164 THE EFFECTS OF PARASITES ON THEIR HOSTS. 



field and pasture, and afterwards lose them during the subsequent 

 fattening. x 



It is also a well-known fact that liver-rot (Distomum hepaticum) 

 and verminous inflammation of the lungs (Strongylus) are much com- 

 moner among our horned cattle in some years than others. We even 

 know of epidemics of this kind which have in many districts almost 

 destroyed the cattle for a long time. Wet seasons especially have 

 this pernicious result, since long-continued rainy weather assists the 

 transportation of the young brood or of the intermediate host, and 

 greatly increases the possibility of infection. 



Even among human parasites there is one, on the occurrence 

 of which a damp season has an undeniable influence. This is the 

 Filaria Medinensis of the tropics, the periodically varying degree 

 of whose occurrence has long excited the attention of observers. The 

 register of the native general hospital in Bombay shows, according to 

 Carter, 2 that during the years 1851 and 1858 the maximum of sixty- 

 three cases occurred in August, the minimum of twelve in February, 

 and forty-four in the month of May. The results of observations in a 

 military hospital ought to be still more reliable, since the soldiers 

 would at once seek relief on the discovery of the trouble. And these 

 results are so far different, that e.g., in Sattara, a garrison town 100 

 miles from Bombay three-fourths of all the cases were treated be- 

 tween March and June. According to the observations of seven years, 

 the maximum occurs in May (125 cases) and in June (102), and the 

 minimum in January (11). We may therefore conclude that it is in 

 the two months at the end of the dry season and beginning of the 

 wet that the disease most frequently manifests itself. From many 

 cases of infected sailors who have spent only a short time in a place 

 it has been determined that the Guinea-worm requires ten or twelve 

 months for its perfect development, and we can therefore further con- 

 clude that it is during the rainy season that this parasite finds 

 entrance. 3 



The local and endemic occurrence of parasites must be discussed 

 from the same point of view. We should expect, a priori, that the 

 frequency of helminthiasis would be in inverse ratio to culture and 

 civilisation. Filthy and careless habits, the frequent eating of raw 



1 Bloch has already recognised this fact (" Abhandl. liber die Erzeugung der Eingew. 

 Wurmer," p. 10, 1782), and has correctly found its reason in the altered nourishment. In 

 other cases the parasites acquired in youth, during a particular time, continue on into 

 later life. Thus, for instance, the Pdystomum of the bladder and the Opalinw of the 

 rectum in frogs, both date from the time of the larval condition. See Zeller, Zeitschr. f. 

 wits. Zool., Bd. xxvii., p. 238, 1876, and BcL xxix., p. 352, 1877. 



3 Ann. and Mag. Nat. Hist., ser. 3, vol. iv., p. 110, 1859. 



8 For further details see Vol. II. 



