Bile enhances the infectivity of third stage larvae of Dictyocaulus filaria.

TITOY, G.A.P. & SCHWAN, E.V. 1994. Bile enhances the infectivity of third stage larvae of Dictyocaulus filaria. Onderstepoort Journal of Veterinary Research, 61:395-396 Incubating metacyclic larvae of Dictyocaulus filaria in ovine bile solution prior to per os infection significantly enhanced their infectivity to sheep. INTRODUCTION Although there are several reports on the effect of bile on the motility of third stage larvae of Dictyocaulus viviparus (Jmgensen 1973, 1975, 1981), no information seems to be available concerning its effect on the subsequent infectivity of Dictyocaulus larvae. The faeces of a sheep with patent experimental infection of D. filaria were collected and first stage larvae recovered by the Baermann technique (Boch & Supperer 1983). These larvae were cultured to infective third stage larvae at 21 oc for 7 d for use in the experiment. Bile was collected from freshly slaughtered sheep and stored at -20 oc after its dry matter content (9, 1 %) had been determined (Smyth & Haslewood 1963). Eight 5-month-old Dorper sheep, which had been raised under conditions that precluded unintentional nematode infectionand which were kept thus for Accepted for publication 20 September 1994-Editor the duration of the experiment-were allocated to two groups by use of random tables, and infected (day 0). Animals in group 1 (n = 4) were each infected per os with 1 500 untreated third stage larvae of D. filaria, whereas sheep in group 2 (n = 4) each received 1 500 larvae, pre-incubated for 60 min at a concentration of 1 000 larvae/mQ in 4% (w/v) bile solution at room temperature (Jmgensen 1973) . Sheep 8 of group 2, which died on day 21 after infection as a consequence of severe parasitic bronchopneumonia, yielded 140 D. filaria adults of mixed sex at necropsy. The larval infectivity rate in this sheep was 9,3 %. After the expected prepatent period of 28 d, faecal samples from the other sheep were examined daily. The three remaining sheep of group 2 became positive on day 29, whereas only one sheep of group 1 developed a patent infection. From day 41 onwards, one sheep after another stopped passing first stage larvae of D. filaria, until day 49 when the last animal of group 2 became negative. The difference between group 1 and group 2 was highly significant (0,01 > P > 0,001).