Endoparasite control in cats and dogs – can it be achieved without drugs?

 
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Surveys across Europe have shown that the public have a poor understanding about the need for endoparasite control in their pets, including the risks endoparasites pose, such as zoonosis, and about routine preventative treatment. (ESCAPP 2017; Matos 2015; Zanzani 2014) Deworming treatment frequencies are also irregular, and below the minimum roundworm treatment frequency (four per year) recommended by parasitologists. (McNamara, 2018, Matos, 2015) Anecdotally, some owners choose not to use preventive endoparasiticide medicines because of worries about adverse effects or harm to the environment. 

This module focuses on the role of non-drug strategies in preventing morbidity and mortality due to common endoparasites affecting UK dogs and cats, to outline what can be achieved without the use of medicines as well as any limitations of this approach. It aims to help vets in their conversations with clients about parasite management.

By doing this module, you will learn about:

  • the common parasites affecting cats and dogs in the UK, and what risks they pose to humans and animals

  • the non-drug measures to prevent disease and death associated with endoparasites, and how effective they are

  • the different approaches to drug use for endoparasite control

  • the practicalities, and pros and cons, of the ‘test and treat’ approach to endoparasitic infections.

The module is in three parts. The first part describes the organisms routes of infection and diseases; the second part outlines the main non-drug preventative measures; and the third part sets out the different approaches to parasiticide use (blanket; test-and-treat; and risk-based).

Click here to see related modules on parasite control

Dogs and cats in the UK can be infected with a wide range of endoparasites in various body systems. The two main subgroups are helminths and protozoal parasites.

  • The most common helminths of concern in the UK are roundworm (Toxocara spp), lungworm (Angiostrongylus vasorum) and tapeworm (Echinococcus granulosusTaenia spp). (ESCAPP 2017) 

  • Protozoa are single-celled parasites and comprise several species, such as Giardia spp, Tritrichomonas foetusIsospora spp and Toxoplasma gondii, which cause important diseases in dogs and cats. (ESCAPP 2018)

A study in the UK found Toxocara to be the most common intestinal nematode in untreated adult cats and dogs, with 5.3% of dogs and 26.0% of cats infected. (Wright et al., 2016) All other intestinal nematodes had a prevalence of under 5.0%. The largest source of potential Toxocara egg contamination is puppies and kittens because of transplacental and transmammary infection. Untreated adult cats and dogs potentially shed eggs intermittently throughout their lives. (Overgauuwand Van Knapen 2013)

Although high intestinal nematode burdens can cause morbidity in cats and dogs, the main issue is the zoonotic potential of T. canis and T. cati. (Halsby 2016) As well as causing visceral and ocular toxocariasis in humans, trends worldwide show that exposure to Toxocara infection is linked to a variety of chronic disease syndromes in people including cognitive dysfunction, asthma, dermatitis and epilepsy. (Luna 2018, Holland 2017, Pinelli 2007, Buijs J 1997) The most common route of human infection is by the ingestion of embryonated eggs, and younger children are mainly affected. (Halsby 2016) The number of cases of toxocariasis reported in England and Wales has been relatively consistent since 2000 although it is difficult to estimate the true number. (Halsby 2016) Laboratory data suggest there are fewer than 10 cases per year. (Halsby 2016) However, this is likely to be an underestimate because toxocariasis is difficult to diagnose, can present in a variety of different ways and is not a notifiable disease. (Holland 2017)

A. vasorum has spread rapidly over the past 10 years across the whole of the UK and there has been an increase in reported cases of angiostrongyliasis in domestic dogs, with about 21% of practices across the country having seen at least one case in a 12-month period. (Kirk 2014) Post mortem surveys of foxes, which act as wildlife reservoir hosts, in 2005 and in 2014 showed that the overall prevalence of A. vasorum infection rose from about 7% (Morgan 2008) to about 18% (Taylor 2015)  and extended to regions previously clear of infection such as Northern England and Scotland.

Infection occurs through consumption of slugs and snails, and paratenic hosts such as frogs and birds. The consequences of infection for dogs can be severe, with respiratory disease, coagulopathies, neuropathies and sudden death all possible outcomes. (Koch 2009) A. vasorum is not notifiable, so there are no accurate figures for confirmed clinical cases or deaths in the UK. Mortality rates of around 13% have been reported. (Koch, 2009; Chapman et al. 2004) However, both these figures are from referral hospitals, so are likely to be lower in general practice. There is no evidence that environmental control of slugs and snails is an effective method of preventing exposure to the parasite. It is unlikely to be effective because molluscicides promote the release of the parasite on the death of the mollusc as they decompose. It is also not possible to prevent exposure in the wider environment unless there is persistent mollusc control on a country-wide scale which has its own adverse environmental impact.

