226 (23)

Microscopic examination of cerumen, collected with a curette or swab, can be used to look for ectoparasites such as Otodectes cynotis (Fig. 22 : 19), Demodex cati (Fig. 22 : 20), Demodex gatoi, Notoedres cati, Trombicula autumnalis larvae, and larvae of the tick Otobius megnini '^,3,7,8,15'¹⁸. It must be carried out very carefully, even if there is little cerumen present, because most cases of otitis extema in the cat are caused by parasites¹⁸. Microscopic examination in chloral lactophenol or liquid paraffin will usually reveal the mite, Otodectes cynotis, in various stages of its life cycle (adult, nymph, larva, egg). In some cases in which a hypersensitivity reaction is probably involved, demonstrating mites is not easy as it may only take a few parasites to initiate clinical signs^l5,17. It is also possible that when inflammation becomes too severe, the parasites leave the ear canal or get destroyed⁸.

Cytological examination of cerumen is the next step^l'^{3,5,7,8,11,19}. Cerumen should be collected on a swab and placed carefully on a microscope slide. The slide is dried and stained with a rapid stain, ready for examination at a magnification of between 400 and 1000.

Non-parasitic, erythemato-ceruminous otitis: cytology reveals an abnormally elevated number of yeasts of the genus Malassezia, and bacteria. Malassezia is a yeast found in smali numbers in the ear canal of normal cats, especially in the presence of cerumen²⁰. When present in large numbers, it is pathological. Malassezia pachydermatis is easily recognised by its characteristic peanut shape (Fig. 22 : 21). Bacteria seen are usually cocci.

Suppurative otitis: cytological examination of pus reveals round cocci of variable size and appearance, and / or elongated rods. These indicate bacterial colonisation, a common finding in the ear canal ^l9. On the other hand, the presence of degenerating neutrophils, with phagocytosis of bacteria, indicates that the body is responding to infection ¹⁹ (Fig. 22 : 22). Morę rarely, Candida albicans may also be isolated from the ear canal of a cat. Its presence there is always pathogenic. If atypical yeasts are seen on cytology, fungal culture should be performed to identify the organism ^l9.

Bacteriology of exudate should be performed in all cases of chronic or recurrent suppurative otitis and in cases where cytology reveals the presence of rods. A sterile swab must be delicately inserted down to the horizontal ear canal. To avoid contamination, the swab should be inserted through the sterile cone of an auroscope attachment². Bacteria most freąuently isolated in feline otitis extema are Staphylococcus intermedius and Streptococcus spp. (gram + cocci) with Pseudomonas spp. and Proteus spp. (gram - rods) being found less commonly ⁸.

A second auroscopic examination should be carried out at this stage of the diagnostic process. If the ear canal is cluttered with inflammatory debris, cerumen and / or pus, it should be cleaned out, at the surgery, to allow further auroscopic examination ³. Cleaning should involve cleansing, non-irritant substances, the selection of which depends on the type of secretion and lesions present. The ingredients should be diluted in tepid water and flushed into the ear canal, using an adapted enema bag. Syringes attached to flushing and suction devices can also be used *\ Cleaning should be continued until the ear canal is relatively elear and free of debris. Once the ear canal has been dried (e.g. with absorbent paper), the second auroscopic examination can be carried out. This allows evaluation of the ear canal wali, further inspection for foreign bodies or tumours, and assessment of the patency and transparency of the tympanic membranę. A normal tympanic membranę is madę up of two parts¹³: the pars flacida is dorsal, white, quite thick, and well-vascularised; the pars tensa is ventral, bigger, semi-transparent and shiny ¹³.

Radiography and CT scanning of the tympanic bullae are useful in chronic otitis externa and for invasive tumours of the ear canal (Figs 22 : 23,24). Recent studies in the dog suggest that otitis media should be looked for routinely in cases of chronic otitis, even if no rupture of the tympanic membranę can be seen ²¹. The presence of pus in the tympanic bulla can be the source of chronic infections because topical antibiotics do not penetrate deeply enough into the ear canal ^1,2,21. The tympanic membranę could heal rapidly and mask the development of this deep infection ²¹. Radiography can also be performed after injection of radio-opaque, liquid contrast media into the ear canal²².

Paracentesis followed by cytology and bacteriology will probably be a diagnostic test of the futurę²³.

Biopsies of nodules and uleers may be necessary. Nodules may indicate tumours (e.g. squamous celi carcinoma, ceruminous gland tumour, mast celi tumour) or opportunistic infections (e.g. mycobacterial infections, cryptococcosis) in immunodeficient cats. Ulcerated lesions may be the first signs of an auto-immune dermatosis (e.g. pemphigus foliaceus)^IA7.