Parasiticides for cats and dogs: a rational approach. Module 3. Adult dogs

Why we have produced this module

Control of parasites in dogs is a key responsibility for veterinary professionals. The area is complicated because dogs need tailored therapy, there is an overwhelmingly large choice of parasiticide products on the market and very intense promotional activity by drug companies. The aim of this module is to help veterinary professionals use parasiticides rationally. It covers the five main parasites that affect dogs in the UK and the parasiticide drugs used to treat them and outlines the different approaches to parasite control. The module will help readers become familiar with the full range of parasiticide products through integration of the comprehensive Parasiticide Guide.

Several types of parasite affect UK dogs. The main aims of a parasite control programme and use of parasiticides are to control the parasites with zoonotic potential and those that cause or transmit significant disease in dogs. In the UK, these are fleas, roundworm (Toxocara canis), ticks, tapeworm (Echinococcus granulosus) and lungworm (Angiostrongylus vasorum).

The ubiquitous nature of T. canis and fleas together with their disease and zoonotic potential make regular flea treatment and at least 3-monthly roundworm treatment the basis of parasite control programmes in adult dogs. (ESCCAP guideline 2009; ESCCAP guideline 2010)

By far the most commonly seen ectoparasite on cats and dogs is the “cat flea” Ctenocephalides felis. Other fleas that may be found on dogs include the “dog flea” Ctenocephalides canis, the “human flea” Pulex irritans and the “rabbit flea” Spilopsyllus cuniculi.

Cat fleas have become the most successful 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.

Flea control is important because they are responsible for several conditions that present in veterinary practice. They include flea allergic dermatitis, irritation and anaemia in heavy infestations. Fleas emerging from pupae may bite people leading to skin reactions, but people do not become infested. Fleas can also transmit diseases: cat fleas are a vector for Haemoplasma felis, which can cause immune-mediated haemolytic anaemia in cats; they can also act as vectors for significant zoonoses including Bartonella henselae (cat scratch disease) and Rickettsia felis (spotted fever). Fleas also act as the intermediate host for the flea tapeworm Dipylidium caninum.

To prevent flea infestations, adult fleas must be killed on the pet before they can lay eggs, which means within 24 hours of arrival on the animal. So clients need good advice about effective products that kill fleas (i.e. that have an insecticide/adulticide effect), and on how to use them correctly and at the right frequency. It is important to manage clients’ expectations about flea control so that they understand that even with correct use of products, it takes several weeks to control an infestation. In one randomised controlled trial comparing imidacloprid 10% spot-on with fipronil 10% spot-on, used monthly for 3 months in naturally-infested dogs in their homes in Florida, flea counts on the dogs reduced by 90-95% within 1 week of treatment, but the same reduction in flea counts in the home environment did not occur until 1 month after treatment. (Dryden et al 2000) There was no difference in efficacy between the two products; fleas in the environment were not eliminated completely after 3 months’ use of either product.

Insect growth regulators and environmental insecticides together with daily vacuuming are adjunctive measures that can be used to help control environmental flea populations. It has been proposed that their use in combination with insecticides might limit the development of resistance to insecticides among fleas. (Dryden et al 2000)

Even if fleas are killed before they lay eggs it does not prevent them from biting. So for dogs with flea allergic dermatitis, a product with rapid killing or bite prevention effects is desirable to limit exposure to flea saliva.

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. This is why specialists recommend that all susceptible pets in the household should be routinely treated for fleas at an effective frequency. (ESCCAP guideline 2009)

Products for preventing and treating flea infestations

Flea products are available as spot-ons, sprays, impregnated collars and tablets. There is a huge range of products on the market containing a variety of active ingredients. Some contain an insecticide in combination with an insect growth regulator. Many are also active against ticks. Use the Parasiticide Guide to get a feel for the full range.

