soon as an unusual number of cases are reported along the route of any dairyman it is apparent on the card and attention may be given at once to the dairy and the farms supplying the dairy with milk.

In order to properly account to each dairyman the cases having used milk supplied by him it is necessary to take into consideration not only the source of the milk used regularly by the patient during the thirty days prior to the onset of illness but also of that used occasionally. Frequently it will be found that a family receiving its regular milk supply from a certain dairy will on occasions, when the regular supply is not sufficient for the needs of the day, obtain milk from some other dairy, directly or through the corner grocery store. The milk obtained on one of these occasions may be infected and so responsible for the case. The source of milk used at places other than the regular one for taking meals also should be ascertained if possible.

Cases resulting from infection in the milk are by no means confined to persons who use milk as a beverage. The cream or milk used on cereals, fruits, or even in coffee may convey the infection. In the summer of 1906 in Washington there were six cases in one family of eight. persons all of which were attributed quite definitely to infected milk. None of the members of this family drank milk, but they all used cream on fruits and cereals. Of course the chances of contracting the infection from milk is greater among persons who use milk freely.

a

George Newman sums up the characteristics of milk-borne epidemics as follows:

(a) There is a special incidence of disease upon the track of the implicated milk supply. It is localized to such areas.

(b) Better-class houses and persons generally suffer most.

(c) Milk drinkers are chiefly affected and they suffer most who are large consumers of raw milk.

(d) Women and children suffer most, and frequently adults suffer proportionately more than children.

(e) Incubation periods are shortened.

(f) There is a sudden onset and rapid decline.

(g) Multiple cases in one house occur simultaneously.

(h) Clinically the attacks of the disease are often mild. Contact infectivity is reduced and the mortality rate is lower than usual.

In the different outbreaks due to infected milk it is interesting to note how greatly the proportion of persons affected among the users of the milk varies. In some outbreaks the proportion is as great as 25 per cent; for instance, in the epidemic at Palo Alto, Cal., in 1903, which was traced to infected milk by Fish, Mosher, and Snow. Of the 900 persons who used milk from the infected supply, 232 had typhoid fever. In other outbreaks the proportion is as low as 1 or 2

George Newman, Bacteriology and the Public Health, 1904.

per cent. Several conditions no doubt influence the proportion of persons affected, the most important of which must be the amount of infection in the milk. In the Palo Alto epidemic it was determined that the milk became infected through the water used for washing the cans and also at times for diluting the milk. This water was obtained from a creek which received the drainage from several houses in which there were patients with typhoid fever. The water of the creek for two or three weeks must have been quite heavily charged with typhoid bacilli, so that probably the majority of the milk cans washed in this water received some of the organisms.

It is easy to understand how a milk supply thus almost if not quite continuously infected for several weeks may cause the infection of a large proportion of the persons who use that milk; but when the infection is introduced into the milk at irregular intervals for a like period, as would be expected when the infection is conveyed on the hands or clothing of persons or by flies, etc., a very small proportion of the consumers of the milk may become infected.

The susceptibility of the people supplied with infected milk, of course, would affect the proportion. In a community where typhoid fever had been prevalent for years, and in which there would be a number of persons rendered relatively immune by previous infection, we would expect less susceptibility than in a community where the disease had never prevailed.

That it takes susceptibility plus exposure to infection for the disease to occur was well shown by an instance in the course of a milk outbreak in the District of Columbia in the fall of 1906. In a children's home having about 100 inmates 7 children came down with typhoid fever within a period of two or three days. The way in which the milk was delivered to and served at the institution made it practically impossible for the 7 children affected to have drunk milk from any one can, or one day's delivery, from which at least 75 per cent of the children did not drink. Thus of 75 children almost certainly drinking infected milk only 7 had the disease. It is conceivable that in such an instance the typhoid bacilli in the can or cans of infected milk were not uniformly distributed through the bulk of the milk, but were in clumps so that only one or two of the children drinking from a 5-gallon can of milk actually received any of the bacilli; but such a view is contrary to what is known by actual test about the distribution of bacteria such as the B. typhosus in a liquid such as milk, so it is much more reasonable to conclude that all of the 75 children received some of the bacilli and the escape of the majority was due entirely to lack of susceptibility at the time the organisms were ingested.

The establishment of milk as the causative factor in an outbreak of typhoid fever is based on the following points:

(a) A sudden and marked increase in the number of cases along the route of some dairyman, without a corresponding increase in the number of cases among persons living in the same sections of the city but supplied with milk from other sources. In a town supplied largely or entirely by one dairyman a sudden increase in the number of cases would not implicate the milk unless other facts pointed to it and other factors could be excluded, but in large cities, where the people of practically every square are supplied with milk by two or more dairymen, an increase in the number of cases distinctly on the route of a given dairyman is quite easily determined. This fact alone is evidence that the milk is responsible, and if an investigation reveals that at a time corresponding to the period on which the group of cases along the dairyman's route must have been infected there was at the dairy or one of the dairy farms a patient with typhoid fever whose discharges could readily have reached the milk, the chain of evidence is sufficiently strong to justify the assumption that the outbreak was due to the milk supplied by this dairyman, especially if the cases can not positively be proven to have been due to some other factor.

