Ringworm in Dog and Cat: Protect Your Pet From This Fungus

Dermatophytosis or Ringworm is a cutaneous infection with a keratinophilic fungus. Most cases of canine and feline Ringworm are caused by Microsporum canis, Trichophyton mentagrophytes or the geophilic species M. gypseum. In cats, 98% of cases are caused by M. canis. In dogs, the prevalence of infections caused by each of the three common aetiological agents varies geographically. There are numerous other species of Ringworm that may infect domestic dogs and cats from wild animal, soil or human sources.

Simultaneous infection of dogs by more than one dermatophyte species may occur. Of combined infections , those caused by M. gypseum and T. mentagrophytes have been the most common.

Dogs and cats may carry many saprophytic moulds and yeasts on their haircoats and probably on dermatitic skin as well. The most common fungi isolated from the haircoats of clinically healthy dogs are Alternaria, Cladosporium and Aspergillus spp.

Ringworm may also be isolated from the haircoats of normal dogs and cats living in contaminated environments as transient flora; in cats this phenomenon has been referred to as the ‘fomite carrier’ state. M. canis may cause a persistent subclinical infection of long-haired cats. M. gypseum may be cultured from the feet of dogs that show no clinical signs.

The prevalence of Dermatophytosis or Ringworm in dogs is low, although this varies geographically, being higher in tropical and subtropical climates. In cats, the situation is more difficult to define. In free-roaming cats and those congregated in multi-cat facilities, the prevalence can be quite high (approaching 100% in some situations).

Ringworm spread between animals by direct contact or by contact with infected hair and scale in the environment or from fomites. The source of M. canis infections is usually an infected cat. In most Trichophyton infections, dogs and cats are suspected of being exposed by direct or indirect contact with rodents. M. gypseum and T. terrestre are acquired by dogs and cats from digging in contaminated areas.

Infections with anthropophilic species are acquired as reverse zoonoses by direct contact with infected persons. Ringworm infections of pets and wild rodents involve the hair shaft and follicle (except for T. terrestre). Infected hair shafts are fragile and dislodged hair fragments containing infectious arthrospores are the most efficient means of transmission to other hosts. This material may remain infectious in the environment for many months.

Young animals are predisposed to acquiring symptomatic Ringworm infections; exposure in healthy adult animals does not always lead to active infection. Excessive bathing and grooming, warm humid environments and long hair coats may predispose to infection after exposure. Ringworm infections in healthy dogs and shorthaired cats are usually self-limiting, with the infection clearing up within 8 weeks in most cases . Immunodeficient pets are at greater risk of acquiring infections and their infections may be more widespread and more prolonged. Glucocorticoid therapy is particularly likely to increase susceptibility to Ringworm by means of inhibiting local inflammation. Recovery from infection requires a healthy cell mediated immune response.

Clinical Signs of Ringworm

Ringworm infections in both dogs and cats are highly variable in the clinical picture presented to the veterinary surgeon. Some of the clinical presentations of Ringworm are:

• Circular patch of hair loss, ± scale, ± inflammation. Singular or multiple
• Folliculitis: localized, regional or generalized
• Hair loss: Widespread to patchy
• Seborrhoea sicca with hair loss
• Miliary dermatitis in cats
• Kerion reaction – often M. gypseum
• Nodular dermatophytosis (Pseudomycetoma)
• Onychomycosis: nail bed inflammation and nail deformity
• Subclinical

Symptoms of Ringworm in Dogs

Dogs may show foci of alopecia with follicular papules, scales and crusts. Ringworm should be considered in any papular or pustular follicular eruption. Demodicosis and Ringworm may be clinically indistinguishable but can be differentiated reliably by a skin scraping. Superficial spreading folliculitis, with its spreading rings of erythema and exfoliation , is often mistaken for Ringworm.

Facial folliculitis and furunculosis may mimic an autoimmune skin disease. Nodular skin lesions (kerions) are a common presenting sign of M. gypseum. Onychomycosis causes chronic ungual fold inflammation, with or without footpad involvement, or the claw alone may be infected, which causes claw deformity and fragility.

Symptoms of Ringworm in Cats

Culturing for fungi should be considered a part of the minimum database in the work-up of almost every feline skin disease because the lesions caused by dermatophytes are so variable.

