibility of reinfection by larval worms developing from eggs passed in the feces of these sheep was not entirely removed, though greatly minimized, the results obtained do not necessarily indicate that the worms present at the end of the period of observation were all present when the experiment was begun. The experiment, however, while it proves nothing as to the length of life of the adult stomach worm, demonstrates the futility of attempting to rid sheep entirely of stomach worms by simply keeping them away from pasture. TREATMENT OF PASTURES. The maximum period during which the larval stomach worms are able to survive on pastures is not definitely known, but it has been found that pastures on which infested sheep had grazed were apparently still infectious after a lapse of nearly eight months, namely, from October 25, when the infected sheep were removed, to June 16, when the pastures were tested by placing in them some lambs which had been raised under special precautions to avoid previous infection. In cultures made September 14, 1906, from the feces of an infested sheep and kept thereafter in the laboratory, most of the larvæ were dead but some were still alive, though very sluggish, on June 5, 1907, nearly nine months later. Cultures in which the embryos were allowed to develop to the final larval stage, after which they were kept in cold storage at temperatures below freezing-in some cases as low as 12° F.-still contained some living embryos after two or three months, while in other cultures eggs and embryos not yet developed to the final larval stage were killed within a few hours when exposed to temperatures below freezing. These experiments show that pastures may remain infected for several months after sheep are removed from them, and that the infection is not destroyed by cold weather. They also show that during a winter with more or less freezing weather there will be little or no increase in the amount of infection in pastures occupied by infested sheep. The eggs passed in the feces of the sheep will either be killed at once by freezing, or, on account of low temperatures above freezing, will remain dormant or develop so slowly that they are killed later by frost before they have reached the final larval stage, which is resistant to cold. At the same time, while the infection of pastures may not be increased during the winter, the infestation of the sheep may be added to by their picking up from time to time larval worms which, prior to the beginning of cold weather, had already developed to the stage in which they are able to withstand freezing. If sheep, goats, and cattle are kept out of a pasture for a year, it is fair to assume, upon the basis of our present knowledge, that all, or practically all, larval stomach worms will have died within this time. There is also little doubt that the period required for practically accomplishing disinfection of a pasture may be considerably shortened by plowing it up and placing it under cultivation. There are thus two ways by which a pasture may be disinfected, one by excluding sheep or other ruminants for at least a year, and the other by turning the pasture into a cultivated field. In view of the fact that any sheep which may be placed on disinfected fields or pastures will probably not be entirely free from infestation, it is not of much consequence whether every larval stomach worm in the pastures is dead or not. The approximation to this point which is attained by vacating pastures for a year or by plowing them up is sufficient for practical purposes. METHODS OF PREVENTING THE DISEASE. Taking up the question of preventing stomach-worm infection, it appears that at the present time the only method of handling lambs born of infested ewes that can be guaranteed to keep the lambs free from stomach worms is a method which is so impracticable that it is not likely to come into general use. As soon as born the lambs would have to be taken away from their mothers and raised by hand, never allowed to suckle, and be kept by themselves in places not previously occupied by sheep, cattle, or goats, all of which being subject to stomach worms are liable to leave infection wherever they happen to have been, which, it has been noted, may persist for many months. The possibility of the embryos of stomach worms reaching these places by drainage from infected areas, in hay cut from infected meadows, or in barnyard manure used for fertilizer, would also have to be excluded. Furthermore, the milk fed to these lambs, whether from sheep, goats, or cattle, would have to be pasteurized or sterilized in order to avoid the possibility of infection from this source, as some of the tiny embryos of stomach worms which might be present on the skin, wool, or hair of the animal from which the milk is drawn would be very likely to get into the milk during the process of milking and thus ultimately find their way into the stomachs of the lambs. Simple filtration of the milk would not insure the removal of the worms. The experiment of filtering liquids containing embryonic stomach worms has been tried, using filter paper of the kind commonly used in laboratories for ordinary filtration, with the result that the worms