Fungi include yeasts, molds, and mushrooms. Many species can infect animals and people.
Some fungi reproduce by spreading tiny spores into the air. These spores can land on a pet's skin or be inhaled into the lungs, so fungal infections often begin in these locations. Most spores do not cause infection. Except for a few skin diseases, fungal infections are rarely spread from one animal to another. An important exception is ringworm, a fungal skin infection that can pass between animals and from animals to humans. Many fungal infections develop slowly and might not cause problems for months or years.
Some fungi normally live on body surfaces or in the intestines without causing harm. Sometimes they cause infections of the skin, claws, sinuses, or mouth. These usually are mild unless the animal’s immune system is weak. In animals with weak defenses, fungal infections can spread rapidly and become life‑threatening.
Normal balances between fungi and bacteria can be disturbed. For example, normal intestinal bacteria help limit fungal growth in the intestines. When antibiotics kill many of these bacteria, fungi can overgrow in the intestines and cause mild symptoms, like diarrhea. As the bacteria return to normal levels, balance is restored and symptoms usually fade.
Several antifungal drugs exist, but fungi are hard to kill, so treatment can take a long time. For skin or surface infections, antifungal drugs can be applied directly to the skin. For deeper infections, antifungal drugs must be given by mouth or injection, often for several months.
