How it works

You can read or listen to our modules. There is a play button in each heading so you can listen while you read. Or you can listen to the whole module in one go or download it as a podcast. You'll find a play and download button for the whole module at the bottom of this page. At the end of the module there is a quiz, so you can test your knowledge and receive a CPD certificate. You will have to hit the play button on the quiz video and enter your name and email address before you start the quiz.

Several types of parasite can affect dogs in the UK. The main aims of a parasite control programme and use of parasiticide medicines are to control the parasites that can cause or transmit significant disease in dogs, or be transmitted to humans. In the UK these are: fleas; roundworm; ticks; tapeworm; and the lungworm Angiostrongylus vasorum. By the end of this module you will:

  • know the five most important parasites that affect dogs in the UK;
  • understand why dogs are at risk;
  • understand why parasite protection is important for humans and dogs;
  • know the basic parasite protection dogs should get.

Fleas and roundworm can affect all dogs anywhere in the country and so all dogs need protection against these parasites. 

By far the most commonly seen external parasite on dogs is the “cat flea” Ctenocephalides felis. Cat fleas are very successful parasites due to their rapid reproduction, persistent pupal environmental stage and ability to parasitise a wide range of hosts including cats, dogs, rabbits, foxes and hedgehogs. Many owners believe that fleas are only a summer problem, but fleas may be active all year round perhaps because of central heating and warmer winters.

Control of fleas is important because they can cause irritation and can lead to several diseases in dogs. These include flea allergic dermatitis, and anaemia in heavy infestations. Fleas also transmit the flea tapeworm Dipylidium caninum, which can infect dogs and sometimes humans. Fleas emerging from pupae in the environment may bite people leading to skin reactions, but people do not become infested. But fleas can transmit bacterial diseases to people, including Bartonella henselae (the cause of cat scratch disease) and Rickettsia felis (the cause of spotted fever). 

Fleas are present across the whole of the UK and may be brought into homes on pets and on pet owners’ or visitors’ clothes. It is therefore practically impossible to avoid exposure, which in turn can lead to household infestation. As it can take 3 months or more to get rid of a flea infestation, prevention is usually better than cure. There is a wide range of parasiticide medicines for controlling fleas. See the module "Making sense of flea and tick products".

Dog roundworm (Toxocara canis) is a worm that lives in the intestines of dogs. It can be transmitted to humans and potentially cause serious disease, and so it has significant public health implications. Roundworm is present throughout the UK. Most puppies become infected before birth via the placenta (see the module on puppy worming). Puppies may also become infected through their mothers’ milk. Dogs of all ages can become infected at any time by ingesting roundworm eggs in the environment or through eating infected prey such as rodents.

Roundworm infection in dogs is lifelong because roundworm larvae remain dormant in the dog’s body tissues. This does not usually affect the dog. However, from time to time, the larvae may become active and complete development to adult worms. It means that roundworm eggs may be intermittently shed in the faeces, helping the spread of roundworm. 

People can become infected with roundworm through ingestion of eggs in contaminated soil, or on unwashed fruit, vegetables and toys, or through transfer of eggs from the coats of pets. Children aged 2–4 years are most commonly affected by roundworm infection. Washing hands before eating and removing faeces from the environment help prevent infection and the spread of roundworm. 

Dogs should also be wormed regularly with a wormer that is effective against roundworm, to reduce the shedding of roundworm eggs. Worming at a minimum frequency of every 3 months is effective at preventing worm shedding. While 3-monthly worming is sufficient for most dogs, monthly worming is recommended for hunting dogs (because they are more likely to become newly infected with roundworm) or dogs that have contact with young children and immunosuppressed people (because these people are more susceptible than others to infection). 

There are many non-prescription wormers for controlling roundworm. They also usually cover tapeworm too. See the module on Making sense of wormers.

There are other types of roundworm that can affect dogs. They include hookworm (Uncinaria stenocephala), whipworm (Trichuris vulpis) and Toxascaris leonine. These are not transmitted to humans but they can sometimes cause gastrointestinal problems in dogs. The minimum 3-monthly worming recommendation for control of dog roundworm (Toxocara) should also adequately control these parasites. When there are outbreaks in densely populated areas of dogs such as kennels and breeding establishments there may be a need for monthly worming and strict environmental hygiene. Some products are specifically licensed for control of these roundworms.

