The 19th-century veterinarian who nearly cracked one of medicine’s oldest mysteries

As far as presenting the case for germ theory, James Law’s 45-page report “Influenza in Horses,” which appeared in the Report of the Commissioner of Agriculture for the Year 1872 was a tour de force of the medical geography of the Great Epizootic. It contained detailed information on the “atmospheric conditions” that miasmists turned to over and over again, as well as a synopsis of the results of a study Law had commissioned from a microscopist colleague. It was from that combination of medical geography, meteorological data, and microscopy from which Law drew many of his conclusions.

[Editor’s Note: Miasma theory is the now-debunked belief that vapors found in “bad air” caused diseases. During the second half of the 19th century, it competed with germ theory as the best explanation for illnesses and epidemics.]

Law begins his report with a lengthy history of epidemics in humans and epizootics in animals from the time of the ancient Greeks onward, followed by a medical geography of the Great Epizootic. Next, he spends page after page describing the symptoms of horse flu. In total, these three sections make up a third — 15 pages — of the report. But it’s in the next section of the report, subtitled “Causes of the Influenza,” which itself runs 24 pages, where Law is in full form on the question of whether miasma or germ theory explains the Great Epizootic.

The “Causes of the Influenza” section opens with “the old doctrine of an epidemic constitution of the atmosphere has of late years been gradually waning,” which may have been true in Law’s circle of colleagues, but was not reflective of the wide-scale acceptance of miasma theory by respected veterinarians and physicians in so many locations that were hit by the Great Epizootic. Law recognizes this reality to some extent, acknowledging that “more than any other epidemic malady, perhaps, has influenza retained its claim on an atmospheric causation.” What’s more, he makes clear, those claims assuredly were being applied to equine influenza, and in particular, that associated with the Great Epizootic. But “the epizootic of 1872 affords but the slenderest appearance of support to any of these [miasmic] hypotheses,” Law begins his argument against such claims, continuing “neither soil nor elevation has materially affected it. The prevalence and mortality have been almost the same in the mountains of Vermont and New Hampshire as in the flat, malarious sea-coast of New Jersey, Maryland, and Virginia.”

Law next turns to temperature, knowing that miasmists often argue that warmer temperatures create the “atmospheric disturbances” that cause disease. “The temperature has not exerted any marked influence,” he argues, as “the disease has been general wherever it has reached.” But Law digs deeper, providing pages of data and tables that demonstrate that not only was temperature unimportant as a cause, but also “sudden changes in weather or temperature,” another mainstay of miasma theory, were not associated with the origin of the disease in the Toronto area in September 1872. One table that Law presents shows temperature, wind velocity, and barometric pressure, as well as the amount of rain and snow that fell every day of October 1872: Indeed, the table shows each of those measurements gathered three times each day (6:00 a.m., 2:00 p.m., and 10:00 p.m.). As a comparison, Law provides another table with corresponding data for every October from 1844 to 1872.

“From these tables,” Law argues, “it is manifest that there was no extraordinary state nor extreme changes in the weather during the whole of the month [of October 1872].”

Law was also very clear about what he was not saying. He understood that once a horse was infected, cold weather acting on an already compromised immune system could cause symptoms to be even worse, noting that “nasal and bronchial catarrhs often prevail extensively among horses, as among men, in connection with sudden and extreme variations of temperature, and especially in spring and autumn.” But cold weather was not the cause of the infection.

Another miasmic bugaboo was fog, and Law tackles that in the same manner as he did other putative meteorological causes for the Great Epizootic: by employing data. Here, he grants that “remarkably acrid or fetid fogs have been observed to proceed or accompany some epidemics of influenza,” such as in the 1727 and 1732 outbreaks of influenza in human populations in England, France, and Germany, but that was pure happenstance, he concludes, for “the present equine affection has neither been preceded nor attended by any such phenomenon. Fogs appeared on but three days, 6th, 11th, and 18th of September, 1872, whereas they existed on six days, 1st, 4th, 5th, 13th, 16th, and 19th of September, 1871.”

Again, he distinguishes between causes and aggravating factors. “Fogs and vapors, impregnated with sulphureous gases or other bad smelling or putrefying elements, would undoubtedly undermine the general health, and favor the diffusion of such a disease as influenza,” writes Law, “but the origin and course of the present epizootic, like that of the majority on record, show clearly enough that no such condition is essential to development.” In a similar light, there was no evidence that humidity was a cause of the disease.

What then was the cause of the Great Epizootic? “Is there a specific contagion?” Law asks next. “This is manifestly a question of vital importance with reference to the influence of the above alleged causes.”

