During an epidemic, or in its typical form, influenza presents no difficulty of diagnosis; at other times or in other forms a positive immediate diagnosis may be well nigh impossible. The chief diseases from which influenza has to be differentiated are measles, scarlet fever, and enteric fever, also simple coryza, bronchitis, and miliary tuberculosis. Moreover, influenza may occur concomitantly with any of these diseases, -especially, in infancy, in conjunction with scarlet fever or measles,- and under such circumstances even the discovery of the specific bacillus will not, as it generally does, clinch the diagnosis.
Since conjunctivitis, laryngitis, bronchitis, tonsillitis, frontal headache, etc., are common to it and influenza, a certain diagnosis may not be possible before the fourth day, when the fresh febrile accession and appearance of the rash indicate measles. According to Franke, a streaky erythema limited to the anterior pillars of the fauces is pathognomonic of influenza. Very often it is accompanied by enlargement of the anterior papillae of the tongue. On the other hand, Koplik's spots and general erythema of the pharynx and gums would be diagnostic of measles. Siisswein states that influenza often complicates measles in young children and influences it unfavorably, but the mixed cases may be clinically indistinguishable from measles.
From miliary tuberculosis influenza can generally be distinguished by the coryza and conjunctivitis, and from simple coryza or bronchitis by the sudden onset, rapid course, nervous symptoms, rash, and enlargement of the spleen.
For the first few days a definite diagnosis may be impossible. If the attack of influenza commences with gradual pyrexia, with diarrhea, rose spots, and enlargement of the spleen, as happens occasionally, we may have to wait until the temperature begins to fall before the diagnosis becomes certain. Additional doubt will arise if epis taxis and nervous symptoms, signs common to both diseases, are also present. The most useful guides are then the coryza and conjunctivitis, more characteristic of influenza, while a relatively slow pulse and a positive serum reaction would be in favor of enteric fever. Leukocytosis is absent in both diseases when uncomplicated.-Ed.]