88 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



^I 



Figure 9. — Daughter redia in ovary showing yolk plate- 

 lets (arrow) and developing cercarlae, natural infec- 

 tion ; PFA-3, DH-AzB-E ; X21o. 



infection persists for 2 years and possibly for 

 the life expectancy of the snail, estimated by 

 Butler (1953) to be 5, possibly as much as 10, 

 years. 



EXPERIMENTAL INFECTIONS 



Adult Hosts 



Juvenile herring {Larus argentatiks) , ring- 

 billed (Z. delaivarensis) , and laughing gulls (Z. 

 atrieilla), and nestling least terns (Sterna 

 alhifrons), all common along the Gulf coast, were 

 tested for suitability as reservoir hosts for dis- 

 seminating P. acanthus among wild drill popula- 

 tions. 



The observed incidence of natural P. acanthus 

 infection in these species is given in table 6. It 

 is noteworthy that some individuals of all three 

 gull species were naturally infected. Absence of 



FiGUEE 10. — Same redia, X450. 



infection in the least tern nestlings was expected, 

 since they were no more than a day old when ob- 

 tained and had not been exposed to infection in 

 the laboratory. 



Experimental infections were established by 

 two methods : oral or anal administration of en- 

 cysted metacercariae and transfer of mature or 

 iimnature adult worms by pipette to the cloaca of 

 an uninfected bird. 



Table 6. — Incidence of natural P. acanthus infections in 

 juvenile gulls and nestling least terns 



Species 



Herring gull — 

 Ring-billed gull 

 Laughing gull.. 

 Least term 



Number 

 of worms 

 recov- 

 ered 



The results of oral administration of cysts are 

 given in table 7. The data show tliat juvenile 

 herring and ring-billed gulls were readily infected, 

 juvenile laughing gulls were less susceptible, and 

 nestling least terns were resistant. 



