Parasiticides for cats and dogs: a rational approach. Module I. Introduction

Parasite disease management is a key responsibility for veterinary professionals, and the RCVS Practice Standards Scheme requires that practices have an evidence-based parasiticide policy. However, the area is complicated because cats and dogs need tailored management, there is an overwhelmingly large choice of parasiticide products, and very intense promotional activity by drug companies. This module is the introduction to a series of modules on management of the key parasites that affect cats and dogs in the UK. The modules will help with the development of an evidence-based parasiticide policy.

The risk of infection to the animal and need for parasiticide therapy depend on various factors including the age of the animal, its lifestyle (such as whether it eats offal or molluscs), location (affecting the endemicity of parasites) and whether the animal lives with susceptible humans (such as young children, or people with immunosuppression). The use of parasiticide drugs therefore needs to be tailored to the animal. The choice of parasiticide product also depends on factors such as whether the animal is bathed or swims, and on tolerance, convenience and compliance.

In general, in the UK cats and dogs need routine cover for fleas and roundworm. The animal might also need cover for ticks and/or tapeworm and/or lungworm (Angiostrongylus vasorum), depending on whether it is at risk of these infections. Broader parasite cover is needed for animals travelling outside the UK. (ESCCAP ectoparasite and worm control guidelines)

Some parasites are ubiquitous (i.e. all cats and dogs are potentially exposed throughout the UK) and not increasing in prevalence. An example is roundworm (Toxocara spp), to which all cats and dogs are exposed and which provides a constant zoonotic risk. Although the prevalence of this parasite remains fairly constant, the incidence of toxocariasis disease in humans is affected by factors such as lifestyle, the way we manage the risk of transmission, and the number of pets we keep.

By contrast, many parasites are on the increase in the UK, either in terms of numbers, or geographical distribution, or both. This is true of fleas, ticks, Angiostrongylus vasorum and Babesia canis


The distribution of endemic parasites can change and exotic parasites may become established in the UK. For example, parasites such as certain tick-borne diseases, heartworm, Leishmania infantum and Echinococcus multilocularis are spreading across Europe and may be contracted by travelling pets. There is also an increasing trend for importing puppies and rescue dogs from abroad. So it is important to know where to find up-to-date information on parasite distribution. There are several helpful sources available (see below). Using these together with anecdotal reports of cases diagnosed in practice, it is possible to develop a picture of the parasite risk both locally and in areas where clients are likely to travel. 


ESCCAP UK & Ireland: The European Scientific Counsel for Companion Animal Parasites (ESCCAP) is an organisation comprised of parasitology and  public health experts from across Europe. ESCCAP’s role is to develop guidelines for the control of parasites in pet animals. The UK & Ireland branch of the organisation provides practical advice to veterinary professionals and pet owners on protecting pets from parasitic diseases. The web site ( has separate areas for professionals and pet owners. It contains scientific information about parasites, parasite management guidelines, information leaflets on protecting pets at home and abroad (including information on pet travel regulations) and maps showing parasite distribution in Europe. The information is free.


Angiostrongylus vasorum

  • Angiostrongylus distribution studies. (Kirk et al 2014; Taylor CS et al 2015).

  • The Small Animal Veterinary Survellance network (SAVSNET) a general review

Lungworm disease in cats

Parasiticide medicines for animals belong to one of three supply categories:

  • POM-V - available only on veterinary prescription

  • NFA-VPS (non-farm animal – veterinary, pharmacy, suitably-qualified person [SPQ]), can be supplied from pharmacies and suitably qualified sellers

  • AVM-GSL (authorised veterinary medicine – general sales list) can be sold from any retail outlet.

Most flea and tick products (ectoparasiticides) contain one or more ingredients that have insecticide and/or acaricide effects. Insecticides and acaricides work by interacting with the nerve cell receptors of parasites, so disrupting central nervous system activity, leading to death. The products can be used to kill parasites already on the animal’s body. However, as most products (apart from shampoos and powders and certain tablets) remain on, or in, the body of the animal for several weeks, they can kill parasites newly acquired by the animal and so prevent infestation over a period of time, usually 1 month.

Some products contain an insect growth regulator alone or together with an insecticide. Insect growth regulators prevent female insects from laying viable eggs and/or prevent the larvae from developing into adults. When used on their own, insect growth regulators have a preventive effect only.

Some products contain pyrethroids which have a tick-repellent as well as insecticide and acaricide effects. The repellent effect prevents ticks from taking a blood meal.

