mean very little to find that 20 or 30 per cent. of the patients had been in contact with previous cases. This percentage of traceable contacts would mean a great deal more, however, in a larger community where there had been perhaps only one case to each 10,000 inhabitants.

FACTORS OTHER THAN CONTACT. In the effort to trace out contact between cases one must not lose sight of the numerous other possible factors in the spread of the disease, paying most attention to those factors which seem most probably important, but not forgetting to gather information concerning even the seemingly least important. Factors which must be considered are food and water supply, insects, paralysis of domestic animals, relation to water courses, dust, sewage disposal, general hygienic conditions, previous health, etc.

FOOD AND WATER SUPPLIES.

It is impossible in this space to discuss the relation of all these factors to the spread of poliomyelitis. Moreover, their importance is as yet largely undetermined. Food and water supplies have quite generally been eliminated as possible sources of general infection, although Wickman cites one group of cases apparently infected by their common milk supply.

HYGIENIC CONDITIONS.

Previous health appears to have no appreciable influence in determining infection. The influence of insanitary conditions of life is particularly difficult to determine, as it is usually impossible to make more than a rough estimate of the proportion of people in any community who live under what may be called insanitary conditions.

It would seem that, in general, the disease is more prevalent among those

classes of people that live in rather crowded, insanitary surroundings; but the incidence of cases among the lower social strata is not sufficiently disproportionate to justify attaching any great importance to general hygienic conditions as a factor in infection.

INSECT TRANSMISSION.

The probability of insect transmission of the disease is strongly suggested by several epidemiological facts already established. One of the most striking of these facts is the seasonal incidence of epidemics. In this latitude epidemics occur almost without exception in the warm season, from May to November, the season when insects are most prevalent and most active. It is of interest to note in this connection that the epidemics reported from the southern hemisphere have occurred between January and April, a period corresponding seasonally to our late summer and fall months. Another fact which suggests insect transmission. is the geographic distribution of epidemics. Generally speaking, epidemic poliomyelitis is a summer disease of cold countries. In Europe, Norway and Sweden, Holland, Germany and Austria. have suffered most; in this country the states which have suffered most are those included in the northeast quadrant.

A further indication of the probability of insect transmission is the distribution of the disease in relation to density of population. Apparently density of population bears no constant relation to the prevalence of epidemic poliomyelitis. Wickham noted this in Sweden in 1905, and statistics for the United States, so far as they are available, confirm this observation. Indeed, it has been noted both in Sweden and in the United States that epidemics of poliomyelitis are most severe in small towns and rural communities,

the larger cities as a rule suffering less in proportion to population.

Since the first considerable epidemic in this country occurred in and around New York City in the summer of 1907, and epidemics all over the country have been more common since that time, it is naturally suggested that the disease has spread from New York. Yet if that is the case the spread has been remarkably slow considering the constant communication between New York and other parts of the country, and still more remarkable irregular in its progress. In 1907 the region of greatest prevalence was in and around New York City, extending to Massachusetts. In 1908 there were epidemics in Massachusetts, Minnesota, Wisconsin, and at least two small outbreaks in Iowa. In 1909 the epidemics reached their height in Massachusetts, Minnesota and Nebraska. In 1910 the disease has been less prevalent in Massachusetts and Nebraska, but has been epidemic in Iowa, Pennsylvania, District of Columbia, Virginia, Connecticut, and other widely separated states. If the disease has been disseminated from New York along routes of travel, it is hard to understand why it has progressed so irregularly, skipping wide areas of thickly settled country, and why it has spread so slowly, becoming epidemic in the District of Columbia, for example, three years subsequent to the epidemic in New York.

These facts are strongly suggestive of the existence of some as yet unrecognized biologic factor, possibly an insect, the presence of which in a community is necessary or at least favorable to the spread of epidemic poliomyelitis.

