AGGIO CAT*PL GUIDE

Intestinal parasites and infections in cats: diagnosis, transmission and prevention

Why can a cat have a negative faecal test despite an infection? How does Giardia differ from Tritrichomonas, and why can similar signs of diarrhoea require different treatment? Understanding transmission helps in choosing tests and maintaining effective hygiene, both in a single-cat home and in a cattery.

This article explains where intestinal infections come from, how they are diagnosed and what helps reduce recurrence. It does not replace a diagnosis or individual advice from a veterinarian.

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Infection does not always mean disease
Roundworms, hookworms and tapeworms
Giardia duodenalis: life cycle and transmission
Giardia: treatment and environmental hygiene
Tritrichomonas blagburni: a large-intestinal infection
Cystoisospora and Cryptosporidium
Toxoplasma gondii: infection and oocyst shedding
Clostridium perfringens and disease diagnosis
Faecal diagnostic methods and their limitations
Small- and large-intestinal diarrhoea
Intestinal recovery after infection
Microbiota, probiotics and supportive care
Prevention in catteries and multi-cat homes
Key conclusions
Sources and further resources

Infection does not always mean disease

A cat may shed infective forms of a parasite despite having a good appetite, being active and passing well-formed faeces. Conversely, detecting an organism does not prove that it is responsible for diarrhoea. This applies to Giardia, Tritrichomonas and the bacterium Clostridium perfringens, among others, which are also found in animals without symptoms.

Laboratory results must therefore be assessed alongside the cat’s signs, living conditions and the test method used. The organism’s name on a report alone does not determine treatment.

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Roundworms, hookworms and tapeworms

The feline roundworm (Toxocara cati) lives in the small intestine as an adult. Cats can become infected by swallowing eggs that have matured in the environment, or by eating a rodent carrying parasite larvae. Kittens can also become infected through their mother’s milk. The feline roundworm is not transmitted across the placenta; in this respect it differs from the canine roundworm Toxocara canis.

Newly shed roundworm eggs are not immediately infective. They first need to mature under suitable conditions, so prompt litter-tray cleaning reduces the risk. Heavy burdens can cause vomiting, diarrhoea and poor weight gain, and occasionally intestinal obstruction. Faecal tests may be negative before the parasites start producing eggs.

Hookworms attach to the intestinal lining. In large numbers, they can damage tissue and cause blood loss and anaemia. The risk of infection with particular species depends, among other things, on the region where the cat lives or travels.

The tapeworm Dipylidium caninum is transmitted through fleas: the cat becomes infected by swallowing an infected flea. Infection with other tapeworms, such as certain species of Taenia, is associated with hunting and eating prey. Routine faecal testing may miss a tapeworm because segments and eggs are shed irregularly. Sometimes segments resembling grains of rice can be seen around the anus or on bedding. With D. caninum treatment must be accompanied by flea control to reduce reinfection.

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Giardia duodenalis: life cycle and transmission

Giardia duodenalis is a parasitic protozoan that mainly colonises the small intestine. It exists in two forms: the trophozoite, an active form that attaches to the intestinal surface, and the cyst, which enables transmission. Cats become infected by swallowing cysts originating from an infected animal’s faeces. These may reach the mouth through water, objects, contaminated paws or grooming around the anus. Cysts can also be carried mechanically into the home on footwear.

Giardia and small-intestinal damage

Giardia can disrupt the function of the cells lining the intestine and their microvilli, the tiny projections on their surface that assist digestion and absorption. Some animals also develop shortened intestinal villi. The severity of changes varies, and many cats remain asymptomatic carriers. When infection causes disease, the most common signs are soft faeces and diarrhoea, sometimes accompanied by weight loss.

Giardia and the risk to people

Giardia has different genetic variants. Cats often carry assemblage F, which is mainly associated with this species, but variants that infect people may also occur. An ordinary positive result does not identify which variant is present, so it cannot establish the risk to people on its own. Whatever the result, handwashing and litter-tray hygiene remain essential.

Diagnosing Giardia

Cysts can be sought by flotation: in a suitable solution, they rise to the surface of the sample, allowing examination under a microscope. Antigen tests detect components of the parasite, while PCR detects its genetic material. Cysts are shed intermittently, so a single sample may not be enough. Results must be interpreted alongside clinical signs, previous treatment and the situation of other cats in the home or cattery.