Pyrethrins are naturally occurring compounds from the Chrysanthemum plant that have insecticidal properties. They also have a repellent effect. Pyrethrins are a common ingredient in products on general sale. Pyrethroids (deltamethrin, flumethrin, permethrin) are synthetic versions of pyrethrins. Topical products (spot-ons) for dogs containing high concentrations of permethrin are dangerous for cats, which metabolise permethrin less well than dogs. Cats can develop toxicity if a canine permethrin product is applied inappropriately or via secondary contact with a dog treated with a spot-on product containing permethrin. So it is best to avoid use of permethrin-containing products on a dog that shares a home with a cat. There are cat collars on general sale that contain small amounts of permethrin that are appropriate for use on cats.

There are dozens of endoparasiticide (worming) products on the market. Most endoparasiticides work by paralysing the worms through effects on the nervous system.

Drugs belonging to the class called macrocyclic lactones are active against worms and insects. They are: eprinomectin, milbemycin, moxidectin, selamectin.

*this is a general guide. you should check the SPCs of specific products for licensed indications.

The products that can be sold over the counter are aimed at controlling fleas, ticks, roundworm and tapeworm infestations. Some POM-V products have the same ingredients as products available over the counter. However, products that are available only on veterinary prescription are different in the following respects:

  • different formulations (e.g. fipronil spray; injectable products);

  • certain combinations of active ingredients (e.g. fipronil plus S-methoprene, an insect growth regulator*;

  • different active ingredients (e.g. moxidectin, milbemycin);

  • combination products with broader parasite coverage (e.g. including heartworm, lungworm).

*Since this module was published, fipronil plus (S)-methoprene products have become available over the counter.

In order to get a marketing licence, parasiticide products need to have demonstrated efficacy according to certain standards set by the European Medicines Agency. (CPMP 2008; EMA 2002 canines; EMA 2002 felines) Laboratory tests are used to establish efficacy, followed by field studies to confirm efficacy and safety under conditions representing practical use. Products applied topically are also tested for sunlight and water stability.

For insecticides, immediate efficacy is proven if the product reduces adult flea count (following deliberate infestation) by at least 95% within 48 hours of applying the treatment. To show the product has a 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) the count reduction needs to be shown at  4-weekly intervals (2-weekly in the last month of the claimed effectiveness period). (CVMP 2008) 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). (CVMP 2008)

For tick products, immediate killing effect is proven if tick count (following deliberate infestation) is reduced by at least 90% within 48 hours of applying the treatment. To show long-term efficacy (more than 4 weeks) a 90% reduction in tick count needs to be shown through 4-weekly testing (2-weekly in the last month of the claimed effectiveness period). (CVMP 2008) A tick repellent effect means that no tick will attach to the animal, and ticks already on the animal will leave the animal soon after treatment and should not be detectable on the animal after 24 hours following administration of the product. (CVMP 2008) The presence of ticks is tested 24 hours after treatment followed by a 4-weekly challenge to test for long-term persistent efficacy (2-weekly in the last month of the claimed period of effectiveness).

100% effect not guaranteed. It is important to know that no tick products are guaranteed to prevent the attachment of ticks or transmission of disease, and so pet owners should check the animal’s coat for ticks every 24 hours and immediately remove any found.

Kill time. For many of the newer flea and tick 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 earlier than 48 hours (EMA 2015)

For endoparasiticides, two dose confirmation studies are required involving a minimum of 6 controlled and 6 treated animals. (EMA 2002 canines; EMA 2002 felines). The difference in parasite counts between treated and control animals should be statistically significant (p <0.05) and effectiveness should be 90% or higher. For parasites with implications for public health (e.g. Echinococcus spp.) the efficacy standard may be higher (i.e. up to 100%); field studies are not usually required for such parasites. (EMA 2002 canines; EMA 2002 felines)

The authorised indications of commercial products depend on the spectrum of activity of the active ingredient(s), and also on what effects have been demonstrated for the product to get a marketing authorisation. This means that different products with identical ingredients can have a different range of authorised indications. For example, Anthelmin Plus tablets for dogs contains febantel, praziquantel and pyrantel embonate and is authorised for tapeworm, roundworm and hookworm whereas Dinelix Plus tablets for dogs contains the same ingredients but is authorised for tapeworm including Echinococcus, roundworm, hookworm and whipworm. The authorised indications are listed in the product’s summary of product characteristics and in the package leaflet and/or on the package.

There are several ways of using parasiticides:  as impregnated collars, spot-ons, shampoos and sprays, and as oral formulations.

The active ingredients in impregnated collars are slowly and continuously released in low concentrations, spreading from the site of contact over the entire skin surface. Impregnated collars have an effect that lasts several months.

Spot-on solutions are applied to the skin on one to four spots or in a continuous line (depending on the product) on the back of the neck of the animal. Most spot-on products distribute across the skin and hair of the animal over the following days. They are convenient and easy to use but their effects can be reduced by bathing and swimming (the SPCs of spot-on products give details of how the efficacy is affected by bathing). Spot-on products containing selamectin or moxidectin are absorbed through the skin and work systemically, so once absorbed are not affected by bathing and swimming.

