There are two cases which show that those who live in close relation to lepers may develop the disease; in one, a leper returned to Ireland and his brother, who had never been in a leprosy country, but who had occupied the same bed with the leper and worn his clothes, developed the disease in about five years. A similar case is reported from Germany.
As showing that even with intimate contact, infection is rare, it is stated that of 225 healthy Hawaiians, living in the same houses with lepers, only 4½% contracted leprosy. Even when married to lepers only 9 out of 181 healthy people contracted leprosy from their leprous mates.
In Japan, 7% of children of lepers contract the disease, 3.8% of those married to lepers and 2.7% of people living in the same house with lepers.
Just as with tuberculosis, in which all evidence points to the predominance of infection in early life and its infrequency in adult life, so does it seem to be true of leprosy. Among 10,000 lepers in the Culion leper colony, Denny notes that 35% were brothers and sisters, 27% were cousins, 11% were children of lepers, 7% parents of lepers, and only 1% husband and wife. This would indicate that the relationships involving intimate contact in childhood are etiologically most important.
One of the strongest proofs that leprosy is at least feebly contagious is that based on the disappearance of the disease following isolation of the lepers. The best instance is that of Europe, in the thirteenth and fourteenth centuries, where, with 20,000 leper asylums for isolation, the disease disappeared by the fifteenth century. In Norway, there were 2833 cases in 1856, while in 1907, there were only 438 left.
At the end of 1913 there were only 285 cases, 181 of these being interned and 104 in their own homes. The reduction is attributed to isolation.
This might have occurred without isolation because Hansen in investigating the descendants of 160 known Norwegian lepers, who immigrated to the North-western States of America, was unable to find trace of a single leper among their descendants.
This and other facts militate against the views that leprosy may be inherited and the idea is generally held that if a child be taken away from its leprous surroundings after birth there is little or no likelihood of its developing leprosy.
Again, it is a well-recognized fact that leprosy is more than twice as common among men than among women. It is probable that the greater opportunity for contact with lepers by man is the explanation of the greater frequency.
Views as to Mode of Transmission.—It may be stated that nothing definite is known. There has been an idea that itch mites might transmit the disease but no proof has been advanced. Lebouf found leprosy bacilli in the stomachs of flies, which had been feeding on leprotic ulcerations, and did not find acid-fast rods in flies which had fed on persons with nerve leprosy or upon those not showing open lesions. He thinks that flies may deposit faeces containing bacilli about the nasal orifices or upon wounds of well persons, bringing about thereby their infection.