It is evident, therefore, that bacterial growth in substances used as food is not necessarily injurious and may in some cases increase the palatability of food without destroying its wholesomeness. Little or nothing is known about the correlation of visible signs of decomposition with the presence of poisonous products, and it is at present impossible to say at what point in the process of decomposition a food becomes unfit to use owing to the accumulation of poisonous substances within it. There seems to be no connection between the natural repugnance to the use of a food and its unwholesomeness. Under ordinary conditions the nauseous character of very stale eggs is proverbial, and yet few nitrogenous foods have so clear a health record as eggs or have been so infrequently connected with food poisoning outbreaks.
It might seem tempting to conclude on the basis of the available evidence that spoiled or decomposed foods possess poisonous qualities only when certain specific bacteria, like the B. botulinus already discussed, have accidentally invaded them and formed definite and specific poisons. But we have no right to assume that the everyday decomposition products of the banal bacteria are in all cases without injurious effects. Even though no sharply defined acute form of poisoning may be laid at their door, it does not follow that an irritating or perhaps slightly toxic action of the ordinary decomposition products is altogether absent. Our present knowledge of the nature and degree of danger to be apprehended from the use of spoiled food is imperfect and unsatisfactory. That fact, however, does not release us from the obligation to continue measures of protection based even to a limited extent on experience.
CHAPTER IX
POISONING OF OBSCURE OR UNKNOWN NATURE
While many and diverse causes of food poisoning have been discussed in the foregoing pages, there remain certain affections definitely connected with food that are still of obscure or doubtful causation.
MILKSICKNESS OR TREMBLES
This disease, common to man and some of the higher animals, is characterized by a definite symptom-complex, the salient features being excessive vomiting and obstinate constipation accompanied usually by a subnormal temperature. Many cases result fatally. At the present time it is known to occur only rarely in some of the southern and central western states in this country, but during the period of pioneer settlement it was quite common in districts that are now seldom affected. A great many references to milksickness are found in the writings of the early travelers and physicians in the Middle West, one observer predicting that "some of the fairest portions of the West in consequence of the prevalence of this loathsome disease must ever remain an uninhabitable waste unless the cause and remedy can be discovered." In certain regions it is estimated that "nearly one-fourth of the pioneers and early settlers died of this disease." The mother of Abraham Lincoln fell a victim to this malady in 1818 in southern Indiana.
The disease appears to be usually contracted in the first instance by grazing cattle or sheep that have access to particular tracts of land; "milksickness" pastures are, as a rule, well known locally for their dangerous qualities. Milksickness is communicated to man through the medium of raw milk, or butter and possibly of meat. Although some of the earlier observers make the statement that the disease is self-propagating and can be passed on without limit from one animal to another, later experiments cast doubt on this view.[114]
Many different theories have been advanced to account for the origin of the disease. The belief that mineral poisons such as arsenic or copper might be taken up by grazing animals and eliminated in the milk finds no justification either in analytical or in clinical data. Many plants, known or suspected to be poisonous, have been accused of furnishing the substance that imparts the poisonous quality to the milk of animals suffering from trembles, but there is no agreement as to the responsible species. Feeding experiments with suspected plants have in no case given unambiguous results. While some facts have been supposed to indicate that living micro-organisms are the cause of milksickness, other facts are opposed to this view, and the most recent experiments in this direction did not lead to conclusive results.[115] The true cause of milksickness is at present quite unknown.