670 INTESTINAL PROTOZOA AND HOST-PARASITE RELATIONS 



satisfactory results. Wagener (1924) has reported positive precipitin tests in cats in- 

 fected with E. histolytica, and Scalas (1923) has obtained positive intradermal reac- 

 tions, but these researches need confirmation. 



Acquired resistance of the parasite. — The human experiments with Endamoeba 

 histolytica carried out by Walker and Sellards (1913) prove that one strain of amebae 

 may be more aggressive in one host than in another, thus demonstrating that it is 

 differences in the susceptibility of the host rather than in the aggressivity of the 

 parasite that account for the severity of the infection. The work of Baetjer and Sel- 

 lards (1914) and of Wagener and Thomson (1914) with kittens, however, indicates 

 that strains of amebae differ in virulence and that specimens from acute human cases 

 bring about infections in kittens more easily than those from chronic human cases. 



Emetin-resistant cases of amebiasis have been reported from time to time which 

 suggest that there may be strains of E. histolytica that are resistant to this drug. At 

 present it is only possible to express an opinion on the subject because of lack of 

 evidence. Apparently in the so-called "emetin-resistant cases" the drug fails to reach 

 the large intestine, and hence the parasite, because of some physiological idiosyncrasy 

 of the host. 



The carrier condition, latency, and relapse. — Walker and Sellards (1913) separated 

 carriers into those who are infected and pass cysts but have never exhibited symp- 

 toms, i.e., contact carriers, and those who have recovered from an acute attack of 

 amebiasis but still carry the organism in the intestine and pass cysts, i.e., convalescent 

 carriers. Apparently when a person is once infected he remains infected for many 

 years, possibly throughout the rest of his life, although what appears to be a con- 

 tinuous infection may be the result of reinfection. 



During the carrier period when no symptoms are evident the amebae are said to 

 be latent. Reproduction, however, appears to continue during periods of latency but 

 at a reduced rate, and cysts or trophozoites may be found in the stools if daily exam- 

 inations are made. Such a carrier may at any time suffer a relapse, a condition that 

 is not uncommon in amebiasis. The relapses are no doubt due to some condition in 

 the host that lowers resistance and enables the parasite to increase more rapidly. 



Host-parasite specificity. — Endamoeba histolytica does not appear to be a regular 

 inhabitant of the intestine of any other animal but man. Spontaneous infections have 

 been described in monkeys, cats, dogs, and rats. Amebic liver abscesses have been 

 noted in monkeys, cats, and dogs. Young cats appear to be particularly susceptible 

 to infection with this species of ameba. Various species of laboratory animals, includ- 

 ing rats, mice, guinea pigs, and rabbits, have been infected, but usually with diffi- 

 culty. Infected cats do not pass cysts and hence are not dangerous. Rats, on the 

 other hand, do pass cysts, but are so rarely infected as to be of practically no im- 

 portance. It is thus evident that although Endamoeba histolytica is able to live in a 

 number of lower animals its true host is man, and only under especially favorable con- 

 ditions is it able to establish an infection in some other species of host. 



Prevention and control. — Amebiasis is a preventable disease just as are typhoid 

 fever, bacillary dysentery, cholera, etc. Contact and convalescent carriers who may 

 pass as many as three hundred million cysts in a single day are more dangerous than 

 hosts suffering from acute infections, since the cysts represent the infective stage, 



