Parasiticides for cats and dogs: a rational approach. Module 5. Adult cats

Why do this module?

Control of parasites in cats is a key responsibility for veterinary professionals. The area is complicated because therapy must be suited to each cat, 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 cats 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 cats. 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 cats. In the UK these are:


The ubiquitous nature of fleas and roundworm 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 cats. (ESCCAP guideline 2009; ESCCAP guideline 2010)

The ectoparasite most commonly seen on cats and dogs is the “cat flea” Ctenocephalides felis. Cat fleas are 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 can 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 including flea allergy dermatitis, irritation, and anaemia in heavy infestations. Fleas act as the intermediate host for the flea tapeworm Dipylidium caninum. 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 the zoonoses Bartonella henselae (cat scratch disease) and Rickettsia felis (spotted fever). Fleas emerging from pupae may bite people leading to skin reactions, but people do not become infested.

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 or months to control an infestation. For example, in a 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, although flea counts on the dogs reduced by 90-95% within 1 week of treatment, the same reduction in flea counts in the home environment did not occur until 1 month after treatment. (Dryden et al 2000)

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)

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)

Flea products are available as spot-ons, sprays, impregnated collars, tablets and an injection. 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 (acaricide). Use the Parasiticide Guide to see 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, adult flea count (following deliberate infestation) must be reduced by at least 95% within 48 hours of applying the treatment. To show sustained efficacy for up to 4 weeks the same reduction in flea count needs to be seen at weekly intervals; and for sustained efficacy for more than 4 weeks the reduction needs to be seen 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 (meaning that they either sterilise eggs, or inhibit egg hatching and the formation of cocoons).

In these modules we use the term roundworm for ascarids, which include dog and cat roundworm (Toxocara spp), hookworm and whipworm. Toxocara cati is commonly known as the cat roundworm.

T. cati is the most common intestinal roundworm affecting cats. 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 cats may be infected by ingesting embryonated eggs, the most important route of feline infection is transmammary infection of kittens. Cats may also become infected by consuming paratenic hosts such as rodents. Transmammary infection results in kittens shedding large numbers of eggs if the kittens are not wormed (for more on this see module 4 on kitten worming). From the age of 6 months the prevalence of patent infection reduces significantly due to immunity, but mature cats may shed eggs intermittently leading to a prevalence of patent infection of up to 26% in adult domestic cats with outdoor access, and even higher in feral and stray populations (Wright et al 2016; Overgaauw & Van Knapen 2013). Infection in adult cats 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, which takes 3 to 7 weeks, is needed for infection, so fresh faeces do not present a zoonotic risk. People can become infected through ingestion of embryonated eggs in contaminated soil, or unwashed fruit, vegetables and toys or through transfer of eggs from the coats of pets. In humans, 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; Halsby et al 2016) The number of cases of human toxocariasis reported in England and Wales has decreased in the last decade or so to fewer than ten cases per year. (Halsby et al 2016)

It is not known what proportion of zoonotic cases are attributable to T. cati as opposed to T. canis but the zoonotic potential of T. cati has been established and in urban environments, environmental contamination due to T. cati can be at least as great as T. canis from dogs. (Fisher 2003; Holland 2015)

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 – Kittens and puppies are the largest source of potential roundworm infection. (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 3-monthly worming 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 and is recommended for hunting pets (which might become infected through ingestion of paratenic hosts), and for pets that have contact with young children and immunosuppressed people. (ESCCAP 2010)
  • Minimising environmental contamination with cat and dog faeces – UK local authorities have adopted 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. However, removal of all cat faeces is impractical.
  • Cleaning hands before food consumption reduces the risk of egg ingestion.

Emodepside, fenbendazole, piperazine, pyrantel, and the macrocyclic lactones (eprinomectin, milbemycin, moxidectin, selamectin) are active against roundworm and are included in the many products on the market for the control of roundworm in cats. Use the Parasiticide Guide to see what’s available.

