Properties of Dermatophilus dermatonomus zoospores in relation to the transmission of mycotic dermatitis

Abstract

Zoospores from cultures of Dermatophilus dermatonomus were examined with respect to properties considered relevant to their role as the infective agent of ovine mycotic dermatitis. An account is given of nutritional and gaseous requirements for motility, and of several factors which inhibit motility, including salts at relatively low concentrations, pH values outside the range from 6 to 8, osmotic tensions above 0.4 osmole, and the presence of other aerobic organisms. Froin these and other observations it is concluded that the zoospore is ill equipped for prolonged survival in the external environment. The viability of the zoospores is greatest in the first hours after their release from the mycelium. This quality is associated with a highly impermeable cell membrane, a complete independence of external nutrient sources, and a tendency to germinate rapidly even under conditions relatively unfavourable for growth. The zoospores exhibit a negative aerotaxis which seems likely to facilitate their attainment of the skin surface prior to invasion. It is suggested that the probability of a susceptible skin area becoming infected would be high when the sheep in an infected flock were wetted by rain, and even greater if some vector or mechanism were available to expedite the transfer of zoospores from sheep to sheep. The incidence of infection would then depend on the number and extent of the susceptible skin areas on individual sheep.