In order to get a marketing licence, products for treating and preventing flea infestation need to have demonstrated efficacy according to certain standards set by the European Medicines Agency. (CPMP 2008) For insecticides, demonstration of immediate efficacy usually requires that adult flea count (following deliberate infestation) is reduced by at least 95% within 48 hours of applying the treatment. To show short-term residual efficacy (up to 4 weeks), the same reduction in count needs to be shown at weekly intervals; and for long-term efficacy (more than 4 weeks) at 4-weekly intervals (2-weekly in the last month of the claimed effectiveness period). For many of the newer flea products there is evidence that their onset of action (speed of kill) is sooner than 48 hours. The European guidelines for testing parasiticides are being revised and so all future new products will undergo testing that will include assessment of speed of kill sooner than 48 hours. (EMA 2015

Products containing an insect growth regulator need to show an effect on flea metamorphosis (sterilisation of eggs, or inhibition of egg hatching and the formation of cocoons).

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.

Toxocara canis is the most common intestinal roundworm affecting dogs; as a zoonotic parasite it has significant public health implications. It is ubiquitous and infection is lifelong due to arrested larval stages in tissues. Although dogs may be infected by ingesting embryonated eggs, the most important route of canine infection is transplacental. Dogs may also become infected by transmammary infection or by consuming paratenic hosts such as rodents. Transplacental and transmammary infection ensure a high prevalence of puppies shedding large numbers of eggs if they are not wormed (see module 2 in this series, which is about puppy worming). Although immunity leads to a significant reduction in the prevalence of patent infection from the age 6 months, mature dogs shed eggs intermittently. This leads to a prevalence of patent infection of 3.5% to 35% in adult domestic dogs (Wright & Wolfe 2007; Overgaauw & Van Knapen 2013). However, roundworm infection in adult dogs is of low pathogenicity and even large worm burdens are well tolerated.


When roundworm eggs are first shed in faeces the eggs are unembryonated and not infective. Progression to the infective embryonated stage is required for infection and takes 3-7 weeks so fresh faeces do not present a zoonotic risk. Once T. canis eggs embryonate they are capable of infecting humans if the eggs are ingested. Infection can potentially lead to debilitating illness (visceral larval migrans), ocular damage (ocular larval migrans), and neurological defects (neurological larval migrans). Children aged 2-4 years are most commonly affected by roundworm infection, although clinical disease in adults does occur. (Overgauuw & Van Knapen 2013)

It has been proposed that people can be infected by eating the undercooked meat of paratenic hosts such as wild game (Sturchler et al 1990), but the most common route of human infection is through ingestion of embryonated eggs. Zoonotic infection through oral ingestion of embryonated eggs can occur through accidental or deliberate consumption of contaminated soil (geophagia), or unwashed fruit, vegetables and toys (pica) or through transfer of eggs from the coats of pets.

Reducing environmental contamination with eggs and reducing exposure to eggs are the basis of human toxocariasis control. It requires a combination of measures:

  • Regular deworming of cats and dogs – Puppies and kittens are the largest source of potential infection. Treatment of puppies should start at 2 weeks of age, be repeated at 2 weekly intervals until 2 weeks after weaning and then monthly until they are 6 months old. (See module 2 on puppy worming and module 4 on kitten worming) Adult cats and dogs continue to shed eggs intermittently, and it has been shown that use of an effective anthelmintic every 3 months significantly reduces Toxocara spp egg shedding (Wright & Wolfe 2007). Therefore, specialists recommend 3-monthly worming as the minimum frequency for cats and dogs. (ESCCAP 2010) Monthly worming will reduce egg output even more because the pre-patent period (the time needed for infective larvae to become egg-producing adults) of the worm in adult dogs is 30 days. Monthly worming to reduce environmental contamination is recommended for hunting pets (which might become infected through ingestion of paratenic hosts), and for dogs that have contact with young children and immunosuppressed people. (ESCCAP 2010

  • Minimising environmental contamination with cat and dog faeces – UK local authorities are taking this threat increasingly seriously and are adopting measures to reduce faeces in the environment including clearly visible and convenient disposal bins, imposing fines for dog fouling, banning dogs from children’s playgrounds and sports fields, and covering sand pits when not in use to try to prevent faecal contamination from cats.

  • Cleaning hands before food consumption reduces the risk of egg ingestion.

The benzimidazoles (febantel, fendendazole), emodepside, the macrocyclic lactones, nitroscanate, piperazine and the tetrahydropyrimides (pyrantel, oxantel) are active against roundworm. Roundworm-only treatments are available as granules and liquids (containing fendendazole or piperazine). Many preparations are authorised for tapeworm as well as roundworm.

