Evidence Suggesting that Fasciola gigantica Might be the Most Prevalent Causal Agent of Fascioliasis in Northern Iran

Abstract

Following the occurrence of two large outbreaks in Gilan province, northern Iran, the public health importance of human fascioliasis has increased significantly. The mixed infections of Fasciola gigantica and F. hepatica within a given domestic animal individual suggest a very complicate picture of possible circulation ways of the parasite and the possibility for humans to be infected with both species. Elucidating these circulation ways is very critical for understanding the epidemiology and transmission of the disease and being ascertain how animals and humans enter the different liver fluke circulations in this endemic zone. The main objective of the present study was to investigate the distribution and natural infections of local lymnaeids, environmental characteristics related to the disease transmission and determining the most prevalent fasciolids and definitive hosts in human endemic areas of Gilan province. Bandar-Anzali and Rasht are the most important endemic areas with most of the cases of human disease during the epidemics and inter-epidemic periods. Sheep raising is not normal in these regions, while cattle is the most common definitive host. According to the data obtained from slaughterhouse observations in Bandar-Anzali and Rasht, the main fasciolid in local cattle is F. gigantica. Of 928 adult liver flukes collected from 13 infected livers of cattle, in Rasht and Bandar-Anzali slaughterhouses, 91.1% were diagnosed as F. gigantica and 8.9% as F. hepatica. L. gedrosiana and L. palustris were the most prevalent lymnaeid snails in this endemic zone. It appears that L. truncatula is not prevalent in Bandar-Anzali and Rasht and surroundings of these endemic cities. Of 4830 different snails studied, only seven L. gedrosiana were found to be infected with larval stages (rediae and cercariae) of Fasciola sp. Experimental infections of 15 common laboratory mice by metacercariae, obtained from those naturally infected snails, were carried out and all trematodes recovered at necropsy, 8 weeks post-exposure, appeared to be F. gigantica based on morphology. The high temperature, moisture and rainfall during the year, especially in Bandar-Anzali, support the establishment and transmission of the disease in the zone. Although the species involved in human infections in endemic areas of Gilan has been usually referred to as F. hepatica, the results of this study as well as some interesting epidemiological evidences related to the disease, support that Fasciola gigantica might be the main causal agent of fascioliasis in this important endemic zone.