"Review of Snow, On the Mode of Communication of Cholera"
(14 September 1849): 466-70
In this essay, which is a modest contribution to medical literature on cholera, the author avows himself a believer in the now generally received doctrine that this disease is capable of being propagated by human intercourse. He takes, however, an entirely novel view of the mode in which it may become diffused among the inhabitants of a town or city. One of the difficulties which has been opposed to the admission of a contagious miasm in cholera is, that persons living in a remote quarter from the infected, have been suddenly attacked by the disease and died. There has been no proof of intercourse or communication, either directly or indirectly. In order to account for these cases, non-contagionists allege that cholera may spring up any where: heat, moisture, and organic matter, are all that they require for its production. But this view is not satisfactory: the same local agencies were at work in the same localitiies [sic] in 1840 as in 1849, and yet in the former year there was no cholera, while in the latter the inhabitants of the districts are speedily decimated by this disease. Something, therefore, must be superadded; for heat, moisture, and organic matter, even aided by ochlesis ["a morbid condition induced by the crowding together of sick persons under one roof"; Dunglison, Dictionary of Medical Science (1860), 643], whatever the Board of Works may say, will not of themselves produce cholera; a fact further attested by the circumstance that many quarters of a town where these conditions are met with escape the disease. What is this superadded something which renders putrescent effluvia so destructive to life in one year, while they only prove a harmless nuisance in another? What is this mysterious principle which, apparently associated with such effluvia, slaughters its thousands with the same symptoms, and with the same degree of malignancy, on the banks of the Ganges, the Danube, the Bosphorus, the Neva, the Thames, the Seine, and the Mississipi [sic]? It is not yet in our power to give an answer to these questions. Speculation is rife on the sub-[466/467]ject, but nothing satisfactory has been hitherto elicited.
Contagionists endeavour to account for the occurrence of isolated cases, where there is no proof of intercourse, by supposing that the atmosphere becomes a medium for the conveyance of the miasm, and that, although respired by all, it produces its effects only on those who are predisposed. Dr. Snow strikes out a new path; he believes that it is not the air, but the water, which is the medium of conveying the choleraic poison, and that the great prevalence of the disease in certain localities, and its occasional appearance in adjoining districts irrespective of intercourse, may be thereby accounted for. He believes that the discharges of the affected contain the choleraic poison, and that these discharges are excited by the application of some local irritant to the mucous membrane. The mode in which he believes cholera may be communicated will be understood from the following extract:--
"Having rejected effluvia and the poisoning of the blood in the first instance, and being led to the conclusion that the disease is communicated by something that acts directly on the alimentary canal, the excretions of the sick at once suggest themselves as containing some material which, being accidentally swallowed, might attach itself to the mucous membrane of the small intestines, and there multiply itself by the appropriation of surrounding matter, in virtue of molecular changes going on within it, or capable of going on, as soon as it is placed in congenial circumstances. Such a mode of communication of disease is not without precedent. The ova of the intestinal worms are undoubtedly introduced in this way. The affections they induce are amongst the most chronic, whilst cholera is one of the most acute; but duration does not of itself destroy all analogy amongst organic processes. The writer, however, does not wish to be misunderstood as making this comparison so closely as to imply that cholera depends on veritable animals, or even animalcules, but rather to appeal to that general tendency to the continuity of molecular changes, by which combustion, putrefaction, fermentation, and the various processes in organized beings, are kept up."
"Whilst it is matter almost of certainty that intestinal worms are in this way communicated, it is never possible to trace the communication from one person to another: hence, if this be the mode of the propagation of cholera, there must often be great difficulty in detecting it. That a portion of the ejections or dejections must often be swallowed by healthy persons is, however, a matter of necessity. The latter even are voided with such suddenness and force that the clothes and bedding scarcely fail to become soiled, and being almost devoid of colour and odour, the presence of the evacuations is not always recognized; hence they become attached unobserved to the hands of the person nursing the patient, and are unconsciously swallowed, unless care be taken to wash the hands before partaking of food: or if the person waiting on the sick have to prepare food for the rest of the family, as often happens, the material of communication here suggested has a wider field in which to operate; and where the patient, or those waiting on him, are occupied in the preparation or vending of provisions, the disease may be conveyed to a distance, and into quarters having apparently no communication with the sick." (pp. 8-10.)
