Host-Parasite Relationships 537 



TRANSFER OF PARASITIC 

 PROTOZOA 



Protozoan parasites reach new hosts in various ways. Active migra- 

 tion may lead to invasion of aquatic hosts — Endosphaera (66) of ciliates, 

 Amyloodinium (74), and Ichthyophthiriiis (69) of fishes. Contact transfer 

 is the characteristic method for some parasites — oral contact for Enta- 

 moeba gingivalis and Trichomonas tenax; transfer in coitus for Tricho- 

 monas vaginalis, Tritrichomonas foetus, and Trypanosoma equiperdum. 

 Con tarn inative transfer, in which cysts or spores are ingested with food 

 or drink, is the usual method for Coccidia and for intestinal flagellates, 

 amoebae and ciliates. 



Transfer by vectors is characteristic of blood parasites. Vectors include 

 blood-sucking flies (Trypanoso7na gambiense), mosquitoes (malarial para- 

 sites), bugs [Trypanosoma cruzi), fleas {Trypanosoma lewisi), ticks 

 {Babesia bigemina), leeches {Trypanosoma rotatorium), and apparently 

 vampire bats {Trypansoma hippiciim). Transfer by vectors may be a 

 mechanical process during which the parasites undergo no significant 

 changes. In other cases, the parasite passes through a phase of the life- 

 cycle before it is again infective for the final host; this cyclic, or infective, 

 transfer is characteristic of malarial parasites and various trypanosomes. 

 Some vectors inoculate the parasites directly into the tissues of the host 

 during feeding. In contrast to this method, Trypanosoma cruzi is voided 

 from the hind-gut of its vector and reaches the tissues of the vertebrate 

 by contamination of a woimd or invasion of a mucous membrane. 



The case of Histomonas meleagridis, which causes "blackhead" in 

 turkeys, seems to be unique in that the flagellates are said to be trans- 

 ferred in the eggs of an intestinal nematode, Heterakis gallinae (94). 



Congenital infections may follow placental or ovarian transfer of 

 parasites. Placental transfer, involving the passage of parasites through 

 the placenta, has been reported for Plasmodium vivax, P. malariae, and 

 P. falciparum of man, and occasionally also for certain trypanosomes in 

 experimentally infected laboratory animals. Ovarian transfer, involving 

 the direct invasion of eggs by parasites, occurs in such invertebrates as 

 female ticks infected with Babesia bigemina. 



Lacteal transfer, from females to suckling young, has been described 

 in a few trypanosome infections, and this possibility should be con- 

 sidered in interpreting cases of supposedly placental transfer. 



GEOGRAPHICAL DISTRIBUTION OF 

 PARASITES OF MAN 



The protozoan parasites of man include species which invade the 

 vascular, epithelial, and other tissues, and also a number which live 

 in the lumen of the digestive tract. The digestive tract is parasitized by 