Canine Taenia spp tapeworms are found throughout the UK. Apart from T. multiceps, which can lead to CNS signs in young ruminants (‘gid’), infection is well tolerated in intermediate hosts and the greatest consequence of cyst formation is economic. This is because cysts will lead to meat and offal condemnation. Food Safety Agency freedom of information data showed considerable economic loss from liver condemnation because of cysts for T. hydatigena and carcass condemnation from T. ovis cysts.

In contrast E. granulosus tapeworm is zoonotic with hydatid cysts developing in the bone, liver, central nervous system and heart. The cysts form after ingesting eggs passed in the faeces of dogs. Foci of E. granulosus infection in dogs and of human hydatid disease are present in Wales, the Welsh border, Herefordshire and the Western Isles of Scotland. However, the FSA has found that the incidence of E. granulosus was much more widespread in England than previously thought. Postmortem inspections in abattoirs across Britain have found cases, with a particularly high incidence on the Welsh border and North Midlands. In 2015, FSA data showed the incidence of hydatid cyst rejections in sheep and cattle offal to be 0.12% in cattle and 0.30% in sheep across England and Wales. While it is possible that some of these condemnations may have been misidentified T. hydatigena cysts, the figures still present a substantial risk that dogs will be exposed to infection through offal fed directly in hunts, kennels and farms and through unprocessed diets. It also makes it likely that other foci of E. granulosus infection exist in dogs outside of Wales. It is currently unknown to what extent canine infection is occurring, and diagnosis in dogs is difficult because faecal flotation is an insensitive method for detection. (Craig 2014)

G. duodenalis is a flagellate protozoan of the intestinal tract. The life cycle of Giardia is direct, with infective cysts being passed in the faeces. Transmission is therefore by the faecal-oral route, with the potential for environmental, food and water contamination. (Caccio 2005) G. duodenalis can infect many mammals including humans, livestock and pets. Many infections are subclinical. Prevalence of subclinical infection is high in cats and dogs, particularly in pets aged under 1 year. (Bouzid et al., 2015)

The most common clinical sign of giardia infection is chronic, sometimes intermittent, small bowel diarrhoea that can be mucoid, foul smelling and accompanied by flatulence. Lethargy and weight loss may develop even when the appetite is good. Less commonly, there may be large bowel involvement with tenesmus and the presence of mucous and fresh blood in the faeces. Although there is some debate around subclassification of G. duodenalis, it is currently split into 8 subgroups or assemblages. Humans are usually infected by assemblages A and B; infection with other assemblages is very rare. Dogs are mainly affected by assemblages C and D, and cats by assemblage F. However, cats and dogs may be infected by assemblages A and B if they are exposed to environmental contamination from human infection. (Ballweber 2010)

T. foetus is a similar flagellate protozoan to Giardia, infecting the large intestine of cats. The life cycle of T. foetus in cats is direct via the faecal-oral route. Trophozoites are ingested from fresh faecal contamination, either from the environment or from grooming faeces from the coat. Cross contamination of fresh faeces is thought to occur most commonly in shared litter trays. Clinical disease is most prevalent in breeding colonies, and in young and pedigree cats. Dense stocking conditions in breeding situations leads to an increased likelihood of faecal-oral transmission through faecal contamination in the immediate environment. Many infected cats are subclinical carriers, with clinical cases being more common in cats aged under 12 months. (Yao and Koster, 2015)

Clinical outbreaks in litters of kittens can occur because of untreated clinical cases either self-curing or remaining as carriers, and intermittently shedding the organism. Colitis with malodorous chronic diarrhoea is the most common clinical sign. Blood may be present in the stool with large volumes of mucous. There can be chronic weight loss, suggesting a degree of small bowel involvement. (Yao and Koster, 2015) It is unknown whether T. foetus is capable of zoonotic infection.

