Treatment of internal parasites in dogs and cats

How to deworm properly and why not all treatments are the same

Most dog and cat owners have heard of internal parasites, especially those known as “worms”. However, when it comes to treating them, questions often arise: will any antiparasitic work? Does it need to be repeated? Is it the same for puppies as for adults? Why do parasites sometimes reappear despite having been treated correctly?

This article does not aim to explain what parasites exist or how they are diagnosed — topics we have already covered in other content — but rather to focus exclusively on internal parasite treatment. The aim is to help you understand how these parasitic infections are actually treated, the logic behind the different products available, and why the protocol must be adapted to the age and lifestyle of each animal. The importance of a methodical approach is better understood when one looks at the numbers: in a European scientific review, the overall prevalence of intestinal parasites in owned dogs has been reported to range from approximately 12.5% to 34.4%, and in owned cats from 10.1% to 22.8%, with great variability depending on region, lifestyle and diagnostic method; Even in well-cared-for animals, ‘zero risk’ is more of a wish than a reality, as various epidemiological studies point out.

From a veterinary point of view, deworming is not an automatic or routine procedure. It is a preventive and therapeutic medical decision that, when done properly, protects the animal’s digestive health, growth, nutritional status and even overall health in the long term. In fact, when a large number of faecal samples are analysed under real conditions, the problem becomes even more tangible: in a retrospective study by a reference laboratory in Madrid, with 15,899 samples collected between 2013 and 2023, it was found that 26% of dogs and 21.4% of cats tested positive for at least one parasite, with protozoa (e.g. Giardia duodenalis) very common in dogs. These data from a study in Madrid help to understand why prevention cannot be improvised.

Treating internal parasites: the essentials every owner should understand

What treating internal parasites really means

When we talk about “treating” internal parasites, we tend to think only of eliminating worms that are already present in the intestine. However, in veterinary medicine, antiparasitic treatment almost always has a dual function: to cure an existing infestation and, at the same time, to prevent new infestations. This is because many animals are continuously exposed to parasites throughout their lives, especially those that go outdoors, live with other animals, or live in environments where parasites are common. In these cases, spacing treatments too far apart makes it easier for the parasite to complete its cycle and reappear clinically or subclinically.

From a practical point of view, curative treatment eliminates adult parasites present at the time, while preventive treatment seeks to “interrupt” the process before the parasite load becomes significant. This approach is closely linked to a technical concept that deserves a simple explanation: the pre-patent period, which is the time between the animal becoming infected and the parasite beginning to produce eggs or detectable forms. In several common intestinal parasites, this period is around or exceeds four weeks, which is why European guidelines insist that, in high-risk animals, a monthly interval may be consistent with the biology of the parasite.

In practice, this means that owners should not simply ‘give a tablet when it’s time’ without knowing why it’s time, but should understand that the guideline is intended to anticipate the maturation of the parasite. A very simple way to put this theory into practice is to set reminders and review them at least once a year with your vet, because the risk changes when walks, trips, cohabitation with other animals or feeding habits change.

Why not all antiparasitics work the same

One of the most common mistakes is to think that all antiparasitic treatments are equally effective against all parasites. In reality, each treatment acts on certain groups of parasites and may be ineffective against others. The most “treacherous” difference for the owner is that of tapeworms: a dog or cat may be regularly dewormed for roundworms, but still maintain (or restart) tapeworm infections if the product does not include specific coverage for cestodes or if a flea problem persists.

This is easily understood with a clinical concept: if the parasite uses a “vehicle” (e.g. a flea), treating only the intestine without cutting off the vehicle is like drying the floor without turning off the tap. In this regard, the ESCCAP guidelines explicitly describe that the transmission of some tapeworms is related to the ingestion of fleas or chewing lice, and point to pre-patent periods of around three weeks in Dipylidium caninum, which explains why it can “come back” quickly if the external factor is not controlled.

The most common mistakes in deworming

In veterinary practice, failures in antiparasitic treatment are rarely due to a problem with the medication itself. Much more often, the origin lies in errors in the approach to the protocol, such as using an incomplete product, not repeating the dose when the parasite’s cycle requires it, not adjusting the animal’s weight correctly, or not taking the environment into account. Weighing the animal before administering the treatment and not relying on ‘last time’s weight’ is a simple measure that significantly improves effectiveness, because the dosage of most antiparasitic drugs is strictly adjusted to the weight range.

