Dermatofitosis - (La Realidad del Día a Día)

How to really gauge the true prevalence of dermatophytes as causative agents of integumentary disorders.

It is well known that dermatophytes are opportunistic infectious agents. Thus, colonization and infection of the skin is achieved only when there are conditions for their development. These conditions are presented related to alteration of the skin barrier, the fungistatic activity of the sebaceous gland secretion and cell-mediated immune response, as the primary defense mechanisms against these fungal agents.

As a result, clinical evidence shows that the main patients affected are puppies usually less than one year, elderly people or adults with immune disruption and without obeying to a specific age range, other proven susceptible breeds like the Yorkshire terrier and Jack Russel.

Dermatophytes are fungi with affinity for the cornified epithelium and keratinized skin structures. Ringworm is an infection that affects the skin: the stratum corneum, hair follicles, hair and nails, caused in most cases by keratinophilic fungi of the genera Microsporum and Trichophyton. Those can be divided into three groups based on their natural habitat: geophilic, zoophilic and anthropophilic.

Most clinical cases in small animals are caused by Microsporum canis, Trichophyton mentagrophytes and Microsporum gypseum. Microsporum canis and T. mentagrophytes are zoophilic dermatophytes, while M. gypseum is geophilic. Anthropophilic species (e.g., T. rubrum) occasionally can infect dogs and cats causing a reverse zoonosis.

It is commonly an overstated diagnosis because in most cases it is established by clinical presumption, associated with the presence of erythema, peeling, relative pruritus and alopecia. These characteristics correspond to the "classic," focal or multifocal presentation, and it is attributable to M. canis, while M. gypseum (nosepiece) and T. mentagrophytes (initially nails), tend to produce different clinical lesions, both due to the tegumentary location, as well as their initial appearance.

As for distribution by species affected, about 90% of the true clinical pictures of ringworm in cats, are caused by M. canis. Regarding canine ringworm, about 85% of positive cases are caused by M. canis, 10% by M. gypseum and the remainder is due to T. mentagrophytes (contact with rodents or their environment) and other least frequent fungi.

Compatible skin lesions with dermatophytes are presented into two basic forms:

1.  Local or focal alopecias apparently rounded, almost always antipruritic. These are more common in young animals, usually puppies, with evidence of infection in at least in one family member.

2.  Alopecia injuries, which are regularly caused by M. gypseum or T. mentagrophytes.

There should always be practiced a confirmatory or exclusionary test, with other infectious causes of skin disease, especially with bacterial folliculitis and demodicosis, which constitute the most common clinical appreciation mistake. 90% of primary or secondary (pustules, follicular papules, epidermal bolsters) compatible skin lesions, are caused by bacterial infections (pyoderma). About 8% or more are generated by demodectic mites and only 2% tested positive for dermatophyte fungi.

The diagnostic procedures that should always be implemented are surface or periphery scraping of the injury and direct microscopic observation - A trichogram to detect compatible structures damaging hairs in ectothrix location.

When microconidia are detected on direct microscopic observation, these usually come from environmental fungi: Alternaria, Aspergillus.

It should be noted that without expertise, direct microscopic observation of scraping carries an additional margin of error in the diagnosis of ringworm.

Confirmatory diagnosis is obtained exclusively by the mycological culture.

It is indisputable that the clinical expression of ringworm is extremely variable and pleomorphic, and this tends to cause misdiagnosis. Some patients show alopecia lesions with papular eruptions, scaling and crusting. Some cases come without pruritus, while others can be intensely pruritic. This may lead to allergy diagnosis and to the hasty introduction of a glucocorticoid therapy that can be harmful and complicate a real ringworm.

In addition to the most common dermatologic expression, there are other forms, which sometimes are underestimated in the clinic, such as kerion (fungal inflammatory erosive nodule), nodular granulomatous dermatophytosis and nasal bridge folliculitis. These can simulate vesicular skin disease (autoimmune) distorting the correct prescription.

The varied clinical presentation of dermatophytosis makes it imperative to implement all complementary diagnostic tests available: scraping and trichogram for direct microscopic observation, the Wood lamp and mycological culture to establish real and accurate diagnosis with identification of the causing dermatophyte.

The ringworm is a public health problem that should call the attention and social responsibility of us veterinarians dedicated to small animals as for potential contagion by other immunocompromised canine or feline family members. Therefore it is necessary to establish prevention and control measures to manage it proactively.

The ringworm, as anthropozoonosis, represents the most common infectious disease in domestic cats. This species remains the most zoonotic risk representing especially for children in pediatric age. In this regard it is important to note, first, that most of scalp ringworm in children are caused by M. canis (Benavides et al. 1991) and on the other hand, that the cat is an important reservoir of this dermatophyte.

Puppies are the central focus of control, because when considering the age distribution, we can see that most cases of ringworm occur in animals less than one year, which is also consistent with the work done by Sparkes et al. (1993).

Several studies indicate that the incidence decreases progressively with age, as during puberty, there is a modification of the fatty acids of the skin, causing a fungistatic action (Torres, Rodriguez 1987).

Prevention must be based on adequate control of infection in dogs and cats, as reservoirs, and potential carriers and disseminators of the disease (Carlotti et al. 2004).

It is recognized that in most animals the disease heals spontaneously between 2 and 4 months in dogs and cats, respectively, but to prevent zoonotic disease extent it is recommended to treat the process (Carlotti 1996).

In conclusion, this tegumentary infectious disease always requires a reliable diagnosis to implement just the proper treatment in proven cases and to establish a real statistical incidence of this important skin disease.

References

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