1. Infection dose of the parasite Echinostoma paraensei (Trematoda, Platyhelminthes):and cercarial output by the intermediate host snail Biomphalaria glabrata (Gastropoda, Mollusca)
K Adams, T Allen, G Ballon, C Barnett, M Frey, C Gray, K Mitchell, J Nolan, M Quetawki, B Smith, X-X Wang, D Whalen II, T Kennedy, CM Adema.
ABSTRACT
The non-human parasite Echinostoma paraensei is a useful model for study of host-parasite interactions and general biology of digenetic trematode parasites such as liver flukes and schistosomes, which are of great medical and economic significance. Completion of the lifecycle of digenean parasites relies on obligatory development and asexual reproduction in an intermediate host snail to yield free-living cercariae that can infect the next host. Once infected, snails may release cercariae for the remainder of their life. Intramolluscan parasite development is detrimental to the snail host; trematodes may effect immune suppression, parasitic castration (cessation of reproduction), tissue damage and modification of host metabolism for reallocation of host nutrients and energy toward parasite development. While the likelihood of parasite transmission may benefit from release of many cercariae, parasite survival depends on causing modest pathology to avoid death of the host snail and this may limit the extent of cercarial production. To investigate the effect of infective dose on parasite output, juvenile Biomphalaria glabrata snails (4-8 mm shell diameter) were exposed to either 1-5 or miracidia of E. paraensei. Snails were monitored for parasite development. Cercarial emergence was recorded after patency, and compared for low and high infection doses.
RESULTS
OBSERVATION OF INTRAMOLLUSCAN STAGES IN M LINE SNAILS
EFFECT OF HIGH EXPOSURE DOSE
Snails in the high dose experienced a significantly higher rate of mortality (P1,9=0.01)
There was no significant differences in the number of cercariae that were shed between the high and low dose treatments.
The rate of shedding did increase in the high dose treatment over time. This was a weak relationship F1,39=3.31, P=0.0756, R2=0.07.
CONCLUSIONS
A higher infection rate significantly increased mortality in the intermediate host. Higher mortality could be associated with increased pathology caused by the elevated numbers of developing parasites.
Infection rate did not significantly affect number of cercariae shed. A possible explanation is that there is an optimal infection rate in snails: too many cercariae and the snail dies, too little and the parasite fails to infect the definitive host. The variance in number of cercariae shed from each snail was also high
There was a marginally significant increase over time in the rate that cercariae were shed from the snail. The variance in number of cercariae shed each day was also high.
Metacercariae (in M line B. glabrata)
Final host
2nd intermediate
host
Egg
1st intermediate host
Cercariae
Miracidium
FUTURE DIRECTIONS
There was a lot variance in individual snails and on any given day in the number of cercariae shed.
A power test could be used to determine the sample size necessary for future experiments.
To determine if infection dose affects the number of cercariae shed from snails.
To determine if the rate of shedding changes over time
Daughter Rediae
Mother Rediae
(M line w.) sporocysts in heart (also observed from BB02)
Fig.1. Photographic representation of the development of intramolluscan larvae of E. paraensei in M line B. glabrata as 1st intermediate host and 2nd intermediate host. Scale bars:100 mm.
Please note, this project only focused on parasite development as it relates to the (invertebrate) snail hosts, it did NOT involve the definitive host (hamster).
OBJECTIVES
To determine whether infection dose of Echinosotoma praraensei affects mortality of the intermediate host, Biomphalaria glabrata.
To compare whether high or low infection rates is correlated with the number of cercariae shed from B. glabrata.
To determine whether infection dose affects the rate of cercarial emergence.
ACKNOWLEDGEMENTS
The students appreciate the course facilities offered, this includes the snails and numerous parasites that were used in the study.
MATERIALS AND METHODS
Laboratory strain (M line) of Biomphalaria glabrata was obtained from a population maintained in the lab for ~30 years.
Two groups of 12 snails were exposed to miracidia of E. praraense
High dose: miracidia/snail
Low dose: miracidia/snail
Parasite development was monitored weekly by observation and dissections.
Snails were dissected on a weekly basis and inspected for parasite development under a light microscope. All different intramolluscan stages of the parasite were recorded.
At patency, snails were shed on a daily basis by placing snails individuals in a well for 2 hours
After 2 hours, snails were removed and the ceracriae were fixed with 70% EtOH.
Number of cercariae between each treatment was analyzed with a two-tailed t-test.
A regression analyses was used to determine if the rate of shedding changed over time.