Ectoparasites – deciding on best treatment for individual animals

Societies devote a considerable amount of resources to pet care. Last year, in the US alone, the overall spending on the pet industry was nearly $72.56 billion (£59.85 billion) and, of this, $18.11 billion (£14.94 billion) was spent on veterinary care (American Pet Products Association, 2019).

In the UK, meanwhile, approximately £24.5 billion is spent on dogs and £9.6 billion on cats each year (King, 2018).

The majority of pets – regardless of the level of care they receive and the growing increase in veterinary care spending – experience distress due to ectoparasitic infestation at some point in their lives. Ticks, fleas, mites and flies are the main ectoparasites (also known as external parasites) that attack pets, and can cause severe itching, pruritus and serious skin damage. Ectoparasites are also efficient Trojan horse vectors, and play a key role in the transmission of serious infections to companion animals and humans.

Parasiticides are an essential component of any parasite control regimen. According to Zion Market Research (2019), the global animal parasiticides market was valued at about US$8.65 billion (£7.13 billion) in 2018 and is expected to reach US$12.85 billion (£10.6 billion) by 2025.

A major proportion of the parasiticides prescribed in small animal practices is used for the management of ectoparasite infestations in dogs and cats. Many treatment options are available, with an excellent record of safety and efficacy. However, control of ectoparasite infestations remains a considerable challenge due to the absence of clear treatment standards, and the high prevalence of ectoparasites in dogs and cats in the UK (Abdullah et al, 2019), and many other parts of the world.

Therefore, the demand for ectoparasiticide drugs is projected to increase as the population of pets increases – and due to increasing awareness of parasitic diseases, anticipated increase in the prevalence of ectoparasites due to global warming and the growing number of veterinary practitioners.

Parasite management is – and has been mainly based on – a one-size-fits-all approach, without considering an individual pet’s lifestyle and risk factors for infestation, among other variables. The promising potential of providing antiparasitic treatment to individual pets, according to a specific individual pet’s characteristics, has attracted the attention of veterinarians and parasitologists in recent years. Despite the attractiveness of individualised treatment, we must remain realistic about our expectations of what can be gained from implementing this approach.

Why are ectoparasites important?

Ectoparasites – such as ticks, fleas, mites and flies – are common problems in dogs and cats. Ticks transmit many tick-borne diseases (TBDs) to pets and people. TBDs – such as anaplasmosis, ehrlichiosis and many others – can have serious implications on the health of the pet and the owner.

Clinical symptoms of most of these diseases are non-specific, which makes it difficult for the health care providers to accurately and timely confirm the diagnosis, resulting in poor treatment outcome.

The cat flea (Ctenocephalides felis) and the dog flea (Ctenocephalides canis) are the two most common flea species infesting dogs and cats. Also, the European hen flea, Ceratophyllus gallinae (Figure 1), which normally infests poultry species, can attack dogs and cats. Fleas are blood-feeder insects and can consume up to 15 times their bodyweight in blood daily.

Dogs and cats can become sensitised to flea salivary antigens, leading to flea allergy dermatitis (FAD). Pets affected with this condition exhibit excoriation, scaling or crusting, miliary lesions and papules. Additionally, fleas can cause problems by acting as vectors for the transmission of the tapeworm Dipylidium caninum and many serious pathogens. For example, Bartonella henselae has been frequently identified in the cat flea (Shaw et al, 2004; Lappin et al, 2006) and, as a consequence, cats infested with cat fleas are at a high risk of infection with this pathogen (Guptill et al, 2004).

A number of mite species can affect and seriously compromise the health and welfare of dogs and cats, such as the follicular mite (Demodex species), sarcoptic mange (Sarcoptes scabiei var canis), the ear mite (Otodectes cynotis), the harvest mite (Neotrombicula [Trombicula] autumnalis) and the walking dandruff mite (Cheyletiella species).

Travelling to certain Mediterranean countries might expose dogs and cats to severe – potentially fatal – diseases, such as leishmaniosis and heartworm (Dirofilaria immitis) transmitted through the bite of sand flies and mosquitoes (Figure 2), respectively. Some insects are not necessarily interested in biting pets, such as bed bugs (Figure 3); however, they attack pets if they cannot find their preferable human host.

Current practice in management of ectoparasite infestations

Many tick and flea preventive products are on the market to help control these stubborn pests and prevent the spread of pathogens they can transmit to animals and humans. Knowing what kind of ectoparasiticide product to use – and how to use it – is vital to the success of the treatment intervention.

