Experimental studies on the cercaria and metacercaria of a strigeoid trematode, Posthodiplostomum minimum

Abstract

It is probable that very young Physa became infected with the cercariae of P. minimum centrarchi. Snails infected in nature produced 840 to 8,940 cercariae per 24 hours, and the maximum longevity of cercariae was 32 1 2 hours. Cercariae were found to be infective for at least 24 hours after emergence. Cercariae did not emerge at 15 °C, but did at 18 °C and higher; they were not infective at 15 °C, but were at 18 to 27 °C. Of 14 species of fish exposed to the cercariae of the centrarchid line only sunfish and bluegills became infected. Fathead minnows were experimentally infected with the minnow line of cercariae. Metacercariae taken from a species of one fish family and transplanted into a species of a different fish family did not survive as long as when transplanted into a fish of the same family. Because of the host specificity involved in the metacercarial stages subspecific names are proposed: Posthodiplostomum minimum minimum for the minnow line, Posthodiplostomum minimum centrarchi for the centrarchid line. Development and migration of the metacercaria was studied from one hour post infection until full development was reached. The larvae utilize blood vessels, at least in part, during migration to the final site. The parasite cyst is probably formed by secretion through the worm cuticle and it eventually separates from the worm to form a capsule. A very thin host cyst composed of fibrous connective tissue became apparent at about 27 to 29 days in liver tissue, and the adjacent liver cells took on a compressed appearance. The worms did not excyst without partial digestion of the parasite cyst, and agitation. Fathead minnows exposed to abnormally large concentrations of the cercaria of Posthodiplostomum m. minimum died in 16 hours to 5 days. There was marked hyperemia at the bases of the fins, but most of the fish did not show a nervous response to the effects of the infection. In abnormally heavy infections metacercariae were found in unusual locations—liver, spleen, kidneys, ovaries, musculature and eyes. And in one instance the body wall burst, liberating many larvae without killing the fish. The ability of the metacercariae to withstand pepsin solution became apparent at about 26 to 44 days of development. Pepsin solution at pH 2 was more deleterious to the metacercariae than Ringer's solution at the same pH. The lower range of pH tolerance of the larvae was from pH 3 to pH 7. Trypsin at pH 8 was slightly more deleterious to the metacercariae than Ringer's solution at the same pH. The saline tolerance range of the metacercariae was from 0.65 to 1.25% Ringer's solution. Penicillin-Streptomycin had no ill effects on metacercariae in concentrations of 1,000 units each per ml or lower, and in fact increased their ability to survive.