Friday, March 14, 2014

Ringworm (aka dermatophytosis)

Dr Patrizia Danesi (left) teaches a group of Italian veterinary students about dermatophytes with the aid of some plush toys. 
Ringworm gives everyone the heebie-jeebies, and I blame the name. Firstly, its not a worm, which goes some way to removing the freak-out factor - its fungal. Lots of different fungal species cause ringworm or dermatophytosis. And secondly, it can be staggeringly beautiful (see below).

M. canis (courtesy of the wonderful Dr Danesi).
The problem with dermatophytes is that they are zoonotic (i.e. infections can be transmitted from animal to human, and vice versa). In most cases clinical signs are mild.

Dermatophytes are fungi that are adapted to eat keratinous debris on the stratum corneum, hair shaft or claw (nails if you are a human). Microsporum spp and Trichophyton spp account for most animal disease, and despite its name, M. canis is best adapted to the cat. Cats are more susceptible to ringworm than dogs, which explains why most patients I've seen with ringworm are cats. Long-haired cats are at increased risk.

Younger animals are more commonly affected than older pets, most likely due to lack of specific immunity or poor development of innate immune mechanisms like sebaceous lipids in the epidermis.

Transmission is via contact with other affected animals, contamination of the environment with spores, or fomite transmission.

Clinical signs include hair loss or alopaecia around the face and ear tips especially, erythema, scale and crusting, itchiness or pruritis, sometimes a sheen to the lesion and rarely there is a nodular form which causes huge, pussy, inflammatory skin lesions to appear.

Diagnosis is challenging as it takes time...the fungus has to be cultured. You can occasionally see fungal spores on direct microscopic examination of hair plucks in liquid parraffin but a negative result doesn't rule out ringworm (i.e. the test has a low negative predictive value). Woods lamps are notoriously insensitive, detecting only M. canis and only in around 50 per cent of actual cases. Infection can also be confirmed by biopsy.

A ringworm lesion in a cat.
The lesions are often self limiting and can spontaneously regress, but we usually need to treat affected animals due to the risk of the infection spreading to other animals or humans. In animals, usually we use a combination of systemic medication (itraconazole, griseofulvin, ketoconazole) usually for 2-4 weeks following resolution of clinical signs, and topical methods (miconazole/chlorhexidine, enilconazole [in dogs] etc.) Clipping hair is recommonded by some textbooks but only a fool would do this in a vet hospital - think of the contamination!

Ringworm lesion on a vet nurse.
Humans seem to respond really well to topical treatment, maybe because we are less hairy? 

But back to animal patients - systemic antifungals aren't the most benign drugs and have side effects include vomiting, inappetance and hepatotoxicity. Some of the patients I have diagnosed have been far too young for systemic antifungals, so topical treatment and environmental decontamination are ideal.

For an excellent online resource on ringworm diagnosis and management, click here. The key is to be proactive and minimise cases by practising good husbandry. 

Bond R., (2010) Superficial veterinary mycoses Clinics in Dermatology 28: 226-236