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XXXVI] universally—as it still is by some—believed to be an hereditary disease. That this belief, in the same sense as that tubercle may be said to be hereditary, was well founded is quite possible; that is to say, that certain physio-pathological qualities predisposing to leprosy may be inherited. But since the discovery of the bacillus it is impossible any longer, if we properly consider it, to believe that the bacillus itself, and therefore the disease it causes, can be hereditary in the scientific sense of the word. Physiological peculiarities and susceptibilities may, but parasites cannot, be inherited. It is true the ovum may be infected by a germ, as in syphilis; but infection is not heredity. That the ovum can be infected at some stage of its existence by the lepra bacillus is proved if it be true that children have been born with the lesions of leprosy on them. But the fact that leprosy is common in the descendants and blood collaterals of lepers is no proof of ovum infection in every, or perhaps in any case; for family liability is quite as explicable by an hypothesis of contagion, or outside infection, as by an hypothesis of inherited infection. Not only may the individuals of a family inherit a family predisposition of susceptibility to the bacillus, but, as a family, the members of it are generally at one time or another closely associated, exposed to the same hygienic influences, liable by contact to communicate each other's parasites, or to acquire the parasites latent in their common surroundings. Because the members of a family simultaneously, or one after another, contract scabies, or ringworm, or typhoid, no one supposes on that account that any of these diseases is hereditary.

Without absolutely denying the possibility of ovum infection, the probability is that such an event is very rare. The age at which leprosy usually appears is against such a supposition. The latency of a germ for twenty, thirty, forty, or even seventy years is an extremely improbable thing and without parallel in pathology. Atavism, or rather, the appearance of atavism, frequently met with in leprosy, is also against such a supposition; for, although we can understand infection of an ovum by a leper parent, we cannot understand the transmission of a germ from a grandparent to a grandchild through a parent who is not, never was, and may never become a leper. Such a thing would imply proliferation of the bacillus in the parent without pathological evidence of its presence.

Even admitting that leprosy is sometimes transmitted by ovum infection, this method of transmission cannot be the only one, or even a common one, for many lepers have no leper ancestors; and, as is well known, the healthy European, coming from a country in which leprosy has not been seen for generations, may acquire leprosy on visiting a country in which the disease is endemic.

If leprosy be communicated generally, or even sometimes, by parent to child by heredity, how explain the striking fact, brought out by Hansen, that of the numerous offspring of 160