I. canis and I. felis are species-specific coccidian protozoa found in the small intestines of dogs and cats respectively. Transmission is by the faecal-oral route, with oocysts being passed unsporulated in the faeces. In warm, humid conditions, sporulation occurs in as little as 2 days, leading to infective oocysts in the environment. (Mitchell et al, 2009)

Infection is often subclinical, even when large concentrations of oocysts are present in the faeces, but projectile small bowel diarrhoea can develop in heavy infections. The risk of severe infection increases as the numbers of infective oocysts in the environment increase. This most commonly occurs when large numbers of cats or dogs occupy a small space, particularly in warm damp conditions. It is not considered a zoonosis. (Mitchell et al, 2009)

Current evidence suggests that T. gondii infects all mammals but only felids act as a definitive host, producing oocysts in the intestine that are then passed in the faeces. Although cats can be infected through faecal-oral ingestion of oocysts, they most commonly acquire the infection by ingesting tissue cysts. This occurs through eating intermediate hosts such as rodents and birds, or by feeding on raw or undercooked meat from infected livestock or, less commonly, on aborted ovine material. Intermediate hosts, including humans, are infected either transplacentally, or through consumption of oocysts. The oocysts are easily disseminated into surface water where they can survive for several months. This means that water, as well as humid soil or feedstuffs, contaminated with cat faeces is a prime source of infection. Carnivorous intermediate hosts may also be infected through consumption of tissue cysts in raw or undercooked meat, and this is a significant source of human infection. (Dubey, 2008)

Clinical toxoplasmosis is rare in cats and most immunocompetent adults will remain subclinical carriers. Kittens infected in utero can show signs of infection after birth; prenatal infections of kittens are frequently fatal. The reasons for clinical manifestations in adult cats are not fully understood but thought to be linked to immunosuppression. This may be secondary to using immunosuppressive drugs or viral pathogens such as feline leukaemia virus and feline immunodeficiency virus. Affected animals show a variety of systemic signs including fever, anorexia, abdominal pain and dyspnoea. Uveitis and central nervous disorders may also develop. (Dubey, 2010)

While healthy human adults have a low risk of developing acute toxoplasmosis if infected, immunocompromised individuals or babies infected in utero can develop severe ocular and cerebral disease that can lead to blindness or death.

Table: Summary of common endoparasites and who they affect

 
 

There are several non-drug preventative measures that can be used to help control endoparasites and which are essential as part of an overall parasite control strategy alongside preventative drug use. For endoparasites such as intestinal protozoa, for which there is no proven preventative drug prophylaxis, non-drug preventative measures and treatment of known infected pets are the only methods of control. 

  • Good hand hygiene helps to act as a barrier against zoonotic parasites that are transmitted by the faecal-oral route. These include Toxocara, GiardiaEchinococcus and Toxoplasma.

  • Washing fruit and vegetables intended for raw consumption will also help to prevent transmission by the faecal-oral route. Contamination of wild berries and home-grown fruit and vegetables with faecal material from cats, dogs and foxes can lead to zoonotic parasitic life stages being present. 

  • Frequent collection of faeces will help to reduce environmental contamination with parasitic life stages. Promoting awareness among owners of the importance of frequently collecting pet faeces and of anti-fouling by dogs is crucial.

  • Daily washing of infected pets with shampoo particularly around the perineum will help remove faecal contamination and infective parasitic life stages from their coats and help prevent transmission.

  • Disinfection of kennel areas and runs using a quaternary ammonium compound will reduce environmental contamination with ova and cysts.

  • Covering sandpits to prevent cats defaecating in them, and the potential for Toxocara and Toxoplasma contamination.

  • Ensuring that raw diets have been adequately cooked or frozen. Tapeworms and protozoa such as Toxoplasma, Neospora and Sarcocystis are transmitted through the consumption of raw meat and offal. Although raw animal feed is inspected to human abattoir standards, cysts can be easily missed, leading to infection in cats and dogs if the raw feed is consumed. ESCCAP recommends that meat should be frozen at -20°C for 7–10 days before use to inactivate protozoan parasites and reduce the risk of transmission. (ESCCAP 2018)

  • Humane stray cat and dog control.Stray cats and dogs act as reservoirs for endoparasite infection, which will then be passed in faeces. Reducing stray cats and dogs through effective microchipping, rehoming and neutering campaigns will help to reduce environmental parasite contamination over time and is the only way to reduce contamination from these sources. Although foxes are often infected, the numbers are proportionally low so controlling fox populations is not as likely to have as great an impact.

There are three main strategies for drug use: treat all pets preventatively; test for endoparasites and only treat infected animals; or use preventive therapy according to risk. The following sections present the arguments around the different approaches.

