While it is a disease humans can catch from cats, the risk is very minimal explains vet Penny Clarke.
Most people know toxoplasmosis as ‘the illness pregnant women can catch from cats.’ While toxoplasmosis is certainly zoonotic (a disease that can spread from animals to humans), it’s less dangerous to people than reports in the press suggest. And cats aren’t actually the main source of infection, even in pregnant women.
What is toxoplasmosis?
Toxoplasmosis describes the disease caused by a protozoal parasite called Toxoplasma gondii (T gondii). Just like bacteria, a protozoan is a single cell organism only visible under a microscope. However, protozoa are bigger than bacteria with a more complex cell structure (making them more similar to plants and animals.)
The complicated lifecycle of T gondii
Some parasites only live on (or within!) one host whereas others (like T gondii) use two hosts within their lifecycle. T gondii has a ‘definitive host’ (a domestic or wild cat) and an ‘intermediate host’ (other warm-blooded animals).
Cats who are keen hunters can catch T gondii if they eat infected prey (such as a mouse). Once in the gut, T gondii produces oocysts (eggs). Cats are the only species in which T gondii can produce their oocysts (making them a crucial part of the lifecycle). These oocysts pass out of the body in cat poo (this only happens for a very short time after the cat is infected). We’ll come back to these oocysts shortly…
While in the cat’s gut, T gondii changes into a tachyzoite (the teenage stage of protozoal life). Tachyzoites migrate around the body causing inflammation and disease. A healthy cat’s immune system can successfully challenge tachyzoites whereas they cause more issues for less healthy cats (those with a weakened immune system).
In response to a cat’s immune defences, tachyzoites transform again. They become bradyzoites (another stage of T gondii’s lifecycle) and ‘hide’ within body tissue forming multiple cysts (pockets of infection).
Back to oocysts…
Many species of animals (such as rodents and grazing animals) can ingest oocysts from soil contaminated with cat poo. Oocysts don’t immediately cause illness in these ‘intermediate hosts.’ Just like in cats, the oocysts need a few days to change into the more harmful tachyzoite form. These tachyzoites migrate through the animal’s body and just like in cats, form tissue cysts, especially in muscle.
Any animal infected by T gondii may have cysts within their body. These cysts are a potential source of T gondii infection to other animals who may eat them. If these animals are part of the human food chain (cows and sheep for example) then cysts within raw or undercooked meat can infect humans.
Which cats are most at risk of catching toxoplasmosis?
It’s important to note that most cats infected with T gondii don’t show any signs of illness at all (their immune system prevents the protozoa from changing into the harmful form). Cats more at risk of illness are:
- Kittens infected while in the womb (infection in a pregnant cat can cause the death of her kittens before birth or shortly afterwards).
- Cats with a compromised immune system (such as those with feline leukaemia virus (Felv) or feline immunodeficiency virus (FIV))
- Outdoor cats who are keen hunters (they’re more likely to eat cysts in infected mice etc.)
- Cats fed undercooked or raw meat
Symptoms of toxoplasmosis
T gondii can cause a variety of symptoms depending on where in the body the damaging cysts form. Some cats show general, unspecific signs of being unwell including:
- Pyrexia (a high temperature).
- Loss of appetite.
- Lethargy (extreme tiredness).
- Weight loss.
- Vomiting.
- Diarrhoea.
- Enlarged lymph nodes (a sign that the body’s fighting infection just like swollen glands in your neck when you have a sore throat)
Other signs of illness relate to where in the body the cysts form:
- Lung cysts can cause breathing difficulties and even pneumonia (lung infection).
- Cysts within the eye cause generalised ocular inflammation.
- Hepatitis (liver inflammation) can cause jaundice (a yellow tinge to skin, gums and around the eye).
- If cysts form within the brain, neurological symptoms can develop including tremors (uncontrolled shaking), balance issues and even seizures.
How can we diagnose toxoplasmosis?
Your vet will start by taking a full history to find out about your cat’s lifestyle (if your cat has only ever lived indoors, it’s unlikely they have toxoplasmosis). They’ll also check your cat over thoroughly for other causes of their symptoms.
To delve a bit deeper, your cat may need tests including:
- A faecal sample to look for oocysts (cats only shed these in their poo for a short while so it’s possible the lab won’t find any).
- Blood tests to look for T gondii antibodies (disease fighting cells) — this has limitations as blood tests don’t distinguish between a new or old infection.
- Biopsy of tissue to look for cysts (we don’t often do this as it’s quite invasive and your cat would need an operation).
If your cat has antibodies to T gondii in their blood, there’s less of a chance they can infect humans as the period of oocyst shedding in their poo has likely passed.
Is there a cure for toxoplasmosis?
Specific antibiotics can target T gondii. We usually associate antibiotics with a bacterial infection but some are also effective against protozoa. Really poorly cats may need to be hospitalised for supportive treatment including intravenous fluids (a drip) and help feeding, sometimes via a tube into their food pipe.
Exactly how dangerous is toxoplasmosis to people?
Before you don a haz-chem suit to cuddle your cat, let’s examine the facts:
- You can’t catch toxoplasmosis from stroking an infected cat. (Studies show oocysts are never found in the fur of infected cats).
- Research shows cats only shed oocysts in their poo for a very short time (and some infected cats never shed oocysts).
- Vets working with infected cats are no more likely to catch toxoplasmosis than anyone else.
- Most human infections are due to eating undercooked or raw meat.
- Oocysts in cat poo take a few days to become infectious — regular litter tray emptying can eliminate this potential source of infection.
Who should take extra care?
Just under a third of the UK population have been infected with T gondii without knowing it. After contact with T gondii, humans develop antibodies to protect against further infection.
Pregnant women
If a pregnant woman hasn’t got antibodies to T gondii and becomes infected, there’s a 20 — 50% chance that infection could pass to her baby. Even if foetal transmission occurs, most of these babies will be unaffected. In a small number of cases, babies can be born with eye problems or issues with their nervous system (brain, spinal cord and nerves). Very rarely, T gondii causes a miscarriage or stillbirth.
Other at-risk people
T gondii can cause encephalitis (brain inflammation) and other illnesses in:
- Babies and young children.
- Elderly people with weakened immune systems.
- Anyone who is immunosuppressed (such as those having chemotherapy, people with acquired immunodeficiency syndrome (AIDS), and organ transplant patients).
What can you do to prevent it?
While the chance of T gondii ever causing an issue is extremely low, there are simple steps you can take to give you piece of mind:
- Anyone at risk should delegate cleaning cat litter trays to someone else (or wear rubber gloves if it’s unavoidable).
- Empty litter trays daily (this means oocysts don’t have time to become contagious).
- Clean litter trays properly with disinfectant.
- Avoid eating raw and undercooked meat (and don’t feed these to your cat).
- Observe scrupulous kitchen hygiene (especially when dealing with meat).
- Wash fruit and veg well before eating.
- Wear gloves when gardening.
- Cover children’s sandpits so cats can’t use them as a convenient litter tray.
- Encourage children not to dig and play in soil where cats may have been to the toilet.
The bottom line…
We certainly need to take toxoplasmosis seriously but remember that the risk of catching it from your cat is extremely low (pretty much nil if you take a few sensible precautions). Even if you’re in a more vulnerable health category, you can still fully enjoy living with your cat (snuggles are definitely not off the cards!)
If you have more questions about toxoplasmosis, chat to your veterinary team or contact your doctor for human-specific advice.