INTRODUCTION TO PARASITOLOGY 



than 100, of which 23 were considered of 

 major importance. Many more are cer- 

 tain to be revealed by future investigations. 



Our thinking about parasites and dis- 

 eases is ordinarily oriented toward either 

 man or domestic animals. In this context, 

 it is convenient to have a special term for 

 hosts other than those with which we are 

 primarily concerned. A reservoir host 

 is a vertebrate host in which a parasite or 

 disease occurs naturally and which is a 

 source of infection for man or domestic 

 animals, as the case may be. Wild ani- 

 mals are reservoirs of infection for man 

 of relapsing fever, yellow fever and moist 

 Oriental sore, while domestic animals are 

 reservoirs for man of trichinosis and 

 classical Oriental sore. Wild animals are 

 reservoirs of infection for domestic ani- 

 mals of many trypanosomes, while man is 

 a reservoir for domestic animals of Enta- 

 moeba histolytica. 



Parasites and diseases may continue 

 to exist indefinitely in their reservoir 

 hosts, and man or domestic animals may 

 become infected when they enter the local- 

 ity where the parasites or diseases exist. 

 Such a locality is known as a nidus (liter- 

 ally, "nest"). This term is used primarily 

 in connection with vector-borne diseases, 

 altho it need not be restricted to them. 



epidemiology, taxonomy, evolution, etc. 

 (see Hoare, 1955). A dei)ie is a natural 

 population within a species. It lies more 

 or less below the subspecies level, but it 

 is not a formal taxon and is not given a 

 Latin name. There are different types of 

 deme. Nosodemes differ in their clinical 

 manifestations. One example is Leish- 

 mania donovani, which has five nosodemes, 

 Indian, Mediterranean, Sudanese, Chinese 

 and South American, which produce differ- 

 ent types of disease. Serodemes differ 

 serologically. These are best known 

 among the bacteria and viruses, but also 

 occur among the animal parasites. Tri- 

 trichonionas foetus, for example, has 

 several serological types or serodemes. 

 Xenodemes differ in their hosts, and topo- 

 demes differ in geographic distribution. 



There are also other types of demes. 

 The population of a parasite species within 

 a single host animal is a i)io)iodenie, and 

 that in a single flock or herd is an agele- 

 deme. Thus, a population of the stomach 

 worm, Haemouchus contortus, in a single 

 sheep is a monodeme, the population in 

 all the sheep of a single flock is an agele- 

 deme, that in all sheep is a xenodeme. 

 The population in all cattle is another 

 xenodeme and that in all goats is a third, 

 the population of H. contortus in all hosts 

 in North America is a topodeme, etc. 



Natural nidi may be elementary or 

 diffuse (Palovsky, 1957). An elementary 

 nidus is confined within narrow limits. 

 A rodent burrow containing rodents, arga- 

 sid ticks and relapsing fever spirochetes 

 or a woodrat nest containing woodrats, 

 kissing bugs and Trypanosoma cruzi is an 

 elementary nidus. In a diffuse nidus the 

 donors, vectors and recipients are distrib- 

 uted more widely over the landscape. A 

 wooded region in which ticks circulate 

 Rickettsia rickettsii among the rodents 

 and lagomorphs is a diffuse nidus of 

 Rocky Mountain spotted fever, as is an 

 area where tsetse flies transmit trypan- 

 osomes among wild game. The nidality 

 of a disease refers to the distribution and 

 characteristics of its nidi. 



The concept of the deme is useful in 

 discussing host-parasite relationships, 



Each of these demes may differ mor- 

 phologically and physiologically, and a 

 large part of the taxonomist's work con- 

 sists in determining the limits of their 

 variation and deciding whether they are 

 really demes or different species. Since 

 the judgments of all taxonomists do not 

 agree, there is some variation in the 

 names which different parasitologists use. 

 Demes are advance guards in the march 

 of evolution, and no sharp line can be 

 drawn beyond which they become subspe- 

 cies or species. Taxonomists have been 

 able to arrive at no better statement of 

 how species are defined than to say that a 

 species is what a specialist on its group 

 says it is. And since some scientists are 

 splitters and others are lumpers, their 

 definitions vary with their temperaments. 

 For most of us, the best rule is the prag- 

 matic one of using those names which 