See the chart below (following the section on  lungworm) to get an overall picture of ingredients in worming products.

Other common intestinal UK roundworms are not zoonotic but may be a cause of enteropathies and diarrhoea in dogs. They include Uncinaria stenocephala (hookworm), Trichuris vulpis (whipworm) and Toxascaris leonine. The minimum 3-monthly worming recommendation for Tcanis in dogs should also adequately control these parasites. Where there are outbreaks in densely populated areas of dogs such as kennels and breeding establishments there may be a need for monthly deworming and strict environmental hygiene. Some products are specifically licensed for control of these roundworms.

There is need to evaluate the risk of ticks, tapeworm and lungworm in canine patients.

Ixodes spp and Dermacentor spp ticks can attach to dogs and cause primary disease such as dermatitis, bacterial infection (tick pyaemia) and, in severe cases, anaemia. They also have the potential to transmit diseases such as Lyme disease (Ixodes spp) and Babesia canis (Dermacentor spp). Although Ixodes ticks are found throughout the UK, their distribution across the country is not uniform, with high densities in the south west and south east of England. Dense foci are also found in the Lake District and Scottish Highlands. Pockets of Dermacentor have been found in Wales and the south east of England (Smith et al 2011) but these may have spread. The emergence of new tick-borne diseases is possible as a consequence of the increased movement of dogs between the UK and Europe. In early 2016 there were several cases of Babesia canis in dogs in Essex. (ESCCAP UK & Ireland news)

Where to find information on tick distribution.

Lifestyle also puts some dogs at greater risk of tick attachment: for example being walked in rural areas, or in pasture that is used by deer or ruminants or that has tall grass. A history of previous tick attachment also suggests increased risk. Use of tick preventative measures should therefore be based on risk assessment. No tick preventative product is 100% effective. As transmission of current UK tick pathogens is thought to take at least 24 hours (ESCCAP UK & Ireland) 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. Therefore, owners walking in high-risk areas should also check themselves every 24 hours to reduce the risk of disease.

Products for killing and repelling ticks

Products are available as spot-ons, sprays, collars and tablets. Use the Parasiticide Guide to get a feel for the full range. For more information on choice, see our guide to tick products: Which tick product.

Three types of tapeworm predominate in UK dogs. All are of limited or no pathological significance in dogs, but Echinococcus granulosus has significant zoonotic potential.

  • Taenia spp – The adult tapeworms are long (up to 500 cm) and dogs become infected by ingestion of tissue cysts in sheep, rabbits and rodents and through eating unprocessed raw diets or offal. Even large burdens of Taenia worms are well tolerated in dogs and there is no zoonotic potential. Owners often find the sight of segments in faeces or crawling around the anus revolting, so monthly tapeworm treatment may be required. Preventing dogs fouling on sheep pasture helps to prevent sheep infection and meat condemnation.

  • Dipylidium caninum – At up to 50cm in length, the adult tapeworm is smaller than Taenia. Fleas and lice act as intermediate hosts and infection occurs either through grooming or by ingestion of infested prey. Infection is well tolerated in dogs. There is zoonotic potential if fleas are accidentally ingested and adult worms develop in the human intestine. However, infection in humans is rare and usually asymptomatic. The pre-patent period (time needed for infective larvae to become egg-producing adults) is 3 weeks, so monthly treatment may not completely eliminate segment production if there is repeated exposure. Control of the parasite in dogs and people therefore mainly depends on flea control.

  • Echinococcus granulosus – Adult tapeworms are only 5-6mm long. Livestock and humans may act as intermediate hosts in which the larvae encyst to form hydatid cysts. Infection in farm animals can result in offal condemnation. In humans infection can lead to serious disease with cysts forming in the bone, liver, central nervous system and heart. Ingestion of the cysts by dogs occurs mostly through scavenging of carcasses or feeding on offal. There are pockets of endemic foci in the UK where dogs have access to carcasses of ruminant livestock: Herefordshire, mid-Wales and the western isles of Scotland have the highest prevalence. Farm dogs and hounds may be particularly at risk. Preventing dogs having access to ruminant 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.

