Host-parasite Population Dynamics of Haematoloechus coloradensis Cort, 1915 (Digenea: Plagiorchiidae)

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Abstract
A naturally occurring population of a digenetic trematode H. coloradensis was monitored in all hosts for 2 yr. The snail host, Physa virgata; 3 genera of dragonflies, Anax, Tramea and Libellula; 2 spp. of the damselfly genus Enallagma; and the frog host Rana berlandieri were collected from 11 ponds in Sierra County, New Mexico [USA] and examined for this parasite. The incidence of infection and number of parasites per host varied seasonally in all hosts. Incidence of infection increased significantly during the early summer each year in all hosts. The intensity of infection also increased during this time in frogs and odonates. The mean parasite incidence of infection in odonates (1972 = 14.7%; 1973 = 17.9%) was approximately 3 times that in snails (1972 = 5.0%; 1973 = 7.2%), while in frogs (1972 = 68.6%; 1973 = 76.2%) it was approximately 4 times that in odonates. Although some environmental factors apparently affect this helminth population directly, the major influence of these factors was on the density of hosts and their seasonal availability to the parasite life cycle. Incidence of infection with H. coloradensis increased from zero with size in smaller hosts, reached a high in medium-sized hosts, and then decreased again with size to zero in the largest host in both R. pipiens (adults) and P. virgata (asexual stages). In odonates the incidence and intensity of this helminth (metacercariae) showed a significant positive correlation with odonate body length. The estimated mean number of eggs, cercariae, metacercariae and new adults per m3 of pond during the yearly activity period of this trematode life cycle, along with the estimated host population densities per m3 of pond, were used to establish number flow diagrams to examine the efficiency of this life cycle at each host level. During 1972 these efficiencies were 0.02% for eggs actually producing successful miracidia, 4.4% for the number of cercariae produced from the eggs produced, 0.08% for the number of metacercariae in odonates produced from cercariae potentially shed and 0.9% for the number of new flukes realized in frogs from the available metacercariae. During 1973 these efficiencies were 0.01%, 3.3%, 0.1% and 0.2%, respectively.