🪱 Parasites in dogs and cats

Real impact on your pet’s health

Talking about intestinal parasites in dogs and cats is not just about addressing a common cause of diarrhoea, but understanding a health problem that is much broader, more silent and more progressive than is often perceived. In clinical practice, a significant proportion of parasitised animals show no obvious signs in the early stages, which allows the infestation to become chronic and the damage to slowly accumulate in the digestive, immune and metabolic systems. This reality makes parasitic infections a major diagnostic and preventive challenge, especially in puppies, geriatric animals and patients with concomitant pathologies. To put it in figures, in Europe, average prevalences of Toxocara have been reported at 14.6% in dogs and 24.5% in cats, and in dogs ≤ 6 months old it can reach 41.2%, which illustrates why prevention cannot be based solely on “whether or not they have diarrhoea” (European prevalence reviews). Understanding how these organisms work, what signs they generate in the body and why regular prevention is key allows owners to make more informed decisions and effectively protect the health and well-being of their pets.

The parasite as a silent aggressor

Organisms highly adapted to survive in the host

Intestinal parasites, both helminths (worms) and protozoa, represent an extreme example of biological adaptation. Over millions of years of evolution, they have developed effective mechanisms to survive within the host’s body without being immediately detected. Their ability to modulate the immune response, adhere to specific tissues or alternate between active and less obvious phases explains why many parasitic diseases can establish themselves silently, progressively and persistently. In Giardia, for example, the situation is complicated because the detected prevalence varies greatly depending on the diagnostic technique: in a large European synthesis in animals with digestive signs, the pooled prevalence was 15.2% in dogs and 12% in cats, and it was observed that ELISA/IFA/PCR could report 2.6 to 3.7 times more positives than traditional microscopy (European meta-analysis). From a practical point of view, this makes it especially important not to wait for clear symptoms to appear before taking action: follow risk-based preventive guidelines and supplement with checks when appropriate.

⏳ Common clinical fact: The problem has been going on for months… even if the symptom appears today

When a dog suddenly starts having diarrhoea, vomiting or lethargy, many owners think of an “acute” cause. However, in intestinal parasitosis, it is common for the damage to have accumulated over weeks or months, and for the visible symptom to be only the “tip of the iceberg”. Detecting it early avoids longer treatments and slower recoveries.

Much more than just the intestine: multi-organ colonisation

Although the intestine is their main habitat, many parasites undergo phases of migration or localisation in other organs and tissues. The lungs, heart, liver, skin, and subcutaneous tissue may be directly or indirectly involved during the parasite’s life cycle. These migrations explain the appearance of signs that are apparently unrelated to the digestive system, such as coughing, exercise intolerance, skin lesions or general alterations in the animal’s condition. Therefore, when a dog or cat has persistent mild respiratory symptoms or recurrent pruritus without obvious cause, it is advisable to assess its overall antiparasitic status and not limit suspicion to local problems alone.

The clinical deception: subclinical and non-specific symptoms

One of the most complex aspects of parasitic infections is their initial clinical presentation. In a high percentage of animals, especially healthy adults, the first months of infestation are asymptomatic or present with vague symptoms: slightly softer stools, episodes of lethargy or minimal weight changes. This is reinforced by a key diagnostic fact: many parasites (including Giardia) can intermittently shed diagnostic forms, which is why control and diagnostic recommendations emphasise the importance of repeated sampling and adapting the strategy to each case (risk-based control recommendations). In daily practice, this justifies not relying solely on the animal’s external appearance or the absence of diarrhoea to assume that ‘everything is fine’.

🐕 Did you know? Dragging the bum along the ground is not always due to anal glands

The famous “hindquarters dragging” is often associated with anal sacs, but it can also be a sign of intestinal parasitosis. Perianal irritation, itching or the sensation of a foreign body can cause the dog to drag its bottom even if the glands are fine. Therefore, if the behaviour is repeated, it is advisable to look further.

The real risk: cumulative and progressive damage

Far from being a harmless process, chronic parasitic infection involves continuous wear and tear on the body. Constant aggression on the intestinal mucosa, competition for nutrients and persistent inflammatory stimulation end up altering digestive, metabolic and immunological functions. And here’s a very “real-life” point: even when a conventional faecal examination is performed, sensitivity can vary greatly depending on the method and the parasite. In Giardia, for example, wide ranges of sensitivity have been reported in zinc sulphate flotation (34% to 88%), which explains why a negative result does not always rule out infection if the clinical suspicion and context fit (comparison of diagnostic methods). From a preventive perspective, implementing regular measures tailored to age, environment and lifestyle is often more effective (and less frustrating) than pursuing advanced infestations.