For Law, the reason this was a question of vital importance was clear: If there was “a specific contagion, which may exist in the body of the sick animal, increase there, and be the means of communicating the malady to an indefinite number of sound stock, all our theorizing on noxious gases and putrid fogs, inclemencies and extreme vicissitudes of the weather, excess of ozone, magnetic disturbance, and the like, will be of small account.” Law begins with a nifty bit of deductive reasoning. Usually outbreaks of equine flu are associated with outbreaks of human flu. That fits just as well with miasma theory — man and horse exposed to the same atmospheric conditions — as with germ theory: closely-related microbes infect man and horse. But in the Great Epizootic, no such outbreak occurred in humans.

More evidence against miasma as a cause came from the abundant data Law presented on the spread of the disease, which shows how uninfected horses fall ill en masse once sick horses arrive in a city. “If it can be introduced in this way into a new locality, well out of the former area of the disease, and spread promptly from the imported sick animal as a center, it must be possessed of a specific contagium,” Law argues. “Were the body merely charged with noxious gases, with decomposing organic matter … these agents would soon pass from the system and lose their noxious qualities by diffusion or decomposition.” As a capper, he adds, “the presence of the sick animal would be no more injurious than a chemical laboratory, a putrid carcass, or an electric machine.”

The only thing remaining was to try and figure out which contagion/microbe/germ caused the Great Epizootic.

After ruling out fungi as the contagion, Law turns his attention to the “granules” that were found in sick horses. Here, he “recognizes in the granules, which exist abundantly in the diseased organs, the morbid agent capable of transmitting the disease.” But how so? These granules were “microscopical particles of variable size and form,” he argued that “often possess … a capacity to appropriate other material for the increase and multiplication of their substance … [They] are reproduced with extraordinary rapidity in the substance of the diseased mucous membrane and at the expense of the vital elements, liquid and solid, of the body.” Law comes tantalizing close to a modern viral explanation for causation.

Still, those granules were visible under a microscope, but the virus that causes influenza was invisible in Law’s day, as it is today (except using electron microscopy), and so why claim that he came very close to our understanding of viral causation?

Following work by others, such as British microscopist Lionel Smith Beale, on the nature of granules, Law notes the granules found in horses sick with flu “either constituted the virulent principle or contained it.” If the disease-causing agent — the “virulent principle” — was contained within granules, it might indeed be so small as to be undetectable. As far as Law was concerned, if that was the case, so be it, for all that mattered was “that there is a specific virulent element, which finds in the body of a susceptible animal the material essential to its growth, its unlimited reproduction, and extensive diffusion.” And that much he was certain of.

Law’s point-by-point, empirically based data-driven analysis applied to the Great Epizootic was hard for miasmists to recover from.

Law even uses the word “virus” in his report. While presenting the preceding argument, he notes again that while the “atmosphere” doesn’t cause disease, disease-causing agents may be spread through the air: “The air may still be invoked as an important medium,” Law notes, “through which the … virus or bioplasm … may be carried to long distances, and infect new animals and localities.” Here, Law, like Sturtevant had done when he met with the Massachusetts Board of Agriculture, was using virus in the broad sense of a living contagion. In addition to laying out his arguments in the Report of the Commissioner of Agriculture for the Year 1872, Law published a somewhat condensed version of the same material in a venue that would have had a much broader readership in the medical community, The Lens: A Quarterly Journal of Microscopy and the Allied Natural Sciences.

Law’s reports provided strong evidence both for germ theory and against miasma theory. In particular, Law’s point-by-point, empirically based data-driven analysis applied to the Great Epizootic was hard for miasmists to recover from. That isn’t to say that miasma theory disappeared after the Great Epizootic. It didn’t: certainly not in the newspapers where it appeared often for decades to come. But the frequency with which the medical community turned away from miasma and toward germ theory increased steadily thereafter. It is not possible to determine exactly when the coup de grâce for miasma theory occurred, but it is safe to say that miasma theory was all but rejected in medicine after Robert Koch, Friedrich Jakob Henle, Friederich Löffler, and Edwin Klebs laid out a series of postulates for how to definitively demonstrate the causal relationship between a disease and the microbe responsible for it.

No atmospheric disturbances, putrid emanations, noxious vapors, swamp gases, and the like: It was now taken as a given that the origin and spread of disease was explained by germ theory. Quarantine, cordon sanitaire, and many other medical measures that seemed beside the point if miasma was causing animal epidemics, but were obvious measures to take if germs were being transmitted from animal to animal, became standard practice. And in part, we have the Great Epizootic to thank for that.

This article is featured on Big Think.