INTRODUCTION

Ringworm is a relatively common disease of guinea-pigs seen in practice, but is often overlooked, or simply dismissed as ‘mites’. In clinical cases, Trichophyton mentagrophytes is the most common isolate. Microsporum canis and other Microsporum species have occasionally been reported, and have been used to induce infections experimentally.

CASE HISTORY

As mentioned previously, thorough and comprehensive history taking is vital, since many diseases in small mammals are related to poor general husbandry. It is not wise to assume the owner knows the specific requirements of their pet, even if they have kept them for many years.

The guinea-pig was purchased at 9 weeks of age from a pet shop. He was kept with another male guinea-pig from the same litter that was also purchased at the pet shop. There were also dogs and cats in the house, but they had no direct contact with the guinea-pigs.

The guinea-pigs were housed in large hutches in the owner’s garage at night, and allowed access to a large run in a private area of the garden during warm days. Their diet comprised a muesli-type guinea-pig mix, fed ad lib, along with constant access to hay. Each evening they were also given a selection of raw vegetable trimmings from the owner’s meal preparations. No preventative healthcare was carried out and there was no previous medical history. There were no other general health problems; appetite, urination and defecation were all normal.

Several weeks previously, the owner noticed a thinning of the hair on the face, specifically around the left eye. No veterinary advice was sought, but the lesions gradually became worse. By the time of presentation, the lesions had progressed and extended along the back. The owner thought the guinea-pig seemed slightly itchy at this point. The other guinea-pig seemed fine.

CLINICAL EXAMINATION

A careful physical examination should always be carried out to evaluate for any other diseases or additional problems. This guinea-pig was bright, alert and vocal on presentation, and general physical examination was unremarkable, other than the dermatological abnormalities.

Dermatological examination showed marked crusting and scaling around the left eye (Fig. 10.1), with evidence of inflammation of the skin below the left ear. There was also extension of patchy alopecia and scaling onto the dorsum (Fig. 10.2).

Figure 10.1 Extensive alopecia, crusting and scaling around the left eye. Notice also the inflammation of the skin below the pinna.

Figure 10.2 Patchy alopecia and scaling of the dorsal body.

The clinical signs associated with dermatophytosis in guinea-pigs vary. Lesions often begin with circumscribed or coalescing oval to patchy areas of non-pruritic scaling and alopecia, usually affecting the nose (Fig. 10.3), ears and face. However, in advanced cases lesions may spread to involve the neck, limbs and body. If untreated, then the lesions can become inflamed and infected with bacteria. This secondary infection will manifest as pustules, papules and crusts, usually with an increase in the pruritus, although the level of irritation does differ between individuals. Dermatophytosis can occur concurrently with Trixacaris caviae infestation; in these cases, the pruritus tends to be severe.

Figure 10.3 Small area of scaling and alopecia on the nose of another guinea-pig. Lesions like this may be the only initial signs of dermatophytosis, although no diagnosis was made in this particular patient.

CASE WORK-UP

Because of the history and the nature of the lesions, dermatophytosis was suspected, although ectoparasitic infestations needed to be ruled out.

Skin/hair scrapes: Multiple skin scrapes were taken from around the left eye and the dorsum. These were mounted in liquid paraffin on a microscope slide with a coverslip and examined under ×40 magnification. No ectoparasites were seen. This absence does not entirely rule out ectoparasites and, in general, it may be useful to repeat the tape strips, or even consider prophylactic antiparasite treatment.

Further investigations aimed towards diagnosing dermatophytosis:

Wood’s lamp examination: This was performed and was negative. Wood’s lamp is an ultraviolet light that should be allowed to warm up for 5–10 minutes prior to use because the stability of the light’s wavelength and intensity is temperature dependent. When exposed to ultraviolet light, hairs invaded by M. canis will show apple-green fluorescence in approximately 50% of isolates. The fluorescence is due to tryptophan metabolites produced by the fungus and hairs should be exposed for 3–5 minutes. Fluorescence is not present in scales or crusts, and illumination of debris should not be mistaken for fluorescent hairs. Trichophyton spp. will not fluoresce under ultraviolet light, therefore Wood’s lamp examination is not often helpful in guinea-pigs and a negative result does not exclude dermatophytosis.

Hair plucks: Examination of hairs from the affected areas may demonstrate infected hair shafts. This is a difficult technique, requiring experience and a high-quality microscope. The hairs should be mounted in liquid paraffin or potassium hydroxide, covered with a coverslip and examined under ×400 magnification. Fungal hyphae may be seen, or arthrospores may be apparent within infected hair shafts. No fungal elements were seen in this case.

Skin/hair scrapes: Scrapes taken from the periphery of lesions can be examined as above for infected hair shafts. In rare cases of Trichophyton spp. infection, hyphae may only be present in the stratum corneum.

Fungal culture: This test enabled definitive diagnosis and identification of the dermatophyte species. Hair pluck and skin scrapes can be used from the lesions. The differences and limitations of in-house versus external testing are described below. In this case, samples were taken for external laboratory fungal culture, and 2 weeks later the results were reported as positive for T. mentagrophytes.

In-house testing: In-house dermatophyte test medium (DTM) kits are available and often used. These comprise a small culture plate containing Sabouraud’s dextrose agar with cyclohexamide, gentamicin and chlortetracycline as antifungal and antibacterial agents. They also contain a phenol red colour indicator. Growing dermatophytes initially use proteins, producing alkaline metabolites that turn the indicator from yellow to red. Often, contaminant fungi use carbohydrates first, producing acidic metabolites which leave the indicator yellow. However, when the carbohydrates are used up, protein metabolism then causes the red colour change. Black or grey colonies are not clinically significant, whatever the colour change.

Hairs (and scale) from the lesion should be collected and inoculated onto the agar, with the lid replaced loosely to allow entry of air. Some dermatophytes take longer than others to produce the red colour change (10–14 days), so the test kit needs to be checked daily for 10 days. In addition, some non-pathogenic dermatophytes will cause the red colour change (e.g. Aspergillus spp.).

Since the presence of fungal growth and colour change is not entirely reliable, microscopic examination is essential to prevent an incorrect presumptive diagnosis.

Once grown, the plate should be sent to an external laboratory for identification. This allows the clinician to be sure what species is causing the problem, and may suggest the source of infection. For example, if M. canis was identified you might consider sampling the house cat.

External laboratory testing: Commercial laboratories use similar culture media but without the colour indicator. Some of the additives to the Sabouraud dextrose agar may inhibit the growth of certain pathogens; therefore, laboratories use plain agar, which sometimes makes the diagnosis of dermatophytosis easier. In addition, the greater experience of laboratory technicians at interpreting culture results and the possibility of extended culture under controlled conditions if required enables more accurate identification of the dermatophyte. Even with in-house DTM kits, the dermatophyte should still be sent to a lab for identification, so sending to the lab in the first place may well save time and be more accurate.

The relative costs of in-house DTM kits and external laboratory fungal culture tend to be fairly similar.

Sterile toothbrush: Samples may be obtained for microscopy or culture by grooming the hairs on and around the lesions with a sterile or new toothbrush.

Skin biopsy: Diagnosis can be made with skin biopsies submitted for histopathology. Fungal elements may be demonstrated with PAS (periodic acid–Schiff) or methenamine stains. Biopsies were not required for diagnosis in this case.