Milk-Borne Epidemics

MILK-BORNE EPIDEMICS, various kinds of epidemics that arise due to contamination of milk with pathogenic microorganisms, for which milk is an excellent nutrient medium. Among the pathogenic microorganisms capable of causing milk-borne epidemics are: B. typhi abd., B. paratyphi, microorganisms of the Brucella group, B. tuberculosis, Streptococcus, B. dysenteriae, B. diphtheriae, B. anthracis, and the causative agents of scarlet fever and foot-and-mouth disease. Among the microorganisms listed above, typhoid and paratyphoid microorganisms, the Brucella group, streptococcus, and the scarlet fever causative agent most frequently cause epidemics; the others cause epidemics only in rare cases. The first indications of milk as a source of epidemics were provided by English and American authors (Taylor, Ballard, Murchison and others) in the second half of the 19th century. Since then, a significant number of milk-borne epidemics have been recorded. For example, in Germany from 1878 to 1899, out of 650 typhoid epidemics, 110 were milk-borne (Schuder); in the USA from 1918-24, 112 such outbreaks were observed; during the same period, 21 outbreaks of milk-borne scarlet fever were described (Miller). In France, milk-borne epidemics have been described by Ott, Reumaux, Bonnaix, Chantemesse and others. However, in France, milk-borne epidemics occur comparatively rarely due to low consumption of raw milk. Sources of bacterial contamination of milk. The main source of milk contamination is its contamination directly in the dairy farm—from sick persons caring for the sick and carriers among the dairy farm personnel (most often typhoid and paratyphoid) and from sick animals (Maltese fever, tuberculosis, foot-and-mouth disease); contamination of milk with pathogenic microorganisms can occur during milking or during procedures accompanying milking, and in these cases, milk contamination occurs through contaminated hands. Contamination of milk utensils and water, which is sometimes used to adulterate milk or with which utensils are washed, plays a large role in milk contamination. Milk contamination can also occur through flies. Detection of pathogenic microorganisms in milk. Despite the fact that milk is a good nutrient medium for almost all microorganisms, pathogenic microorganisms are found in it very rarely. The reason for this lies partly in the presence of a large number of saprophytes in milk that hinder the isolation of pathogenic microorganisms, and partly in the fact that due to life in an acidic environment and in symbiosis with various microorganisms, the properties of the pathogenic microorganism change so much that its recognition becomes very difficult. Thus, only a positive result of milk examination for pathogenic microorganisms has absolute value, while a negative result has less significance. Features of milk-borne epidemics. Milk-borne epidemics, like water-borne ones, begin with a sudden outbreak. However, milk-borne epidemics usually do not reach the same extent as water-borne ones, due to their limitation to consumers of raw milk. Milk-borne epidemics reach particularly large proportions if contaminated milk reaches collection points where milk from different sources is mixed and then distributed over a wide area. On the other hand, the number of epidemic diseases can be very limited if most consumers use boiled milk. The total number of diseases depends not only on the number of milk consumers but also on associated contact infections. The duration of epidemics varies; it depends on the duration of existence of the source of infection. If contaminated milk is consumed once, such an epidemic ceases quickly. If, however, milk contamination occurs day after day, such an epidemic can drag on for years and reach extensive proportions. With a small number of consumers, such a prolonged epidemic can simulate an endemic disease. Usually, milk-borne epidemics end as soon as appropriate measures are taken to eliminate the source of milk contamination: removal of carriers, boiling or pasteurization of milk, or prohibition of milk sales from the contaminated dairy farm (farms, state farms, collective farms, etc.). A characteristic feature of milk-borne epidemics is that the sick are mainly children and women. Another feature of milk-borne epidemics is the topography of diseases. Their territorial distribution is closely related to milk distribution; they are irregularly scattered depending on the place of residence of consumers of suspected contaminated milk from the