The chill which comes from a sudden extreme fall of temperature, or the standing in a cold draught when wet or perspiring, lays the system open to this as to other microbian invasions.
The electric tension preceding a thunderstorm, to which many of the lower animals are excessively susceptible equally prepares the system to succumb to the germs. It may here be noted that September 1872, the last days of which witnessed the start of the great epizoötic, had no less than eleven thunderstorms, while in September of the previous year there were but two in the vicinity of Toronto. It is just possible that the great and frequent electric tension, lowered the animal vitality, allowing a violent invasion by the hitherto slumbering germ, and gave to the latter that encreased potency which sent it forth on that year of almost unparalleled epizoötic record.
The high barometer and low dew point similarly affect the animal economy and encrease receptivity to disease. Rain fell at Toronto 16 days in September 1872 and but 8 days in September 1871.
Impurities in the air whether originating in volcanic eruptions, telluric emanations, close, filthy overcrowded buildings or compartments or large collections of decomposing organic matter, impair the animal vigor and lay the system open to a more violent attack. For this among other reasons epizoötics of equine influenza are nearly always more deadly in the closely packed city stables than in the pure country air.
Overwork and poor irregular feeding and watering pave the way for debility, prostration and severe invasion.
Sudden vicissitudes of temperature, which are so common in spring and autumn, associated as they are with the shedding and growth of the coat, materially encrease susceptibility and sometimes determine an encreased severity in the attack.
Youth has its influence, even if it means only that the system that has never before been exposed to the poison, retains all its native susceptibility, and has none of that acquired immunity which comes from a previous exposure to the virus and successful resistance.
Acquired immunity must of course be reckoned with. After a non-fatal attack this is usually to be relied on for several years or even for the rest of the lifetime, yet it varies with the individual animals, and, under the baleful combination of a specially potent germ and strongly conducive accessory causes, it may become worn out in a year. Yet the older horses can always be trusted to show a large measure of this immunity, so that in the absence of extraordinary epizoötics it is mainly the young that suffer, and it is only when a country has had no general invasion for a length of time, or when the germ has acquired an unusual pathogenic potency, or when these two conditions conjoin, that the invasion of the equine population becomes universal, as it virtually was in the United States and Canada in 1872–3. Under other circumstances the germ, temporarily shorn of its power, lingers in city and dealers stables, biding its time until circumstances become more favorable for a new general outbreak.
Immunity largely explains the comparative mildness of the last cases in any particular locality. The more susceptible animals are attacked first and most severely, while the partially immune ones, which for a time resist, throw off the disease with greater readiness. The explanation has been sought in a lessening potency of the germ, but though this may hold true of some cases, it manifestly does not apply when slight lingering cases only are left in one locality, and the disease is advancing over the neighboring state with all its original force and vigor.
Incubation. This appears to vary within certain limits. When during an epizoötic a sick horse is brought into a new locality and stable, other cases usually develope in from one to three days. Trasbot gives examples of one day, Salle, Cadeac and others of two, others claim four, seven and even, exceptionally, fifteen days. One reason for an apparently prolonged incubation may be found in the seclusion of the germs in the alimentary canal, so that they escape only when passed with the fæces. The pathogenic potency of individual germs, and the varying susceptibility of the animals exposed must also be taken into account.