Feline Ringworm often appears as irregular patchy alopecia with scale; this is the most common presentation in older kittens and young adult cats. In long-haired cats, this type of lesion is more difficult to detect. Other syndromes include classical circular patches of alopecia with scaling, miliary dermatitis, focal or multifocal pruritic dermatitis, onychomycosis and granulomatous dermatitis. In kittens, scaling and crusting lesions on the face and extremities are common presentations.

Granulomatous dermatitis takes the form of a well circumscribed ulcerated nodule. The lesions occur on cats with generalized M. canis infection. Persistent and subclinical infections may be a problem in long-haired cats and there may be ‘anatomical reservoirs’ in persistently infected cats, in face folds, periocular skin or ungual folds. These cases can be difficult to distinguish from fomite carriers in multiple cat households.

Diagnosis of Ringworm

Wood’s light examination

This technique can be very valuable in the hands of an experienced diagnostician, remembering that it can never be used to rule out Ringworm, as not all infections exhibit fluorescence. M. canis infections may be positive on a Wood’s light examination. False positive examinations may result from greasy scale (follicle casts) and medication.

True fluorescence is quite bright, apple green, and should only be seen within the shafts of infected hairs. It is useful to pluck positive hairs for direct microscopic examination and for culture. Hairs may remain fluorescent during treatment after there are no longer any viable fungal spores in the hair. In Wood’s lamp-positive infections in multiple cat households, a Wood’s lamp may be used to demonstrate the extent of environmental contamination.

Direct microscopic examination of Ringworm

Dermatophytes form hyphae and arthroconidia within and on hair and scale, which may be seen on examination of hairs plucked from lesions. Mineral oil mounts may be sufficient as a mounting medium, but visualization is facilitated by mounting in 10-20% potassium hydroxide for variable periods of time. Even in experienced hands, this technique is time-consuming and may be diagnostic in only a few cases. It may lead to misinterpretation if saprophytic fungal spores are present in the specimen. Dermatophytes never form macroconidia in tissue.

Procedure of direct microscopic examination are:

• Choose hairs that appear damaged or ‘dirty’ from the periphery of the lesion. Scrape or pluck so as to collect the intrafollicular portion of suspect hairs.
• Place the sample into a large drop of clearing agent (10–20% KOH) and apply a coverslip. 
• Examine with low power for fractured hair shafts with clinging debris and disrupted cuticle. 
• Examine these hairs with high power for hyphae and masses of arthrospores.

Fungal culture for Ringworm

Definitive diagnosis of Ringworm is made by culture. Several important principles should be followed to ensure accurate results.

Specimen collection: If done properly, clipping and cleaning the lesions to be cultured will reduce contaminant growth. This is most important in humid climates where saprophyte spores are common contaminants of haircoats.

• Clip hair within and around lesions to 0.5 cm.
• Pat area with alcohol-moistened gauze and allow to dry. 
• Collect hair stubble with haemostats by grasping the hair shafts close to the skin and rolling the hairs from the follicles.
• Select hairs that fluoresce in UV light or are broken and are near active inflammation, when possible
• Include scales in the sample
• Place firmly in contact with the surface of the DTM medium
• Exudates or antiseptics should not be transferred to the medium

Media and incubation

Culture can readily be performed as an in-house procedure using dermatophyte test medium (DTM). DTM consists of Sabouraud’s dextrose agar, phenol red as pH indicator and antimicrobials to inhibit bacterial and saprophytic mould growth. For incubation, DTM containers should be loosely capped at room temperature and protected from UV light and desiccation. They should be inspected daily for a colour change of the medium to red and simultaneous growth of a cottony mycelium. If the colour change occurs later, which may be a result of saprophyte growth exhausting the carbohydrate in the medium, the result will be a false-positive reading.

After 7-10 days of growth, most colonies will begin to produce spores, which will allow species identification. A suspect colony that fails to produce spores or is difficult to identify, as is often the case in Trichophyton spp., should be sent to a qualified diagnostic laboratory.

Zoophilic dermatophyte colonies are white to buff-coloured. Anthropophilic species may be pinkish to yellow. If blue, green, dark brown or black fungal contaminants have overgrown a colony suspected of being a dermatophyte, subculturing will be necessary.

