明确大理地区大片形吸虫的最适中间宿主,为制定防治措施提供依据。收集大片形吸虫虫卵,28 ℃孵育。每日定时吸取虫卵,观察其发育情况,直至孵出毛蚴。将3种实验用螺随机分为感染组和未感染组。感染组螺数量依次为：尖膀胱螺141只、椭圆萝卜螺240只和小土蜗251只;未感染组螺数量依次为：尖膀胱螺124只、椭圆萝卜螺136只和小土蜗87只。感染实验于24孔细胞培养板上进行,每孔加入数量比为1 : 3的螺与毛蚴。感染4 h后将螺放入养螺盆,每盆放置48~60只螺,室温下饲养。未感染组螺直接放入养螺盆,与感染组螺同法饲养。每日挑拣死螺,显微镜下观察大片形吸虫幼虫发育情况,同时记录室温和水温。待螺体内的幼虫发育为子雷蚴（含尾蚴雏形）,收集囊蚴。结果显示,毛蚴于第11天孵出。感染组螺中,尖膀胱螺、椭圆萝卜螺的最长存活时间为72 d,小土蜗的最长存活时间为53 d。期间测定的饲养室温为21 ℃~27 ℃（平均24 ℃）,水温为21 ℃~25 ℃（平均23 ℃）。141只尖膀胱螺均为阴性,至感染后第72天,仍有12只存活;仅1只椭圆萝卜螺为阳性,于感染后第28天死亡,其余239只均为阴性,至感染后第72天,仍有36只存活。共检出112只阳性小土蜗,感染率为44.6%（112/251）,至感染后第53天,全部死亡。未感染组螺中,尖膀胱螺、椭圆萝卜螺的最长存活时间为77 d,小土蜗的最长存活时间为57 d。124只尖膀胱螺均为阴性,至饲养第77天,有8只存活;136只椭圆萝卜螺均为阴性,至饲养第77天,有30只存活;87只小土蜗均为阴性,至饲养第57天全部死亡。所有阳性螺均未观察到胞蚴;仅有的1只阳性椭圆萝卜螺体内可观察到母雷蚴;112只阳性小土蜗体内可观察到雷蚴、母雷蚴、子雷蚴和尾蚴。阳性小土蜗于感染后第11天可见雷蚴,第23天可见母雷蚴,第26天可见含不成熟尾蚴的子雷蚴,第30天可见含成熟尾蚴的子雷蚴,感染40天后死亡的阳性小土蜗软体中发现尾蚴。感染后第40天开始捡获囊蚴,持续11 d,共收获511个囊蚴,其中499个囊蚴（占97.7%）均于次日清晨8 : 00检获,剩余12个囊蚴（占2.3%）于其中4天下午（14：30~16：30）捡获,期间存活的小土蜗有29只,其中24只为阳性,占阳性螺总数的21.4%（24/112）,每只阳性螺逸出尾蚴且形成囊蚴的平均数为21个（511/24）。大理地区大片形吸虫的最适中间宿主为小土蜗。

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.