Page:West African Studies.djvu/247

 his homestead in the dark; but Dr. Manson's suggestion is far more likely to be the correct one. Against it I can only urge that in some districts where I am informed by my medical friends that Filaria perstans is very prevalent, such as Calabar, the Niger, and the Ogowe, sleeping sickness is not prevalent. Dr. Manson says "the fact that the disease can be acquired only in a comparatively limited area, suggests that the cause is similarly limited; and the fact that the disease may develop years after the endemic area has been quitted, suggests that the cause is of such a nature that it may be carried away from the endemic area and remain latent, as regards its disease-producing qualities for a considerable period; even for years." He then goes on to say, "Filaria perstans, so far as is known, is limited in its geographical distribution to Western Equatorial Africa—that is to say, it can be acquired there only—and it may continue in active life for many years after its human host has left the country in which alone it can be acquired. We also know that similar entozoa in their wanderings in the tissues by accident of location, or by disease, or injury of their organs, not infrequently give rise to grave lesions in their hosts. I therefore suggest that possibly Filiaria perstans may in some way be responsible for the sleeping sickness. I know that this parasite is extremely common in certain sleeping sickness districts, and moreover, I have found it in the blood of a considerable number of cases of this disease—in six out of ten—including that described by Mackenzie. There are many difficulties in the way of establishing this hypothesis, but there is a sufficient inherent probability about it to make it well worth following up."

The most important statement that I have been able to