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Giardia: treatment and environmental hygiene

Recurrence in a group of cats often results from renewed contact with cysts. Even a successfully treated cat may swallow them from a litter tray, the environment or while grooming contaminated skin and fur. Therefore, treatment, prompt faeces removal and thorough cleaning must take place together. In multi-cat homes, management must cover the whole group. I explain this in the section on prevention and simultaneous treatment.

Giardia cysts are susceptible to sufficiently high temperatures. During management of an infection, ESCCAP recommends cleaning litter trays daily with boiling water and drying them thoroughly. Steam can be effective on surfaces that tolerate it, provided that a suitable temperature and exposure time are actually achieved. Washing bedding at high temperatures and cleaning bowls also help reduce contamination.

The order matters: first remove faeces and wash the surface, then use an appropriate heat treatment or a product with proven effectiveness, and finally dry everything thoroughly. Not every household disinfectant destroys cysts. High temperatures can damage some materials. Cleaned items must cool before the cat uses them again.

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Tritrichomonas blagburni: a large-intestinal infection

Tritrichomonas blagburni, previously described in cats as T. foetus, is a trichomonad that mainly colonises the large intestine. Unlike Giardia, it does not form typical resistant cysts. Its active forms are susceptible to drying, making transmission on dry footwear much less likely than the introduction of Giardia cysts. They can, however, survive for a time in moist faeces.

Signs and intestinal changes

Tritrichomonas can cause inflammation of the large intestine, damage its surface and disrupt the balance of microorganisms living there. The large intestine does not have the villi characteristic of the small intestine, so trichomonosis should not be described as a disease that destroys villi. Common signs include chronic diarrhoea, mucus, fresh blood and painful straining to defecate. Despite these problems, a cat may retain a good appetite and remain active for a long time.

Diagnosis and the importance of choosing the right medicine

The preferred diagnostic method is PCR on an appropriately collected faecal sample. Direct microscopic examination of a fresh sample is more likely to give a false-negative result; specialised culture is also available. Ask the laboratory in advance how to collect and transport the sample. PCR detects DNA, so a positive result does not always prove that live protozoa are present, particularly after treatment.

Treatment for giardiasis cannot automatically be used for trichomonosis. Metronidazole is not considered effective at eliminating T. blagburni, nor is fenbendazole. Where treatment is needed, ronidazole is preferred, but it is not licensed for cats and can cause serious neurological signs. Not every infection responds to treatment. This medicine therefore requires a diagnosis and close veterinary supervision.

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Cystoisospora and Cryptosporidium

Protozoa of the genus Cystoisospora multiply in the cells lining the intestine and can damage them, particularly in heavily infected kittens. Their oocysts, the forms shed in faeces, need time and suitable conditions to mature and become infective. Prompt faeces removal reduces this risk. Flotation is the most commonly used diagnostic method.

Cryptosporidium can also damage the intestinal surface and cause diarrhoea, particularly in young or immunocompromised cats. Its oocysts are very small, so special staining techniques, tests detecting parasite components or PCR are used to identify infection. The risk of transmission to people depends on the protozoan species and genetic variant.

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Toxoplasma gondii: infection and oocyst shedding

Felids are the definitive hosts of Toxoplasma gondii; the form of the parasite shed in faeces can develop in their intestines. Cats can become infected by eating meat or prey containing cysts, or by swallowing mature oocysts from the environment. Most healthy adult cats have no symptoms. More severe disease may occur in very young or immunocompromised animals.

The shedding period and the importance of the litter tray

Cats do not shed oocysts continuously. Shedding most commonly follows the first infection and is relatively brief, usually around 10–14 days. Repeat shedding is possible in certain circumstances. Fresh oocysts are not immediately infective: they usually mature in the environment over 1–5 days. Daily removal of faeces from the litter tray is therefore an important preventive measure.

Interpreting tests and the role of raw meat

Antibody tests assess the body’s response to exposure to the parasite. A positive IgG result does not mean that the cat is currently shedding oocysts. IgM results should likewise not be interpreted in isolation from signs and other tests. Raw meat may contain parasite cysts. A trusted source and correct storage do not remove all risk; appropriate heat treatment offers more effective protection.

