Europe, the disease exhibits the usual well-marked double incidence, the spring outbreak occurring in May and June, the autumnal one in September and October. In the far south the disease may appear as early as January, and may be met with at any period of the year. In Barbados the disease seems to prevail more or less from May to October or November. Whilst the wide range of pellagra throughout the world might lead one to believe that climate exerts no special influence, the very definite seasonal periodicity of the disease shows that climatic factors display an important, though indirect, part in its etiology.
Topographical distribution.—This has been carefully studied by Sambon in Europe, in America, and in the West Indies. His observations tend to show that, like malaria, sleeping sickness, and yellow fever, pellagra is a disease of locality, and is contracted solely within certain rural districts where exist the conditions essential to its propagation.
Sambon points out that the presence of pellagrins in a locality is not necessarily evidence of endemicity. In Italy he visited several Alpine villages where numerous cases had been reported, and in every instance he found that the local pellagra consisted entirely of imported cases. The patients were adult mountaineers, who every spring went to labour in the subjacent affected regions; young children and adults who had never left their mountainous district were not affected. All pellagra cases, in whatever part they occur, may be traced to exposure within the endemic centres of the disease; while individuals who there contract the infection, but leave the infective area, are not a source of danger to those with whom they come in touch, however intimate the association. According to Sambon, pellagra is readily contracted at the appropriate season within its endemic stations, and the period of incubation may be very short about a fortnight. He mentions several instances of large families from healthy districts, in which all or most of the respective members contracted the disease soon after taking up residence in a pellagrous area. On several occasions he met families all the elder members of which were pellagrins, whilst the two or three youngest children were not, owing to the fact that the parents had removed from a pellagrous to a healthy locality before the birth of the latter. Pellagra everywhere presents the same peculiar topographical distribution: its stations or endemic foci are irregularly scattered and always confined to well-defined rural areas either intersected by streams or connected with other water bodies. A striking feature of its distributional peculiarities is the exemption of towns; "pellagra stops at city gates," and when the disease is epidemic the crowded town escapes the scourge.