suspected dairy farm and often correspond to the road along which milk is delivered daily. Milk-borne epidemics of specific infections. Typhoid fever and paratyphoid fever. Of the large number of described milk-borne typhoid epidemics, bacteriological confirmation of milk-borne origin was achieved only in very rare cases. Positive findings were obtained only in a few cases. In all cases where bacteriological confirmation was not possible, the origin of the epidemic was established epidemiologically. Epidemiological evidence of the role of milk includes the following points: 1) the presence of a typhoid fever patient or carrier among the dairy farm personnel and their families (attention should be paid to mild, unexplained cases and childhood diseases); 2) disease incidence among a group of persons who used suspicious raw milk and absence of disease among a group of persons who used the same milk but boiled; 3) cessation of the epidemic after the implementation of appropriate measures. It should be borne in mind that the animals themselves (cows) do not pose a danger in terms of spreading infections, since even after ingesting typhoid microorganisms with food, they do not excrete them in milk. When milk is contaminated with typhoid or paratyphoid microorganisms, the outbreak of diseases begins no earlier than 10 days (Vaughan, Sadov) after consumption of contaminated milk. In raw milk, typhoid microorganisms persist for 2-3 days. Milk does not possess natural bactericidal properties against typhoid microorganisms. Among the milk-borne epidemics of recent years, the most extensive was the 1927 epidemic in Montreal (Canada), which caused 4,849 diseases and 489 deaths, and the 1925 epidemic in Anklam (Germany), which caused 303 diseases and 36 deaths. The typhoid fever epidemic in Montreal—a large well-equipped city—lasted from March 1927 to September of the same year. Since the possibility of water or any other common food origin of the epidemic was completely excluded, the commission that studied the epidemic turned its attention to milk. It turned out that at least 75% of all the sick received milk from the same large dairy farm, and that among children's institutions, those supplied with milk from this same farm were most affected. Since in this case the main mass of diseases outside children's institutions was also of young age (35.5% of patients were under 10 years old, 32.2% from 10 to 20 years old), there were grounds to suspect infection through milk. Upon examination of the suspected dairy farm, it was found that not all milk arriving at the farm was pasteurized, and since milk was partially delivered from an area where typhoid fever was constantly present, the possibility of contamination of milk with typhoid bacilli could not be excluded. The measures taken in Montreal consisted of organizing supervision over the suspected farm. The typhoid fever epidemic in Anklam began in mid-June; in early July it reached its greatest development and persisted, declining throughout July and early August (see diagram). The rapid increase in the number of cases suggested a common source of infection. Since the water supply did not arouse any suspicion, attention was turned to the dairy business. In the city there were three dairies, of which the largest processed up to 30,000 liters of milk per day. It turned out that almost all cases were concentrated in the area supplied by this dairy. Upon examination of this dairy, it was found that 1) not all milk arriving from 107 farms was pasteurized and 2) pasteurization was performed imperfectly. And since among the personnel on two farms three bacillus carriers were found, the assumption of milk-borne origin of this epidemic was considered sufficiently justified. To stop the epidemic, in addition to the usual measures, measures were taken to eliminate shortcomings in milk pasteurization technique, and bacillus carriers were removed from work on the farm. The Anklam epidemic was distinguished by particular severity—with a mortality rate from 11% to 34% (in hospitals). In the USSR, a milk-borne paratyphoid outbreak was described by Bashenin and his colleagues. The outbreak began in September 1927 in a colony for the blind at Klyazma station. In total, 46 children out of 130 colony residents fell ill within 5 days. Based on the clinical course and bacteriological examination of patients, paratyphoid B was established. Food infection, especially milk-borne, was suspected. When examining meat and milk, the paratyphoid B bacillus was found in the latter.

