Cornell Mushroom Blog

This post was written by a plant pathology grad student and former farmer who took PLPA 309 this semester.

The word “Zoonoses” evokes elephant trunks and zebra muzzles, but the reality is less picturesque, it is the term for diseases normally occurring in animals which can be transmitted to humans. Ringworm, or Tinea as it is known in people, is such a disease, and has nothing to do with worms. The term comes from the Latin word Tinea, which refers to worm-like moth larvae that chew circular holes in woolen blankets—resembling the circular skin lesions typical of this condition.

Ringworm is caused by a group of 40 species of fungi that belong to the odd ascomycete order Onygenales. These dermatophytes (literally “skin plants”) don’t actually invade living tissue, but the outer, keratin-rich layer of hair and skin and nails. Keratin is a fibrous structural protein, one of the most abundant and stable animal proteins on earth, which is targeted for breakdown by fungal keratinases. These are the only fungi which have evolved a dependency on humans or animals for survival on keratin. Micosporium canis, Trichophyton verrusosum, and Trichophyton metagrophytes are the most important species responsible for human infections.

Ringworm is quite contagious, and can be spread from direct contact with an infected animal, soil, hair or skin shed from an infected animal, or fencing, tack, or grooming tools. The spores are viable in shed skin or hair for months or even years. Any part of the body can be infected but exposed areas of the face, neck or arms where there is likely to be abrasion to aid fungal colonization is the most common. Human to human spread is rare. Microsporum canis moves from dogs to humans. And many human infections come from our feline friend, the domestic cat, which in turn can get it from rodents they prey on. Up to 90% of cats display no visible ringworm symptoms, so it can be unknowingly passed on—one nurse infected five babies in a neonatal nursery before the connection with her cat was made.

Your doctor can check for ringworm with a simple office test using a Wood’s light (a filtered UV light) to reveal the fungal spores as they emit bright bluish-green fluorescence. The ringworm fungi are opportunistic pathogens, often starting on young or compromised animals at weaning. Children ages 4 to 11 are most often infected, with a higher incidence on males which isn’t seen in animals.¹ Now I know why both my sons had this and the girls avoided it while doing the same things! The rash and itching associated with ringworm is due to the fungus’s manner of producing enzymes to digest its food externally, rather than internally, as most other organisms do. These fungi have developed to survive on their animal hosts without causing excessive immune reactions, but on human skin, an inflammatory response may develop from exposure to the enzymes released by the fungi and a raised red, itchy rash is often the result. This mimics a bacterial infection, and is often misdiagnosed as such. Often bacteria do invade this irritated area, making treatment more confounding and painful, may I add from experience. Since this is not a reportable disease, and often no treatment by a physician is sought, the exact prevalence is not certain.

In cattle, ringworm infection often occurs during fall or winter months when animals are moved into barns and there is crowding, higher humidity, and less sunlight. Ringworm infection often starts around the eyes and can cause scaling and a grayish crusting of the head in calves, and chest or brisket of older animals. The moisture around the eyes enables the fungal spores to adhere and start to grow, and because of cattle’s use of their head to determine their rank in a group, it is often abraded and more easily colonized. Most feeding systems require the animals push their heads through an opening to eat which facilitates spore transmission and spread. It is usually a self-limiting disease of 1 to 4 months, but treatment is sought when it’s time for the county fair and a calf isn’t allowed to go because it is a communicable disease (and is your child’s only calf) or it becomes a threat to stressed animals. A scrub brush with dilute bleach is often prescribed and oral or topical antifungal treatments work, but must be continued for long periods of time and can be harmful to pregnant animals. Some work has been done using wood rotting fungi which have antifungal activity against these dermatophytes with Gleophyllum trabeum being the most effective.³

A vaccine has been developed for cattle and confers almost 100% protection against T. verrucosum using an attenuated form of the fungus. The best protection results from using a live, virulent strain.^6,7 The vaccine was developed by studying the antigens produced during spore formation and early hyphal growth—these compounds stimulate a cell-mediated immune response rather than antibody formation more consistent with a chronic disease.⁵ The vaccine approved for cats doesn’t have the same effectiveness and although it reduces symptoms, the fungus isn’t completely controlled. Our vet clinic doesn’t even offer this for cats.

With the popularity of petting zoos, fairs, and other areas of contact of young children with animals in public settings, there is a movement to educate vendors, staff, and visitors of the possibility of zoonotic diseases like ringworm. Simple precautions such as hand washing, especially before entering areas where food is sold, can keep this from being a public health problem.⁴ On farms, it is harder to control, but with an understanding of modes of transmission, sanitation, and vaccination when appropriate, the disease’s incidence on calves, dogs, cats and kids can be reduced.

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