Though long known, it was not until about 1890 that it was emphasized that the disease occurs in epidemic form. At this time Medin reported his observations on an epidemic of forty-three cases which occurred in and around Stockholm in 1887. Since then, according to Frost (1911), epidemics have been observed with increasing frequency in various parts of the world. The largest recorded epidemics have been those in Vermont, 1894, 126 cases; Norway and Sweden, 1905, about 1,500 cases; New York City, 1907, about 2,500 cases. Since 1907 many epidemics have been reported in the United States, and especially in the Northern States east of the Dakotas. In 1912 there were over 300 cases of the disease in Buffalo, N. Y., with a mortality of somewhat over 11 per cent.
In view of the sudden prominence and the alarming spread of infantile paralysis, there have been many attempts to determine the cause, and the manner in which the disease spreads and develops in epidemic form. In the course of these studies, the question of possible transmission by insects was naturally suggested.
C. W. Howard and Clark (1912) presented the results of studies in this phase of the subject. They dealt especially with the house-fly, bedbug, head, and body lice, and mosquitoes. It was found that the house-fly (Musca domestica) can carry the virus of poliomyelitis in an active state for several days upon the surface of the body and for several hours within the gastro-intestinal tract. Mosquitoes and lice were found not to take up or maintain the virus. On the other hand, the bedbug (Cimex lectularius) was found to take the virus from the infected monkeys and to maintain it in a living state within the body for a period of seven days. This was demonstrated by grinding up in salt solution, insects which had fed on poliomyelitic animals and injecting the filtrate into a healthy monkey. The experimenters doubted that the bedbug is a carrier of the virus in nature.
Earlier in the same year, Brues and Sheppard published the results of an intensive epidemiological study of the outbreak of 1911, in Massachusetts. Special attention had been paid to the possibility of insect transfer and the following conclusion was reached:
"Field work during the past summer together with a consideration of the epidemiology of the disease so far as known, points strongly toward biting flies as possible carriers of the virus. It seems probable that the common stable-fly (Stomoxys calcitrans L.) may be responsible to a certain extent for the spread of acute epidemic poliomyelitis, possibly aided by other biting flies, such as Tabanus lineola. No facts which disprove such a hypothesis have as yet been adduced, and experiments based upon it are now in progress."
As stated by Brues (1913), especial suspicion fell upon the stable-fly because:
1. The blood-sucking habits of the adult fly suit it for the transfer of virus present in the blood.
2. The seasonal abundance of the fly is very closely correlated with the incidence of the disease, rising rapidly during the summer and reaching a maximum in July and August, then slowly declining in September and October.
3. The geographical distribution of the fly is, so far as can be ascertained, wider, or at least co-extensive with that of poliomyelitis.
4. Stomoxys is distinctly more abundant under rural conditions, than in cities and thickly populated areas.