CONDITION
Intestinal Parasites
Intestinal parasites are organisms that live in the digestive tract of dogs and cats, drawing nutrients from the host and sometimes causing visible signs. The types most commonly encountered in the United Kingdom include roundworms and tapeworms in both dogs and cats, and hookworms and whipworms, which here are found mainly in dogs. Protozoa such as Giardia also occur. The specific parasites vary by species, age, and lifestyle. Many owners arrive on this page after observing changes in stools, such as segments that resemble grains of rice, visible worms, mucus, or diarrhoea, or because a routine faecal sample has returned a positive result. In other cases, there may be no outward signs at all, particularly in adult animals with low parasite burdens. Young animals, those with outdoor access, or pets in multi-animal households tend to encounter parasites more frequently. A heavy burden can make a puppy or kitten seriously ill, and some of these parasites, including the common roundworms, can also infect people. This page explores what different parasites look like when they cause signs, how they are transmitted, the diagnostic methods used to identify them, and the range of treatment and prevention approaches available. The goal is to help you understand what may be happening and how these infections are investigated and managed over time.
Why this matters now
Intestinal parasites can be acquired at any life stage, but puppies and kittens are particularly vulnerable because puppies can be infected with roundworm before birth, kittens can be infected through their mother's milk, and their immune systems are still developing. Adult animals can pick up parasites from contaminated soil, grass, or prey, or through contact with infected faeces in environments where many animals congregate. Indoor cats tend to have lower exposure than those with outdoor access, though some parasites can be carried indoors on shoes or by intermediate hosts such as fleas. Several of these parasites can infect people. The roundworms Toxocara canis and Toxocara cati pass eggs in faeces. The eggs become infective after one to four weeks in soil or sand, and people are infected by swallowing them, most often young children. The larvae can migrate through the body, and in the eye they can cause loss of vision, usually in one eye, which can be permanent. Echinococcus granulosus, a tapeworm that dogs acquire by eating raw offal from infected sheep, can cause hydatid cysts in people who swallow its eggs from dog faeces; this is mainly relevant in sheep-farming areas. Some strains of Giardia can infect people, while others are specific to dogs or cats. Hookworm larvae in contaminated ground can penetrate human skin and cause an itchy, winding rash. A related tapeworm, Echinococcus multilocularis, can cause serious illness in people. There have been no known cases acquired within Great Britain. It is a notifiable disease there, so a suspected case is reported to the Animal and Plant Health Agency, and dogs entering Great Britain from most countries are required to have a tapeworm treatment containing praziquantel between 24 and 120 hours before arrival.
The course varies widely depending on the species of parasite, the number of organisms present, and the animal's age and immune competence. An adult dog harbouring a small number of tapeworms may remain outwardly normal for months, and infections of this kind often persist at low levels unless treatment is given. The picture differs in puppies and kittens. A young puppy with a heavy roundworm burden may show signs within days to weeks, and very heavy burdens in newborn puppies can cause death. Worm infections can also make young dogs more vulnerable to parvovirus, which causes similar signs. Reinfection can occur whenever the animal is re-exposed to eggs or larvae in the environment.
Signals & patterns
Early signals
Soft or loose stools
Faeces may become less formed or more frequent, sometimes with mucus coating the surface. This can reflect irritation of the intestinal lining by the parasites or their movement through the gut wall.
Visible worms or segments
Roundworms may appear as pale, spaghetti-like strands in vomit or faeces, while tapeworm segments can resemble grains of rice near the anus or in bedding. These are often the first sign an owner notices, particularly in young animals or after a new arrival.
Dull or rough coat
The coat may lose its usual sheen or feel coarser to the touch, reflecting reduced nutrient absorption as parasites compete for resources in the intestine. This change tends to develop gradually and may be subtle in well-nourished adults.
Increased appetite without weight gain
An animal may eat normally or more than usual but fail to put on weight, because calories and protein are being diverted to support the parasites. In young animals, this can show as slower growth compared to littermates.
Bloated or distended abdomen
Puppies and kittens with roundworm infections often develop a pot-bellied appearance, even when the rest of the body looks thin. This reflects both the physical presence of worms in the intestine and gas accumulation from altered digestion.