Other common feline intestinal UK roundworms are not zoonotic but may contribute to enteropathies and diarrhoea. These include Ancylostoma tubaeforme and Toxascaris leonine. Three-monthly worming of cats to control T. cati  should also adequately control these parasites. (ESCCAP guidelines 2010)

As well providing protection against fleas and roundworm, there is need to evaluate and minimise the risk of ticks, tapeworm and lungworm in feline patients.

Ixodes spp and, rarely, Dermacentor spp ticks can attach to cats leading diseases such as dermatitis, bacterial infection (tick pyaemia) and, in severe cases, anaemia. Ticks also have the potential to carry diseases such as Lyme disease (Ixodes spp) and Babesia canis (Dermacentor spp). Lyme disease is rarely recognised in cats; whether this is due to cats being relatively resistant to exposure or resistant to disease or because it is under-reported and underdiagnosed in felines is not known. Cats do not become not infected with B. canis but may act as reproductive hosts by maintaining and transporting infected ticks. Tick preventative measures should be used on cats that are frequently exposed to ticks. (ESCCAP guidelines 2009)

All but one of the products licensed for protection of cats against ticks are spot-ons and sprays containing fipronil (an insecticide and acaricide). Use the Parasiticide Guide to see the full range. The exception is a collar that contains flumethrin (a pyrethroid), which has acaricide and tick-repellent effects. Many products licensed for tick control in dogs contain high concentrations of pyrethroids, which are toxic to cats. Flumethrin in the cat collar (and permethrin in over-the-counter cat flea collars) is present in a concentration that is safe for use in cats.  

According to the current European guidelines (in use since 2008) on testing tick products before marketing, immediate efficacy means that tick count (following deliberate infestation) must have been reduced by at least 90% within 48 hours of applying the treatment. (CVMP 2008)

Products that are marketed as tick repellents have had to demonstrate a repellent effect in trials, showing no ticks were detectable on the animal 24 hours after treatment. (CVMP 2008)

No preventative product is 100% effective and so, If possible, cats should be checked every 24 hours and any ticks found removed with a tick hook.

Two types of tapeworm predominate in UK cats: Taenia taeniaeformis and Dipylidium caninum.

  • Taenia taeniaeformis – Cats become infected by ingestion of infective rodents. However, even large burdens are well tolerated by cats and there is no zoonotic potential. If tapeworm segments are seen around the anus, monthly tapeworm treatment might be needed.
  • Dipylidium caninum – Fleas and lice act as intermediate hosts. Infection occurs either by grooming or by ingestion of infested prey, but infection is usually well tolerated in cats. Adult worms can develop in the human intestine if fleas are accidentally ingested. However, infection in humans is rare and usually asymptomatic. As the pre-patent period (time needed for infective larvae to become egg-producing adults) is only 3 weeks, monthly treatment may not completely eliminate segment production if there is repeated exposure. Control of the parasite in cats and people therefore mainly depends on flea control.

Dichlorophen, fenbendazole and praziquantel are active against tapeworm and included in products authorised for use in cats. Use the Parasiticide Guide to see the full range.

The lungworms that affect cats are not the same as those that affect dogs. Aelurostrongylus abstrusus and Eucoleus aerophilus are both true lungworms of cats and are thought to be endemic in most European countries including the UK. Preventing exposure to lungworms is difficult in outdoor cats.

A. abstrusus has an indirect life cycle with cats as definitive hosts and slugs or snails as intermediate hosts. First stage (L1) larvae  pass out in the faeces of cats into the environment and penetrate slugs and snails, in which they develop to the infective L3 stage. (Elsheikha et al 2016) Various animals can act as paratenic hosts including reptiles, amphibians and birds through ingestion of infected gastropods. Hunting cats are therefore at greater risk of infection than non-hunting cats. Adult worms live in the lung tissues and are commonly of low pathogenicity and infection can be asymptomatic. The most common clinical sign in infected cats is a mild to moderate chronic cough but there may be other respiratory signs including sneezing, wheezing, mucopurulent nasal discharge and dyspnoea.  