readily passed through the filter. THE BARE-LOT METHOD. Methods of preventing stomach-worm infection less stringent than this have been tried with imperfect results. In the bare-lot method of raising lambs, devised by Dalrymple, the ewes and lambs are kept in an inclosure as free from vegetation as it can be kept, and are fed and watered from raised racks and troughs. Dalrymple found that when this method was used the lambs did not escape infection, and we have had similar experience. PASTURE ROTATION. In 1908 at the Experiment Station of the Bureau of Animal Industry near Washington, ten inclosures were constructed in a large field which had not been occupied by sheep or other ruminants for many years. Into inclosure No. 1 were placed a number of ewes and lambs which from the time the latter were born had been kept in separate pens except when turned together at intervals into another pen in order to enable the lambs to suckle, this pen being cleaned after each suckling period.. Prior experiments had indicated that the eggs and newly hatched embryos of the stomach worm would not develop to maturity if swallowed by sheep, and that the embryos must first develop to the final larval stage before they are able to complete their development when swallowed. In view of the fact that more or less time is required for the development from the egg stage which is found in the fresh feces of infested sheep to the final larval stage in which the young worms are ready to be swallowed, it was thought that the lambs might be kept for a time with the ewes in inclosure No. 1 without danger of infection, the danger point being reached when the sheep had been in this inclosure long enough for embryos to develop to the infectious stage from eggs in the feces of the infested ewes. In order to determine when the danger point would be approached cultures were made from the feces of an infested sheep two days before the ewes and lambs were placed in inclosure No. 1. These cultures were kept outdoors near the inclosures and examined from day to day, other similar cultures being made at intervals of two to three days. As soon as it was found that the stomach-worm eggs present in any of the cultures had hatched and that the embryos had developed to the final larval stage, the lambs and ewes were moved to inclosure No. 2 in order to avoid infection with embryos in the in 10765-10- -18 fectious stage, which the test cultures indicated would appear in this stage in inclosure No. 1 a couple of days later. When the sheep and lambs were moved to inclosure No. 2, test cultures were again used as a guide to determine when the animals should be moved to inclosure No. 3, and so on throughout the entire series, the lambs alone finally being placed in inclosure No. 10. When the lambs were killed and examined after they had been in this inclosure about six months-namely, from July to December—some of them were found to be infested with stomach worms. The infection of the lambs was evidently due to one or both of two things, namely, they became infected from larval stomach worms on the skin and wool of the ewes, whence the parasites were taken into the mouth by the lambs while suckling, or they became infected from worms which developed to the infectious stage in the various inclosures more rapidly than the test culture indicated. In either case it is clear that during the summer lambs can not be entirely protected from infection if kept with infested ewes, even though the flock be placed on fresh pasture at intervals much shorter than would be possible under practical conditions. In the experiment the sheep averaged less than nine days in each inclosure. Relatively few embryos in the test cultures had developed to the infectious stage when the sheep were changed from one inclosure to the next, the development of the majority proceeding much more slowly; but as it was desired in this experiment to keep on the safe side the sheep were moved as soon as the first appearance of embryos in the final larval stage was noted in the cultures. Even though the periods during which the sheep remained in the various inclosures had been lengthened to correspond to the periods required for larvæ in the infectious stage to become abundant in the cultures, the changes would still have been more frequent than would be practically possible, owing to the large number of clean pastures that would be required. THE SUCKLING-PEN METHOD. Dalrymple's plan of having a special suckling pen has also failed to prevent entirely stomach-worm infection in lambs from infested. mothers. In the latest experiment with this method which has been carried out in this Bureau the ewes and lambs were placed in two small pastures which were free from infection at the beginning of the experiment, not having been occupied by sheep or other ruminants for a period of several years, the lambs occupying one pasture and the ewes the other. Until the lambs were weaned they were allowed with the ewes at frequent intervals for suckling in a small, bare pen from which all droppings were removed after each period of occupancy. After weaning, the lambs remained in the pasture which they had occupied before until some months later, when they |