  1. Treat all pets continuously

Continuous parasiticide cover against all clinically relevant roundworms and tapeworms will drastically reduce numbers of infective ova and larvae in the faeces, reduce clinical disease and zoonotic risk without the need for a tailored strategy, and drive sales of parasiticides. However, there are several potential drawbacks that make routine blanket chemoprophylaxis unsuitable for many UK pets.

  • The potential for development of resistance: The risk of resistance developing in companion animal endoparasites is limited by wildlife and livestock reservoirs of infection. The arrested stages of some parasites such as Toxocara spp cannot easily be eliminated by routine treatment with anthelmintics and this also limits selection pressure for resistance. Nevertheless, resistance can occur. Examples are heartworm resistance developing to macrocyclic lactones in the USA (Bowman 2012) and hookworm resistance to pyrantel in urban Australian dogs. (Kopp et al, 2007) Therefore, to limit cases of resistance becoming more widespread, constant chemoprophylaxis should only be employed when its use can be justified to mitigate significant disease risk, such as in heartworm-endemic countries or for ubiquitous zoonotic parasites such as Toxocara. Resistance can only be limited by allowing infective eggs to persist in the environment (refugia), which in the case of Toxocara represents an unacceptable zoonotic risk. The international veterinary parasitology organisation ESCCAP therefore recommends a minimum treatment frequency of four times a year for Toxocara in all adult cats and dogs to reduce shedding over time and reduce overall environmental contamination. The frequency can be increased to monthly for pets at high risk of infection (from hunting or eating raw diets) and those living with people at highest risk from infection (young children and immunosuppressed individuals). (ESCCAP 2017) For purely indoor cats, it is possible to eliminate all faecal contamination if cats use litter trays exclusively. If cats and dogs have outdoor access, some faecal residue will remain even if it is picked up, making routine deworming an essential component of control.

  • Potential lack of diagnosis and surveillance: If a blanket recommendation is made and followed by most people it may lead to a reduction in routine diagnosis of parasitic disease and surveillance of parasite distributions, which may put pets and the wider public at greater risk.

  • Problems with compliance: Some owners find continuous treatment of their pets difficult or they may wish to avoid drug management or prefer to use the minimum required. Making treatment recommendations that the client cannot or will not follow will leave them unprotected. Agreeing an option that offers necessary protection and is acceptable and manageable to the client is crucial. For those who cannot adminster the treatment, this might mean attending the surgery for parasiticide treatment.

  • There is no routine prophylactic treatment option for protozoal infections: No products are licensed or have been proven to be effective as preventative treatments for protozoal infections. They can therefore only be employed to treat existing infections as part of a wider control strategy.

2. Routine testing of all pets for parasites followed by treatment of those found to be infected

Basing preventive parasiticide use on the results of routine diagnostic testing is another option. It consists of regular faecal egg counts or antigen testing. The argument for this strategy is that it reduces the promotion of resistance and environmental contamination. Routine diagnostic testing gives information about parasite distribution and prevalence, which is crucial for assessing disease risk and the need to control specific parasites in different parts of the country. However, this strategy also has several drawbacks:

  • Not treating parasite-free pets does not help decrease resistance: As explained earlier resistance of parasites to parasiticides is only reduced if parasites not exposed to anthelmintics are released into the environment to create refugia. This is not achieved by not treating pets testing negative for parasites. To reduce resistance, deworming pets is needed at a frequency that allows some shedding of ova and larvae into the environment. This is hard to justify when there is significant human health risk, such as with E. granulosus and Toxocara spp.

  • Pets will already have been exposed to infection before using a parasiticide: If treatment is used only when positive animals are identified, then infection has already occurred. This means that people or pets may have already been exposed to pathogens before treatment is started.

  • Diagnosis of parasites is often more expensive to the client than treatment: It can be difficult to justify the expense of diagnostic tests for parasitic infection when simple, cheap prophylactic measures are available. However, this iargument is being weakened by the availability of simple cheap diagnostic tests for some parasites, such as Angiodetect blood testing for A. vasorum. (Bird, 2018)

  • Shedding of parasitic stages in the faeces is often intermittent: Even if infection is present, faecal flotation tests can still be negative because of intermittent shedding. Some infections such as A. vasorum or Ancylostoma caninum may also cause life-threatening disease without patent infection being present. However, this argument is also become weaker because of the introduction of commercial faecal and blood antigen testing. (Bird, 2018)

These factors make ongoing individual screening of pets for parasites an impractical and ineffective basis for parasite control. It does not mean that testing has no value because it can aid countrywide surveillance and allow targeted treatments for the elimination of protozoal infections such as Giardia and Tritrichomonas in breeding establishments and multi-pet households.

