INTRODUCTION TO PARASITOLOGY 



belong here too, altho they verge on the 

 symbiotic. The bacteria which produce 

 these vitamins in the large intestine of 

 swine are more nearly mutualistic, since 

 the pigs cannot absorb the vitamins thru 

 the colon wall but must re-ingest their 

 feces to obtain them. The same is true 

 of rabbits, and is undoubtedly responsible 

 for their coprophagy. 



The bizarre protozoa which swarm 

 in the rumen and reticulum are almost 

 certainly mutualistic. Their host can get 

 along without them, but they may benefit 

 it by providing a better type of protein 

 than it ingests. In addition, they are an 

 important source of volatile fatty acids, 

 and they smooth out the carbohydrate fer- 

 mentation process. 



Commensalism is an association be- 

 tween host and parasite in which one part- 

 ner is benefited and the other is neither 

 benefited nor harmed. Many intestinal 

 bacteria such as Escherichia coli are 

 normally commensals, as are many in- 

 testinal protozoa such as Entamoeba coli 

 and Trichomonas spp. 



The next two terms both refer to po- 

 tentially pathogenic parasites. Parasito- 

 sis is the association between two organ- 

 isms in which one injures the other, 

 causing signs and lesions of disease. 

 Parasitiasis is the association between 

 two organisms in which one is potentially 

 pathogenic but does not cause signs of 

 disease. 



The difference between parasitosis 

 and parasitiasis is quantitative. In para- 

 sitiasis the host is able to repair the 

 damage caused by the parasite without 

 noticeable injury, while in parasitosis it 

 cannot. As Whitlock (1955) put it, "Para- 

 sitiasis is a state of balance. Parasitosis 

 is a state of imbalance. " Applying the 

 concept to ruminant helminths, Gordon 

 (1957) said, "Helminthiasis is almost uni- 

 versal and continuous, helminthosis is 

 more restricted and sporadic. However, 

 one shades imperceptibly into the other in 

 subclinical infestations." The same or- 

 ganism can cause either parasitosis or 

 parasititiasis, depending upon the number 



present or upon the nutritional condition, 

 age, sex, immune state, etc. of the host. 

 Failure to recognize this distinction may 

 cause many false diagnoses--the mere 

 presence of a potentially pathogenic spe- 

 cies of parasite does not necessarily 

 mean that it is causing disease. 



The carrier state furnishes a good 

 example of parasitiasis. Carriers are 

 animals which have a light infection with 

 some parasite but are not harmed by it, 

 usually due to immunity resulting from 

 previous exposure, but which serve as a 

 source of infection for susceptible ani- 

 mals. Thus, adult sheep and cattle may 

 be lightly infected with gastrointestinal 

 nematodes without noticeable effect, but 

 their lambs and calves may become heav- 

 ily parasitized from grazing with them. 

 The condition in the adults is parasitiasis; 

 that in the young is parasitosis. Adult 

 chickens rarely suffer from coccidiosis 

 because they have recovered from a clin- 

 ical or subclinical attack when young. 

 However, they are usually still lightly in- 

 fected and continue to shed a few oocysts; 

 they have coccidiasis. Cattle which have 

 aborted as a result of Brucella infection 

 may continue to shed the bacteria in their 

 milk without ordinarily suffering further 

 clinical attacks. The aborting cow has 

 brucellosis, while the carrier has bru- 

 celliasis. 



These endings can also be applied to 

 the names of the disease agents, as has 

 already been done above. Thus, Haenion- 

 chiis coiitortus may cause haemonchosis 

 or haemonchiasis, Taenia may cause 

 taeniosis or taeniasis, Histomo>ias mel- 

 eagridis may cause histomonosis or his- 

 tomoniasis, depending on the circum- 

 stances. 



It was mentioned earlier that the so- 

 lutions different parasites have made of 

 their problems of living have varied in 

 satisfactoriness. We might consider this 

 in regard to type of parasitism. Symbio- 

 sis is a highly specialized type of associ- 

 ation which occurs only in certain groups. 

 Mutualism is a much looser association, 

 also fairly uncommon. It could well be a 

 step on the road to symbiosis. The most 