Your vet advises you – how to know if the treatment has “worked”

When we suspect poor response or resistance, there is a simple tool: the Faecal Egg Count Reduction Test (FECRT), which compares the number of eggs before and after treatment. It is inexpensive, can be done in the clinic and avoids “going round in circles” changing products without knowing what is happening.

Many apparent relapses are actually reinfections. Therefore, when an owner says “it hasn’t worked”, the vet’s job is often to translate that impression into a practical question: “have we eliminated the parasite, or have we eliminated the parasite but the animal is reinfected?”. In animals that hunt, eat raw meat or have access to rodents, the answer usually lies in their habits rather than in the tablet.

The role of the environment and cohabiting animals

Treating an animal correctly without taking its environment into account is, in many cases, insufficient. In households with multiple animals, the risk of cross-transmission is high, and prevention works best when applied consistently. A practical measure is to synchronise the deworming dates for all animals in the household and, if there is a confirmed flea-associated tapeworm problem, to maintain systematic flea control for as long as necessary to break the cycle.

What types of anti-parasite treatments are available and how to choose them

Treatments for the most common intestinal worms

The basis of any internal deworming programme is treatment targeting the most common intestinal worms. These parasites are easily transmitted through the soil, the environment or contact with other animals and are particularly common in young animals. For this reason, deworming schedules by age propose intensive guidelines in the early stages: in dogs, for example, ESCCAP recommends starting at 2 weeks of age and repeating every 14 days in the early stages, continuing until a more stable pattern is achieved. In cats, the proposed start is from 3 weeks of age with a similar logic of repetitions, before moving on to monthly guidelines until 6 months of age. These details are particularly useful because they turn a general recommendation (“deworm puppies”) into a concrete plan.

In practice, administering the antiparasitic after a light meal and at a quiet time improves digestive tolerance and reduces the risk of stress-induced vomiting, which is especially important in sensitive puppies and kittens.

Treatments that include protection against tapeworms

Tapeworms require a specific approach. Unlike other intestinal worms, their life cycle usually involves an intermediate host, such as fleas. Therefore, in animals with fleas or at risk of ingesting them, flea control is no longer an “extra” but becomes part of the internal treatment. In addition, for certain risk profiles, ESCCAP is very explicit about intervals: for example, it states that dogs that consume raw meat or insufficiently treated offal should receive treatment against tapeworms at 4-week intervals, because the risk is not occasional but sustained.

Broad-spectrum treatments: when they make sense

Broad-spectrum treatments combine different active ingredients and cover several types of parasites in a single administration. They are particularly suitable for animals at higher risk of exposure or when the aim is to simplify the protocol to improve owner compliance. This is not a minor detail: in surveys and studies of perception and practice, a significant proportion of owners do not follow the guidelines considered “optimal”, and it has been found that owners with regular veterinary visits tend to comply better with the recommended protocols, which connects directly with the idea of simple and sustainable plans. In other words, sometimes the best medicine is the one that is administered well throughout the year, as discussed in studies on practices and compliance.

Forms of administration and adherence to treatment

Nowadays, internal antiparasitic treatments are not limited to traditional tablets. There are chewable formulations, internal-acting pipettes and combination treatments that cover both internal and external parasites. From a clinical point of view, the best treatment is one that the animal accepts and the owner can administer regularly and correctly. The choice of format is not a cosmetic detail, but a tool to avoid failure: a perfect plan that is not followed becomes an ineffective plan.

Medical fact – when the side effect… is the parasite itself

After deworming, some dogs or cats may experience occasional vomiting, diarrhoea or loss of appetite. Sometimes it is not an “allergy to the medication”, but a digestive reaction to the expulsion of parasites. If it is intense, if there is blood or if it persists, then yes: call the vet. 

The special case of intestinal protozoa

Intestinal protozoa, such as Giardia or coccidia, are not worms and do not respond to the same treatments. Furthermore, even when treated correctly, difficulties may persist because some animals continue to shed cysts or are easily reinfected if the environment is contaminated. In general terms, it is important to state clearly that it is not always a case of “drug resistance”; sometimes it is simply that “the parasite remains in the domestic ecosystem”. This point is repeatedly raised in clinical reviews on giardiasis, which emphasise that management includes treatment plus environmental measures. And so as not to idealise, it is also important to know that efficacy may vary in real conditions: a study in France evaluated fenbendazole and described that, after treatment, high cyst excretion may persist, suggesting reinfection and/or limited efficacy in that population, which reinforces the role of hygiene and veterinary follow-up.

