Studies on the epidemiology of ovine dermatophilosis

John Reginald Edwards

Ovine Dermatophilosis is a bacterial dermatitis affecting the skin and fleece of sheep and is caused by the organism Dermatophilus congolensis. The aim of this thesis was to study aspects of the epidemiology of the disease in the south coast region of Western Australia. A series of epidemiological studies and field experiments were conducted on 50 farms and two research stations between 1983 and 1989. Dermatophilosis was present in the fleece in most flocks of lambs and hoggets. The prevalence in lamb flocks in three years was 5.1%, 6.9% and 1.6% and in hogget flocks was 8.6%, 10.4% and 9.4%. Dermatophilosis was shown to be an important source of financial loss. Total regional losses in 1983/84, 1984/85 and 1985/86 were $1.7m, $1.9m and $1.7m respectively. The main losses were due to the association with blowfly strike and in particular, the cost of jetting to prevent body strike. Other losses were due to reduced value of wool, reduced body weight, deaths due to Dermatophilosis, losses from culling, cost of treatment and tanning losses. A technique for measuring the severity of ovine Dermatophilosis in natural infections was developed and evaluated. The severity was estimated by measuring the area and length of scabs of Dermatophilosis and calculating the total volume of scab in the fleece. The distance between the tip of the fleece and the top of the scab was used to calculate the approximate date when infection occurred. This date has application in the investigation of clinical cases where it can be used retrospectively to identify the events which led to transmission. Dermatophilosis was produced experimentally by a method that simulated natural infection and was predictable and suitable for use in field experiments. The risk of Dermatophilosis was directly related to the average annual rainfall and this was the only farm risk factor association present. The colour and condition of the fleece were the only associations with individual sheep risk factors. The relationship between Dermatophilosis and farm of origin, mean fibre diameter, fibre diameter variation and yield was studied in three experiments. In contrast to other studies, increased fibre diameter was associated with a decreased risk of infection with Dermatophilosis. Wetting, close contact and the presence of infected sheep were essential for the transmission of D. congolensis. Dermatophilosis may be prevented by avoiding these conditions. Addition of zinc sulphate to dipping fluid reduced transmission of Dermatophilosis at concentrations between 0.125% and 0.5% and prevented transmission at 1.0%.

Studies on the epidemiology of ovine dermatophilosis

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