Local alterations: where the parasite lives, the symptom appears

Digestive system: the epicentre of the problem

Most intestinal parasites exert their primary action on the digestive tract. Acute or chronic diarrhoea is the most common sign and may be accompanied by vomiting, flatulence, poorly digested stools, the presence of mucus or blood, and progressive weight loss. These manifestations reflect malabsorption processes, mucosal inflammation, damage to the intestinal villi, or continuous blood loss, depending on the parasitic species involved and the infestation load. In practical terms, there are two messages that can often save weeks of trial and error at home:

1. avoid chaining together dietary changes or “intuitive treatments” if the symptoms recur, as this can mask the real pattern; and
2. optimise sample collection: when indicated by the vet, providing serial samples (e.g. over several days) increases the chances of detecting intermittent shedding and choosing the correct treatment (approach recommended by control guidelines).

One example that “escapes” typical suspicion is large intestine diarrhoea in young cats: in a series of 1,606 faecal samples, Tritrichomonas foetus was detected in 10.6%, and the prevalence rose to 14.9% in cats under one year of age (study on T. foetus in cats). This is a valuable clue when there is mucus, urgent defecation, and relapses despite dietary changes or “routine” treatments.

⚠️ Medical fact: Occasional vomiting may be intestinal in origin, not gastric.

Some dogs with parasites experience sporadic vomiting, often in the morning or on an empty stomach, without other noticeable signs. This occurs because intestinal irritation and altered digestive transit can cause secondary gastric reflexes. If vomiting recurs and obvious causes are ruled out, the intestine —and its possible parasites— deserves to be checked.

Respiratory system: when the parasite migrates

During certain stages of their life cycle, some parasites pass through the respiratory system. This migration can cause a persistent dry cough, exercise intolerance or rapid breathing, especially in young animals. In many cases, these signs are interpreted as respiratory infections or allergic processes, delaying the correct diagnosis. Contextualising (access to the outdoors, cohabitation with other animals, busy parks, hunting habits, etc.) greatly changes the prior probability, and with it the decision to perform specific tests.

Skin and perianal region: visible signs of the problem

Anal pruritus, dragging the hindquarters and excessive licking of the perineal area are classic signs which, although not exclusive to parasites, should raise suspicion when combined with soft stools or loss of body condition. At the skin level, infestation can result in a dull coat and recurrent dermatitis, due to nutritional and inflammatory effects. In terms of environmental prevention, three simple measures often make a difference in reinfestations: collect faeces regularly, clean elimination areas thoroughly (especially in puppies) and prevent the animal from drinking from puddles or ingesting debris/faeces during walks (especially relevant for protozoa such as Giardia).

Mechanical, inflammatory or haemorrhagic damage

Not all parasites cause damage in the same way. Some cause mechanical damage by attaching themselves to the mucosa, others induce persistent chronic inflammation, and others feed on blood, causing continuous microhaemorrhages. This diversity explains why protocols should not be “one size fits all”: European control guidelines recommend individualising the strategy according to risk and lifestyle, and in some profiles (e.g., households with young children or immunocompromised individuals) they propose stricter control plans and/or periodic testing (recommendations by risk group).

Systemic impact and general symptoms

Much more than a digestive problem

When parasitic infection persists over time, its effects transcend the intestine. Chronic loss of nutrients, proteins, iron and vitamins can lead to anaemia, growth retardation and progressive muscle wasting. In puppies, this is not theoretical: the fact that Toxocara is particularly common at an early age (with figures reaching 41.2% in dogs ≤ 6 months in European compilations) fits with why we see animals “that eat but do not thrive” if the preventive plan is not adjusted (prevalence data by age). At the same time, sustained immune activation can promote states of immune imbalance and predispose to secondary infections or digestive relapses.

Compromised general condition

Fatigue, apathy and reduced exercise tolerance are common manifestations in moderate to severe infestations. These signs, often attributed to age or stress, actually reflect the metabolic cost of living with a persistent parasite burden. Observing subtle changes in daily behaviour — reduced interest in play, early fatigue, changes in appetite — provides clues for early detection, especially when the animal “does not seem ill” but is not quite itself.

