Stuart Fitzgerald
Dr Stuart Fitzgerald (MVB MANZCVS, University College Dublin)
Veterinarian deworming her dog

Something which sends a shiver down the spine of most dog owners is the idea that their beloved pet might have a worm infestation. However, many people do not really understand how common these parasites are, or the effects they can have on the health of both dogs and humans.

Every complex species on the planet is afflicted by a greater or lesser number of parasites – simpler organisms which have evolved in parallel to take advantage of the ecosystem on or inside their host’s body. The term ‘worm’ gives a false impression of some homogenous population of earthworm-like creatures, when in fact worms affecting dogs come in a great variety of shapes and sizes.

As diverse as their appearance is the way in which these parasites are transmitted. Millennia of evolution have resulted in a range of methods, from being spread through faeces to species whose larval stages ‘hitch a ride’ on an intermediate host, such as flea, which subsequently transfers the infecting organism into a new host. Reliance on an intermediate host may, however, limit the geographic spread of a particular species of worm.

For example, Paragonimus westermani, a type of parasite called a fluke, has a particularly complex life cycle through which it may infect a range of species from humans to tigers. Due to the fact that it relies on intermediate development in crustaceans and molluscs, this parasite is confined to certain warm, moist parts of the world, being most prevalent in the paddy fields of Southeast Asia, where many workers become infected, and in turn spit out the larval stages that go on to reinfect creatures below the surface of the water.

Human Health Concerns

Woman kisses her dog

As indicated above, humans can and do become infected with worms that they have picked up from infected animals. Given our particularly close relationship with dogs, we must take care not to allow our pets to develop heavy worms burdens and therefore to put our own health at risk. Many of the species of worms we commonly see cause minimal disruption to their definitive host: it is not in the parasite’s interest to kill the host, as this will limit its own survival, and its opportunities to reproduce.

However, severe health effects are much more common when a non-definitive host is infected. This is the case with the most common parasite of dogs, Toxocara canis, which is discussed below. Guidelines on deworming frequency are drawn up largely with human, rather than canine, health in mind, and adherence to these guidelines is especially important for any dogs owned by families with young children, who are more susceptible to developing severe complications due to infestation, and who also tend to be less fastidious about hygiene when handling their pets.

How Common Are Worms in Dogs?

Unless bred in a pathogen-free laboratory setting, it is safe to assume that all dogs carry a worm burden. Complex mechanisms developed by the parasites ensure that transmission occurs to newborn pups in the womb, while feeding from their mother, and in the environment, however hygienic it may be. Many of the species of concern spend much of their lives in a quiescent state in an adult host, as immune responses of the adult result in worms becoming subdued, or walled-off in intestinal, muscular, nervous, or mammary tissues. As part of the massive disturbance to normal physiology during pregnancy, elevated levels of glucocorticoids in a pregnant bitch cause suppression of her immune system.

This suppression allows reactivation of any dormant worms throughout the body, which then flock to the womb and mammary glands in particular, where they may be passed on to the pups during pregnancy, birth, and suckling. The worms’ life cycle begins again, ensuring their persistence in the next generation of dogs.

Types of Worms

Dog Roundworm

Helminths Toxocara canis, Dog Roundworm

As mentioned above, there are marked geographic differences in the species of worms one is likely to encounter. The following discussion is relevant to pets in the UK and Ireland, although these parasites will also be seen in many other regions.

Our changing climate, together with more relaxed international travel rules, mean that some of the species currently considered ‘exotic’ or foreign to our shores may appear more frequently in future.


As the name suggests, this family of worms do indeed bear some similarity to the earthworm, with which everyone is familiar. Roundworms are also known as nematodes. This grouping includes a wide range of species, including:

  • Ascarids
  • Hookworms
  • Whipworms
  • Lungworms


These are the most ubiquitous worms, having mastered the transmission method from mother to puppy outlined above. This group includes Toxocara canis, Toxascaris leonina, and Toxocara cati – all of which preferentially locate in the gastrointestinal tract of the host. Pups born to a mother that has not been recently dewormed may develop truly massive worm burdens and have the typical pot-bellied appearance seen in many cases of neglect or abandonment.

The sheer number of worms can cause gastrointestinal obstruction or torsion in severe cases, which can be fatal. More commonly, signs are less severe and include vomiting, diarrhoea, failure to thrive, and anaemia due to the damage to the gut that the parasites inflict. Long white worms may be seen in vomit or faeces from these pups, and they may often be alive and wriggling when ejected. These adult worms are not infectious; rather, it is the eggs passed in faeces, which hatch in the environment before being ingested by the next host, which are of concern.

Both in dogs and humans, infestation can result in a phenomenon called visceral larval migrans, where immature worms migrate through organs other than the gut. This often involves the lungs, causing signs of coughing and breathlessness. In humans, however, signs can be much more severe. It is believed that worm larvae have a harder time navigating through a human host, and they can sometimes invade the nervous system or the eye. This scenario arises in young children in particular, with larval invasion of the eye causing lumps to develop in the retina. These lumps can be very difficult to distinguish from an ocular tumour called a retinoblastoma, and as a result many children each year have surgery to remove eyes which are mistakenly believed to be cancerous.