Later signals
Persistent or bloody diarrhoea
As the parasite load increases or the intestinal lining becomes more damaged, diarrhoea may become more frequent or contain visible blood, particularly with whipworms in the large bowel of dogs. Bloody diarrhoea with vomiting and lethargy in a puppy that is unvaccinated or not fully vaccinated also fits canine parvovirus. The two cannot be separated at home, and untreated puppies with parvovirus can die within two to three days.
Weight loss despite eating
Progressive loss of body condition can occur when nutrient malabsorption becomes significant, particularly in animals carrying multiple parasite species or those with compromised immunity. The ribs and spine may become more prominent over weeks to months.
Weakness or reduced activity
In the UK, marked weakness from worms is uncommon in adult animals. It is seen mainly in puppies and kittens with heavy burdens, and in dogs imported from, or that have travelled to, areas where blood-feeding hookworms such as Ancylostoma caninum occur. A young animal that is very dull and weak, not eating, has pale or white gums, is vomiting repeatedly, or has a swollen belly that is painful to touch can have severe anaemia, dehydration or a blockage. Very young puppies with heavy burdens can die from these effects.
Vomiting with visible worms
Heavy roundworm burdens can cause vomiting, sometimes with intact worms visible in the material. In most young animals this reflects irritation from the worms rather than a blockage. Blockage of the intestine by a mass of worms, or an intussusception where one part of the intestine telescopes into the next, is uncommon. Repeated vomiting, a painful belly, no faeces being passed and increasing dullness are the pattern that goes with it, and a blockage cannot be ruled out without an examination and often imaging.
Click to read about the biological mechanisms
How this is usually investigated
Investigation usually begins with a detailed history covering the animal's age, environment, access to hunting, travel history, and any observed changes in faeces, appetite, or body condition. A physical examination may reveal findings such as poor coat quality, abdominal distension in young animals, or weight loss. When parasites are suspected, investigation centres on faecal analysis, though the timing and method of testing influence what can be detected.
Faecal analysis
Complete blood count
Physical examination
Chemistry panel
Options & trade-offs
Management typically involves a combination of anthelmintic treatment to reduce or eliminate existing parasites, adjustments to the environment or routine to lower reinfection risk, and in some cases ongoing preventive medication. The approach chosen reflects the type of parasite identified, the animal's age and lifestyle, the presence of other animals or vulnerable people in the household, and the practicalities of administration. Different households find different combinations workable, and protocols often evolve over time as circumstances change.
Anthelmintic treatment courses
Medications such as fenbendazole, pyrantel, or praziquantel are given over one or more doses to kill adult parasites and, in some cases, larval stages. The choice of drug and duration depends on the parasite species identified or suspected. Some protocols involve repeated dosing to account for the time it takes for larvae to mature into egg-laying adults.
Trade-offs: Treatment eliminates parasites present at the time of dosing but does not prevent reinfection. Animals with access to contaminated environments or prey may require further treatment if exposure continues. Some parasites are more easily cleared than others, and not all drugs are effective against all life stages.
Regular preventive medication
Monthly or quarterly dosing with broad-spectrum anthelmintics reduces the accumulation of parasites in animals with ongoing exposure. These products often combine activity against several worm types and may also address external parasites such as fleas, which can carry tapeworm larvae, or lungworm species such as Angiostrongylus vasorum that are of concern in the UK.
Trade-offs: Preventive protocols require consistent administration and do not eliminate the need for faecal monitoring in some situations. Cost and the animal's tolerance of the formulation, whether tablet, spot-on, or chewable, affect long-term adherence. Not all products cover all parasite types, and selection depends on local parasite prevalence and individual risk.
Environmental management
Reducing access to contaminated soil, promptly removing faeces from gardens and litter trays, and controlling intermediate hosts such as fleas or rodents lowers the chance of reinfection. In multi-animal households, treating all susceptible animals together can reduce cycling of parasites between individuals.
Trade-offs: Complete elimination of environmental contamination is difficult, particularly outdoors, and some parasite eggs remain viable in soil for months or years. Cats that hunt and dogs with access to shared green spaces face ongoing exposure that hygiene measures can reduce but not eliminate.
Treatment of breeding animals and neonates
Pregnant or lactating females may be treated to reduce the transmission of roundworm larvae to puppies or kittens through the placenta or milk. Young animals are often given anthelmintic treatment at intervals during the first months of life, timed to coincide with the development of larvae acquired perinatally.
