The release and survival of Dermatophilus dermatonomus zoospores

Abstract

Observations were made on the release of Dermatophilus dermatonomus zoospores from mycotic dermatitis scabs, and on the duration of their survival in the scabs. Soil samples from properties with affected sheep were tested for their ability to transmit mycotic dermatitis and for their capacity to permit the survival of zoospores and the growth of D. dermatonomus. The zoospores can only emerge from the scab when it is wetted and before they have lost through age their capacity for motility. After an initial delay, the zoospores are released rapidly during several hours. The infection must remain active or the scabs must remain dry to enable the zoospores to persist in lesions. These observations support the view that a high transmission rate would be more likely when there were already some actively infected sheep in the flock. The dependence of the release of zoospores on wetting, and the effects of heat on survival and on bacterial competition, suggest that transmission would be favoured by cool wet weather. None of the soils collected was infective for the skin of susceptible sheep, and none of those tested was able when unsterilized to support growth, survival, or continued infectivity in the wet state. Of eight soils contaminated with scab material and kept dry, however, six were still infective after 4 months (the longest period tested). It is concluded that although contaminated soil may cause some infections, the survival of the species D. dermatonomus and the initiation of mycotic dermatitis outbreaks are more likely to depend on the existence of active infections.