Geographical correlation of schizophrenic birth excesses and MS prevalence to spirochaetal diseases The gradient of MS prevalence  and schizophrenic birth excesses [16, 28] entirely spares the tropical belt where human treponematoses prevail . In subtropical zones between the 37° latitudes, there exists an additional climatic effect diminishing the prevalence rates of MS worldwide. This 'rule' of variation by latitude, however, is violated in Europe, Australia and New Zealand where circumpolar migratory seabirds reportedly introduce Ixodes uriae and Borrelia garinii  from the MS hot spots in the north [12, 15]. The arrows represent the migratory routes and distribution of seabirds if a number of species such as puffins (shearwaters), seagulls and terns are taken together [56–59]. The sites from where samples in search for Borrelia garinii were collected by Olsen et al.  are marked with a red dot and include the Egg and St. Lazaria Islands (Alaska), Gannet Island in Newfoundland (Canada), Iceland, the Falklands, the Faroes, Bonden Island (Sweden), Cape Sizun (France), the Crozet Islands (South Africa), and Campbell Island (New Zealand). There are just a few subtropical areas including Madeira, Morocco and Tunisia from where the presence of B. burgdorferi s.l. has been documented. Two protective factors could thus explain the rarity of MS and schizophrenic birth excesses in developing countries: acquired immunologic resistance against spirochaetes in the tropics, and higher temperature being inversely related to tick-borne spirochaetal transmission in subtropical zones.