Exercise 10 discussion

Discussion of Diagnostic Exercise 10

Cathy Shilton DBIRD Veterinary laboratories, Berrimah NT

Disease due to Demodex bovis infection in cattle in Australia is usually limited to low numbers of small dermal or subcutaneous nodules, important only with respect to damage to hides used for leather. The nodules are formed when mites invade a follicle, the hair falls out and the epidermis subsequently closes over the follicle, which becomes cystic as it distends with proliferating mites (Murray 1976). The cyst may ultimately rupture, setting up a local inflammatory response. Secondary bacterial infection of affected follicles may also occur. It is thought that in most cases infection is acquired at an early age from the dam, with the disease taking months to manifest. It is apparently most common in adult housed cattle in the late winter to summer months, differing from the canine counterpart in which clinical disease is most common in young immunocompromised individuals (Radostits et al 2000). Lesions occur predominantly on the neck and anterior thoracic regions, with most animals developing less than 100 nodules that generally disappear after several weeks. It has been hypothesised that the distribution of lesions is related to increased sweat gland activity in the predisposed regions, with the mixture of sweat and sebum favouring mite motility (Matthes 1994).

Severe generalised demodicosis, as in the present case, is considered rare in cattle. In Africa, severity of infection was thought to be related to poor nutritional state and increased environmental temperature (Yager and Scott 1993). In a severe case reported from Kenya, the affected animal was a Friesian heifer that had been treated with glucocorticoids, had severe tick and flea infestations and was found at necropsy to have several concurrent diseases (Mbuthia et al 1994). At necropsy the steer in this case was found to have mild unilateral pyelonephritis from which Arcanobacterium pyogenes was cultured; no uroliths were noted at field necropsy. There was also generalised peripheral lymphadenopathy due to marked lymphoid hyperplasia, with large numbers of neutrophils within sinuses and a few cortical pyogranulomas with central bacterial club colonies. Some of the dermal follicular cysts were markedly infiltrated with neutrophils and contained bacterial colonies, therefore it was assumed that the suppurative lymphadenitis was a result of secondary bacterial dermatitis. Conversely, the systemic bacterial infection raises the possibility of partial immunodeficiency. The animal was described by the field vet who performed the necropsy as being in relatively poor condition, but as can be seen in the picture of the live animal, body condition wasn’t dreadful. The disease occurred over the dry season when excessive temperature and poor pasture quality could be issues that resulted in stress, but there is no obvious reason why this steer would be more affected by these husbandry factors than the rest of the herd.

Yager JA, Scott DW. The skin and appendages. In: Pathology of Domestic Animals, 4th edn. Jubb, KVF, Kennedy, PC, Palmer N eds. Academic Press Inc., San Diego, 1993:531-738.

Radostits OM, Gay CC, Blood, DC, Hinchcliff KW. Diseases caused by arthropod parasites. In: Veterinary Medicine: A Textbook of the Diseases of Cattle, Sheep, Pigs, Goats and Horses. 9th edn. Harcourt Publishers Ltd., London, 2000:1387-1415.

Matthes HF. Investigations of pathogenesis of cattle demodicosis: sites of predilection, habitat and dynamics of demodectic nodules. Vet Parasitol 1994;53:283-291.

Mbuthia PG, Kariuki DI, Mulei CM.. Generalised demodicosis in a Friesian heifer from a zero-grazing unit.Vet Parasitol 1994;51:337-343.

Murray MD. Demodectic mange of cattle (letter). Aust Vet J 1976;52:49.