There is some confirmation of this view in the fact that puerperal fever is thus conveyed by accoucheurs to females in the puerperal state: and we entertain no doubt that the prevalence and fatality of cholera, as well as of many other diseases, may be ascribed to a want of cleanliness on the part of those who are in contact with the sick.
In spite of infinitesimal dilution, the author believes that the cholera may be spread over a town by the use of water contaminated by sewers, which carry off the dejections of patients. We do not understand him to assert that the use of water accidentally impregnated with the contents of drains and sewers will produce cholera, unless cholera dejections exist in it; indeed, there are numerous facts which are adverse to such a view. For many years past, and while the cholera was as remote as the banks of the Caspian sea, water of this foul description was habitually consumed by the poor in many quarters of this metropolis; but cholera did not make its appearance. Dr. Snow's theory, therefore, is that the choleraic poison, however diluted, must be contained in the water in order to produce the disease in an individual using it. He tells us that so far as his inquiries have extended, he has found that in most towns in which the malady has prevailed to an unusual extent, this means of its communication has existed.
In support of this opinion the author quotes a series of cases which recently [467/468] occurred in Surrey Buildings, Southwark:--
"The two first cases on the 20th and 21st may be considered to represent about the average amount of cases for the neighbourhood, there having been just that number in the adjoining court, about the same time. But in a few days, when the dejections of these patients must have become mixed with the water the people drank, a number of additional cases commenced nearly together. The patients were all women and children, the men living in the court not having been attacked; but there has been no opportunity hitherto of examining into the cause of exemption, as the surviving inhabitants had nearly all left the place when the writer's attention was called to this circumstance." (pp. 14-15.)
The facts mentioned in this extract, however, clearly admit of another explanation. There is nothing to prove that the additional cases, commencing about the same time, absolutely depended on the drinking of the water. Other causes, irrespective of the water, may have been in operation, especially as the persons were living in close proximity to the affected. It might turn out on inquiry that the men living in the court, who were not attacked, used the same water. We do not say that the disease may not thus be communicated to the healthy, but we consider that the facts here mentioned only raise a probability, and furnish no proof whatever of the correctness of the author's views. The experimentum crucis would be, that the water conveyed to a distant locality, where cholera had been hitherto unknown, produced the disease in all who used it, while those who did not use it escaped.
The fatal cases which occurred at Albion Terrace, in the Wandsworth Road, have excited great public notoriety. Dr. Snow thinks that their occurrence is to be explained by a reference to his views of the mode of communication of cholera.
"In Albion Terrace, Wandsworth Road, there has been an extraordinary mortality from cholera, which was the more striking, as there were no other cases at the time in the immediate neighbourhood; the houses opposite to, behind, and in the same line, at each end of those in which the disease prevailed, having been free from it. The row of houses in which the cholera prevailed to an extent probably altogether unprecedented in this country, constituted the genteel suburban dwellings of a number of professional and tradespeople, and are most of them detached a few feet from each other. They are supplied with water on the same plan. In this instance the water got contaminated by the contents of the house-drains and cesspools; the cholera extended to nearly all the houses in which the water was thus tainted, and to no others."
"The first case of cholera occurred at No. 13, on July 28th (two days after the bursting of the drain), in a lady who had had premonitory symptoms for three or four days. It was fatal in fourteen hours. There was an accumulation of rubbish in the cellar of this house, which was said to be offensive by the person who removed it; but the proprietor of the house denied this. A lady at No. 8 was attacked with choleraic diarrhœa on July 30th: she recovered. On August 1st, a lady, age 81, at No. 6, who had had some diarrhœa eight or ten days before, which had yielded to her own treatment, was attacked with cholera; she died on the 4th with congested brain. Diarrhea commenced on August 1st, in a lady aged 60, at No. 3; collapse took place on the 5th, and death on the 6th. On August 3rd, there were three or four cases in different parts of the row of houses, and two of them terminated fatally on the same day. The attacks were numerous during the following three or four days, and after that time they diminished in number. More than half the inhabitants of the part of the terrace in which the cholera prevailed were attacked with it, and upwards to half the cases were fatal. The deaths occurred as follows; but as some of the patients lingered a few days, and died in the consecutive fever, the deaths are less closely grouped than the seizures. There was one death on July 28th, two on August 3rd, four on the 4th, two on the 6th, two on the 7th, four on the 8th, three on the 9th, one on the 11th, and one on the 13th. These make twenty fatal cases; and there were four or five deaths besides amongst those who were attacked after flying from the place."
