Fasciola gigantica transmission in the zoonotic fascioliasis endemic lowlands of Guilan, Iran: experimental assessment.

The lowland flatlands around the city of Bandar-Anzali, at the Caspian Sea shore, Guilan province, are an endemic area where Fasciola gigantica appears to be the fasciolid species involved and past outbreaks affecting around 15,000 people and the highest human infection rates in Iran have been reported. Fascioliasis transmission in that area has been experimentally analysed for the first time, by means of assays of monomiracidial (Group A: 120 snails) and pentamiracidial (Group B: 96 snails) infections of local Radix lymnaeid snails with a local cattle F. gigantica isolate. Ribosomal DNA ITS-2 sequencing proved that Lymnaea (Radix) gedrosiana should henceforth be considered a synonym of Radix auricularia, the haplotype found in Bandar-Anzali being identical to that found in many European countries. Survival rates at day 30 postinfection and metacercarial productivity (both higher in Group A) and longevity of the shedding snails (higher in Group B), were affected by the miracidial dose, whereas prepatent period, patent period, infection rate, and the percentages of shedding and infected non-shedding snails did not. The higher percentage of shedding snails in Group A (51.0% versus 37.7%) counteracts the higher number of metacercariae produced in Group B (243.9 ± 259.2 versus 157.2 ± 153.2). High numbers of shedding snails in both experimental groups passed less than 100 cercariae, and 16% in Group A and 35% in Group B produced more than 300 metacercariae, while only four snails (8%) in Group A and two snails (10%) in Group B shed more than 500 metacercariae. Most metacercariae (94.7% in Group A and 85.1% in Group B) were recorded during the first 15 days of patent period. The comparison with results from other fasciolid/lymnaeid systems indicates that the F. gigantica/R. auricularia system of Guilan is highly susceptible and compatible. Results obtained suggest that increased lymnaeid vector populations and not polymiracidial snail infections most probably underlay the human outbreaks in the past. The climatic analyses suggest a long fascioliasis transmission period in the Bandar-Anzali area covering from May to the end of November regarding permanent water bodies, a transmission season window widening in rice fields and irrigation canals, and a shortening of the transmission period from only end of August to beginning of November in temporary water bodies.