3. Basing preventative therapy on risk assessment

Finally there is the option of using preventative therapy according to risk of infection. The risk of a pet being infected with endoparasites that are likely to cause morbidity or have zoonotic potential can be assessed by considering key risk factors:

  • Does the pet hunt?: Predation of paratenic hosts will increase the risk of Toxocara spp infection. For pets that catch prey, monthly rather than 3-monthly, deworming is recommended by parasitologists to reduce zoonotic risk (ESCCAP 2017). Pets may also become infested with tapeworms through this behaviour.

  • Are there children in the household?: Children aged 2–4 are most susceptible to Toxocara infection. Therefore, cats and dogs living in households with children should be dewormed monthly to reduce Toxocara spp ova output. Although fresh faeces do not pose a direct risk, if pets are not dewormed adequately there is risk that infective ova will build up in gardens. It has also been shown that T. canis ova can embryonate in the coats of dogs. (Wolfe and Wright 2003)

  • Is there a history of slug or snail consumption, or lungworm infection?: Known consumption of slugs and snails or eating grass that may contain small slugs and snails increase the risk of infection with the lungworms A. vasorum and Crenesoma vulpis. A history of lungworm infection also increases risk of infection because there is no lasting protective immunity. In these cases, monthly prophylaxis (the licensed regimen) against A. vasorum with a licensed product containing moxidectin or milbemycin oxime should be advised (ESCCAP 2017)

  • Lungworm endemic area?: A. vasorum has a very patchy focal distribution so local cases being identified in a practice suggests other dogs in the area may be at risk.

  • E. granulosus endemic area?: Herefordshire, Mid Wales and Western isles of Scotland are still considered endemic for the zoonotic sheep strain, so all dogs living in these areas (unless lead-walked and on cooked diets) should be treated with praziquantel every 4–6 weeks to suppress egg production.

  • Does the pet have access to raw food/offal/carcasses?: E. granulosus is found throughout the country at meat inspection and not all affected carcasses originate in Wales. This implies that other foci of infection may be present in the UK with the potential for human infection through ingesting eggs passed in the faeces of dogs. (Craig 2014) Deworming dogs with access to fallen livestock, raw offal or unprocessed raw diets with praziquantel every 4–6 weeks is therefore recommended to safeguard human health. (ESCCAP 2017) These activities, and hunting in cats, will also lead to Taenia tapeworm segment shedding, which people often find unpleasant and which can erode of the human-animal bond. Treatment with praziquantel at least four times a year in dogs that have access to raw food, offal or carcasses is recommended to reduce segment shedding. Also preliminary data indicate that this frequency helps reduce zoonotic transmission of E. granulosus over time (Craig, 2014)

Strategies for endoparasite control are complex and include routine deworming, non-drug preventative strategies and diagnostic testing. The key points when considering endoparasite control are:

  • Pet owner education so that endoparasite risk to human and animal health is appreciated, and that control measures are understood and implemented. Owners need to understand that absence of visible parasites in faeces does not need mean absence of parasites.

  • Non-drug strategies for endoparasite control are essential as part of overall parasite control programs but also for the control of parasites for which no proven preventative drugs exist.

  • Routine preventative drug treatments are essential for worm control in cats and dogs. This may be blanket treatment in the case of ubiquitous zoonotic helminths such as Toxocara, or targeted treatment based on risk. Blanket treatment of all helminths is inappropriate as a control strategy.

  • Diagnostic tests are useful for monitoring the success of control strategies and for allowing targeted treatment of protozoa. However, diagnostic testing for all endoparasites is inadequate as a control strategy.

The limitations of sole reliance on blanket treatment, and testing before treatment means that a risk-based approach to treatment is appropriate in most cases. Clients should be given a clear recommendation on the frequency of the parasiticide administration needed for their pets based on the risk of infection and the likelihood of achieving good compliance.

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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, BVMS BSc MSc MRCVS
Veterinary Practitioner and Head European Scientific Counsel Companion Animal Parasites (ESCCAP) UK and Ireland. The draft was circulated unsigned to practising first-opinion vets. 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. 

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