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Dog roundworm (Toxocara canis) is the most common parasitic worm that affects dogs in the UK. It can cause serious illness in puppies and it can be transmitted to, and cause serious disease, in humans. By the end of this module you will: 

  • understand the lifecycle of Toxocara canis
  • understand the public health reasons for controlling Toxocara
  • know why regular worming of adult dogs is needed
  • make sense of non-prescription wormers through knowledge of the active ingredients
  • understand the difference between prescription and non-prescription wormers.

Roundworm is present throughout the UK. Dogs can become infected by ingesting active roundworm eggs in the environment (e.g. in contaminated soil) or by consuming prey (such as rodents) that carry roundworm. But dogs most commonly become infected as puppies because the roundworm larvae can travel from the mother to the puppies via the placenta. Puppies can also become infected through their mother's milk. If puppies are not adequately wormed they can become very unwell and possibly die because large numbers of roundworms may develop in their intestines. Unwormed puppies will also shed a large number of roundworms in their faeces, which risks re-infecting the mother and infecting humans.  For these reasons it is very important to worm puppies. (See the module on puppy worming). Once a dog is infected, roundworm can remain dormant in the dog's body and so it is important to continue to worm dogs regularly after puppyhood to prevent the spread of roundworm.

  • When roundworm eggs are first shed in the dog's faeces the eggs are unembryonated and not capable of causing infection.
  • To be capable of infection the eggs need to develop to the embryonated stage, which takes 3 to 7 weeks if conditions are right. This means there is no risk of contracting roundworm from fresh faeces.
  • If faeces are left in the environment, the eggs can embryonate and can survive for months in soil or in sand and are capable of causing infection if ingested. This is why it is important not to leave dog faeces in the environment.
  • Active roundworm eggs in the environment can be ingested by dogs or other animals (e.g. mice, rats).
  • Once ingested by the dog, the roundworm larvae hatch from the eggs and migrate in the blood to the liver and lungs.
  • The larvae then migrate into the tissues where they become encysted and can remain dormant for years. If a dog has roundworm infection it does not usually cause problems to the dog and there may be no signs that it is infected. At any time, the roundworm can reactivate and be deposited in the faeces.

Active roundworm eggs can be accidentally ingested by people, for example on unwashed fruit or vegetables, or through contact with toys and other objects that have been contaminated. It is also possible that any roundworm eggs on a dog's coat might be accidentally ingested by humans that handle the dog.

If roundworm eggs are ingested by a human they may cause infection. In most people, there will be no symptoms and the body's immune system will fight off the infection. But, rarely, serious illness can develop as a result of roundworm larvae migrating to various parts of the body. For example, migration of larvae to the eyes can result in blindness; migration to the heart or brain can result in death. In humans, disease caused by roundworm is called toxocariasis or toxocarosis. Children aged 2 to 4 years are most commonly affected, but disease can also occur in older children and adults. People who have a weak immune system because of disease or because they take medicines that suppress the immune system might be more likely to develop roundworm infection.

From time to time, encysted roundworm can become active so that the dog sheds roundworm in the faeces allowing the parasite to be spread to other animals and humans. The following actions are all important for preventing the spread of roundworm:

  • Cleaning hands before eating
  • Picking up dog faeces
  • Worming puppies (see the puppy worming module)
  • Giving dogs a worming treatment every 3 months. This is the minimum frequency of worming that has been shown to significantly reduce shedding of roundworm eggs in the faeces. Monthly worming will reduce egg output even more and is recommended for hunting pets (which might become infected through ingestion of prey), and for dogs that have contact with young children and immunosuppressed people.

Other common roundworms in the UK that may cause disease in dogs are hookworm (Uncinaria stenocephala), whipworm (Trichuris vulpis) and Toxascaris leonine. The minimum 3-monthly worming recommendation to control dog roundworm should also adequately control these parasites. Where there are outbreaks of these other roundworms in areas densely populated with dogs (e.g. kennels and breeding establishments) there may be a need for monthly deworming and strict environmental hygiene. Some worming products are specifically licensed for control of these roundworms as well as dog roundworm (Toxocara canis).

Worming products contain endoparasiticide drugs. The following endoparasiticide drugs are all active against roundworm and are ingredients in wormers that are for sale over the counter (NFA-VPS or AVM-GSL).







  • A few worming products are licensed for control of roundworm alone.

Beaphar worming cream (piperazine)

Beaphar worming granules (fenbendazole)

Bob Martin Clear Worming granules and Easy to Use Dewormer granules (fenbendazole)

Johnson's Puppy Easyworm (piperazine)

Piperazine citrate worming granules (piperazine)

  • All the other products are licensed for control of roundworm and tapeworm.
  • Some contain fenbendazole or nitroscanate, which on their own are active against roundworm and some forms of tapeworm.  
  • The others contain one or two of the drugs that are active against roundworm plus a drug that is effective against tapeworm (either dichlorophen or praziquantel).
  • Praziquantel is the only drug that is active against all the important forms of tapeworm in dogs.

The table shows non-prescription products licensed for control of roundworm and tapeworm. Some are also licensed for control of hookworm and whipworm and a certain type of lungworm (Oslerus osleri). There is no evidence that any one product is more effective than any other in controlling roundworm. 

The following endoparasiticide drugs are active against roundworm and are ingredients in prescription-only wormers:






They are usually in combination with other drugs in products that are licensed for the control of a broad range of parasites including roundworm (e.g. fleas, ticks, lungworm). Examples of the brand names of prescription-only wormers are: Advocate, Milbemax, Nexgard Spectra, Profender, Stronghold.


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.


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

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

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.