A lumpy skin disease (LSD) investigation was carried out on a three-year-old Angus cow at Coonabarabran in June that was displaying extensive skin lesions.
The cow was part of a mob of 30, grazing native pasture mixed with digit grass (Digitaria eriantha subsp. eriantha) and had no access to known photosensitising plants. She had displayed the skin lesions for several months but had never been unwell and no other cattle in the mob were affected. The cow was in calf and in her third trimester of gestation at the time of examination.
Skin lesions were located on the face, neck, rump, caudal hind limbs and tail head. Lesions ranged from hair loss and scurf, to both irregular and circular ulcerations of varying sizes. There were smaller raised circular nodules, 1cm in diameter, present on the skin of the caudo-dorsal udder (Figures 1, 2, and 3). No lice were seen grossly and no other significant abnormalities on physical examination were noted. The cow had a body condition score 4/5.
Differential diagnoses included photosensitisation, mycotic dermatitis, mange (chorioptic, demodectic, sarcoptic), hypersensitivity, lice (such as Bovicola bovis and Linognathus vituli), the exotic disease LSD, dermatophilosis and bacterial dermatitis.
Skin biopsies (fixed and fresh) were sampled from affected areas using lignocaine and a biopsy punch. Blood (EDTA and plain clotted) was also collected. A skin scraping was performed and was negative for mites, however the examination of the slide was delayed due no access to a microscope on site.
Tests conducted by the Australian Centre for Disease Preparedness were negative for LSD (Capripox virus) on Taqman assay of blood and skin biopsies and ELISA performed on the serum.
Histopathology of the skin biopsy performed at Elizabeth Macarthur Agricultural Institute diagnosed an eosinophilic dermatitis. There was no histological evidence of ectoparasites.
There was also a sparse but predominant growth of Staphylococcus hyicus on culture of the skin. Fungal culture was negative and no fungal elements were noted on histopathology.
Eosinophils accumulate in large numbers and are the main leukocyte present in inflammatory reactions against parasites and other allergic reactions.1 Therefore, the cause of the eosinophilic dermatitis in this case may be related to ectoparasitism or a hypersensitivity (possibly to insect bites).
It would be expected that an eosinophilic dermatitis would be pruritic although no scratching or rubbing behaviour had been noticed by the owners.
The histological absence of ectoparasites does not necessarily exclude them as a possible cause, however the history and clinical skin lesions are not typical of those described for lice or mite infections in cattle in Australia.
Lice populations are higher in winter than summer. Cattle louse infestations are often asymptomatic. Heavy louse infestations cause irritation and rubbing, which may cause loss of hair.
Chorioptic mange predominantly affects housed dairy cows and is most common in winter. The mite displays a preference for the base of the tail, escutcheon and legs. Asymptomatic infections are far more common than obvious dermatitis. Heavy infestations may be associated with skin irritation and rubbing, which can lead to exudation and scab formation.2
Demodex mites live in hair follicles. The hair falls out and the follicles enlarge to form nodules. Lesions occur mainly on the dorsal half of the body. Nodules can be palpated over the neck, withers, brisket, shoulder and flank regions. The condition is usually mild and there is no pruritis. Mites are numerous within hair follicles and easily seen in skin scrapes or on histopathology.2
Sarcoptes mites prefer thin-haired areas of the skin such as the neck, the lumbar area adjacent to the tail, the inner surface of the thighs, the escutcheon and udder. It causes severe pruritis and large crusty lesions with hair loss and skin may become thickened and wrinkled.2
Psoroptes ovis is primarily a parasite of sheep (the cause of sheep scab), though it can become adapted to cattle. It does not occur in Australasia. Psoroptic mange is intensely pruritic. Lesions are common around the poll, withers or base of the tail and are characterised by dry crusts, scales, skin thickening and alopecia. Constant pruritis is associated with loss of condition due to reduced feed intake.2
Staphylococcus hyicus is a normal inhabitant of the skin of cattle, though it has been reported to be associated with mange lesions.3
Eosinophilic dermatitis has also been associated in cattle grazing vetch (Vicia spp.)4, however the cow in this case had no access to vetch in the pasture and there was no hay supplemented.
A macrocyclic lactone was used to treat possible ectoparasite infestations but the owners noted no response within the month following treatment. The cow's subsequent calf born was also noted by the owner to have similar skin lesions on the ear pinnae and in the perianal region (Figure 4).
The owner reported topically spraying the skin lesions on the cow and calf with cetrimide/diethyl toluamide (DEET) (an antiseptic and insect repellent) and then the lesions resolving a short time later. This coincided with a seasonal change to dry hot weather.
The cause of the eosinophilic dermatitis was not determined in this case. A hypersensitivity potentially due to a biting insect or plant is suspected due to the lack of response to ectoparasite treatment and self-resolution of clinical signs with a change in seasonal conditions. The cetrimide treatment may have given a temporary relief from biting insects to allow a hypersensitivity to resolve.