Merck Manual

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Pet Owner Version

Infections Caused by Fungi


Manuals Staff

Reviewed/Revised Feb 2020 | Modified Nov 2022

Fungi are a type of organism that can infect animals and people. Yeasts, molds, and mushrooms are all examples of fungi.

Some fungi reproduce by spreading microscopic spores. These spores are often present in the air, where they can be inhaled or come into contact with an animal’s body surfaces. Consequently, fungal infections usually begin in the lungs or on the skin. Of the wide variety of spores that land on the skin or are inhaled into the lungs, most do not cause infection. Except for a few skin conditions, fungal infections are rarely passed from one animal to another. Because many fungal infections develop slowly, months or years may pass before a problem becomes noticeable.

Certain types of fungi are present on body surfaces or in the intestines. Although normally harmless, these fungi sometimes cause localized infections of the skin and claws, the sinuses, or the mouth. They seldom cause serious harm, except in animals with a weakened immune system. In this situation, fungal infections can be very aggressive, spreading quickly to other organs and leading to death.

Sometimes, the normal balances that keep fungi in check are upset, and infections develop. For example, the bacteria normally present in the intestines limit the growth of certain fungi in the area. When antibiotics are given, those helpful bacteria can be killed—allowing the fungi to grow unchecked. The resulting overgrowth of fungi can cause signs, which are usually mild. As the bacteria grow back, the balance is restored, and the problem usually resolves.

Several drugs are effective against fungal infections, but the structure and chemical makeup of fungi make them difficult to kill. Antifungal drugs can be applied directly to a fungal infection of the skin or other surface. In some types of fungal infections, antifungal drugs must be given by mouth or injected. Several months of treatment are often needed.

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