Culturing asymptomatic animals

Brush culturing is the preferred method of obtaining specimens from asymptomatic animals. A clean toothbrush is satisfactory for this technique. The animal’s haircoat is brushed thoroughly for 3 minutes. The bristles are then impressed directly into the culture medium in several sites.

Onychomycosis

When Ringworm is suspected as a cause of chronic paronychia, special culture techniques may be needed. In many cases the hair surrounding the ungual fold may be infected and may be cultured as for elsewhere on the body, taking special care to clip and clean to reduce contaminant growth. However, in dogs, geophilic fungi may contaminate pre-existing foot lesions, so it may be necessary to correlate cultural findings with histological demonstration of fungi in hair or claw. Otherwise, repeated isolation of fungus from the lesions may be regarded as evidence of cause. If the claws alone are affected, a scalpel blade may be used to shave fine pieces from the proximal end of clipped or surgically excised specimens for culture.

Histopathology

Examination of biopsy samples is not as sensitive as culture in the diagnosis of Ringworm. However, in cases where the true significance of culture results is questioned, demonstration of the organism in biopsy specimens is more definitive. Histological examination is most useful in detecting the nodular forms of Ringworm, i.e. kerion and granulomatous ringworm. With nodular lesions, it may be impossible to culture the organisms causing the inflammation from hair and scale.

Shaved, clipped or surgically excised specimens of claws may be submitted for histological examination in cases of paronychia, onychorrhexis or onychomadesis. If fungal organisms are present, they will be readily visible within the substance of the claw.

Treatment of Ringworm

Most animals with Ringworm should receive both topical and systemic therapy. In healthy animals with small individual lesions, the prognosis is good for resolution without therapy; however, resolution can be hastened by treatment, and topical treatment will reduce the likelihood of contagion. Long-haired cats and all animals with numerous lesions or widespread disease should always be given systemic therapy until culture results are negative.

Cats with M. canis infection can remain culture-positive long after they have resolved clinically. Trichophyton infections should always be treated systemically as well, since spreading and generalized infections are common. Kerions and granulomatous lesions do not require specific antifungal therapy unless more widespread disease is also present. The kerion should be cleansed gently to remove exudate and avoid potentiation of scarring.

Topical therapy for Ringworm

The goals of topical therapy are to help hasten clinical resolution in animals receiving systemic treatment and to reduce environmental contamination. When sparsely haired sites are infected and when the infection is localized, topical therapy alone may be sufficient, but care must be taken to ensure that more widespread disease is not present. Baths and rinses will remove the scale, crusts, exudate and infected hairs, reducing the potential for spread of infection to other animals and people. Agents useful in topical treatment are summarized in Table-1 below.

Agent	Formulation	Frequency
Sodium hyposulphite (bleach)	1:10 in water	twice/week
Lime-sulphur dips	2-4% in water	twice/week
Miconazole	lotion, spray, shampoo rinse	twice/week
Clotrimazole	cream, lotion	as directed
Enilconazole	Clinafarm	twice/week
	Imaverol 0.2% emulsion	twice/week

The decision about whether to clip around dermatophyte lesions should be made on an individual basis. Clipping can spread infection on an animal and severely contaminate the environment in which the procedure is performed. On the other hand, careful disposal of clipped hair reduces the release of infected hair in the home environment, facilitates application of topical therapy and may stimulate new hair growth and shed of infective hairs during therapy.

Systemic therapy

Drugs and dosage regimens are summarized in Table-2.

Drug	Dosage
Griseofulvin (microsized)	50-120 mg/kg divided BID
Griseofulvin (ultra-microsized)	10-15 mg/kg divided or as a single daily dose
Itraconazole	5-10 mg/kg once daily
Itraconazole pulse therapy	5-10 mg/kg daily for one week, rest one week, repeat
Terbinafine	30 mg/kg daily

Treatment with any systemic antifungal should be continued until two successive brush cultures are negative separated by 2 weeks. The first culture can be taken after 3-6 weeks of therapy depending on the agent being used.

