CLIMATOLOGY 



1331 



cold climates, and therefore is well known to occur in Europe and 

 America. 



In the tropics it principally appears in places situated at high 

 altitudes, or where there are cool seasons, in which case it disappears 

 in a most remarkable manner as soon as the warm season sets in. 



In Africa it is said by Hirsch to be endemic in Nubia, and has been 

 carefully studied in Egypt by Sandwith, where he says that it has 

 been definitely recognized since 1836, and where a few cases are 

 reported every year in the spring months. It is also reported 

 from Tripoli, Tunis, Algeria, and Morocco. In Asia it appears to 

 occur only in epidemic form in Asia Minor and Syria, but is believed 

 to be endemic in Persia, and possibly also in Afghanistan, because 

 it is said to be regularly imported into India by caravans from this 

 country. In India an endemic area exists on the west of the Indus, 

 stretching from Beloochistan in the south to Yusufzai, Hazara, and 

 Kashmir in the north, and then passing eastwards along the ranges 

 of the Himalayas, where it is especially prevalent at Kulu, and also 

 passing southwards into the district of Rawal Pindi. In Ceylon 

 we have met with a few cases in European sailors, who have, however, 

 acquired the disease elsewhere. It occurs in Indo-China ; in Northern 

 China, where it is endemic, becoming epidemic every spring; and in 

 Japan, where it occurs yearly in the province of Hiogo. It does 

 not occur in Australasia or Oceania. In tropical America it is 

 endemic in Mexico, rare in Central America, absent in the West 

 Indies, the Guianas, Colombia, and Venezuela, and is rare in Brazil, 

 though it is common in Peru and Northern Chili. 



This climatology depends upon the louse and the air temperature 

 which is best for the parasite to develop therein. 



The Louse. — ^As Pedicuhcs corporis belongs to man, it can go 

 wherever man goes, but typhus fever does not, being absent in the 

 tropics during the hot seasons. 



The Parasite.— The fact that most observers believe that a low 

 temperature is necessary for the development of the virus in the 

 louse is borne out by the climatology, being limited to the cold and 

 temperate climates, and only appearing in tropical or subtropical 

 climates during the cold weather or in the hills. 



In Mexico it is found on the lofty plateau, in India in the hills, in 

 Egypt and Northern Africa in the cool season, and it will be remem- 

 bered that the clothes louse is exposed to the effects of air tempera- 

 tures. 



Epidemiology. — ^Epidemics are caused by anything which favours 

 the propagation of and dissemination of lice. The principal factors 

 which do so are (i) massing together of people of all classes; 

 (2) retaining these masses under conditions which render personal 

 cleanliness and clean clothing difficult or impossible. Such con- 

 ditions are typically produced in times of war and famine, and exist 

 endemically among the poor. To these we must add a third 

 condition, (3) an atmospheric temperature not too high — i.e., a 

 Temperate Zone temperature. All this has been well exemplified 



