Relationship between egg output and parasitic burden in lambs experimentally infected with different doses of Dicrocoelium dendriticum (Digenea).

The relationship between egg elimination and parasitic burden was studied in two groups of 12 lambs experimentally infected with 1000 and 3000 Dicrocoelium dendriticum metacercariae, respectively. Half the animals in each group were slaughtered 2 months post-infection (p.i.) and the other half 6 months p.i. In order to detect and follow elimination of D. dendriticum eggs by the lambs, faeces samples collection started one and a half months p.i. and continued fortnightly until the end of the experiment. Egg elimination was first detected between days 49 and 79 p.i. (mean = 59 +/- 1.6 SE). Mean eggs per gram (epg) was higher in the lambs infected with 3000 metacercariae (347.2 +/- 42.4 epg) than in those infected with 1000 (194.8 +/- 14.4), although no significant differences were detected between both groups using the Student 't' test. Egg elimination was higher in the faeces samples taken in the afternoon (mean = 357.8 +/- 47.6 epg) than in those from the morning (mean = 215.7 +/- 21.3). The percentage of metacercariae which became established as worms was higher in the animals dosed with 1000 metacercariae (21.6%) than in those infected with 3000 (16.3%). The number of worms recovered on necropsy of each animal varied between 30 and 2063 (mean = 346.6 +/- 80.5) and their length between 2.6 and 7.1 mm (mean = 5.2 +/- 0.1). The mean number of parasites for lambs infected with 3000 metacercariae (489.3 +/- 163.1) was higher than that obtained from those dosed with 1000 (215.7 +/- 41.4), although more worms were collected in some cases from the lambs infected with the latter dose than the former. In general there was an increase in the number of epg eliminated as days p.i. and parasitic burden increased. A positive relationship was observed via the correlation coefficient between the number of epg eliminated by each of the lambs throughout the experiment and that of worms recovered. This relationship was more intense on considering only the number of epg eliminated between days 120 and 180 p.i.