Parasiticides for cats a dogs: a rational approach. Module 2. Puppy worming

Roundworm control in UK puppies

The dog roundworm Toxocara canis is an intestinal nematode with ubiquitous presence in the UK. It can cause significant disease in puppies and can be transmitted to humans to cause toxocariasis, an important zoonosis. Roundworm control is the most important consideration in puppy parasite management.

There is no strict definition of the term roundworm. In these modules we use it for ascarids, which include dog and cat roundworm (Toxocara spp), hookworm and whipworm. T. canis is commonly known as the dog roundworm.

Adult roundworms live in the small intestine and shed eggs into the environment via the faeces of infected dogs. When first shed the eggs are unembryonated and not infective, so fresh faeces do not represent a zoonotic risk. Development of the eggs to an infective embryonated egg (containing third-stage [L3] larvae) is needed for infection (Schnieder et al 2011), a process that takes 2 to 7 weeks under optimum conditions. 


Although dogs may be infected by ingesting embryonated eggs or infected paratenic hosts, the most important route of infection in puppies is via the placenta. Puppies may also become infected via their mother’s milk. The transplacental and transmammary routes ensure a high prevalence of T. canis infection in puppies not regularly treated with an endoparasiticide.

In puppies younger than 3 months, larvae are released from ingested eggs in the small intestine. They cross the gut wall and migrate to the lungs via the liver. Once in the lungs the larvae migrate to the trachea, are coughed up and swallowed, and complete development to adult worms in the small intestine.

In dogs older than 6 months the hatched larvae are more likely to migrate to various tissues including the liver, central nervous system, lungs, heart and muscle. They encyst at these sites and can remain dormant for years. At any time, some of the dormant larvae may reactivate, migrate back to the gut, complete development to adults and start producing eggs. In pregnant bitches dormant larvae become activated around 3 weeks before parturition and migrate through the placenta to the foetal liver. After birth the larvae migrate to the puppy’s lungs and subsequently to the intestine. Larvae passed to puppies during suckling do not migrate but proceed directly to adulthood in the gut.

In dogs older than 3 months, the liver-lung pathway of larval migration becomes less likely and tissue migration becomes more likely.

Roundworm infection in puppies may be subclinical or may lead to mild signs such as a pot-bellied appearance and a transient cough. Puppies infected with high numbers of worms by the transplacental route may present with lethargy, poor weight gain and failure to thrive. Severe infections in neonatal pups can result in sudden death at a few days of age due to mass lung migration, intussusception or intestinal obstruction.

Roundworm infection occurs when people (usually young children) ingest infective eggs in contaminated soil or sand or from unwashed hands or vegetables or toys that have been in the garden, or from direct contact with pets. Between 2.5% and 30% of humans in European countries are seropositive for Toxocara antibodies (Overgaauw & van Knapenn 2013). After ingestion, hatched larvae penetrate the small intestine wall and are carried in the circulation to the liver where they may lodge and cause a granulomatous response. Some migrate to other tissues, including lungs, striated muscle, heart, brain and eye. Infection is usually asymptomatic and the larvae die without causing serious problems. But there can be generalised symptoms (such as fever and urticaria), eosinophilia and focal signs related to the migration of larvae (such as bronchospasm or granulomatous swelling in the eye) and occasionally death due to heart or brain involvement. Rarely, larvae survive in the tissues for years, causing symptoms long after the initial infection.

Some people are more vulnerable than others to infection: young children are more likely than others to accidentally ingest infective eggs; people who are immunosuppressed might be at increased risk of infection.

Good hygiene (including house training puppies and handwashing) and reducing environmental contamination with eggs are central to the control of human toxocariasis. Toxocara spp eggs are very resistant to adverse environmental conditions and can survive for years. There is no practical way of reducing the number of eggs in the environment once they are present, so preventing initial contamination (by picking up faeces from gardens and public areas) is crucial. 

The aim of deworming puppies is to prevent roundworm disease in the puppies and to suppress T. canis egg output until they are 6 months old. There are many sources of environmental contamination with Toxocara spp eggs, but puppies and nursing bitches are the most potent source of T. canis infection. Puppies have been recorded with faecal egg counts as high as 107,500 eggs per gram (Jacobs and Fisher 1993). The eggs can reinfect the nursing bitch as well as contaminate the environment. This cycle of infection can lead to lifelong establishment of infection as well as ongoing environmental contamination.