The dog's lifestyle might expose it to ticks, tapeworm and lungworm, so protection against these parasites, in addition to fleas and roundworm, might be needed.

Ticks can attach to dogs leading to diseases such as dermatitis, bacterial infection and, in severe cases, anaemia. Ticks also have the potential to carry bacterial diseases such as Lyme disease and babesiosis (a cause of serious anaemia in dogs). Lifestyle puts some dogs at greater risk of tick attachment: for example being walked in rural areas, or in pasture that is used by deer, cows or sheep, or that has tall grass. If an owner has ever found a tick on the dog, it suggests the dog is at risk of ticks attaching in the future and it may be worth using a tick preventative product regularly, particularly in areas known to have Lyme disease or babesiosis. But no tick preventative product is 100% effective and so pets should be checked for ticks every 24 hours and any found removed with a tick hook. Ticks will also attach to, and feed on, humans and can transmit Lyme disease to them. Therefore, owners walking in high-risk areas should also check themselves every 24 hours to reduce the risk of disease. 

Most non-prescription flea products also protect against ticks. See the module on Making sense of flea and tick products.

There are three main types of tapeworm that can affect UK dogs: TaeniaDipylidium caninum and  Echinococcus granulosus. They do not usually cause much of a problem to dogs, but one of these, Echinococcus granulosus, can cause serious disease in humans.

Taenia – Adult Taenia tapeworms are long (up to 500 cm). Dogs become infected by eating tissue cysts in sheep, rabbits and rodents or through eating unprocessed raw diets or offal. Dogs are usually unaffected, even by large numbers of worms and the worm poses no danger to people. However owners may not like the sight of worm segments in faeces or crawling around the dog’s anus, so monthly tapeworm treatment may be required. It is important to prevent dogs fouling on sheep pasture to prevent sheep infection and meat condemnation.

Dipylidium caninum – At up to 50cm in length, the adult Dipylidium caninum tapeworm is smaller than Taenia. Fleas and lice can transmit this worm and so infection occurs either through grooming or by ingestion of infested prey. Infection is usually well tolerated in dogs but the worms can be transmitted to humans if fleas are accidentally ingested, and adult worms may develop in the human intestine, although this is rare and does not usually cause symptoms. The time needed for infective larvae to become egg-producing adult worms is only 3 weeks so even monthly worming may not completely stop worm segment production if there is repeated exposure to this worm. Control of this parasite in dogs therefore mainly depends on flea control.

Echinococcus granulosus – Adult Echinococcus granulosus tapeworms are only 5–6mm long. The worm can be transmitted to farm animals and humans. Infection in farm animals can lead to cysts in the organs result in offal condemnation. Ingestion of the cysts by dogs occurs mostly through scavenging of carcasses, or feeding on offal. In humans, infection can lead to serious disease with cysts forming in the bone, liver, central nervous system and heart. There are small areas in the UK where this tapeworm is known to be present – including Herefordshire, mid-Wales and the western isles of Scotland. Dogs that have access to livestock carcasses, for example farm dogs and hounds, may be particularly at risk of infection. Preventing dogs having access to livestock carcasses, unprocessed raw diets or undercooked or raw offal will prevent infection, but the remoteness of some of the locations can make the clearance of potentially infected carcasses difficult. 

Praziquantel is the treatment of choice for dogs for controlling Echinococcus granulosus. In areas of the country known where the worm is known to be endemic, treatment of dogs with praziquantel monthly is recommended. Elsewhere, for dogs fed raw offal or that have access to sheep or cattle carcasses, parasite specialists recommend treatment with praziquantel at least every 3 months.

‘True’ lungworms, which can live in the lungs of dogs, include Crenesoma vulpis and Oslerus osleri. They do not usually cause serious disease in dogs and there is little need for routine control measures. However, Oslerus osleri can build up in large numbers in breeding and racing establishments. In these circumstances, increased environmental hygiene and treatment with a product authorised to control this lungworm may be needed.

Another parasite, Angiostrongylus vasorum is commonly known as ‘lungworm’ because it causes respiratory problems. But strictly speaking, it is a heartworm. It is an important parasite because it can cause serious disease in dogs. Adult worms live in the right chamber of the heart and pulmonary artery. The worm larvae migrate through the lungs, causing respiratory signs such as coughing.  For more on this, see the module on Lungworm in dogs.

