430 ANlSrUAL REPORT SMITHSONIAN INSTITUTION, 1956 



conclusions, perhaps with one or more possible but questionable excep- 

 tions. The ability of pig ascarids to migrate to the liver and lungs 

 of human beings and to produce hepatic and pulmonary symptoms 

 and lesions places this parasite in the category of a human pathogen, 

 even though it probably does not develop, or develops only rarely, to 

 fertile maturity in the intestine of man. 



MIGRATING LARVAL PARASITES 



Visceral larval migrans. — Since many nematode parasites wander 

 extensively through the body before getting to their ultimate location, 

 it is not surprising that immature migrating worms are trapped from 

 time to time in various organs and tissues through which they pass, 

 without reaching their ultimate destination. It appears to be well es- 

 tablished, moreover, that with repeated invasions of migrating larval 

 nematodes the tissues through which they pass may become sensitized 

 to them and the parasites tend to become arrested and trapped there. 

 In fact, certain liver and lung lesions that are sometimes seen in ani- 

 mals and man appear to be due to a reaction to repeated assaults by cer- 

 tain nematode larvae. In the main, visceral larval migrans, as this con- 

 dition is called, has been associated with the larvae of the zoological 

 family Ascaridae, species of which occur in man, swine, and pet ani- 

 mals. The tissue reaction is characterized by small inflammatory foci, 

 observed as a rule in the liver and lungs, in which the infiltrating cells 

 are predominantly eosinophilic leucocytes. This reaction has been as- 

 sociated in man with larval Ascaris lumbricoides and also, more re- 

 cently, with the larvae of carnivore ascarids accidentally acquired by 

 man. Some of the earlier association of Ascaris lumbricoides larvae 

 with visceral larval migrans in man is considered by some recent inves- 

 tigators to have been due perhaps to the larvae of one of the dog 

 ascarids, Toxacara canis. Recent observations have shown that the 

 larvae of these ascarids and probably also those of the closely related 

 cat ascarid, T. cati, are involved in the production of the visceral le- 

 sions in man. It would appear, therefore, that migrating nematode 

 larvae in a strange host, in this case carnivore ascarid larvae in man, 

 can evoke liver and lung lesions that migrating larvae in a natural host 

 apparently evoke as a result of a previous sensitization. At any rate, 

 these studies, still in the early stage, have pointed up the danger to 

 human beings, and especially children, from parasites of pet animals 

 and probably also other animals, the eggs of which may contaminate 

 food or water or be ingested in other ways. 



Cutaneous larval migrans. — This condition involves the skin and is 

 related to visceral invasion by strange parasites. In cutaneous larval 

 migrans the causative parasites enter the body through the intact skin. 

 The larvae are trapped there and produce by their movements under 