ESCCAP guidelines recommend that puppies should be dewormed at 2 weeks of age and then every 2 weeks until 2 weeks after weaning, then every month until they are 6 months old. (ESCCAP guidelines 2010).

The prepatent period of most roundworms in adult dogs is more than 4 weeks, and so deworming every 4 weeks will prevent most patent canine roundworm infections (Fahrion et al, 2008). However, the transplacental and transmammary routes of Toxocara infection can produce patent infection every 2-3 weeks, which is why young puppies need deworming every 2 weeks.

As bitches often become reinfected during the suckling period, roundworm treatment should be started in nursing bitches at the same time as their puppies. (ESCCAP guidelines 2010)


Puppies may be infected through the ingestion of eggs from other common roundworms such as hookworm and whipworm.  

Uncinaria stenocephala (“Northern” hookworm) has no significant zoonotic potential and regular deworming to control Toxocara will usually be sufficient to control any clinical signs caused by it. In kennelled environments where there is an outbreak of U. stenocephala use of a product specifically licensed for treatment of this roundworm should be considered. Ancylostoma caninum is a hookworm that is transmitted to puppies through lactation and can lead to anaemia and even death in litters of puppies. It is endemic in southern Europe and tropical climates and may be present in some parts of the UK. Some products are specifically licensed for use against this hookworm. Use the Parasiticide Guide to search for products licensed for use against hookworm and the product SPCs for specific information about the type of hookworm for which products are licensed.

Trichuris vulpis (whipworm) is rarely seen in young puppies because of its long prepatent period (6-12 weeks). Some products are licensed for use against whipworm.

Drugs that are active against roundworm (check the SPCs for authorised uses of commercial products)

benzimidazoles (febantel, fenbendazole) 


macrocyclic lactones (eprinomectin, milbemycin, moxidectin, selamectin)



tetrahydropyrimidines (oxantel, pyrantel)

Any parasiticide product licensed for treatment of endoparasites (including Toxocara spp) must have been shown in clinical trials to reduce worm counts by at least 90% (for more on this, see Module 1). On this basis, any product licensed for the treatment of adult roundworm in puppies used at the recommended frequencies can be expected to prevent levels of infection likely to cause significant disease. The few published comparative trials do not suggest any important differences between the efficacy of different puppy worming regimens on reducing roundworm counts, and so preventing disease in puppies (Schenker et al 2006; Jacobs 1987; Fisher et al 1994) However, the results of one small trial involving 44 puppies suggested that 2-weekly dosing with piperazine was less effective than febendazole or febantel + pyrantel in reducing egg shedding and therefore the risk of reinfection. (Fisher et al 1994)

Products licensed to treat T. canis in puppies from the age of 2 weeks:

Combination products licensed for use in puppies from the age of 2 weeks

There are many combination products available over the counter and on prescription that are authorised for use from the age of 2 weeks: they contain febantel + pyrantel + praziquantel; milbemycin + praziquantel; or milbemycin + lufenuron and are licensed for use against other parasites as well as roundworm. Many more are authorised for use from the age of 6-8 weeks. Use the Parasiticide Guide to look at the range of products available to treat roundworm.

The summaries of product characteristics (SPCs) of endoparasiticides authorised for use in puppies indicate that adverse effects are rare at recommended doses. To give the correct dose it is essential to weigh the puppy accurately for each dose.

The benzimadazoles (fenbendazole and febantel) and the tetrahydropyrimidines (oxantel and pyrantel) can cause self-limiting gastrointestinal signs. Macrocyclic lactones (such as moxidectin, selamectin and milbemycin) can rarely cause lethargy, systemic neurological signs (tremors, ataxia) and gastrointestinal upset. Collies and related breeds may be more sensitive than other breeds to the effects of macrocyclic lactones. The SPCs for products containing milbemycin caution that the tolerance of the products in young puppies from Collie-type breeds has not been investigated.

