Directly the soil may transmit the actual contagia (micro-organisms) of disease either by means of the subsoil water or its air. In the former case the disease-causing material gains access to the drinking water of wells, springs, or rivers; in the latter case it may be borne to the surface of the soil by currents of the ground-air or by insects, and then inhaled as dust, or gain access to food.
Certain disease-producing micro-organisms have been proved to be capable of living for some time in the soil. The chief of these found in the soil are the bacilli of tetanus (lockjaw), of anthrax, of malignant oedema, and of enteric (typhoid) fever. There are reasons for thinking also that the micro-organisms causing diphtheria, rheumatic fever, and epidemic diarrhœa, and possibly some other diseases, may occasionally live in the soil. In some diseases as enteric fever, cholera, dysentery and anthrax, the contamination of the soil can be shown to be derived from a patient suffering from the same diseases. In others, and particularly in tetanus, the same chain of evidence is obtainable.
(1) The conditions favourable to the production of malarial diseases have been generally considered to be the presence of a certain proportion of dead organic matter, the exposure of the soil to alternations of heat and moisture, with a limited access of air, and a temperature of at least 65°F. Though most common in marshy districts, and in recent alluvial soils, malaria may develop in connection with any geological formation. That it may be removed by drainage of the subsoil, is well known. The true nature of the connection between soil and malaria is stated on page [220].
(2) According to observations made by Pettenkofer in Munich, attacks of enteric (typhoid) fever are connected with fluctuations of the subsoil-water. He states his conclusions as follows:—
“Between the fluctuations of subsoil water and the amount and severity of enteric fever there is an unmistakable connection in this wise, that the total number of cases of and deaths from enteric fever falls with a rise of the subsoil water, and rises with fall of it; that the level reached by the disease is not in proportion, however, to the then level of the subsoil water, but only to the variation in it on each occasion; or in other words, that it is not the high or low level of the subsoil water that is decisive, but only the range of fluctuation.”
His observations have not been confirmed in this country; and the coincidence between excess of enteric fever and lowness of ground-water has been explained by the fact that under these circumstances the water in wells is low, and the area of drainage and the consequent risk of contamination are proportionately increased. There can be no doubt that the most common origin of enteric fever is from the infection of water or milk by infective matter from a recent case of the disease. This does not exclude the fact that enteric fever in this country is more prevalent in hot dry autumns, in which the ground-water is low. Probably under such conditions the contagium of the disease multiples more rapidly in the soil, in privies and other polluted places, and consequently the risks of infection of water and food as well of infection by dust carried from the contaminated spot are greatly increased.
(3) In regard to cholera, Pettenkofer holds similar views. He believes that the contagium of cholera can only be developed when there is a damp porous subsoil to receive the infected stools from a cholera patient; the damp porous subsoil forming a second host in which the poison of cholera must pass through one stage of its existence, before it is again capable of producing the disease. Such an essential relationship of the soil is not borne out by observations in India; and in England cholera has been repeatedly shown to be due to contamination of food (e.g. oysters) or water by the stools of preceding cholera patients, without the intervention of any agency of the soil.
(4) It has been repeatedly stated that a damp soil favours the prevalence of diphtheria. I have shown elsewhere, however, that this is not true, and that the greatest epidemics of diphtheria have occurred in exceptionally dry years, especially when several years of exceptionally small rainfall have succeeded each other; and have suggested that this may be associated with an intermediate stage in the life-history of the diphtheria-bacillus in the soil. A low ground-water and a comparatively high temperature of the soil go along with deficient rainfall, and would probably favour the multiplication of this bacillus in the soil.
(5) In rheumatic fever I have similarly shown that the supposed connection between damp soil and this disease is erroneous, the disease being most prevalent, both in this and other countries, in years of exceptional drought.
(6) Epidemic or Summer Diarrhœa has been supposed to have a special relationship with soil-temperature, Ballard having found that the summer rise in the mortality from this disease does not commence until the mean temperature recorded by the four-foot earth thermometer has attained somewhere about 56°F. The soil-temperature may be accepted as a convenient index of the conditions causing this disease. The disease I have elsewhere shown occurs most severely with a high temperature of the air and a deficient rainfall, and its fundamental cause is an unclean soil, the particulate poison from which infects the air, and is swallowed most commonly with food, especially milk.