Some products are available by prescription only, while others can be obtained over the counter. Many of these products are topical (spot-on), applied directly to the pet’s skin; however, some other products are given orally or available in the form of collars. For example, a slow-release collar containing imidacloprid (providing insecticidal activity) and flumethrin (providing repellent activity) can be used in dogs and cats to control fleas and ticks for up to eight months. This collar has been also shown to reduce the risk of transmission of Leishmania infantum by sand flies to dogs in endemic areas.

Many anti-flea products are available on the market, and these are divided into adulticides to kill adult fleas, and insect growth regulators to prevent the development of eggs and larvae. Some pet owners might think pets at risk for flea or tick infestation should be treated only during the flea or tick seasons (spring and summer) using appropriate preventive drugs. On the other hand, some professional bodies recommend more regular or year-round tick and flea control.

There should not be a minimum or maximum limit for the duration of flea or tick control plan. Flea and tick control should be tailored to pets based on an individual pet’s characteristics, and environmental and lifestyle factors.

The design of the housing environment for the pets can support and maintain the flea life cycle. Therefore, besides treating the infested pet, reduction of the flea population (flea eggs, larvae and pupae) in the house is required via thoroughly cleaning the pet’s sleeping areas, and vacuuming floors, carpets and furniture.

Dogs and cats develop leishmaniosis after being bitten by sand flies. This disease can be fatal, and treatment is complex (that is, it requires chemotherapeutic and immunomodulatory products) and not always successful. Therefore, prevention is best.

The risk of transmission of Leishmania by sand flies and disease development in the infected pets can be mitigated via the combined use of a repellent and prophylactic treatment using the immunotherapeutic drug domperidone (Wylie et al, 2014).

Also, dogs can be protected from sand flies by using a collar containing a repellent, which also controls infestation with ticks for several months. One of the key biological strategies to control mosquitoes (the vector of heartworm disease) is to remove the potential breeding sites, such as standing water, because all mosquito pre-adult fly stages (eggs, larvae and pupa) require water to develop.

Parasite control is not ‘one size fits all’

One pet may react differently to a product than another. Certain pet-related factors can affect the type and dose of the ectoparasiticide product, such as age, species, breed, lifestyle, pregnancy and health status of the pet, as well as any other medications the pet is receiving. The use of targeted treatment is already happening in practice.

Management of certain ectoparasite infestations requires individualised and tailored treatments, based on the burden of infestation, animal susceptibility/response and lifestyle conditions. For example, the severity of clinical signs of FAD, such as pruritus and associated skin lesions, varies from animal to animal depending on the flea salivary antigen load inoculated into the skin during the flea bite. For particularly more sensitive animals, treatment of FAD may require elimination of the flea population, but in less reactive animals, clinical improvement can be achieved even if the flea population has not been completely eliminated. Dogs living in kennels represent another example where targeted treatment can be practised because kennel dogs are at an increased risk of tick and flea infestations, and prophylactic treatment in this setting is therefore recommended.

Veterinarians are always advised to be cautious when considering flea/tick treatment of very young and very old pets, or in certain species and certain breeds. For example, fipronil has high safety margins in cats and dogs, but should not be used in rabbits. Pyrethroids are efficacious against ticks, and crucial for preventing exposure of dogs and cats to Leishmania via sand fly bites in endemic countries. Licensed products are safe at therapeutic doses, but cats do not tolerate therapeutic doses for dogs.

The pyrethroids permethrin and deltamethrin are toxic to cats and products containing pyrethroids should, therefore, not be used in combination, but are safe to use in combination with parasiticides from other classes. Hence, it is best to avoid the use of permethrin-containing products on a dog that shares a home with a cat. It is worth mentioning flumethrin is an ingredient in a tick treatment and prevention collar, which is safe for use in dogs and cats.

How much can we expect from individualised treatment?

Prophylactic treatment of ectoparasite infestations is useful when the benefits (reduction in illness and transmission of infections) supersede the risks (over-prescription, excessive treatment and financial/compliance implications). Veterinarians should assess the balance between the benefits and the risks by accounting for the individual pet characteristics, such as environmental and lifestyle factors, and the potential risk factors for infestation. A risk-based approach can be useful if a gain exists from targeting the treatment to individual pets that will benefit most from the ectoparasiticide treatment.

