Skin lesions were noticed on 4 of 63 calves at weaning of a group of Shorthorn calves near Burren Junction NSW in March 2011. This followed a particularly wet summer and abundant mixed natural pasture growth in the region.
The 6 to 8 month old calves were well grown and in general appeared in good health. On two of the affected calves, lesions were restricted to the ventral abdomen and legs. On the other two, lesions were more generalised and severe.
The skin lesions were markedly hyperkeratotic. The lesions on the ventral abdomen were broad and flat, with hyperkeratosis between 5mm and 1cm thick. The lesions on the body and legs were pyramid shaped with a base of up to 2cm diameter and the hyperkeratosis extending away from the skin surface in a horn-like manner. If the lesions were plucked from the animal, the hair pulled out with the horn-like growth and a dry granulation bed was exposed beneath. The affected calves were not pruritic.
The two weaners with generalised lesions were smaller and appeared ill-thrifty when compared with their cohorts. Dermatophilosis (Dermatophilus congolensis infection) was suspected, although other bacterial or fungal pathogens were considered possible.
A skin scraping was taken and submitted with samples of the skin lesions (scabs) to the State Veterinary Diagnostic Laboratory. The most severely affected heifer was also sampled to determine if she was immunocompromised due to being persistently infected with bovine pestivirus.
Gram stain examination of the skin scraping was negative for Dermatophilus congolensis, however a scraping from the scab was positive. A profuse predominant growth of Dermatophilus congolensis was cultured from a scab. The heifer tested negative for pestivirus antigen.
Radostits et al describe dermatophilosis as a sporadic disease of cattle in temperate climates. While the bacteria may persist in healthy carrier animals, it is not highly invasive and rarely breaches the natural barriers of healthy skin. Intercurrent disease or stress predisposes an animal to clinical infection.
Widespread above average rainfall occurred throughout north western NSW over the 2010 - 2011 summer period, including the property where this case occurred. Dermatophilosis in sheep was diagnosed by District Veterinarians on a number of properties in the North West Livestock Health and Pest Authority during January and February 2011. However, dermatophilosis in cattle is a rare occurrence in the region.
It is likely that prolonged pasture wetting contributed to this case of dermatophilosis. Over much of summer, native grasses in the area were dense and to a height of approximately 60cm. Consistent rainfall resulted in the pasture being wet for prolonged periods. The legs and ventrum of young calves would have been persistently exposed to wetting and the skin may have become macerated, allowing Dermatophilus bacteria to invade. It is possible that the two calves with generalised lesions were immunocompromised, although bovine pestivirus was ruled out as a precursor in the worst affected heifer.
In this case, the skin scraping was Gram stain negative for Dermatophilus congolensis. However a Gram stain performed by the laboratory on a scab was positive. When typical scabs are present, samples of the scabs (see Image 3) should be submitted whole to the laboratory for Gram stain examination to confirm the diagnosis. Culture is then rarely required.