In order to understand how indoor cats get ringworm we need to refer to the life-cycle of this fungus. We are not talking about a worm but a plantlike growth that invades the hair and hair follicles. You probably know that already. This disease is transmitted by contact with ringworm spores.
A ringworm spore may come from the soil or by contact with infected hair of dogs and cats which is normally found on carpets, brushes, combs, toys and furniture. For an indoor cat living alone there should normally be no direct contact with other animals. If they are living with other cats and dogs the spore will most likely come from them directly or indirectly via bedding etc..
In a single cat home the spores that I refer to must be picked up by a person and brought into the home. So the spores could be on the person or the person could be infected with ringworm herself. They may become infected by contact with another animal in a different household or anywhere else where they might have bumped into a cat or dog. They may have picked it up from another person or another person’s clothing and even from a chair that an infected person sat on. It can also be picked up at swimming pools. Ringworm is highly contagious. Children are particularly likely to catch the disease.
In other words the likely way the spores came into the house is through humans living in the house with the cat. Once the animal or person is in contact with spores threadlike tubes called hyphae develop from them. These hyphae penetrate into the hair shaft causing it to weaken. This takes from 4-14 days. The hair breaks leading to patches of hair loss and a degree of inflammation of the hair follicle. The surface of the skin forms a scab. As the infection progresses spores are produced that are carried by fragments of hair as they break off. This spreads the infection.
Ringworm only grows in the outer dead parts of the skin. They also feed on the dead keratin on the epidermis of the skin. So ringworm grows on the keratin of the skin and on hair and nails and claws which are also made of keratin. The infection only very rarely spreads into the deeper living tissue of the skin.
The fungus produces enzymes which defuse into the living skin below the epidermis causing it to become inflamed. This is what we see when we look at ringworm on the skin. We see a circular patch of inflamed skin and scabs.
As ringworm spores are released from the infected skin they are then picked up as mentioned above by a cat, dog or a human who may become infected themselves.
It is therefore quite possible that a full-time indoor cat living alone can become infected with ringworm and the vector will usually be the cat’s human guardian and caretaker.
My former male cat who I adopted from my deceased mother’s estate had ringworm and he gave it to me. That was many years ago and I’ve had ringworm ever since. I suppress it with UVB light when it is manifest but it is normally not on my skin and months and years go by when there is no sign of it. However, I am very conscious of the possibility that I can transmit it to my current cat. Ringworm is hard to treat in cats. I have a page on that which you can read if you click on this link. However treatment needs to be holistic. The whole place needs to be treated and spores can live for up to a year. They need to be eliminated to preven reinfection.