The first point to make is that it’s probably sensible to make a presumption that your cat has worms if she is an indoor/outdoor cat. She’s likely to hunt and eat mice and in doing so ingest worms. The particular worms I am referring to are roundworms (ascarids). They are the most common worm parasite in cats and there are two common species.
They occur in 25-75% of adult cats. It is reasonable to make a presumption that outdoor cats have this worm because they ingest the eggs by (1) contact with the soil where the eggs are and licking their feet or (2) eating a host animal such as a mouse in which the encysted larvae lives. If the cat spends lots of time outside and hunts a lot she may acquire a heavy infestation.
Also there is a greater likelihood of picking up fleas if your cat is an outside cat. If your cat has fleas it is likely she’ll have tapeworms because the flea carries immature tapeworms in their intestines. The cat eats a flea when grooming and therefore ingests the tapeworm.
Hookworms are not as common in cats as in dogs but again can pick up the parasite through their feet by the parasites penetrating the skin or rarely through eating mice which are hosts to the larvae.
You can see that outdoor cats are quite likely to have worms and therefore a monthly treatment is advised.
My cat lives in a garden enclosure and no longer eats prey. I don’t give him deworming pills. This is a personal choice and some people would say a bad choice.
How do I know if my cat has worms? Signs
Other than a sensible presumption as mentioned above, there are some signs.
Diarrhoea, anaemia, weight loss and getting weaker. If the infestation is heavy the cat’s faeces may be bloody and dark (tarry-black). A diagnosis by a vet is made by seeing eggs in the cat’s faeces.
Heavy infestation: pot-bellied, thin, severe illness and possible death. Vomiting, diarrhoea, anemia, pneumonia. Stool or vomit may contain the worm. They look like earthworms or moving strands of spaghetti.
Tapeworm segments can be seen on the cat’s bum. They may move if moist or look like rice when dried up. These segments are passed in the faces. They might also be on the cat’s stool.
Human health consideration
There is also a human health issue worth noting. Roundworms can cause a disease in people: visceral larva migrant. It is one of the top zoonotic diseases. The primary culprit is the dog version of the worm: Toxocara canis but Toxcara cati can also cause human disease. Children are most at risk because they like to put things in their mouth even earth.
When a person eats a roundworm egg the larvae develops in the person but they don’t develop into worms but wander around the internal tissue of the person causing general illness such as fever, anemia, pneumonia and other illnesses.
Migrating larvae can get into a person’s eye causing ocular larva migrans which can lead to the loss of an eye.