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Clostridium perfringens and disease diagnosis

Clostridium perfringens also occurs in healthy cats. A positive PCR detecting this bacterium alone does not prove that it caused the diarrhoea. The veterinarian considers the signs, the bacterium’s ability to produce toxins and other possible causes. It should not be confused with Clostridium botulinum, which can produce botulinum toxin.

Do not feed your cat food from a swollen, leaking or suspiciously ‘clicking’ tin. This means an unopened container whose distorted lid flexes with a click when pressed, rather than the normal sound of opening a tin. Do not taste the product or assess its safety by smell alone. The appearance of a tin cannot identify the cause of damage or the organism involved, but it is sufficient reason not to feed the contents. Keep a photograph of the packaging and the batch number for a complaint.

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Faecal diagnostic methods and their limitations

No single test detects all parasites. The method, sample quality and whether the cat was shedding eggs, cysts or oocysts at the time of collection all matter. Samples from the same cat, collected on different days or examined using different techniques, may therefore produce different results.

Method	Main use	Important limitation
Flotation	Eggs of many worms, and cysts and oocysts of some protozoa	Effectiveness depends on the solution and technique
Centrifugal flotation	Concentrating the sample, which may improve parasite detection	Does not detect all parasites
Sedimentation	Heavier parasite eggs, particularly those of some flukes	Does not replace routine flotation
Direct fresh wet mount	Active, motile forms of some protozoa	Infections are easily missed; a fresh sample is essential
Antigen test	Components of a specific parasite, such as Giardia	Coverage depends on the test
PCR	Genetic material of a specific organism	Does not always confirm a living organism or that it caused the signs
Specialised culture	Selected live organisms, such as Tritrichomonas	Requires specific collection and transport conditions

Centrifugal flotation helps concentrate the sample, while sedimentation detects forms that settle more readily to the bottom. The choice depends on the parasite being sought. A single negative result does not always exclude infection, particularly before egg shedding begins or when cyst shedding is intermittent.

In a cattery, pooled sampling from a mother and her litter can be agreed with the veterinarian or laboratory. A negative result does not, however, replace individual investigation of a sick kitten. If a particular cat has diarrhoea, it is best to test a sample from that cat.

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Small- and large-intestinal diarrhoea

The appearance of faeces and the pattern of defecation can help suggest which part of the intestine is affected. They are not enough to diagnose the cause.

Feature	More typical of the small intestine	More typical of the large intestine
Volume of faeces per bowel movement	Larger	Smaller
Frequency of bowel movements	Unchanged or slightly increased	Markedly increased
Painful straining	Less common	More common
Mucus and fresh blood	Less characteristic	More characteristic
Weight loss	More common with malabsorption	May be absent

Signs can overlap, particularly when disease affects more than one part of the intestine. The table is a guide to observation, not a basis for choosing treatment yourself.

Do not delay veterinary advice if diarrhoea is accompanied by lethargy, loss of appetite, repeated vomiting, a substantial amount of blood or signs of dehydration. Kittens need especially prompt help, as they can become weak rapidly.

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Intestinal recovery after infection

After an infection is controlled, the intestines do not always return to normal immediately. Inflammation, malabsorption or changes in the microbiota, the community of microorganisms living in the digestive tract, may persist. Recovery varies between individuals: one cat may improve within a few days, while another may need longer treatment for chronic changes.

However, persistent diarrhoea should not simply be assumed to mean that ‘the gut is rebuilding’. If treatment does not produce the expected improvement or signs return, go back to the veterinarian. It may be necessary to assess treatment effectiveness, check for reinfection or investigate another disease.

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Microbiota, probiotics and supportive care

Dysbiosis is a harmful disruption of the composition or function of the microbiota. It may accompany infections, antibiotic treatment, dietary changes and intestinal disease. Not every difference in microbial composition means disease, however.

A probiotic contains live microorganisms that provide a documented health benefit when given in adequate amounts. A prebiotic is a substance used as a nutrient source by particular intestinal microorganisms, providing a health benefit. A synbiotic combines live microorganisms with a suitably selected nutrient source of this kind. Not every product containing bacteria, or every type of fibre, meets these criteria.

