The experiments in which transmission of the disease was attempted by flies hardly parallel the observations within the herd. It is not unusual to observe large numbers of flies feeding on the exudate. Slight disturbances may interrupt feeding and cause the flies to disperse and within a short period alight about the eyes of other cows.
The experiments, however, bring out the fact that the bacterium will not remain viable for even a few minutes in the digestive tract of the fly. Its life on the external surfaces of the fly is extremely short and in our observations has not exceeded 3 hours. The latter fact strengthens the opinion that in the main the infection is not dust-borne since the bacterium soon dies when not in contact with the eye.
Two other points are of considerable significance. The ability of the organism to maintain itself on the eye for considerable periods after the acute symptoms have subsided may explain the reappearance of cases during the warmer months. The organism can exist in the eye throughout the winter and with the warm weather flies may transmit it to other susceptible individuals and thus a nucleus of an epidemic may be established.
The presence of the organism in the nasal passages in the incubation stage and early in the disease in two of our experimental animals affords an explanation for the appearance of the disease in sporadic cases in the colder months. It is assumed that nasal exudate as a fine spray may be forcibly expelled and directly reach the eyeball of a normal individual. It has been shown that small quantities of culture sprayed on the cornea are capable of giving rise to the characteristic disease. The irregularity of the elimination of the organism through the nostril may be explained by the effect of inflammation on the tear duct. In experimental cases a small quantity of bouillon containing the culture was dropped or sprayed on the cornea. Doubtless the bacilli are deposited on the mucosa of the tear duct. Here they may multiply and set up an inflammation and thus gain access to the nasal passage. To what degree the virus is spread by the forcible expulsion of nasal secretion containing lacrimal fluid cannot be determined. The elimination of the bacilli from the nasal passage in our experimental inoculations leads us to believe that in the main the phenomenon is associated with early infections. The examination of the nasal passages of a large number of well established cases with negative findings tends to corroborate this view.
Of interest to those concerned with the treatment of animal diseases is the readiness with which the inflammation subsides when treated with 1:40 zinc sulfate.