Upon examination for bacillary carriage among the kitchen staff and the family of the milkwoman who supplied the colony with milk, two cases of paratyphoid were found in the milkwoman's family, from which in one case the paratyphoid B microbe (Schottmüller type) was isolated from the feces, identical to the strains isolated from the children. - Dairy products, such as butter and cheese, are also dangerous in terms of spreading typhoid fever. Typhoid microbes can persist in butter for up to 24 days and in cheese for up to 14 days (Miller). Zorge (Sorge) reports a large typhoid epidemic caused by Limburger cheese contaminated by the hands of an ill saleswoman. Brucellosis. This includes diseases caused by microbes that are closely related but different in origin of the Brucella abortus group: Bruce's Micrococcus melitensis (Brucella melitensis) and Bang's Bact. abortus (Brucella abortus). Both of these microbes cause in humans Malta fever, or undulant fever (see Malta fever). In cases of animal disease and in cases of asymptomatic carriage, agglutinins (Zammit) and the microbes themselves (on blood culture) can be found in animal milk. In the spread of Malta fever, the carriage of Brucella group microbes by people, usually excreted in urine, is undoubtedly important. This carriage is especially dangerous among personnel of dairy farms (milkers) and among persons caring for livestock, due to the possibility of them infecting animals. Butter and cheese (goat, sheep) have great epidemiological significance. - Tuberculosis and foot-and-mouth disease. The tubercle bacillus enters milk almost exclusively from diseased cows, and in marketed retail milk the tubercle bacillus is found in 10-20%. Since it has been experimentally established (Ravenel; 1901) that the bovine tubercle bacillus is pathogenic for humans, and since infection occurs mainly through milk, milk from diseased cows is dangerous and should not be consumed even in pasteurized form, since pasteurization does not always kill the tubercle bacilli. - The causative agent of foot-and-mouth disease also enters milk from diseased cows, but infections of humans from cow's milk are rare, probably due to the low susceptibility of humans to it. Septic sore throat. The causative agent of epidemic septic sore throat is a special variety of hemolytic streptococcus. The disease spreads through milk, with the cow in this case being a passive or active (human streptococci can multiply in the cow's udder) carrier of human microbes (Savage). The first description of a septic sore throat epidemic was made by Winslow (Winslow; 1912) regarding an outbreak in Boston that resulted in 1,400 cases. Since then, 1,000 cases have been reported in Baltimore, 10,000 cases in Chicago, etc. - Scarlet fever. Milk plays a significant role in the spread of scarlet fever. Attention was first drawn to this by Taylor and Hart in the 1870s. In subsequent years, many milk-borne scarlet fever epidemics have been described, especially in England and the USA. Of recent epidemics, the typical one was the epidemic in Birslime in 1923. The epidemic resulted in 280 cases over 9 weeks. Epidemiological investigation suspected milk from the distributor. Upon examination of this distributor, it was found that on one of the farms supplying milk to the distributor, there were 2 cases of scarlet fever: one recent, and the other 2 weeks old. The sale of contaminated milk was stopped, and the population was advised to use boiled milk. After these measures were taken, the epidemic ended quickly. - Dysentery. There are no reports in the literature of finding dysentery microbes in milk, but milk can undoubtedly be one of the ways of spreading dysentery diseases. The sources of milk contamination and the nature of epidemic development are the same as in typhoid fever. Dysentery microbes can persist in milk from 3 to 27 days. Cases where the role of milk was established epidemiologically have been described by Zinsser and Finney. In butter and cheese, dysentery microbes can persist for up to 9 days. Diphtheria. The role of milk in the spread of diphtheria is established with certainty. Individual positive findings of diphtheria bacilli have been described in the English literature. An example of a milk-borne diphtheria epidemic can be the Frankfurt epidemic of 1903, where whipped cream was considered the probable source of infection. Contamination of milk can occur not only from carriers of diphtheria bacilli, but also from diseased cows, in which diphtheria bacilli can be found on the udder and nipples (Birger). - Cholera. Cholera outbreaks of milk origin were described by Koch in 1884 and by Simpson in 1887. Epidemiological investigation of these outbreaks proves the infectious role of milk. There are no reports in the literature of positive findings of cholera vibrios in milk. The rarity of milk-borne cholera epidemics is probably explained by the sensitivity of the cholera vibrio to acid that develops in milk. In sterile milk and in butter, the cholera vibrio persists for a long time, in cheese for 1 day. - Anthrax. The causative agent of anthrax can theoretically be transmitted through milk from a diseased animal, but since it appears in milk only in the late stage of the disease, when the diagnosis is clear and the animal has been removed, such transmission is practically an exception (Miller). Anti-epidemic measures. Contamination of milk with pathogenic microbes occurs mainly in the dairy farm itself; therefore, to prevent contamination, dairy farms must meet maximum sanitary requirements in terms of premises and cleanliness of utensils, as well as in terms of the serving personnel (the milker's hands) and the condition of the animals, and both personnel and animals must undergo proper periodic medical examination for disease or carriage. - In the fight against milk-borne epidemics, early notification of the first cases is very important; therefore, information about suspicious cases is also very important. Milk that has been received for distribution should subsequently be protected from contamination by flies and dust and from the multiplication of bacteria that have entered it; the degree of the latter depends on two factors: the storage period and the storage temperature of the milk. To prevent bacterial multiplication, milk should be cooled immediately after milking and stored and transported at a low temperature (10°) (see also Milk).