When in addition it is taken into consideration that the ordinary roasting or broiling of a piece of meat is often not sufficient to produce a germicidal temperature throughout, the argument that a heat-resistant toxin is present in such cases is not conclusive. It must be remembered also that in some outbreaks those persons consuming raw or partly cooked meat have been affected while at the same time others eating well-cooked meat from the same animal have remained exempt; this would seem to indicate the destruction of living bacilli by heat, since the toxic substances formed by these organisms are heat-resistant. The view that a definite infection occurs, is favored, too, by the fact that the blood-serum of affected persons so frequently has an agglutinative action upon the paratyphoid bacillus. This would not be the case if the symptoms were due to toxic substances alone. Altogether the rôle of toxins formed by B. enteritidis and its allies in food outside the body cannot be said to be established. The available evidence points to infection as the main, if not the sole, way in which the bacilli of this group are harmful.
Sources of infection.—The main sources of enteritidis-suipestifer infection are: (1) diseased domestic animals, the infected flesh or milk of which is used for food; (2) infection of food by human carriers during the process of preparation or serving. To these may be added a third possibility: (3) contamination of food with bacteria of this group which are inhabitants of the normal animal intestine. Considering these in order:
1. Diseased animals: The majority of the meat poisoning outbreaks are caused by meat derived from pigs or cattle. [Table III] gives the figures for a number of British[77] and German[78] epidemics.
| B. enteritidis | B. suipestifer | Belonging to This Group but Undifferentiated | |||||
| British | German | Total | British | German | Total | British | |
| Pig | 1 | 1 | 2 | 3 | 5 | 8 | 4 |
| Ox or cow | 3 | 9 | 12 | 2 | 3 | 5 | 5 |
| Calf | 0 | 7 | 7 | 2 | 2 | 4 | 0 |
| Horse | 0 | 1 | 1 | 0 | 1 | 1 | ... |
| Chickens | 1 | 0 | 1 | 0 | 1 | 1 | ... |
Occasional outbreaks have also been attributed to infection through eating rabbit, sheep, goose, fish, shrimp, and oysters. Especially noteworthy is the relative rarity of infection from the meat of the sheep.
More definite information is needed respecting the pathological conditions caused by these bacteria in animals and the relation of such conditions to subsequent human infection. A rather remarkable problem is presented by the relation of B. suipestifer to hog cholera. This bacillus, although not now considered the causal agent of hog cholera, is very commonly associated with the disease as an accessory or secondary invader, and is frequently found in the internal organs of swine after death. It might be supposed that in regions where hog cholera is prevalent human infections would be more common than in other districts, but this seems not to be the case. No connection has ever been demonstrated between outbreaks of hog cholera—in which B. suipestifer is known to be abundantly distributed—and so-called B. suipestifer infections in man.
Suppurative processes in cattle, and especially in calves, have given rise to poisoning from the use of the meat or milk of the infected animals. It has been often demonstrated that bacteria of the enteritidis-suipestifer group are associated with inflammation of the udder in cows and with a variety of septicemic conditions in cattle and other domestic animals as well as with manifestations of intestinal disturbances ("calf diarrhea," etc.).[80] The frequency with which poisoning has occurred through the use of the meat of "emergency-slaughtered" animals has been already mentioned. K. F. Meyer[81] has reported an instance of accidental infection in a laboratory worker caused by handling a bottle of sterilized milk that had been artificially contaminated with a pure culture of B. enteritidis for experimental purposes. The strain responsible for the infection had been isolated from the heart blood of a calf that had succumbed to infectious diarrhea.
2. Human contamination: In a certain number of paratyphoid food infections there is some evidence that the food was originally derived from a healthy animal and became infected from human sources during the process of preparation. In addition to the instances already mentioned (Reinhold et al., p. 67) the Wareham (England, 1910) epidemic[82] was considered by the investigators to be due to infection of meat pies by a cook who was later proved to be a carrier of paratyphoid bacilli. The evidence in this case, however, is not altogether conclusive. Söderbaum[83] mentions a milk-borne paratyphoid epidemic occurring in Kristiania which was ascribed to infection of the milk by a woman milker. Sacquépée and Bellot[84] report an interesting paratyphoid outbreak involving nineteen out of two hundred and fifty men in a military corps. The patients fell ill on different dates between June 14 and June 21.
It was found that an assistant cook who had been in the kitchen for several months had been attacked a little before the epidemic explosion by some slight malady which was not definitely diagnosed. He had been admitted to the hospital and was discharged convalescent. The cook, on being recalled and quarantined, stated that some days before June 10 he was indisposed with headache and anorexia. He had nevertheless continued his service in the kitchen.... B. paratyphosus B (B. suipestifer) was repeatedly found in his stools in August, September, and October.... In all probability, therefore, the outbreak was due to food contaminated by a paratyphoid-carrier who had passed through an abortive attack of the fever.[85]