Blowfly strike and the skin-fleece microenvironment of sheep

Abstract

Blowfly strike or cutaneous myiasis is currently one of the major problems confronting the sheep industry in Australia. This thesis was undertaken to study blowfly strike, especially that occurring on the body of the sheep. Emphasis was placed on examining the interaction between sheep and blowflies and the role of the skin and fleece microenvironment in predisposing sheep to body strike. Long-term field studies were conducted on four Department of Agriculture Research Stations over three years with the aim of investigating the sheep-blowfly problem under field conditions. The results, based primarily on the data collected from Mount Barker Research Station (MtBRS), demonstrated that the field incidence of body strike was sporadic and that it occurred less frequently than breech strike. An analysis of the data failed to demonstrate a significant relationship between the occurrence of body strike and breech strike. Dermatophilosis was also found to be the most important of the skin and fleece conditions predisposing sheep to body strike. Samples collected from struck sheep confirmed that Ludlia cupvina (Wiedemann), the Australian sheep blowfly, was the most important of the primary blowfly species involved in fly-strike on sheep. CalViphova albifvontalis Malloch, the Western Australian brown blowfly, was also found to be an important primary species striking sheep at MtBRS. An investigation of the sheep-blowfly interaction in the field demonstrated that the majority of blowfly strike was recorded during the autumn and spring months each year. From the relationship between the relative abundance of primary blowflies and species involved in strike it was demonstrated that the presence of L. cupvina in the blowfly traps was probably sufficient indication that blowfly strike could occur. L. cuprina was shown to be present and presumably capable of striking throughout most months of the year except when the mean daily maximum temperature in any month (Tm.max) fell below a critical level (range 15 to 17°C). However C. albifvontalis, which was detected irrespective of Tm.max , throughout the entire year was only active in strike when present in relatively high numbers. An examination of the climatic factors associated with outbreaks of body strike demonstrated that the number of body strikes was dependent on both Tmax and the occurrence of rainfall in the week preceding strike. Of the two climatic factors rainfall was the most important. As a measurement of the lifetime susceptibility of an animal to body strike, attempts were made to derive a value for the repeatability of this condition. Unfortunately the low incidence of body strike prevented the derivation of a realistic estimate. As an alternative, the repeatability of dermatophilosis and fleece rot abnormalities shown to predispose animals to body strike was investigated. Even with this approach it was possible to derive a meaningful value only for the repeatability of fleece rot (0.26). To obviate the problems encountered in studying blowfly strike in the field, investigations were undertaken to simulate field fly wave conditions in the laboratory. Techniques were developed for the exposure of sheep artificially infected with Dermatophilus congolensis, to laboratory-reared L. ouprina blowflies in a controlled environment. The results from the first laboratory experiment demonstrated that gravid L. ouprina clearly preferred wet areas to dry areas of fleece on sheep. The inability of the blowflies to differentiate between the adjoining wet dermatophilosis-affected areas and the control areas of fleece, highlighted the need to specially separate test areas on experimental sheep. In a subsequent laboratory experiment wet dermatophilosis lesions were shown to be more attractive to gravid L. cuprina than wet control areas. The wet dermatophilosis lesions were also found to provide sufficient protein for the development of L. cuprina larvae to the second instar stage. As a consequence of the laboratory studies, a field study was conducted to relate these findings back to the field situation. The subsequent field study confirmed that dermatophilosis was the most important skin and fleece condition predisposing sheep to body strike. The bacteriological examination of sites adjoining the struck areas demonstrated that it was not always possible to isolate the bacteria associated with each skin and fleece condition visually recorded at each site. The approach adopted in this thesis of combining field observations with laboratory studies has led to the refinement of a hypothesis describing the interrelation of sheep, blowflies and climate as well as the role of bacteria and protein in both oviposition and the development of strike. It has also resulted in the identification of dermatophilosis as an important condition predisposing sheep to body strike in the field and allowed the subsequent study of this association under controlled conditions