Spread of infective Dictyocaulus viviparus larvae in pasture and to grazing cattle: experimental evidence of the role of Pilobolus fungi.
Four calves experimentally infected with Dictyocaulus viviparus were made Pilobolus-free by hygienic measures and by feeding them irradiation sterilized feed. Two of the calves were only administered laboratory cultured Pilobous sporangia daily. As a result, the faeces from one pair contained D. viviparus larvae and Pilobolus sores, and the faeces from the other one pair contained D. viviparus larvae, but no Pilobolus spores. Two identical plots were used for deposition of the two kinds of faeces, and one of them remained free of Pilobolus fructification. Herbage sampling and the use of tracer calves revealed that on this plot the larval contamination and the infectivity of the pasture were greatly reduced. A mean larval count of 1321 near the faecal pats (0-5 cm) in the plot where Pilobolus was observed was reduced to 69 per kg of herbage on the Pilobolus-free plot. At a distance of 100 cm from the pats, a reduction from 99 to 3 larvae per kg herbage was found. Each plot was grazed by four parasite-free tracer calves for 3 days. During the subsequent stabling period of these calves, the lungworm larval excretion of those from the Pilobolus-free plot was reduced by 90% and the clinical symptoms were milder than those which grazed the plot which contained the fungus. The mean post mortem worm counts after 4 weeks of stabling showed a reduction from 167 to 25 worms. A more marked effect of Pilobolus fungi on the transmission of D. viviparus infection is to be expected under field conditions where calves are grazing more selectively than in the present study.