    In E. granulosus endemic hotspots treatment with praziquantel at least every 6 weeks is recommended to reduce zoonotic risk. (ESCCAP 2010) Elsewhere, for dogs fed raw offal or that have access to sheep or cattle carcasses specialists recommend treatment with praziquantel at least 3-monthly.

Dichlorphen (not Echinococcus), febantel, fenbendazole, nitroscanate (not Echinococcus), praziquantel. Praziquantel is the drug recommended in the UK for control of Echinococcus. (ESCCAP 2010)

All the products are available as oral formulations. Most are available in combination with other drugs to give a broad parasite coverage, but there are products containing praziquantel alone (including a solution for injection). Use the Parasiticide Guide to get a feel for the full range.

See the chart below to get an overall picture of ingredients in worming products.

The UK has no zoonotic lungworms and the ‘true’ lungworms (which live in bronchi) such as Crenesoma vulpis and Oslerus osleri have limited pathogenic significance in dogs and there is little need for routine control measures. However, O. osleri can build up in large numbers in breeding and racing establishments. In these environments, increased environmental hygiene and treatment with a product authorised to control this lungworm may be needed.

In contrast to the true lungworms, Angiostrongylus vasorum (strictly a heartworm, but commonly known as ‘lungworm’ because it causes respiratory signs) can be highly pathogenic in dogs. Adult worms live in the right chamber of the heart and pulmonary artery and larvae migrate through the lungs. Infection can also predispose to spontaneous coagulopathies. A. vasorum is increasing in prevalence and geographic range, spreading north across the UK. (Kirk et al 2014; Taylor CS et al 2015) Dogs in known endemic areas should be treated monthly with a parasiticide licensed for use against A. vasorum. In areas where the presence of A. vasorum is less certain, a risk-based assessment can be made. Slugs and snails are intermediate hosts. Dogs that are known to eat slugs and snails, or eat grass that may contain small slugs are also at greater risk. Whether it is possible for the infection to be transmitted via slime trails is not known. Foxes are a reservoir host and so the risk to dogs is affected by prevalence in the local fox population. (Taylor et al 2015) Lungworm is covered in detail in the module Lungworm (Angiostrongylus vasorum) in dogs: reducing the risks

Products authorised for use in the prevention of Angiostrongylus vasorum are of two types, both containing a macrocyclic lactone:

  • moxidectin + imidacloprid spot-on; also authorised to cover fleas and roundworm (and some other parasites, but not ticks or tapeworm)

  • milbemycin + praziquantel tablets; also authorised to cover roundworm and tapeworm, including Echinococcus (and other parasites, but not fleas or ticks)

The choice of product depends on the required parasite coverage and on preference for an oral or topical formulation. There are no UK products containing moxidectin or milbemycin alone. Control of lungworm, treatment and prevention, is covered in detail in the module Lungworm (Angiostrongylus vasorum) in dogs: reducing the risks

Use the Parasiticide Guide to see what’s available.

Parasiticides are used to reduce pet morbidity and mortality and to prevent the spread of infection. The aim of parasite control programmes is to use the products at a frequency that optimises the benefits, while limiting the risk of harms (including adverse effects in the animals on which they are used, on humans and the environment), the risk of promoting resistance and the financial cost. When deciding how often to use these parasiticides one of three strategies may be used: continuous use; use based on routine testing for the presence of parasites; or risk-based use.

Continuous use of parasiticides against fleas, ticks and all clinically relevant roundworms and tapeworms is a popular strategy in some countries as it aims to reduce clinical disease and zoonotic risk without need for a tailored strategy, and it drives product sales. However there are several drawbacks with this strategy.

  • Development of resistance – The risk of resistance developing in companion animal parasites is limited by wildlife reservoirs of infection and, in some parasites such as Toxocara, because arrested larval stages are not affected by regular parasiticide use. Nevertheless, resistance can and still does occur. Examples are heartworm resistance to macrocyclic lactones and suspected flea resistance to ectoparasiticides in the USA (Bowman 2012; Cole and Dryden 2014) and hookworm resistance to pyrantel in Australia. (Kopp et al 2007)

  • Loss of confidence in the veterinary profession – It seems logical that if blanket parasiticide therapy is recommended the public might consider veterinary advice irrelevant or perceive that it is driven by product sales rather than evidence. This in turn could reduce compliance with parasite control measures.

