Opisthorchis viverrini: the relationship between egg production, worm size and intensity of infection in the hamster

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Worm recoveries, egg production and worm lengths were determined in golden Syrian hamsters given between five and 200 Opisthorchis viverrini metacercariae. Animals given five, 10 or 20 metacercariae showed similar worm recovery rates of 38%, 36% and 37%, respectively, whereas animals given 50, 100 or 200 metacercariae gave recoveries of 49%, 65% and 54%, respectively. The distribution of worms between right, left and middle liver lobes was similar for animals with between one and 43 worms but different for animals with between 64 and 154 worms. A curvilinear relationship between eggs found per gram of faeces per worm (epg/worm) and eggs/worm/24 hours and worm burden was observed at 60 days post-infection, with worm egg output decreasing as worm burden increased. Regression analysis of the logarithmically transformed data gave regression coefficients of r = 0·62 for log 10 epg/worm v. worm burden and r = 0·71 for log 10 eggs/worm/24 hours v. worm burden. When the same analyses were performed substituting worm density (worms/gram liver tissue) for worm burden, little or no difference was seen in the correlations obtained. A similar relationship was also observed between worm length and worm burden with mean worm length appearing to decrease with increase in worm burden. These results clearly show that worm crowding in the hamster host can have a profound effect on worm egg output and growth. The possible mechanisms underlying these phenomena are discussed and their potential relevance in human opisthorchiasis considered.