Considering, on the other hand, the evidence against insect transmission, the most striking is that presented by laboratory experiments already cited, viz, the low degree of infectiousness of the blood;

the apparent dissemination of the virus through the body by the lymph stream rather than the blood; the demonstrated infectiousness of the nasal and buccal secretions; the possibility of infecting animals through the normal mucosa of the respiratory and digestive tracts. Epidemiological studies have failed to give evidence of the prevalence of unusual insects or of common insects in unusual numbers in epidemic foci; they have failed to give any evidence of an extrinsic period of incubation; they have failed to show that infection is confined to places rather than persons; and have, indeed, shown the possibility of healthy persons acting as carriers of infection. Any insect to merit consideration as an obligatory factor in the transmission of poliomyelitis must be of almost world-wide distribution and perennial prevalence, for poliomyelitis has occurred in all latitudes from Australia to Canada, and, while epidemics have been confined almost exclusively to the warm months, scattered cases have been reported in the United States in every month of the year. On the whole, the evidence at present available is against the theory of any insect being a necessary or important factor in the spread of the disease; but on this, as on other points, undoubtedly more evidence is neededanother indication of the necessity for field studies.

PARALYSIS OF DOMESTIC ANIMALS.

As regards the relation of paralytic diseases of animals to epidemic poliomyelitis, it has been noted in connection with a number of epidemics that domestic animals, especially chickens, dogs, horses, hogs, cattle and sheep, were found in the same community to be suffering from paralytic diseases clinically similar to the disease prevailing among human beings. The earliest observations of this kind of

which I am aware were recorded by Caverly (12) in his report of an epidemic occurring at Rutland, Vt., in 1894, when he noted paralysis of chickens and dogs. One of these chickens, examined by Dana (14), of New York, showed lesions of the lumbar cord resembling the lesions of acute anterior poliomyelitis.

So far as I have been able to ascertain from an incomplete review of the literature, this is the most suggestive evidence yet presented of a close relation between fowl paralysis and human poliomyelitis. The pathology of the paralytic disease of animals has evidently not been sufficiently studied, but the bulk of the pathological evidence now available is against the assumption of a close etiologic relation between such affections and epidemic poliomyelitis. Numerous attempts have been made to inoculate laboratory animals other than monkeys with the virus of human poliomyelitis, the results being uniformly negative except for the inoculations of rabbits, previously referred to.

The reports of paralysis among domestic animals in localities where poliomyelitis is prevalent have certainly been quite striking. Paralysis among domestic animals is, however, quite common and may be due to divers causes, and it may be that the numerous reports of it from such localities are due more to increased interest in the matter than to any unusual prevalence of paralysis of animals in such localities. The most careful investigation of this point by the Massachusetts State Board of Health (1) showed that the distribution of paralysis among animals did not correspond to the distribution of human poliomyelitis.

DUST.

The occurrence of epidemic poliomyelitis in the hot, dry, dusty season has given rise to the surmise that dust may be in

some way a factor in the spread of the disease. This surmise has been strengthened by the grouping of cases along dusty thoroughfares, observed in several localities, and by the cessation of several epidemics shortly after the dust had been abated by rainfall or sprinkling of streets.

It

Other observations in support of the causative relation of dust to epidemic poliomyelitis are, the greater incidence of the disease among children at the age when they are likely to crawl and play in the dust, and the greater incidence among males, who are out of doors in the dust, than among females, who are more intimately exposed to infection. through contact with sick persons. has been suggested, in view of the occurrence in horses of a disease resembling poliomyelitis, that the infective agent in dust is horse manure. The excessive prevalence of dust has not, however, been found constantly to coincide with the prevalence of poliomyelitis. It is true. that the disease is more prevalent in the late summer and fall months; it is also true that dust is generally more prevalent at this season, but the coincidence is not sufficient to establish the relation of cause. and effect.

SUMMARY.

Epidemic poliomyelitis must, in the light of present knowledge, be regarded as most probably transmissible by direct. contact. Its spread, to be sure, does not exactly follow the routes and the laws which we should expect in the case of a disease transmitted by direct contagion; but it is to be remembered that infection of the human body with any micro-organism is a fairly complex biological phenomenon into which there may enter many factors other than the mere bringing together of the body and the germ.