At home, practical measures are usually simple but consistent: frequent washing of drinking bowls and feeders, prompt removal of faeces, cleaning of surfaces where the animal lies down and, in selected cases, a bath at the end of treatment to reduce cyst contamination of the coat.

Deworming protocols according to the age and profile of the animal

Puppies and kittens: why the protocol is different

During the first months of life, the risk of parasitic infection is particularly high. Many parasites can be transmitted from the mother, and the animal’s immune system is still immature. Therefore, protocols at this stage are more intensive and require repeated treatments. In cats, for example, ESCCAP recommends starting at 3 weeks and continuing with repeat treatments at short intervals before moving to monthly treatments until 6 months of age. It also mentions that a specific regimen for the mother (e.g. with emodepside spot-on around 7 days before giving birth) can reduce the lactogenic transmission of Toxocara cati to kittens, which exemplifies how treatment is thought of as “family-based” and not just “individual-based”. 

Strictly following the recommended schedule and not delaying doses, even if the animal “seems to be fine”, is essential at this stage, because the goal is not only to cure, but to allow for the most stable growth and digestive development possible.

From six months onwards: maintenance and prevention

In healthy adult animals, the goal becomes prevention. The frequency of treatment should be adapted to the animal’s lifestyle, as not all animals are at the same level of risk. When communicating this information, it is important to explain it clearly and precisely: some animals, due to their habits, are at risk every day (hunting, raw meat, contact with rodents, colonies, crowded parks), while others are at risk only sporadically. Therefore, guidelines can range from longer intervals for lower-risk animals to monthly treatment for higher-risk groups, based, among other arguments, on the pre-patent period of some parasites and the objective of reducing environmental excretion.

At the same time, some international guidelines and advice emphasise the general principle of “individual risk”: treat if you live in areas or conditions where fleas, ticks or worms are widely present, and define the optimal interval with your vet. It is even mentioned that, in the US, CAPC recommends year-round treatment due to the widespread presence of parasites in many regions; although Europe is not homogeneous, this logic of personalisation helps to understand the reasoning behind modern preventive plans.

Adapting the protocol to lifestyle

Factors such as urban or rural living, access to the outdoors, contact with other animals, hunting habits or living with children directly influence the deworming strategy. Therefore, there is no single schedule that is valid for all animals, and communicating these aspects to the vet is key to defining a truly effective plan. A practical recommendation that usually works very well is to review the plan at least once a year, coinciding with a routine visit, because an animal’s lifestyle changes more than it seems: a young dog that starts going to the mountains, a cat that starts going outside, a family that adopts a second animal or adds a baby to the household.

Cases requiring individualised treatment

Pregnant or lactating females, geriatric animals, or those with chronic digestive diseases require an even more personalised approach. In these cases, veterinary monitoring is essential to ensure both the efficacy and safety of the treatment, avoiding automatic decisions and adjusting the plan to actual tolerances and risks. In addition, the environmental impact of the use of parasiticides is now also considered, and a cautious approach is recommended, balancing animal health, public health and sustainability, as indicated by the positions of veterinary associations.

“Your vet advises you” – when we raise the bar

Some European guidelines suggest that households with young children (≈5–6 years old), elderly people, or immunocompromised individuals should consider a more stringent strategy: monthly deworming or periodic faecal testing and treatment based on findings. This is a way of adapting the plan to the actual risk in the household, rather than the “average” risk.

Source: https://www.esccap.org

The treatment of internal parasites is a key part of modern preventive medicine. It is not a matter of administering a product occasionally, but of following a consistent, tailored and regularly reviewed strategy. A well-designed protocol improves digestive health, reduces reinfestation and protects both the animal and its environment. At Eurovet Veterinary Clinic, we advocate personalised deworming, based on the age, lifestyle and real needs of each pet, as the best guarantee of effective and lasting protection.