  • Adverse effects - If more parasiticides are used, more animals will have adverse effects to parasiticides and more parasiticide substances will be deposited in the environment.

  • Lack of diagnosis and surveillance – If a blanket recommendation is made and followed by most people then routine diagnosis and surveillance in practice may decrease. This loss of knowledge could place pets and the public at greater risk of disease.

Basing parasiticide use on the results of routine diagnostic testing as an alternative to routine parasiticide use is a strategy used in some countries. It consists of regular faecal egg counts, flea combing and tick surveillance. For example, in Denmark vets are prohibited from supplying or prescribing antiparasitic drugs without a diagnosis, except in exceptional circumstances. (Danish ministry of environment and food 2015) The argument for this strategy is that indiscriminate parasiticide use might promote resistance and pollute the environment. Routine diagnostic testing helps build a picture of parasite distribution which is crucial in assessing disease risk and the need to control specific parasites in different parts of the country. However, this strategy also has several drawbacks.

  • Diagnosis of parasites is often more costly to the client than treatment – It can be difficult to justify the expense of diagnostic tests for parasitic infection when simple, more affordable prophylactic measures are available, although this might be countered by the availability of cheaper diagnostic tests.

  • Shedding of parasitic stages in the faeces is often intermittent – Even though an animal is infected faecal egg counts may be negative because eggs are only shed intermittently. Some infections such as A. vasorum can cause life-threatening disease without patent infection being present.

A risk-based approach means tailoring parasiticide therapy for each animal based on the risk of infection due to location and lifestyle and whether it lives with susceptible humans (such as young children and people with immunosuppression). It involves giving clients a clear recommendation on suitable products for their pets and the appropriate frequency of use. The limitations of the above two approaches makes a risk-based use of parasiticides for control of parasites in UK pets seem logical and it is the approach promoted by UK parasite specialists (ESCCAP UK & Ireland).

The risk of a pet being infected with parasites that are likely to cause morbidity or have zoonotic potential can be assessed by answering a series of simple questions. The answers to these questions may be obtained by asking the client directly, asking them to fill in a questionnaire or by reviewing the pet’s history. A full picture may develop over several visits to the vet rather than at one consultation or through “flea and worm clinics”.

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

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.

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.

Question 4: Does your dog eat slugs, snails or grass? Such behaviour together with knowledge of local A. vasorum prevalence will indicate if the dog is at increased risk of infection with A. vasorum (‘lungworm’)

Having established the dog’s risk of parasite infestation, it is possible to consider product choice. No single product for dogs covers all the key parasites (fleas, ticks, roundworm, tapeworm and lungworm). There are many scenarios (see the illustration below) when more than one product will be needed for the desired parasite coverage. Other factors to consider in product selection are the choice of formulation and drug interactions. These will be discussed in module 6.

Single combination product or multiple products


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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 parasiticides for adult dogs.

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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. 


Bowman DD. Heartworms, macrocyclic lactones, and the spectre of resistance to prevention in the United States. Parasit Vectors 2012; 5: 138.

Cole TB, Dryden MC. Insecticide/acaricide resistance in fleas and ticks infesting dogs and cats. Parasit Vectors 2014; 7: 8.

Committee for medicinal products for veterinary use, 2008. Guideline for the testing and evaluations of the efficacy of antiparasitic substances for the treatment and prevention of tick and flea infestation in dogs and cats.

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.

European Medicines Agency. Guideline for the testing and evaluation of the efficacy of antiparasitic substances for the treatment and prevention of tick and flea infestation in dogs and cats. Draft. 2015.

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

Kopp SR et al. High-level pyrantel resistance in the hookworm Ancylostoma caninum. Vet Parasitol 2007; 143: 299-304.

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.

Taylor CS et al. Increased prevalence and geographic spread of the cardiopulmonary nematode Angiostrongylus vasorum in fox populations in Great Britain. Parasitol 2015; 142: 1190-5.

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