A study of chart No. 3, Bulletin No. 35, Hygienic Laboratory, United States Public Health and Marine-Hospital Service, shows strikingly the increase in the number of cases on the routes of the three dairymen in Washington whose milk supply during the summer and fall of 1906 was for a time infected. One dairyman (No. 4) had among his customers in June, 1 case; in July, 13; in August, 6; in September, 3, and from October 2 to 21, 32 cases. This sudden and marked increase in cases during October along the route of this dairyman was not accompanied by any corresponding increase in the number of cases among the people living in the sections of the city over which his route extended but obtaining milk from other dairymen.

(b) The demonstration of the B. typhosus in the suspected milk. When this is done, the chain of evidence is, of course, complete. But frequently it can not be done, because in the period of usually three or four weeks-covering the incubation period, diagnosis, and report of the cases elapsing from the time of infection of the cases and the recognition of the outbreak, the B. typhosus has disappeared from the milk.

If cases of typhoid fever are not discovered to account for the infection of an implicated milk supply, it is well to examine bacteriologically the stools and urine of all persons who handle the milk at the farms and the dairy. In this way the source of the infection may be found in the discharges of some person who has the disease

in an ambulant and unrecognized form, or of some one who has had the disease months or even years before (chronic bacillus carriers).

"

Besides the large groups of cases of typhoid fever caused by infected milk, there must be in large cities frequently single cases or small groups of cases which are due to infection in the milk and yet can not be traced to that source. In a community where factors other than milk were operating to cause a rather extensive prevalence of typhoid fever, five or six cases occurring within a few days among the customers of a dairyman supplying several hundred families with milk would direct some suspicion toward that milk supply,..but if this small group of cases should not be followed by an unusually large number of cases on the route of this dairyman and no typhoid cases were found on the dairy farm or at the dairy, these 5 or 6 cases would be placed by the investigator among those due to causes undetermined or to causes other than milk. In many such instances, however, these groups of cases are doubtless due to infection introduced in one of the many possible ways-hands, clothes, flies, water for washing cans, etc.-into a part of the dairyman's output of milk for perhaps only one day.

In cities having milk supplied by a number of dairymen, if several of these small groups of cases among customers of different dairymen occur at about the same time, a list of the farms supplying each of the suspected dairies should be studied, and if it is found that two or more of these dairies receive milk from any one farm, an investigation should be made of that farm, and in this way the source of the infection for the several groups of cases may be determined.

MEASURES TO PREVENT THE DISSEMINATION OF THE INFECTION OF TYPHOID FEVER IN MILK.

(a) The prevention of the introduction of infection into milk.This at once suggests itself as the proper measure; but the difficulty of carrying it out practically becomes evident when we consider the number of farms from which the milk supply of the average American city is obtained, the liability of cases of typhoid fever occurring on these farms, and the numerous ways in which the infection may be conveyed from the patient to the milk. New York City's milk supply, according to Darlington, is derived from 35,000 farms, and shipped from 700 creameries, located in 6 States. It is easy to appreciate how difficult and expensive it would be to keep up a sufficiently thorough supervision of the multiple sources of that city's milk supply. It is practicable to accomplish much toward the prevention of the infection getting into the milk after the milk is delivered to the city. The following requirements are suggested:

1. Location of the dairies in good surroundings.

2. The prevention of the handling of the milk by persons who are in contact with typhoid fever patients or who themselves are liable to be discharging the B. typhosus in their excreta. It does not seem unreasonable to require the owner of a store in which milk is sold and in which there is a patient with typhoid fever to either remove the patient to a hospital or some other house or to close up the business until the danger from that patient is passed.

3. Exclusion of flies and other insects so far as possible, by screening, etc.

4. Sterilization of bottles and cans returned from houses before being again filled with milk, or the use of paper bottles which would not need to be returned.

5. The sealing of the bottles or cans of milk so that they may not be infected in the course of delivery.

(b) The destruction of infection in milk.-This at the present time seems to be the cheapest and the most practicable method to prevent the spread of typhoid infection in the milk supply of cities. In exceptional instances when a dairy receives its supply of milk from only one or two farms over which a thorough supervision may be exercised, efforts to prevent the infection reaching the milk may be attempted. But for the general supply of cities, Pasteurization of the milk after it has been placed in the bottles or cans for distribution is the best measure. Supplement this with an intelligent supervision over the depots and stores where milk is sold and milk as a causative factor of typhoid fever in cities would be practically removed.