Feline Endoparasites; what is inside my cat?!

Broadly, parasites of cats can be divided into two main categories; parasites we consider as worms, and parasites we call “protozoa”. The first category includes some fun shapes and objects: roundworm, hookworm, and tapeworms (whipworms also exist but are fairly rare). Roundworms are the most prevalent species in cats. The protozoa include IsosporaGiardia and Toxoplasma as the main offenders. Let’s take a closer look at what these mean for your cat. 

Roundworms: Toxocara cati (also known as Toxocara mystax), Toxascaris leonine

What are they? These are part of the class “Nematodes”; they are large, white worms, which can be up to 10cm in length! They settle in the small intestine.

How does my cat get infected? Cats can become infected when they eat infected animals (like rodents!) with roundworm larvae inside; rats play an important part in their lifecycle, so roundworm infections are commonly seen in hunters. Another way that cats can be infected is transmammary infection; if a mother queen is infected, her suckling kittens may become infected.

Why are they bad? The larvae spend a period of development in the stomach, before migrating to the small intestine, and they can cause inflammation in the stomach wall. Kittens can suffer from gastrointestinal upset; diarrhoea and constipation. If the burden is very high there can be anaemia and blood loss, because roundworms damage intestinal lining. Sometimes, worm larvae may burrow through the tissues, causing damage or cysts in other organs.

Hookworms Uncinaria stenocephalaAncylostoma species

What are they? Nematodes of the small intestine; they are much smaller than roundworms, typically under 1cm.

How does my cat get infected? Commonly, rodents act as a paratenic host; this means that the larvae do not require the rodents in order to develop, however, they can ‘hang out’ (or rather, hang inside) the rodent, until the final host, the cat (or dog), eats it. Once inside the mouth, they are swallowed, entering the gut directly. There can be transmission through the skin but this is rarer.

Why are they bad? These parasites are blood suckers, but usually associated with a low pathogenicity – that is, they don’t cause very severe disease. However, both can cause protein loss and (very occasionally) anaemia, especially in heavy burdens, resulting in lethargy and tiredness in your cat (or dog!). They can also cause diarrhoea.

Tapeworms (Dipylidium species are the most common infections of dogs and cats)

What are they? They are descriptively named; their long, flat bodies resemble a ribbon, or piece of tape. Such a description is far nicer than the reality of these cestodes!

How does my cat get infected? These tapeworms have an intermediate host of fleas and lice; unlike hookworms and rodents, the tapeworm requires these ectoparasites in order to complete its life cycle. Cats with high flea and/or lice burdens (which we will discuss in the next blog post!) are therefore at higher risk.

Why are they bad? Tapeworms live (again…) in the small intestine. They absorb nutrients the cat eats – all that Sheba is going to waste, I hear you cry! This means that the cat will often lose weight and experience diarrhoea.

Isospora species

What are they? Protozoa are microscopic, single-celled organisms. Isospora are protozoa which cause “coccidiosis”, which is a cause of concern in kittens (and puppies!).

How does my cat get infected? The oocysts (early stage of the life cycle) are excreted, and cats pick it up when they ingest oocysts from faecal matter.

Why is it bad? Nearly all cats are infected – the good news is it rarely causes disease. The risk is that it causes damage to the lining of the small intestine in young kittens; the areas involved in absorption of nutrients are damaged, causing a mucusy-diarrhoea.


What are they? Like Isospora, Giardia are protozoa; these are single-celled organisms which parasitize the small intestine.

How does my cat get infected? Again, they are excreted into faeces and picked up my cats who may be snuffling near faecal matter (or even eating it).

Why is it bad? Typically it causes diarrhoea, but it can be chronic, acute or intermittent.

Toxoplasma cati

What are they? These protozoa cause toxoplasmosis.

How does my cat get infected? Toxoplasma is different to its pals Isospora and Giardia; its dormant stages, known as cysts, encyst in the muscle of rodents. When a cat eats them, they infect the cat and live in the small intestine.

Why is it bad? These parasites rarely cause disease in cats. The bad news is that it is infective to people; the immunocompromised are at higher risk. In pop-culture, Toxoplasma’s claim to fame is that Tommy in “Trainspotting” contracted toxoplasmosis from his kitten, as he was severely immunocompromised. Interestingly, some modern studies suggest that Toxo infection leads to risk-taking behaviour (like speeding!) – but don’t try to use that as a defence in court!

How can I control parasites in my cat?

Good sanitation! Giardia is more common in multi-cat households, and the faecal-oral transmission of the protozoa means that cleaning litter trays regularly and gardens is essential.

Worming programmes; speak to us about what is most appropriate to keep your Felix happy. Hunters are at higher risk, as are cats in multi-cat households. This isn’t a reason to keep a cat who loves the outdoors inside, though!

Good hygiene; regular hand washing is essential for minimising your risk of exposure to the protozoa; Giardia and Toxoplasma are transmissible to humans, as are the roundworms (but this is quite rare!).

Controlling ectoparasites; lice and fleas are essential for infection by Tapeworms – see next week’s post to get the lowdown on ectoparasites.

Be vigilant about your cat’s health; weight loss, lethargy and diarrhoea can all be caused by endoparasites; speak to our vets if you have any concerns.

“Cats are rather delicate creatures and they are subject to a good many different ailments, but I have never heard of one who suffers from insomnia” – Joseph Wood Krutch