The strain matters, not just the bacterial name

A probiotic’s effect depends on the specific strain, a particular group of microorganisms within a species. In a study by Bybee and colleagues in 2011 shelter cats receiving strain SF68, described as Enterococcus faecium, were less likely to have diarrhoea lasting at least two days than cats receiving a placebo. In turn, Torres-Henderson and colleagues in 2017 found that some gastrointestinal signs were reduced in cats receiving amoxicillin-clavulanate. However, they found no difference in microbiota diversity between the groups. These findings concern specific situations, not every type of diarrhoea.

These results cannot be attributed to every product bearing the name Enterococcus. When choosing a product, the strain designation, the number of live microorganisms expressed as CFU, a dose supported by research and viability through the expiry date all matter.

Saccharomyces boulardii is a probiotic yeast studied in selected intestinal disorders. Laboratory findings or research in other species cannot automatically be applied to cats. There is insufficient evidence for a probiotic to replace treatment of giardiasis or trichomonosis in cats. Nor is there a basis for recommending the same predetermined supplementation period, such as 30 or 90 days, to every cat after an infection.

No specific probiotic has been shown to directly rebuild a cat’s intestinal villi. Controlling the cause of illness, appropriate nutrition, hydration and treatment tailored to the animal’s condition remain the basis of recovery.

The section on microbiota and probiotics was inspired by the webinar ‘Probiotykoterapia dla psów i kotów’ (‘Probiotic Therapy for Dogs and Cats’), presented by Adrianna Iwan, MSc, author of the ‘Mikrobiolog z przypadku’ profile (Pets Diag S.A., 30 November 2022). The findings concerning strain SF68 described above come from the linked scientific publications.

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Prevention in catteries and multi-cat homes

Quarantining new cats, risk-based testing, prompt faeces removal, thorough washing and drying of litter trays, flea control and safe feeding help reduce infections. Dirt must always be removed before disinfection because faecal residue impairs the action of many products.

In symptomatic or recurrent giardiasis in a multi-cat home, management must cover the entire group. Cats sharing litter trays may shed cysts without having diarrhoea. A single negative result does not rule out infection because shedding is intermittent, and the cat may still acquire the infection from others. Treating only the animal with symptoms may therefore fail to stop the parasite circulating in the home.

In this situation, simultaneous treatment of all cats in contact may be needed, including those without symptoms or with a single negative result. The veterinarian determines the plan for the whole group, chooses medicines and sets the timing. Litter trays, bedding and the environment must be cleaned at the same time. This approach is included in the ABCD guidelines on giardiasis. It does not automatically mean that every asymptomatic cat with an incidental positive result needs treatment, or that the same rule applies to all parasites.

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Key conclusions

Intestinal parasites and infections require different tests and treatments. Laboratory results must always be considered alongside the cat’s condition. The best results come from combining an accurate diagnosis, appropriate treatment and consistent environmental hygiene.

At home and in a cattery, the aim is lasting control of infection and a return to health, rather than giving one product after another without establishing the cause of the problem.

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Sources and further resources

ESCCAP – Worm Control in Dogs and Cats
ESCCAP – Control of Intestinal Protozoa in Dogs and Cats
ABCD – Guideline for Trichomonosis
CAPC – Fecal Exam Procedures
ABCD – Guideline for Giardiasis. Diagnosis, treatment and management of groups of cats.
CAPC – Giardia. Interpreting results and managing infection.
Bybee S.N., Scorza A.V., Lappin M.R., 2011. Effect of the probiotic Enterococcus faecium SF68 on presence of diarrhea in cats and dogs housed in an animal shelter. Journal of Veterinary Internal Medicine, 25(4), 856–860.
Torres-Henderson C. et al., 2017. Effect of Enterococcus Faecium Strain SF68 on Gastrointestinal Signs and Fecal Microbiome in Cats Administered Amoxicillin-Clavulanate. Topics in Companion Animal Medicine, 32(3), 104–108.
Iwan A., ‘Probiotykoterapia dla psów i kotów’ (‘Probiotic Therapy for Dogs and Cats’). Pets Diag S.A. webinar, 30 November 2022; educational material that inspired the section on microbiota and probiotics.