We must consider, first, that the infect

ing organism is not an unchanging fixed quantity; not a definite thing like a stable chemical compound, but a far more complex and probably very variable factor-a living organism, reacting to all kinds of external conditions. Realizing the complexity of conditions in the environment of the organism, together with our inability even to analyze these conditions, much less to appreciate their effect upon an ultramicroscopic body, we should be prepared to find the organism deviating at times from the course which, with our very limited knowledge, we would lay down for it.

Taking up, on the other hand, the factor of susceptibility to the infection of poliomyelitis, we may assume this factor also to be extremely variable. There are some facts which indicate that only a certain proportion-usually a small proportion of persons exposed to the disease are readily susceptible to infection. In general it has been found that only one, or, at most, a few, of a family have the disease. Assuming that the disease is contagious, the other members certainly have been exposed to infection, and their failure to develop the disease would seem to be due to a lack of susceptibility. Even assuming that the disease is not contagious and that infection is contracted from some other source in the environment, it certainly is probable that in general the members of one family, especially the small children, are likely to be exposed to the same environmental conditions. Whether we regard the disease as contagious or not, the rarity of multiple cases in a family seems best explained by individual variations in susceptibility The occurrence of abortive cases is also an indication in the same direction.

The conditions constituting susceptibility are, of course, unknown except in a broad, general sense. Statistics indicate

that children are more susceptible than adults; that males, especially in later life, are more susceptible than females; the white races more than the negro. The increased incidence of the disease in the summer months, among children, suggests the possible operation of causes similar to those which mark diarrhoeal diseases especially prevalent among children in hot weather.

NECESSITY OF FURTHER STUDIES.

It has been the object of this paper not to explain the spread of epidemic poliomyelitis, but rather to point out the difficulties in the way of explaining it; to attempt an interpretation of known facts chiefly to show the deficiencies in the facts. If the facts already ascertained seem contradictory, it is because they are incomplete. What is needed to harmonize the apparent contradictions is more facts. Laboratory workers have contributed at generous share of knowledge concerning this disease; clinicians all over the counofficer should embrace the opportunity to try are studying it; and every health

contribute his share of the facts which shall explain the spread of epidemic poliomyelitis. There is little chance of making a brilliant discovery in this work. If such a discovery remains to be made, it will be made by one or at most a very few of the many workers engaged. There is a certainty, however, that every accurate observation, every common-sense fact added to the subject will play its part in solving a problem that has already become very serious and shows no indication of becoming less so.

PROPHYLAXIS.

While a discussion of the prophylaxis of epidemic poliomyelitis is not strictly germane to this paper, a few words on

After a careful consideration of the facts of epidemic poliomyelitis as known. at present, it seems to me that health authorities are morally bound to put into effect to the best of their ability certain pretty definitely indicated measures for the prevention of the spread of epidemic poliomyelitis-measures similar to those adopted for the control of other diseases commonly accepted as directly contagious. Without attempting to go into detail, these measures may be given as:

1. Isolation of the patient, with isolation of the contacts so far as practicablecertainly to the extent of excluding members of the patient's family from school for at least two weeks. Exclusion of insects and animals from the room.

2. Disinfection of the secretions of the nose and mouth and of the stools and urine. Disinfection of all articles which might have been contaminated by the patient.

3. Fumigation of premises after re

covery.

In framing our expectations of results from these measures we must consider several circumstances:

1. The disease is already disseminated over a wide area. Experience with other widespread contagious diseases, such as scarlet fever, for the control of which we have to depend solely on isolation and disinfection, has demonstrated that we can hardly expect to eradicate such a disease by present methods, but that much may be done in the way of limiting its spread.

2. Epidemic poliomyelitis presents unusual difficulties in the recognition of even typical cases in their early stage and of abortive cases in all stages.

3. It will be difficult to estimate the effect of preventive measures, since the

[From the American Journal of Urology.]

This report is based upon the study of cases seen at the Massachusetts General Hospital and in private practice. One case, No. 22, was a patient of Dr. A. T. Cabot. The bacteriology and preparation of vaccines has been largely by Dr. H. F. Hartwell and E. C. Streter, to whom my best thanks are due.

In looking over the material it seemed

*Read before the American Association of GenitoUrinary Surgeons, at the Eighth Congress of American Physicians and Surgeons, 1910.