"There are no data for showing how the disease was probably communicated to the first patient, at No. 13, on July 28th; but it was two or three days afterwards, when the evacuations from this patient must have entered the drains, having a communication with the water supplied to all the houses, that other persons were attacked, and in two days more the disease prevailed to an alarming extent." (pp. 15, 17,18, 19.)
With respect to the communication of the disease by the water, Dr. Snow observes--
"There were two or three persons attacked with cholera amongst those who came [468/469] to nurse the patients after the water was condemned, and who, consequently, did not drink it; but these person were liable, in waiting on the patient, to get a small portion of the evacuations into the stomach in the way first pointed out; and there might be food in the houses previously prepared with the tainted water. It is not here implied that all the cases in Albion Terrace were communicated by the water, but that far the greater portion of them were; that, in short, it was the circumstance of the cholera evacuations getting into the water which caused the disease to spread so much beyond its ordinary extent." (p. 21.)
We cannot perceive how the last position is proved, or even rendered probable. The first case which occurred on July 28th. at No. 13,--a house in which there was an accumulation of decomposing organic matter in the cellar, could have had no connection with the choleraic dejections; for the water was then rendered foul only by impregnation with the contents of the drain; and it is clear that whatever will explain the origin of this one case, will explain the occurrence of others in the immediate vicinity. Dr. Snow admits that all the cases were certainly not communicated by the water, and, with this admission, we are at a loss to know why it is assumed that the greater portion of them were. There is, in our view, an entire failure of proof that the occurrence of any one case could be clearly and unambiguously assigned the use of the water. We agree with Dr. Milroy in thinking that the foul effluvia from the state of the drains, afford a more satisfactory explanation of the diffusion of the disease. Any local cause operating in one house, might fairly be considered to extend its influence to those immediately adjoining it; and whatever produced one case of cholera, might suffice to produce twenty in succession. . . .
Dr. Snow admits that the choleraic poison may find its way into the lungs under the following circumstances:--
"It should be observed, that the mode of contracting the malady here indicated does not altogether preclude the possibility of its being transmitted a short distance through the air; for the organic part of the fæces, when dry, might be wafted as a fine dust, in the same way as the spores of cryptogamic plants, or the germs of animalcules, and entering the mouth, might be swallowed. In this manner, open sewers, as their contents are continually becoming dry on the sides, might be means of conveying cholera, independently of their mixing with water used for drinking." (pp. 26-7.)
It is due to Dr. Snow to state that he modestly puts forward his opinions, not as matters of certainty, but as containing some probability in their favour. The assumption that cholera may be diffused by the use of water containing the dejections of patients, or by want of cleanliness among attendants on the sick, cannot be denied; but the facts are yet wanting to place it among those unquestionable truths which the profession is earnestly endeavouring to accumulate in reference to this disease.
Whether his theory be confirmed or disproved by subsequent experience, all will agree in the propriety of the suggestions with which the author concludes his pamphlet.
"If the writer's opinions be correct, cholera might be checked and kept at bay by simple measures that would not interfere with social or commercial intercourse; and the enemy would be shorn of his chief terrors. It would only be necessary for all persons attending or waiting on the patient to wash their hands carefully and frequently, never omitting to do so before touching food, and for everybody to avoid drinking, or using for culinary purposes, water into which drains and sewers empty themselves; or, if that cannot be accomplished, to have the water filtered and well boiled before it is used. The sanitary measure most required in the metropolis is a supply of water for the south and east districts of it from some source quite removed from the sewers." (p. 30)
[Then an endorsement of Snow's interpretation of the index case of the 1848-49 cholera epidemic in London, extensive quotations on this matter, and a conclusion:]
 Notwithstanding our opinion that Dr. Snow has failed in proving that cholera is communicated in the mode in which he supposes it to be, he deserves the thanks of the profession for endeavouring to solve the mystery. It is only by a close analysis of facts, and the publication of new views, that we can hope to arrive at the truth.