Shampoos and sprays that are available over the counter have no residual effect. Spray formulations of fipronil are available on prescription; these leave a film on the coat that persists for several weeks.

Oral formulations. Flea and tick, and worming products are available as tablets; a few roundworm products are available as granules, paste, syrup and suspension.

For optimum efficacy and safety, parasiticide products must be used correctly. This means that product choice must take into consideration the likelihood of client compliance. These are points to consider:

  • Whether the pet is bathed or swims frequently. Products that are partially or completely washed off will need to be applied more frequently (see the relevant product SPC for details), which may be difficult for owners to achieve. A systemically absorbed product may be more suitable.

  • Client preference. Clients will become disheartened if a product is difficult to use and in some pets certain routes of administration may be impossible, even for a veterinary professional.

  • Previous adverse reactions to a product, for example, a local reaction to a spot-on product or a collar, or vomiting after a tablet. Even a one-off adverse reaction may put an owner off using the same product or formulation again.

  • Likelihood of correct use. Although a method of administration may seem obvious to a trained professional, it may not be to a client. Without correct instruction spot-on products might be incorrectly placed on the coat instead of the skin, or even administered orally or rectally. The correct administration of a product should be at least described, and ideally demonstrated, to a client.


Some might argue that everyone in the practice should have their own opinion about the parasite requirements of pet owners and product choice. Such individual clinical freedom in practice is important and there is debate across the whole of veterinary medicine about the conflict between individual clinical freedom and a consistent practice treatment policies. Parasite control advice is a special case in this context, because a conversation about parasite management will often start at reception or in a nurse clinic and end in the veterinary consultation room where the final recommendation is made. Everyone in the practice should be aware of the common parasites that may require control measures, the relevant risk factors, and how to quickly determine the client's needs. This way, crucial information can be obtained before the veterinary consultation, when time is limited. 


  • Clients receive a clear recommendation. Clients are often confused about parasitic threats because of public advertising campaigns and press stories about zoonoses such as toxocariasis and Lyme disease. Unclear or conflicting messages from practice staff may lead to confusion, disillusionment and lack of compliance.

  • A smaller range of products can be stocked. If local parasite risks are established, only items that are needed routinely need to be stocked. This can be done while allowing some flexibility in what is offered. For example, a client who lives in an area where A. vasorum is endemic and whose dog eats slugs needs a product that protects against this parasite. Stocking a milbemycin oxime tablet and a moxidectin spot-on solution (both active against A. vasorum) gives a choice of two different formulations. This means that there is a consistent message about protecting against A. vasorum while allowing some freedom of choice.

  • Practice staff can become familiar with fewer products.

  • Practice morale may improve. An inconsistent message about parasite management may have a negative effect on the client and consequently on practice morale. A consistent message should help promote effective parasite control and lead to satisfied clients whose pets’ health will be improved. This in turn can lead to positive feedback and increased staff morale.

A difficulty with getting to grips with the full range of parasiticide products on the market led to the development of the Veterinary Prescriber small animal Parasiticide Guide. It contains every parasiticide product authorised in the UK for cat, dogs, ferrets and rabbits and has been created using the VMD product information database (which includes every authorised product). and is updated every 3 months. The guide is intended to help with choice; for full information on prescribing and using the products (including the specific authorised indications, dose, contraindications, adverse effects, interactions, handling, water stability etc.), consult the product's summary of product characteristics (available for each product on the Veterinary Medicines Directorate website).

In later modules you will use the guide to help get to know the up-to-date full range of products and select suitable ones for a parasiticide policy. In the meantime, watch this short video showing how it works.


If you prefer, you can listen to the whole audio presetation of Parasiticides module I 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 and dogs.

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

Once complete you will be emailed with the results of your parasiticides for cats and dogs CPD along with the answers so make sure you enter your email address carefully at the head of the form so could can also receive your certificate

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. 


CVMP.  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. Available:

EMA. Step 7 Consensus Guideline, 2002. Efficacy of anthelmintics: specific recommendations for felines. Available:

EMA. Step 7 Consensus Guideline, 2002. Efficacy of anthelmintics: specific recommendations for canines. Available:

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 insect and ticks in dogs and cats. Guideline 3, March 2009. Available:

ESCCAP. Worm control in dogs and cats. ESCCAP guideline 01 second edition, September 2010. Available:

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

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

Smith FD et al. Estimating Lyme disease risk using pet dogs as sentinels. Comp Immunol Microbiol Infect Dis 2012; 35: 163-7.

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