Development of Microscopic Lesions and the Role of Causative Agents of Bovine Parasitic Otitis

Abstract

The early and later stages of development of lesions of Bovine Parasitic Otitis (BPO) were studied by microscopic examination of tissue specimens taken from the wall of the external auditory meatus (EAM).The specimens were from nine, five to nine  month  old bull calves that  had an experimentally induced  BPO in  an ongoing  epidemiological  study of  the disease  at Sokoine  University  of Agriculture (SUA). Early histopathological changes were also studied by inoculating Rhabditis bovis (Rh. bovis) into rabbit EAM. Rhabditis bovis is one of the suspected causes of BPO. The early microscopic lesions observed in the present study were spongiosis or hydropic degeneration followed by focal and later diffuse necrosis of upper epidermal layers of the EAM. Also observed were intraepidermal cleft formation and acantholysis. These are changes that would predispose to easy penetration of EAM epidermis by microorganism. Changes in the dermis/subepithelium were those of acute suppurative inflammation followed by healing and reparative changes in both the epithelial and subepithelial tissues. It is suggested that Rh. bovis, normally a free living soil nematode, predisposes the lining of EAM to bacteria infection through its feeding on some of the bacteria that form the normal flora of the EAM. This feeding behavior upsets the normal competitive inhibition in favou r of building up of pathogenic/opportunistic bacteria to infective levels. At the same time the Rh. bovis excretory products and breakdown products of reproduction may play part, along with exotoxins of the pathogenic/ opportunistic bacteria, in causing the early microscopic lesions in the epidermis. However, the major subsequent inflammatory changes in the lining epithelium arid undertying tissues are largely bacterial   in type. Further studies on the role of the causatives agents of BPO needs to be directed in these suggested mechanisms