Influence of Climate on Susceptibility of Monkeys to Intranasal Infection with Poliomyelitis Virus

Although sporadic cases of poliomyelitis occur throughout the year, the incidence of the disease rises sharply during the late summer (and early autumn months) and, as a rule, decreases with the onset of cold weather. The cause of this seasonal incidence is as yet obscure. The virus is supposedly transmitted from man to man by droplet infection without the intervention of an intermediary host or reservoir upon which changes in climate could exert an effect. That droplet infection is not in itself the factor is clear from the well-known fact that other diseases believed to be transmitted in the same manner have their own peaks of seasonal incidence, many of which vary and happen to be different from that of poliomyelitis. 1 Since the virus of poliomyelitis is less resistant to higher than to lower temperatures, it does not seem probable that the greater occurrence of the disease during the hot months is attributable to an effect of climate on the virus itself. Hence it appears that the host, more than any other factor, should be considered as the agent influenced by climate, although the nature of this possible influence remains unknown. Presumably the climatic effect may be either a local one, i. e., the nasal secretions may perhaps be so changed as to prevent the virus from gaining access to the first cells (olfactory) which it apparently must infect in order to spread further to the central nervous system, or the change may be elsewhere along the course of the virus, which would impede its progress and thus prevent the disease from becoming clinically manifest.

The question which this investigation proposed to examine was whether or not cold weather or changes from warm to cold could in themselves so influence the nasal membranes or secretions as to render infection with the virus of poliomyelitis more difficult. Six Macacus rhesus monkeys were kept outdoors 8 to 9 hours each day during late February and March, when it snowed on 2 days and the temperature varied from 28° to 50°F. During the remainder of the day they were kept indoors in a room in which the temperature is automatically regulated at 72° to 76°F. They were purposely allowed to spend part of the time indoors in order to approximate more closely living conditions of man during cold weather. The monkeys were outside every day for a week before they were given poliomyelitis virus intranasally. One cc. of a 10% suspension of pooled poliomyelitic monkey cords was instilled in each nostril on two occasions 48 hours apart. Subsequent to this treatment they remained outside during the day until the end of the experiment. Seven monkeys which were kept indoors in the same room during the same period as the others were similarly inoculated and the course of the disease in the two sets of animals observed (Table I).