The most common of the canine hookworms is Ancylostoma caninum, although several others, such as Uncinaria stenocephala, are also encountered. Adult worms live in the small intestine, to which they attach using their sharp mouthparts. By feeding on the blood supply to the gut, as well as causing blood loss due to mucosal damage, these worms commonly cause mild-to-moderate anaemia, which again may manifest as ill-thrift in young pups. Vomiting and diarrhoea can also occur. These species reproduce by laying eggs into the gut that are excreted in faeces, and hatch in the environment.

The next host may be infected in one of two ways. Most commonly, the larvae will be ingested, following what is known as the faecal-oral route of transmission. However, owing to their sharp mouthparts, it is also possible for the larvae to penetrate the skin, and to travel under the skin, through the body, to reach the gut. This is known as cutaneous larval migrans, and can cause sores to develop in the interdigital webbing of dogs, usually those kept in close confines under conditions of poor hygiene.


Whipworm infestations are usually caused by Trichuris vulpis, and although signs are typically mild, causing little more than an itchy bottom, some younger pups may experience more severe colonic inflammation, resulting in bloody, mucous-laden diarrhoea. Transmission occurs via the faecal-oral route.


Also colloquially known as the French heartworm, the disease caused by Angiostrongylus vasorum has in recent years been the subject of an intense awareness campaign in the mass media. This roundworm undergoes a period of development in an intermediate host – for example, a frog or slug – before this host is eaten by a dog, the definitive host. The ingested larvae then migrate to the major blood vessels on the right side of the heart, where they reproduce. The eggs which are laid are then washed, in the bloodstream, to the small blood vessels of the lungs. There they hatch, and the ensuing larval stage is coughed up, swallowed, and passed in faeces, ready to infect the next intermediate host. The larvae may cause coughing as they hatch, or signs of infestation may be due to anti-clotting agents secreted by the adult worms.

These anticoagulants may cause bleeding disorders, manifested as blood loss from the nose or gut, bruising of the thin skin of the abdomen, or haemorrhage into the sclera – the white of the eye. Dramatic signs including seizure or sudden death may also be seen. Because of the potentially severe consequences of infestation with this parasite, rigorous attention to deworming protocols is warranted, although it poses no risk to human health.


The other major class of worms to infest dogs in this part of the world is the tapeworm, most commonly Dipylidium caninum. In contrast to the worms described above, tapeworms are flat and broad in shape, with bodies which are segmented. These worms reside in the intestine of their host. The life cycle of Dipylidium involves the passing of worm segments, containing eggs, in the faeces. Alternatively, these segments are capable of independently crawling out through the anus. These may be seen as mobile, white objects resembling grains of rice in the hair coat around the dog’s rear. The eggs are then released from this segment into the coat. These eggs must be ingested by an intermediate host, either a louse, or more commonly a flea, in which they develop into a stage which is infective to dogs. The infected flea or louse is swallowed by the dog during grooming and the larvae are released in the gut, where they mature to adults.

Fortunately, other tapeworms that are found in warmer parts of the world, and which present serious dangers to human health, are not commonly seen in the UK or Ireland. However, it is possible for dogs (and humans) to acquire infection with these other species through eating raw or undercooked meat – something which must be borne in mind when developing home-cooked or raw recipes as an alternative to commercially produced pet food. Signs of a Dipylidium infestation are usually very mild, although they are likely to cause some degree of abdominal discomfort.

Exotic Worms

A mandatory part of the international pet travel arrangements is appropriate deworming, usually both before leaving the country of origin and before returning. Compliance with this should prevent the pet from contracting an exotic infestation, the best-known and most serious of which is probably Dirofilaria immitis, the heartworm. This parasite is found throughout much of the world, including southern Europe, Africa, the Middle East, Australia, the USA, and Canada. This worm depends on the mosquito for its transmission, so although it is not currently endemic in Northern Europe, this may not be the case in the future because of climate change.

Deworming Treatment

Vet giving dog worming treatment pill

Thankfully, there is a wide range of very effective and safe deworming treatments available. These come in a variety of preparations, from injectable, to oral liquids and tablets. Spot-on products are also available, which will generally also treat external parasites through a single application to the skin. However, some of these products have been available, used, and abused for decades, and it is usually worth avoiding the very cheapest products, as these are more likely to be ineffective and to potentially have side effects.

As discussed above, tapeworm infestations present a special case, in that Dipylidium requires an external parasite as an intermediate host. Therefore, any dog seen to have tapeworms must be given an anti-flea treatment at the same time as deworming. Failure to do so is certain to result in treatment failure.

For puppies, a product recommended by a veterinary surgeon should be administered at around 3, 5, and 8 weeks of age. Monthly treatment is generally advised thereafter up to 6 months of age. For older dogs, deworming schedules should be determined based on the dog’s lifestyle, taking into account the environment in which they live, immune status, and presence of children in the house. At a minimum, an effective product should be given every 3 months, although this may increase to monthly for the reasons outlined above. Deworming of bitches during pregnancy is essential, and gives the unborn pups the best chance of starting life one step ahead of these tenacious parasites.