The Parasiticide Guide will show you all the UK products authorised for treating roundworm in dogs together with the minimum age and weight for which the products are authorised. Different products will suit different clients’ needs so when choosing products consider the following:

  • Choice of formulation – Endoparasiticides come in the form of tablets, liquids, pastes, granules and spot-ons. It is worth finding out if the client has a preference because this might affect compliance. The client might find it difficult to comply with a need for frequent dosing or be put off a particular formulation because of a previous experience of an adverse reaction to a product. It is usually easier to give a liquid or paste rather than tablets or granules to a 2-week old puppy. There is no need to continue with the same dewormer used by a breeder.


  • A need for treatment and prevention of other parasites – It may be appropriate to use a product that also covers other parasites such as fleas, ear mites or lice. Puppies are unlikely to be exposed to ticks, tapeworm and lungworm until they venture outdoors but if a puppy has been found to have a tick, is on a raw diet, eats slugs and snails, spends time in a garden, or has visible tapeworm segments then treatment of these parasites will need to be combined with intestinal roundworm treatment. Consider the possibility of exotic parasites (such as Dirofilaria immitis) in imported puppies. The required coverage can be achieved with a single product or a combination of products. Search the Parasiticide Guide to see what’s available. If a combination product is used, it’s important to consider if the treatment frequency needed for roundworm is too frequent for other parasites, in which case separate products might be more suitable.


  • Specific licence claims – Products that treat T. canis are also likely to control other intestinal roundworms, but if a breeding establishment is known to have other roundworms in the environment such as U. stenocephala (hookworm) or T. vulpis (whipworm), then products specifically licensed for activity against these (as stated in the product's SPC) should be used. Use the Parasiticide Guide to help with the search.


  • Cost – the product(s) should be affordable to the client throughout the treatment period. Some products such as puppy suspensions are more economical for treating large numbers of puppies. Combination products may be more economical for treating several parasites in a puppy.


If you prefer, you can listen to the whole audio presetation of puppy worming module using the following podcast. Don't forget that you can also download the podcast to your iPod, music player, tablet or smartphone using the Download link on the right of the audio player.

Goal of activity: Update knowledge; help clinical decision-making

Authors/disclosures: Veterinary Prescriber editorial team/no conflict of interest

Specific learning objectives: to improve knowledge and understanding of the use of puppy worming.

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How we produced this module

Our modules start with a detailed outline and electronic literature search. We commission a collaborating author, who is a specialist in the module topic, to write a draft module. The collaborating author on this module was Ian Wright. The draft is circulated unsigned to a wide range of commentators, include practising first-opinion vets, other topic specialists, the companies that market any mentioned drugs and other organisations and individuals, as appropriate. They can raise points about the interpretation of evidence, ask questions that are important to clinical practice, and present alternative viewpoints. There is a rigorous editing and checking process and the result is a module that is evidence-based, impartial and relevant to clinical practice. The final module is unsigned because it is the result of collaboration. 


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

Fahrion AS et al. Patent Toxocara canis infections in previously exposed and in helminth-free dogs after infection with low numbers of embryonated eggs. Veterinary Parasitology 2008; 177: 186-9.

Fisher MA et al. Studies on the control of Toxacara canis in breeding kennels. Vet Parasitol 1994; 55: 87-92.

Jacobs DE. Control of Toxocara canis in puppies: a comparison of screening techniques and evaluation of a dosing programme. J Vet Pharmacol Therap 1987; 10: 23-9.

Jacobs DE, Fisher MA. Recent developments in the chemotherapy of Toxocara canis infection in puppies and the prevention of toxocariasis. In: Lewis JW & Maizels RM (Eds), 1993, Toxocara and toxocariasis: Clinical, epidemiological and molecular perspectives. Institute of Biology. London  pp: 111-6.  

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

Schenker R et al. Comparative effects of milbemycin oxime-based and febantel-pyrantel embonate-based anthelmintic tablets on Toxocara canis egg shedding in naturally infected pups. Vet Parasitol 2006; 369-73.

Schnieder T et al. Larval development of Toxocara canis in dogs. Vet Parasitol 2011; 175: 193-206.