🧠 Medical fact: ‘Unexplained’ fatigue can start in the gut

A dog with parasites may appear more apathetic, sleep more or tire more quickly on walks, even without other clear symptoms. This is because chronic intestinal inflammation and nutrient loss generate a constant energy cost for the body. It is often attributed to age or heat, delaying the actual diagnosis.

Why each animal reacts differently

The clinical expression of a parasitic infection depends on age, immune status, nutrition, parasite load, diagnostic techniques used and environmental conditions. Two animals infected by the same agent may show anything from no symptoms to severe symptoms. This is very evident in Giardia: in recent studies with large samples, rates can be significantly higher in young animals than in adults (e.g., 18.2% in dogs aged 1–12 months versus 3.8% in dogs older than 12 months in a large series) (data by age group). This variability explains the need to individualise testing and prevention.

Differential diagnosis and prevention: the keys to control

In the presence of recurrent digestive signs, unexplained weight loss or persistent general alterations, intestinal parasitosis should always be included in the differential diagnosis, even in animals that are “apparently well dewormed”, because the actual risk depends on exposure and whether or not the protocol covers what that particular animal needs. In terms of prevention, European guidelines propose a risk-based approach: from periodic faecal checks (e.g. 1–2 times/year for lower-risk profiles) to more frequent plans (e.g. ≥ 4 times/year for certain cat profiles and/or higher-risk situations), adjusting according to lifestyle, cohabitation and health context (European guidelines on control and frequencies). This strategy avoids both under-prevention (when one “trusts” that nothing will happen) and misaligned prevention (when the same thing is always used without covering the actual risk).

Scientific sources and recommended reading

Overgaauw, P. A. M., Vinke, C. M., van Hagen, M. A. E., Lipman, L. J. A. (2020) – Prevalence of patent Toxocara spp. infections in dogs and cats in Europe – Parasites & Vectors – Systematic review and large-scale prevalence analysis demonstrating the high frequency of Toxocara in European dogs and cats, especially in young animals, and its zoonotic relevance – https://pubmed.ncbi.nlm.nih.gov/32381226/

Epe, C., Rehkter, G., Schnieder, T., Lorentzen, L., Kreienbrock, L. (2010) – Giardia in symptomatic dogs and cats in Europe – results of a European study – Veterinary Parasitology – European multicentre study analysing the prevalence of Giardia in dogs and cats with digestive signs and demonstrating diagnostic variability depending on the technique used – https://www.sciencedirect.com/science/article/abs/pii/S0304401710003481

Taylor, L. A., Zajac, A. M., Lindsay, D. S. (2023) – Comparison of three diagnostic tests for the detection of Giardia and Cryptosporidium in dogs – Journal of Veterinary Diagnostic Investigation – Comparative study demonstrating the differences in sensitivity between faecal flotation, antigen tests and molecular techniques in the diagnosis of intestinal protozoa – https://pmc.ncbi.nlm.nih.gov/articles/PMC10078926/

Kurnosova, O. P., Zimina, E. V., Baksheeva, S. V., et al. (2024) – Prevalence of Giardia duodenalis in dogs and cats of different age groups – Veterinary World – Recent epidemiological study showing significant differences in Giardia prevalence according to age, with higher rates in young animals – https://www.veterinaryworld.org/Vol.17/February-2024/16.pdf

European Scientific Counsel Companion Animal Parasites (ESCCAP) (2025) – Worm control in dogs and cats – Guideline 01 – ESCCAP – European reference guide establishing parasite control strategies based on risk, lifestyle and cohabitation, with practical recommendations for veterinarians and guardians – https://www.esccap.org/uploads/docs/biu0jhej_0778_ESCCAP_GL1__English_2025_v21_1p.pdf

European Scientific Counsel Companion Animal Parasites (ESCCAP) (2025) – Worm management scheme – cats – ESCCAP – Practical document detailing deworming schemes for cats according to exposure level, age and environment, with special attention to the prevention of zoonotic risks – https://www.esccap.org/uploads/docs/8obbw02h_0778_ESCCAP_GL1__Worm_Management_Scheme_Fact_Sheet__Cats_v7.pdf