Crookshank believes that encephalitis lethargica is a distinct disease, but that it occurs frequently as an antecedent of or coincident with influenza, together with increase in the existence of poliomyelitis and certain other diseases.
Nevertheless the association in point of time and place between influenza and lethargic encephalitis cannot be lightly overlooked. As we have seen, Flexner’s criticism that encephalitis antedated the influenza is not valid, because the latter was present in 1916. We must await fuller evidence on this subject.
SECTION VII.
Comparison of Influenza with Other Epidemic Diseases.
A certain amount of knowledge concerning the epidemiology of influenza may be gained by a comparison of the epidemic features of that disease with those of other epidemic diseases, particularly measles and the exanthemata, meningitis, the plague, and certain diseases of the lower animals. Influenza is described as a disease with distinctive epidemiologic characteristics, the chief of which are found only in epidemic spreads. Thus one of the fundamental characteristics of these epidemics is supposed to be the primary type of wave, the wave characterized by rapid rise, quasi-symmetrical evolution, and a concentration closely grouped around the maximum. “This is found in no other disease. In no other type of epidemic does the curve rise as rapidly to a peak or fall as swiftly, nor is the epidemic completed in as short a time.”
The secondary type of curve, that which is more frequently found in recurring influenza epidemics, characterized by a more gradual ascent, a still more gradual decline and a longer duration, is found frequently in the curves for other diseases; it is much less characteristic. We shall attempt by a comparison of epidemic influenza with these other diseases to explain the cause for this characteristic primary curve, so as to gain a further insight into the epidemic features of the disease.
There are certain characteristics held by epidemic influenza in common with other diseases. There are certain resemblances between it and epidemic meningitis; in certain ways it resembles measles and there are some points of similarity to the pneumonic form of plague. The fact that it cannot be compared with one of these diseases to the exclusion of the others renders deductions more complicated.
Epizootics.—Soper has written at some length on a comparison of influenza in man with the so-called influenza among horses. The close resemblance in many features is striking.
Epizootics of a disease apparently resembling influenza have been described among horses from before the Christian Era. A disease believed to have been influenza was recorded as having occurred B.C. among horses in Sicily. According to Parkes the epidemic which attacked the army of Charlemagne in 876 attacked at the same time dogs and birds. Finkler describes an epizootic among horses in 1404 A.D. There were other epizootics in 1301, 1711 and 1870 to 1873. In 1901 a severe outbreak occurred in America, and one has also been described by Mathers as occurring in Chicago in the winter of 1915–16. These epidemics of a disease clinically resembling influenza have frequently occurred among horses at the same time with true epidemics of influenza in man. Nevertheless there has been no clear cut evidence to prove that the disease is the same.
Leichtenstern discusses the incidence of respiratory disease among animals, particularly household pets during epidemics of influenza. He comes to the conclusion that human influenza is a disease limited entirely to the human race and having no connection with animal disease. This is particularly true with regard to diseases reported among cats, dogs, canaries and other captive birds. He also believes that the epizootics among horses which have been reported from time to time as occurring with influenza epidemics have nothing to do with the disease in man. The symptoms are frequently very similar, but epizootics have frequently occurred at times when there was no epidemic of disease among humans.