Severe skin lesions in a six-month-old heifer were investigated by a local veterinarian in the lower southeast (SE) of South Australia (SA). Since lumpy skin disease (an exotic disease) was a possible differential diagnosis, samples were sent to Australian Animal Health Laboratory and histopathology performed in SA as well. Treatment with anti-inflammatories and an antibiotic appeared to provide some relief for the animal, but the owner elected to euthanase it anyway.
A weaned six-month-old Angus heifer developed a severely painful and smelly skin disease with thickened lumps of brown keratinized crusts or scabs. These skin lesions were distributed primarily over the dorsal and lateral surfaces of the head, flank, tail and limbs. The animal had a slightly raised temperature and was “quite cranky in the crush”. Where the scabs fell off, the underlying skin was moist with an ulcerated appearance. This animal was the only one affected in a mob of about 300, and lesions were reported to have developed within the previous 10 days.
The local veterinarian had not encountered this clinical presentation before and requested assistance from the Primary Industries and Regions SA (PIRSA) Biosecurity as part of the investigation. Differential diagnoses included dermatophilosis, dermatophytosis, parasitism (ticks, fleas, Besnoitia), and viral dermatitis caused by parapoxvirus (contagious pustular dermatitis) and capripoxvirus (lumpy skin disease, caused by a pathogen exotic to Australia).
The animal was treated with topical iodine and flystrike powder, injections of antibiotics and anti-inflammatories, and made some improvement. However, the producer decided to euthanase the animal. Blood and skin samples were collected for histopathology, as well as other diagnostic tests including culture and PCR to detect parapoxvirus, capripoxvirus, and pestivirus.
Histopathology of the skin is compatible with chronic and severe dermatosis, with epidermal hyperplasia and hyperkeratosis, intralesional bacteria and ectoparasites (lice). Branching filaments of Gram-positive coccoid zoospores suggestive of Dermatophilus are arranged in parallel rows ("train track" appearance) within the hyperkeratotic epidermis and hair follicle external root sheath. Sections with overlying lice are associated with erosive hyperplastic and hyperkeratotic eosinophilic dermatitis.
A diagnosis of severe hyperplastic and hyperkeratotic, eosinophilic and neutrophilic dermatitis caused by lice and Dermatophilus congolensis with secondary Staphylococcus aureus was made. Both bacterial organisms were cultured. In addition, the samples were negative for parapoxvirus and capripoxvirus on PCR, and negative for pestivirus on PCR.
Dermatophilus congolensis are filamentous, branching, and facultative anerobic bacteria that commonly cause skin infections in livestock inhabiting areas of high humidity and rainfall. Cutaneous dermatophilosis has an average incubation period of two weeks, but it can range from a day to one month. Acute lesions are painful but are rarely pruritic. Dermatophilus are usually secondary invaders of mechanically damaged skin (e.g. from lice induced excoriations in this case) with severity of infection exacerbated by prolonged moisture and concurrent diseases or stresses compromising the host's immune system. The affected animal in this case tested negative for pestivirus, thereby excluding pestivirus as playing a role in immunosuppression.
Warm, wet environments are required to activate dormant zoospores into germinating motile filaments that penetrate the epidermis and hair follicles. Dermatophilus invasion of newly-formed keratin results in oedematous and exudative neutrophilic epidermitis with regeneration. Over time, skin infection leads to formation of a thick scab composed of alternating layers of neutrophilic exudate, hyperkeratosis, and epidermal regeneration. Zoospores remain viable in dried skin exudate on animals for years.
The skin damage from dermatophilosis in this heifer predisposed the skin to secondary Staphylococcus infection. However, the underlying lice infestation would have caused skin irritation, scratching and rubbing, further damaging the skin and promoting extensive Dermatophilus invasion and infection. Lice are generally host specific and sensitive to heat, sunlight and humidity; heavy burdens can develop in stressed and/or diseased animals.
This case represented an unusually severe dermatitis of mixed origin. The warm and wet climatic conditions in the lower SE of SA area had been ideal for the development of skin infections over the previous few months of spring. It is helpful to investigate and rule out potentially serious exotic diseases, and private veterinarians can access primary industry funding from PIRSA Biosecurity to do so.