Angiostrongylus appears to be spreading across the UK. Slugs and snails can be carriers of the larvae of this parasite. Dogs that are known to eat slugs and snails, or eat grass that may contain small slugs are at risk of becoming infected by the parasite. Whether it is possible for the infection to be transmitted via slime trails is not known. There are parasiticides licensed for control of Angiostrongylus vasorum but they are only available on veterinary prescription. A vet can make an assessment of the likelihood of infection and advise on whether preventive therapy is needed.

Include the table “Summary of recommendations for parasite control” from module 3 of the VP series.

The following questions will help determine the appropriate level of roundworm protection for a dog, and the need for protection against the other main parasites – ticks, tapeworm and lungworm (Angiostrongylus vasorum), as well as fleas.

Question 1: Is your dog in regular contact with young children, or a person with a condition or on medication that suppresses the immune system? If so, monthly rather than 3-monthly roundworm treatment is recommended.

Question 2: Have you ever found a tick on your dog? A history of tick attachment strongly suggests that the dog’s lifestyle exposes it to ticks. Dogs walked in environments likely to contain ticks (e.g. long grass, pasture) are at risk. A product effective against fleas and ticks is recommended.

Question 3: Does your dog eat raw food or hunt? Dogs that eat raw offal might be at increased risk of Echinococcus infection for which regular treatment with praziquantel is recommended (usually 3-monthly, or monthly for farm or hunting dogs in areas where Echinococcus granulosus is endemic).

Question 4: Does your dog eat slugs, snails or grass? This might increase the chance of the dog being infected with Angiostrongylus. The owner should be advised to discuss this with their vet.

Having established the dog’s risk of parasite infection, it is possible to consider product choice. No single product for dogs covers all the key parasites (fleas, ticks, roundworm, tapeworm and lungworm). Other factors to consider in product selection are the choice of formulation. 

If using non-prescription products, two separate products are needed to cover fleas and ticks and worms. Some prescription-only products cover several of the key parasites in one product. See the modules on "Making sense of flea and tick products" and "Making sense of worming products" for more on product choice.

There are certain things you should check before selling a parasiticide medicine.

  • what parasites the product is licensed to cover. Although different products might have the same ingredients, they might have different licensed uses;
  • what species it is intended for;
  • that the product is suitable for the animal's age ;
  • that the dose is correct (for the animal's weight);
  • how often it should be used;
  • the expiry date.

Remind the owner to read the information on/in the package.

Practical exercise

Become familiar with the parasiticide products stocked for dogs. What different formulations can you offer (e.g. collar, spot-on, tablets)?


You can listen to a complete podcast of the module by using the play button below or use the download link on the right-hand side of the player to download the podcast to your mobile device.


Test yourself

To do the CPD quiz and receive your certificate, you will need to hit the play button on the player below and enter your name and email address before you start.


Dryden MW et al. Control of fleas on naturally infested cats and dogs and in private residences with topical spot on applications of fipronil and imidacloprid. Vet Parasitol 2000; 93: 69-75.

ESCCAP. Ectoparasites part 1: control of parasitic insects and ticks in dogs and cats. Guideline 3, March 2009.

ESCCAP. Worm control in dogs and cats. Guideline 01 second edition, September 2010.

Kirk et al. Angiostrongylus vasorum in Great Britain: a nationwide postal questionnaire survey of veterinary practices. Vet Rec 2014; 175: 118

Overgaauw PAM, Van Knapen F. Veterinary and public health aspects of Toxocara spp. Veterinary Parasitology 2013 193: 398-403.

Smith F et al. The prevalence, distribution and risk associated with tick infestation of dogs in Great Britain. Med Vet Entomol 2011; 25: 377-84.

Smith FD et al. Prevalence, distribution and risk associated with tick infestation of dogs in Great Britain. Med Vet Entomol 2011; 25: 377-84.

Sturchler D et al. Transmission of toxocariasis. J Infect Dis 1990; 162: 571-2.

Wright I, Wolfe A. Prevalence of zoonotic nematode species in dogs in Lancashire. Veterinary Record 2007; 161: 790-1.