Sources and recommended reading

Zanzani, S. A., Gazzonis, A. L., Scarpa, P., Berrilli, F., Manfredi, M. T. (2014) – Intestinal parasites of owned dogs and cats from metropolitan and micropolitan areas: prevalence, zoonotic risks, and pet owner awareness in northern Italy – BioMed Research International – Study of prevalence and awareness among owners (coprology + Giardia coproantigens) with a focus on zoonotic risk. – https://pubmed.ncbi.nlm.nih.gov/24883320/

Barrera, J. P., Montoya, A., Marino, V., Sarquis, J., Checa, R., Carmena, D., Estévez-Sánchez, E., Gómez-Velasco, C., Moraleda, P., Cano, L., Fuentes, I., Miró, G. (2025) – Intestinal parasite prevalences in dogs and cats: a decade of retrospective data from a reference veterinary laboratory in Madrid, Spain – Parasites & Vectors – Retrospective series (2013–2023; 15,899 samples) with prevalences, co-infections and epidemiological trends. https://doi.org/10.1186/s13071-025-07168-1

ESCCAP (European Scientific Counsel Companion Animal Parasites) (2025) – Worm Control in Dogs and Cats (ESCCAP Guideline 01, Seventh Edition – June 2025) – ESCCAP – European reference guide for helminth control (risk-based approach, schedules and complementary measures). – https://www.esccap.org/uploads/docs/biu0jhej_0778_ESCCAP_GL1__English_2025_v21_1p.pdf

ESCCAP Secretariat (n.d.) – Scheme for individual deworming of dogs – ESCCAP – Practical guide to individualised deworming of dogs (by age and risk factors). – https://www.esccapuk.org.uk/uploads/docs/vyt9zsid_Scheme_for_individual_deworming_of_dogs.pdf

ESCCAP Secretariat (n.d.) – Scheme for individual deworming of cats – ESCCAP – Practical guide to individualised deworming of cats (by age and risk factors). – https://www.esccapuk.org.uk/uploads/docs/9tvb43m3_Scheme_for_individual_deworming_of_cats.pdf

Companion Animal Parasite Council (CAPC) (2025) – General Guidelines for Dogs and Cats – CAPC – Recommendations for year-round prevention, frequency by life stage, and a veterinarian-guided approach. – https://capcvet.org/guidelines/general-guidelines/

British Small Animal Veterinary Association (BSAVA) (2022) – Parasite control (Position statement) – BSAVA – Position statement: risk-benefit assessment, lifestyle-adapted control and environmental considerations. – https://www.bsava.com/position-statement/parasite-control/

British Veterinary Association (BVA), British Small Animal Veterinary Association (BSAVA), British Veterinary Zoological Society (BVZS) (2025) – BVA, BSAVA and BVZS policy position on responsible use of parasiticides for cats and dogs – BVA/BSAVA/BVZS – Updated joint document (31/10/2025) on responsible use and minimising environmental impact without compromising health and welfare. – https://www.bva.co.uk/media/6627/bva-bsava-and-bvzs-policy-position-on-responsible-use-of-parasiticides-for-cats-and-dogs.pdf

Massetti, L., Traub, R. J., Rae, L., Colella, V., Marwedel, L., McDonagh, P., Wiethoelter, A. (2023) – Canine gastrointestinal parasites perceptions, practices, and behaviours: A survey of dog owners in Australia – One Health – Survey quantifying compliance gaps and factors associated with correct prophylaxis. – https://pubmed.ncbi.nlm.nih.gov/37415719/

Kaufmann, H., Zenner, L., Benabed, S., Poirel, M.-T., Bourgoin, G. (2022) – Lack of efficacy of fenbendazole against Giardia duodenalis in a naturally infected population of dogs in France – Parasite – Clinical study under real conditions discussing clinical response vs. complete elimination and the role of reinfection. – https://pubmed.ncbi.nlm.nih.gov/36315102/

Smith, R. C., Starkey, L. (2023) – Update on Giardiasis: Diagnostics, Treatment, and Management – Today’s Veterinary Practice – Practical review (diagnosis, treatment and management; emphasis on cyst persistence and hygiene measures). – https://todaysveterinarypractice.com/wp-content/uploads/sites/4/2023/10/TVP-2023-1112_Giardiasis.pdf

World Small Animal Veterinary Association (WSAVA) (2025) – Principles of Wellness – WSAVA – Outreach document for carers: includes parasite control as a preventive pillar within “holistic wellness”. – https://wsava.org/wp-content/uploads/2025/06/Principles-of-Wellness-FINAL.pdf

European Platform for the Responsible Use of Medicines in Animals (EPRUMA) (2024) – Best Practice Guidelines for the Responsible Use of Parasite Control in Pets – EPRUMA – Good practice guide for effective and responsible parasite control (One Health approach and targeted use). – https://epruma.eu/wp-content/uploads/2024/11/Epruma-Parasite-Control-Guidelines1124.pdf