The extent to which ectoparasite control is individualised to meet specific pet needs is being seen as a potentially more efficient parasite control strategy. The idea is based on the assumption that a risk stratified analysis would identify pets with a higher or a lower than average benefit from prophylactic parasite control plans and that this information could inform better treatment decisions.

If we are going to adopt this approach to control ectoparasites, a need exists to develop diagnostic tools and models that can stratify the pets based on the differences in their circumstances, lifestyle and potential risk factors.

The lack of evidence for the effectiveness or superiority of individualised treatment versus non-individualised treatment could obviously be a limiting factor for the uptake of this approach by veterinarians and pet owners. One way to examine the feasibility of this approach is to conduct a randomised trial in which similar treatment components are available, but where pets in one group receive treatment tailored to their needs and the pets in the other group receive non-tailored treatment.

Even in the presence of the best evidence for individualised treatment, physical and logistical barriers will exist to the adoption of this approach. These barriers – such as increased consultation time and owner belief that his or her pet may not benefit from individualised treatments – will have to be addressed. For now, it remains to be seen if and how the profession will explore the individualised parasite treatment approach, and how this approach can improve the management of ectoparasite infestation in pets.

What pet owners want

Some might believe the problem of ectoparasite treatment is bigger than using individualised treatment alone, and requires that more inclusion of pet owners in the decision-making process, and taking their priorities and opinions into account, become the main components of the parasite management plan. How can a pet owner affect decisions made by veterinary professionals? Can veterinarians force their opinions on owners, or should they go with what the owners think the right thing for their pets is – particularly if they are well informed or as informed as they can be?

In the author’s opinion, pet owners should be involved in treatment decisions, but first, veterinarians should provide them with the information needed to make an informed choice and give them adequate time to think about the treatment options available.

We are all professional and/or experts in what we do; however, owners – even well-educated ones – may not understand the needs for using more than one product or how to apply the products on the pet in the correct manner. Without proper consultation with the veterinarian, the owner may struggle to know what is the best for his or her pet, and might not be convinced of the individualised treatment approach.

Many conceptual and practical barriers exist to pet owners’ adherence to treatment recommendations in general – not to mention the adherence to a new parasite control model. These barriers will need to be identified and taken into account while designing individualised parasite control interventions. For example, prescribing an ectoparasiticide with a long duration of action seems to improve the adherence of dog owners to flea and tick control recommendations (Lavan et al, 2018).

Pet owners may become more compliant with the individualised treatment approach if they receive information that provides them with clear, easy-to-comprehend and simple facts. Pet owners will need to be informed about which parasites a certain product protects against, how often they should use/apply the product on the pet, how long it will take for the product to work, and what they should do if the pet develops an adverse reaction to the product.

The implementation of the individualised treatment will require the sources of uncertainty to be identified and addressed. One of the potential problems that may reduce the uptake of the individualised treatment approach is the fact designing a treatment protocol based on a predictive algorithm might be simply seen as a guessing game.

One obvious solution would be to ensure individualised treatment decisions are made based on a range of resources, to select the antiparasitic drugs that are the most appropriate to treat individual pets. These resources include best clinical evidence, a veterinarian’s own experience, peer support, the pet owner’s views and the pet’s circumstances. Most of this evidence is not available as yet.

Summary

Veterinarians generally find ectoparasite control in pets to be straightforward, pampered by plenty of options and a considerable number of modern ectoparasiticide products available on the market. Nonetheless, lack of familiarity with exotic parasites, less-informed pet owners, ignoring the impact of pet characteristics on the treatment outcome with one product versus another, and poor understanding of pet owners’ expectations/opinions are some of the factors that make it hard for a veterinarian to determine the most appropriate ectoparasite management strategy for individual pets.

Improper treatment or prophylaxis can subject the animal to potentially serious diseases (tick-borne infections, for example) whose therapeutic outcome can be uncertain and requires more financial resources. Treatment outcome can vary from pet to pet and from drug to drug. Therefore, matching treatment protocol to the particular needs of the pet receiving the treatment is vital. Identifying the barriers to the implementation of individualised treatment for ectoparasite management is the first step forward in exploring the feasibility of this intervention.

Finally, development of evidence-based guidelines to support veterinarians selecting the most appropriate ectoparasiticide drug for the individual pets will definitely be welcomed.

Conflict of interest

The author has no conflict of interest to declare.