Abstract:

To identify the optimal intermediate host for Fasciola gigantica in Dali so as to provide information for developing control measures, the eggs of F. gigantica were collected and incubated in water at 28 ℃until being hatched to miracidia. The hatched miracidia were used to infect three different snails: Physa acuta, Radix swinhoei, and Galba previa collected from Dali area in a 24-well plate with a ratio of 1 : 3 for snail and miracidia. Three kinds of snails were randomly divided into the infected group and the un-infected group. The number of snail in the infected group was 141 (P. acuta), 240 (R. swinhoei) and 251 (G. previa). The number of un-infected snails was 124(P. acuta), 136 (R. swinhoei) and 87 (G. previa). Four hours after infection, the snails were placed in incubation pots with 48 to 60 snails for each pot at room temperature. The un-infected group was placed directly into a snail basin, which was fed in the same way as the infected group. The dead snails were picked out daily and the development of the larvae in the snail was observed under a microscope. The room temperature and water temperature were recorded daily. When the larvae in the snail developed to rediae stage, a plastic membrane was placed on the surface of water in the pot to collect metacercaria. The results showed that the miracidia were hatched from eggs on day 11. In infected groups, snail P. acuta and R. swinhoei could survive till 72 days and G. previa survived till 53 days. The culture room temperature maintained at 21 ℃ to 27 ℃ (average 24 ℃), and the water temperature was maintained at 21 ℃ to 25 ℃ (average 23 ℃). All 141 P. acuta snails were negative for the infection, 12 kept survived and the rest died on day 72. For infection in 240 R. swinhoei, only one was infected and died on the day 28, 36 were survived till 72 days and the rest died. Total 112 out of 251 infected G. previa snails were positive with an infection rate of 44.6%, but all died 53 days after infection. In un-infection groups, snail P. acuta and R. swinhoei could survive till 77 days and G. previa survived till 57 days. In the un-infection groups, all 124 P. acuta snails were negative and 8 were survived till 77 days, all 136 R. swinhoei snails were negative and 30 survived till 77 days, all 87 G. previa snails were negative and died before 57 days. No sporocyst was observed in all positive snails. In the only one infected P. acuta, mother rediae was observed. The stages of rediae and mother rediae were observed on day 11, 23 respectively. The daughter rediae containing the immature and mature cercaria were observed on day 26, 30 respectively. The cercariae were observed in the body of dead G. previa 40 days after infection. The metacercaria were obtained on the day 40 after infection and collection lasted 11 days. Total 511 metacercaria were obtained. Most of the metacercaria were collected at 8:00 in the morning (97.7%, 499/511) and some were collected at 2:30-4:30 in the afternoon (2.3%, 12/511). During this collection period, 29 G. previa kept alive and 24 of them were infected (21.4%, 24/112). The average of 21 metacercaria were formed from each infected snail.

Key words: Fasciola gigantica, Intermediate host, Galba previa, Physa acuta, Radix swinhoei, Dali