The lifecycle of E. aerophilus is direct although paratenic hosts such as earthworms play a role. Canine and feline respiratory infection caused by E. aerophilus is sporadic across Europe and most cases are subclinical. However, clinical cases in cats have been reported (Barrs et al 2000; Foster et al 2004). The parasite is commonly found in wildlife but knowledge of its distribution across Europe, including the UK, is incomplete. Typical clinical signs associated with infection are coughing and sneezing with varying degrees of severity.

Fenbendazole is licensed for the treatment of A. abstrusus infection and is effective when used daily for at least 3 days. (Elsheikha et al 2016) Spot-on products containing emodepside or eprinomectin are also licensed for controlling A. abstrusus infection.

Spot-on products containing moxidectin or selamectin and oral milbemycin have been shown to effectively reduce larval shedding in infected cats (Elsheikha et al 2016).  It therefore seems likely that the routine use of such products for the control of Toxocara cati may be sufficient to limit the incidence of clinical aelurostrongylosis. Note that this is an unlicensed use of these products.

There is no product licensed for the treatment or prevention of E. aerophilus infection and there is only limited evidence on the efficacy of parasiticide drugs in controlling this parasite. In a controlled trial involving 36 cats, a spot-on preparation containing moxidectin + imidacloprid (Advocate) had close to 100% efficacy against infection (Traversa et al 2012) so this is an option for routine use in cats repeatedly exposed to the parasite (an unlicensed use of the product). 

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 on the animals on which they are used, on humans and the environment), and limiting 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 a need for a tailored strategy, and it drives product sales. However there are several drawbacks with this strategy.

  • Potential for 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 & 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.
  • 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. abstrusus 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 asking the client some 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”. For more on this see module 6.

The following questions will help determine the appropriate level of roundworm protection for a cat and the need for protection against the other main parasites – ticks, tapeworm and lungworm.

Question 1: Is your cat 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 cat? A history of tick attachment strongly suggests that the cat’s lifestyle exposes it to ticks.

Question 3: Does your cat go outdoors/does it hunt. Unless the cat remains indoors it is likely to be exposed to lungworms. Cats that hunt might be at increased risk of tapeworm infection. Treatment with a product that controls lungworms and/or tapeworm may be needed.

Having established the cat’s risk of parasite infestation, it is possible to consider product choice. This depends on many factors including required parasite coverage, formulation, likelihood of correct and consistent use by the client, adverse effects, age restrictions, safety in pregnancy, and cost. A single product or a combination of products may be needed. The Parasiticide Guide will help you select suitable products.

Products that are active against fleas are available as spot-ons, sprays, tablets, collars and an injection. Many also cover ticks. Seresto collar (imidacloprid plus flumethrin) is the only product containing a tick repellent.

There is a large choice of products active against roundworm in the form of spot-ons, tablets, granules, suspension and syrup. Many also contain praziquantel, which is active against tapeworm. It is also possible to use praziquantel alone, as a spot-on or as tablets.

Oral formulations containing fenbendazole and a spot-on containing eprinomectin in combination with S-methoprene, fipronil and praziquantel (brand name Broadline) are licensed for the control of the lungworm A. abstrusus. As discussed above products containing milbemycin, moxidectin and selamectin are also likely to offer protection against this parasite (unlicensed uses).

One spot-on product (Broadline, containing S-methoprene, eprinomectin, fipronil and praziquantel) is authorised to cover all the parasites discussed in this module; it does not contain a tick repellent.

Factors to consider in product selection will be discussed in more detail in module 6.


If you prefer, you can listen to the whole audio presentation of this 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 parasiticides in cats.

Click the button below when you are ready to answer the CPD questions based on the parasiticides for adult cats module from Veterinary Prescriber.

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Some of the following CPD questions make use of our Parasiticide Guide. You may wish to use this link to open the guide in a new window now before attempting the questions.

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

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

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