Trade-offs: This approach reduces early parasite burdens and the risk of heavy infection in young animals but does not prevent exposure from the environment after weaning. Protocols vary depending on the species and the perceived risk in the breeding population.
Common misconceptions
"If no worms are visible in the faeces, the animal does not have parasites."
Many intestinal parasites are microscopic, and even when larger worms such as roundworms or tapeworm segments are present, they may be passed intermittently or in small numbers that go unnoticed. Faecal analysis can detect eggs or cysts that are invisible to the eye, and a lack of visible worms does not rule out infection.
"Indoor cats do not need parasite prevention because they have no exposure."
While indoor cats generally face lower risk, exposure can still occur through fleas brought in from outside, ingestion of infected prey such as mice that enter the home, or contact with contaminated soil on shoes or clothing. The level of risk varies with individual circumstances, but it is not absent.
"A single dose of worming treatment protects an animal permanently."
The drugs used to clear worms eliminate parasites present at the time of dosing and give no lasting immunity. Animals with ongoing exposure require repeated treatment or regular preventive protocols to keep parasite burdens low. The frequency depends on lifestyle and local parasite prevalence.
Related conditions
Giardiasis
Giardiasis is caused by a specific microscopic parasite that lives in the intestine and can produce similar signs to other intestinal parasites, including diarrhoea and poor weight gain. It is one of the organisms often identified during faecal testing when parasitic infection is suspected.
Tritrichomonas Foetus in Cats
Tritrichomonas foetus is a microscopic intestinal parasite seen in cats, and can be considered alongside other parasitic causes of persistent diarrhoea. Like some intestinal parasites, it spreads through direct contact with faecal material. Routine faecal flotation does not detect it and a direct smear misses most infections, so testing typically relies on faecal PCR or culture.
Gastroenteritis in Dogs
Gastroenteritis in dogs can share clinical signs with intestinal parasitic infection, particularly diarrhoea and vomiting, and parasites are one of several possible causes of acute or persistent gastrointestinal inflammation. Faecal testing is often part of the diagnostic process when gastroenteritis is present.
Lymphoplasmacytic Enteritis
Lymphoplasmacytic enteritis describes chronic intestinal inflammation of uncertain cause that can produce signs similar to parasitism, including persistent diarrhoea and weight loss. Ruling out intestinal parasites through faecal examination is typically part of the diagnostic work-up when persistent small intestinal signs are present.
Colitis
Colitis can occasionally be triggered or worsened by certain intestinal parasites that inhabit the large bowel, and parasitic infection is one of the causes considered when a dog or cat shows signs of large intestinal inflammation. Faecal testing may help distinguish parasitic from non-parasitic causes of colitis.
Canine Parvovirus
Canine parvovirus causes lethargy, vomiting and often bloody diarrhoea in puppies that are unvaccinated or not fully vaccinated, and at home it cannot be told apart from heavy parasitism. Untreated puppies can die within two to three days, and concurrent worm infections can increase vulnerability.
Echinococcus Infection
Echinococcus tapeworms live in the intestine of dogs, usually without causing signs, but their eggs can cause cysts in people who swallow them. Echinococcus multilocularis is notifiable in Great Britain, and the tapeworm treatment required for dogs entering Great Britain is aimed at keeping it out.
Intestinal Intussusception
In young animals, heavy roundworm burdens are one recognised cause of intussusception, where one section of intestine telescopes into the next. It is uncommon, and the blockage it causes is not resolved by worming treatment.
The pattern of signs observed, the animal's lifestyle, and any findings from faecal testing shape the conversation about how often to test or treat, and which parasites to focus on. That routine picture does not fit a puppy or kitten that is very dull and weak, not eating, has pale gums, is vomiting repeatedly, has a painful swollen belly, or has bloody diarrhoea. These signs can reflect severe anaemia, a blockage or parvovirus rather than an ordinary worm burden, they cannot be told apart at home, and worming treatment does not address a blockage or parvovirus. In households with young children, multiple pets, or animals that hunt, the risk to people and other animals is often part of that conversation. Understanding the life cycles of the parasites identified can clarify why reinfection occurs and what adjustments to routine or environment might reduce it.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS