HOST-PARASITE SPECIFICITY IN THE GENUS GIARDIA 145 



SOURCES OF SUPPLY 



Surveys of the intestinal protozoa of man indicate that Giardia 

 lamhlia occurs in about ten per cent of the general population. As 

 a rule cysts only are to be found in formed stools. They are very 

 irregular in their appearance, being present in considerable num- 

 bers on one day but absent or few in numbers on other days. Troph- 

 ozoites are passed when stools are loose and likewise are some- 

 what irregular in their appearance. Perhaps the easiest species to 

 obtain is Giardia muris that occurs in rats and mice. Cysts may 

 be found in the fecal pellets of these animals and uninfected rats 

 may be infected by placing them in the same cage with an infected 

 specimen. Heavy infections may be secured by adding infected pel- 

 lets to the food supply of the rats, being careful to supply suffi- 

 cient moisture to prevent the drying out of the cysts. Field mice 

 are frequently found to be infected but are more difficult to obtain 

 for study. 



The trophozoites living in rats or mice may be secured without 

 killing the hosts by giving them a dose of Epsom salts. Active 

 flagellates can then be secured in the liquid fecal material that 

 results from this treatment. 



Giardias do not seem to be present in as many of the other com- 

 mon laboratory animals, such as rabbits, guinea-pigs, cats and 

 dogs. The incidence of infection in domesticated animals such as 

 horses, cattle, sheep and goats is not well known. Thus far, 

 giardias have been reported from a number of species of birds but 

 only two species have been given names, one from the blood of 

 a falcon, G. sangidnis (Gonder, 1910) and G. ardece from the 

 intestine of several species of herons (Noller, 1920). Giardias have 

 also been recorded from the intestine of a shrike, an avocet (Kot- 

 lan, 1922) and from the intestine of the great blue heron and 

 black-crowned night heron (Hegner, 1925). It is interesting to 

 note that all of these birds feed upon animals that are known to 

 be infected with giardias. Falcons and shrikes capture and devour 

 field mice; avocets and herons feed on tadpoles. Perhaps those 

 species found in birds have evolved from those that occur in 

 their prey. Of the giardias that have been reported from cold- 

 blooded animals, only G. agilis of the tadpole appears to be com- 

 mon. 



