Clinical and Epidemiologic Identification.
Up to the present time we have discovered no one characteristic by which we may say that a case or an epidemic is positively influenza. We have had to rely on the general symptomatology, which indeed is sufficiently characteristic, although so nearly like the symptoms of certain other diseases as to make us hesitate to make an absolute diagnosis, and on the epidemic characteristics. The necessity of an absolute criterion in the clinical diagnosis is particularly felt in the presence of an isolated interepidemic case, or a small endemic outbreak. It is at this point that the opinions of epidemiologists diverge, a divergence which results in two schools of thought in the explanation of the endemic source of epidemic influenza. Are the interepidemic cases and the small localized epidemics due to the virus which causes the great pandemics; are they influenza vera, or are they entirely different diseases with similar symptomatology, caused by some other microorganism and should they be designated by some other name? Thus Leichtenstern remarks: “When we go over the records of the years 1173 to 1875, and particularly those of the last century, when the information has been more extensive and more accurate, we find that scarcely a year has passed without news of the epidemic occurrence of influenza at some point or other of the earth. Some of these local and territorial epidemics are merely endemic recurrences of the great pandemics which have left the germ deposited in the various localities. Others of these small epidemics probably have nothing to do with influenza vera, but are local outbreaks of catarrhal fever.”
Contrary to the usual belief, influenza is a disease of quite definite and distinct characteristics, both clinical and epidemiological. The symptoms are clear cut, with sudden onset, severe prostration out of all proportion to the clinical symptoms and to the fever, headache and pain in the back, general body pains, and fever of greater or less degree. There is usually a lack of leucocytosis or a true leucopenia. In uncomplicated influenza there are as a rule no localizing symptoms. There may be a slight soreness of the throat, or a slight cough, but these are at best mild. The fever lasts from three to five days and disappears, while at the same time all of the symptoms clear up with the exception of the profound prostration, which as a rule continues for some time, rendering convalescence surprisingly slow. The pain in the back may remain for a week or so. This is the description of uncomplicated influenza.
The manner of spread of epidemic influenza is constant in a primary epidemic and the epidemic as a whole has certain features which render it characteristic. The sporadic case has as a rule the same quite clear cut clinical symptomatology, but it fails to manifest the one feature most characteristic of epidemic influenza—a high degree of contagiousness. Further, although the symptoms in themselves are characteristic, there is no one pathognomonic sign by which one may say, “this is a case of influenza,” and, finally other disease conditions such as tonsillitis, frequently resemble it so much as to cause error in diagnosis.
This becomes, then, one of the problems in the study of influenza epidemiology. It is a matter of first importance to determine once and for all whether true influenza is with us always, or whether it appears only at the time of the great pandemics. Upon the answer to this question more than upon any other one thing rests our choice of methods of eradication. Any procedures of preventive medicine that may be undertaken on the assumption that the source of pandemic influenza is to be found in one or a few endemic foci, such as the one supposed to exist in Turkestan, would fail utterly should the true condition be that of a universal distribution of a relatively avirulent virus which from time to time from some unknown cause assumes a highly increased virulence.
Before becoming involved in this very complicated question, let us familiarize ourselves completely with the characteristics of the pandemic and epidemic variety of the disease.