(1) Griseofulvin

Treatment with griseofulvin is expensive and long term, and side effects are common. It should be used only when diagnosis is certain. Griseofulvin shows variable and incomplete absorption after oral dosing. Absorption improves when taken with a meal containing fat or when using formulations that include polyethylene glycol. Particle size also greatly affects oral absorption and bioavailability.

Dosages recommended for dogs and cats are not based on modern pharmacological studies. Dosages that have proved to be effective in the largest numbers of cases are higher than manufacturers’ recommendations , and significant toxicities may be encountered. The most frequently reported side effects include vomiting, diarrhoea, and loss of appetite. These can be partially avoided by dividing the daily dosage into two administrations. Bone marrow suppression and neurological signs have occurred, probably as idiosyncratic reactions. Griseofulvin is teratogenic and must never be given during the first two-thirds of pregnancy.

(2) Ketoconazole

Ketoconazole has been shown to be a moderately effective fungistatic drug against M. canis and T. mentagrophytes. Other imidazole and griseofulvin are more effective, and the use of ketoconazole is reserved for those who can not tolerate griseofulvin and for whom other drugs are not affordable. The most common side effect is anorexia. Rarely, hepatic toxicity is encountered.

(3) Itraconazole

Itraconazole is a triazole antifungal agent that would, except for cost and the lack of veterinary licensing, be the treatment of choice for Ringworm in dogs and cats. It is more effective and less toxic than ketoconazole. The drug is dispensed in 100 mg capsules which can be opened and the contents divided and mixed with butter or prescription critical care diet for easy administration to cats. It should be given with food to enhance absorption. Side effects include anorexia, occasionally vomiting; rarely, it may cause liver enzyme elevation. There is no need to monitor liver enzymes routinely; these should be checked if anorexia develops.

Itraconazole can be useful in pulse therapy protocols for treating longhaired cats in a heavily contaminated environment. In these settings, relapse is a common problem , and pulse treatment for several months can be used to reduce the relapse rate.

(4) Terbinafine

This is an allylamine antifungal that is well concentrated in the skin after oral administration. It is useful for treating Ringworm in dogs and cats, but does not have a veterinary licence. Doses reported in the literature and anecdotally for dogs and cats have varied. Side effects include anorexia and, rarely, liver enzyme elevation.

(5) Lufenuron

Lufenuron is a chitin synthase inhibitor used for flea control that has been reported to be useful in canine and feline Ringworm. The original report outlined the use of 50-60 mg/kg given once. Since that report, several treatment failures in challenging situations have been reported anecdotally and higher dosages and more prolonged treatment is recommended. Many dermatologists are combining monthly lufenuron at label doses with initial treatment using itraconazole or terbinafine in cats. Until controlled trials are available, use of lufenuron cannot be recommended for systemic treatment of Ringworm.

Environmental control

In each confirmed case of Ringworm in dogs and cats, there is an environmental clean-up problem to be handled. The veterinary surgeon must be prepared to advise the client about environmental contamination and recommend appropriate measures to prevent spread of infection. The necessity for clean-up is most serious in the case of cats infected with M. canis. Dermatophyte arthrospores, liberated from broken and shedding hairs of infected pets, are quite long-lived.

The most important facet of environmental clean-up is complete mechanical removal of hair by means of vacuuming . Full-strength household bleach, enilconazole preparations or chlorine dioxine disinfectants may be used to disinfect surfaces and utensils. Carpets and furnishings must be vacuumed thoroughly or removed. Clothing and bedding should be thoroughly laundered with bleach or discarded.

Public health considerations

Pet owners

Owners of cats infected with M. canis are at great risk of acquiring the infection. It is the most frequently reported zoonotic agent acquired from pet animals. Worldwide, the numbers of reported cases of human infection with M. canis is increasing. Veterinary surgeons should consider advising people adopting a new cat to have the cat evaluated for ringworm before all members of the household are exposed. Cats acquired from catteries and from animal shelters are of particular concern.

Animal health workers

The occupational risk of acquiring Ringworm is great. In a study of government veterinary surgeons and animal health workers conducted in Great Britain, animal ringworm was the most commonly reported zoonosis; the overall prevalence was 24%.Veterinary surgeons must be vigilant in protecting themselves and their staff from this troublesome and potentially serious zoonosis.