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Comparison of Anti-ectoparasite and Anti-endoparasite Therapies and Control Strategies

Debra J. Woods*, Tom L. McTier and Andrew A. DeRosa

Abstract

In this chapter, we consider the similarities and differences between management of ecto- and endoparasites. We discuss the general approaches of prevention and control of ecto- and endoparasites (historic and current chemotherapies, environmental management/host management), while considering the different challenges faced relating to lifecycle, host distribution, genetics, and selection pressure.

Introduction

The Merriam Webster dictionary defines a parasite as an organism living in, with, or on another organism. "Parasitism" refers to the intimate association between the parasite and host, whereby the parasite obtains part or all of its nutrition or needs from the host and results in an overall negative effect on the host. Simply, ectoparasites live on the outside of the animal and endoparasites on the inside. Microparasites (bacteria, viruses, protozoa) establish infections where it is hard to quantify numbers of infectious agents present, so numbers of infected hosts are quantified, rather than numbers of parasites within each host. Microparasites are small and have rapid generation times relative to their hosts. Macroparasites (nematodes, flies, ticks, etc.) are larger and can be counted; so the unit of study is the individual parasite, not the infected host. Macroparasites are also small and have rapid generation times, but there is less of a difference than between microparasites and host. Epiparasites are an interesting class of parasites whereby a parasite parasitizes a parasite in a host-parasite interaction referred to as hyperparasitism (as referred to in the well-known poem by Jonathan Swift: "a flea has smaller fleas that on him prey, And these have smaller still to bite 'em: And so proceed ad infinitum"). Examples of this are the larvae of the tapeworm, Dipylidium caninum, which infect fleas (Ctenocephalides species) and biting lice (Trichodectes canis). When a dog ingests a parasitized flea/louse when grooming, the tapeworm develops into an adult in the dog's intestine.

Fleas, ticks, and flies are the most visible and treated ectoparasites, but lice and mites also affect health and wellness. Infestation with ectoparasites causes many pathogenic effects, including tissue damage and blood loss due to feeding; hypersensitivity responses following exposure to ectoparasite antigens; secondary infections; and, most importantly disease transmission. Ectoparasites have evolved to fill many niches, but may be considered in terms of their host association. Many mites and lice live almost completely in permanent association with their host and, as such, have fairly low mobility and are open to risk of desiccation and death without the protection of their host. Other parasites, such as fleas, ticks, and flies, are more mobile and relatively resistant to damaging factors when off the host. As a result, the first category of organisms, mites and lice, often has a commensal relationship with the host as opposed to a parasitic interaction. The latter are able to find new hosts relatively easily, so are less impacted by death of a host and therefore likely to impose greater harm to the host. Most medically important ectoparasites have short generation times, large numbers of offspring, and very high rates of population growth [1].

Roundworms are the major infective internal parasite in both humans and animals, although cestodes (tapeworms) and trematodes (flukes) also have a significant impact on health. Helminth infections cause significant long-term, chronic debilitating disease and even death. In humans, it is estimated that around 125?000 deaths occur every year, and these are mainly due to infections with the hookworms, Ancylostoma duodenale and Necator americanus, or the roundworm, Ascaris lumbricoides [2]. In companion animals, endoparasite infections are primarily a disease of younger animals, with peak occurrence in dogs less than 6 months old and cats under 18 months old [3], with prevalence ranging from 5% to 70% worldwide [4]. Clinically, symptoms can vary from zero to critical (emaciation, anemia, death) and the zoonotic risks associated with some helminths are an additional concern. The economic impact of helminth infections on livestock, especially ruminant, production is well recognized [5, 6]; in pigs, it has been shown that the presence of endoparasites induces a reduction in body weight [7]. The mechanisms for the impact of helminths on production include direct tissue damage and diminished function of the affected organs; diversion of energy and protein resources of the host from production toward defense and immune mechanisms and reduced feed intake. In companion animals, there are similar adverse effects on health; unfortunately, roundworm infection is common, due to the ubiquity of infective stage larvae in the environment, and concerns are elevated due to zoonotic health risks.

Approaches for Ectoparasite and Endoparasite Control

Treatment of parasites results in removal of an existing infection, whereas prevention is a process by which infection is deterred. For dog and cat ectoparasite infections, experts generally recommend prophylaxis (year round in some climates) over therapeutic treatment, to effectively manage control of the lifecycle, as well as to reduce the risk of disease transmission from ectoparasite vectors [8, 9]. The benefit from regular preventative treatment is particularly recognized for the control of fleas due to the nature of their lifecycle; an adult flea infestation is only a very small part of the population, which includes immature stages present in the pet's environment. It is critical to control these stages, either by the use of products that target these early lifecycle stages or by regular use of products that eliminate adult fleas on the animal, which will progressively lead to the reduction of environmental lifecycle stages. CAPC (Companion Animal Parasite Council) goes as far as to recommend "avoiding initial infestation altogether by placing pets on life-long prevention programs is the best option for pets and their owners" [8]. Transmission of diseases (i.e., Rickettsia rickettsia and Borrelia burgdorferi) by vectors, especially ticks, in dogs and cats is a major concern, and reducing the ability of a vector to attach and/or feed with an effective ectoparasite control program will reduce the risk of disease transmission. Tick-borne diseases in dogs and cats are becoming increasingly important, with several tick species responsible for the continued spread of multiple diseases. Among the other more important diseases are babesiosis, hepatazoonosis, Ehrlichiosis, anaplasmosis, cytauxzoonosis (cats), and tick paralysis. Although control of internal parasites is the primary concern for horses, ectoparasites can also impact the welfare of horses, either through dermatological effects or nuisance bites, which affect the ability of horses to thrive. The primary ectoparasites of horses are houseflies, stable flies, mosquitoes, and horse and deer flies; ticks, lice, and mites also parasitize horses. The major problem is a limited supply of effective, licensed products for horses [10], combined with the challenges of managing ectoparasite species that are able to live for extensive periods off the animal, requiring frequent treatment. Fly repellents tend to have a very short duration of efficacy, if any, and need frequent reapplication. Taylor's 2001 review [11] highlighted how few pharmaceutical agents are available for treating horse ectoparasites and this situation has improved little in the intervening years.

For livestock, as for companion animals, ectoparasite control is dependent on the parasite lifecycle - do they spend their whole life on the host, like lice; or only spend time on the animal to feed, as for some species of mites, which then return to protected spaces in the environment? For the former, treating just the animal will suffice; for the latter, the environment must also be treated. In a 1992 review [12], Byford et al. gave an authoritative overview of the commercial and health impact of ectoparasite infestation in the United States, focusing on the horn fly, Haematobia irritans, commercially the most important and widespread pest in cattle in the southern United States. Although a complicated condition, the overall implication was that the damaging effect on production and performance of cattle results from an alteration of the total energy balance following ectoparasite infestation. This is a major problem, considering the widespread resistance of horn flies to pyrethroids, probably accelerated by the use of pyrethroid-impregnated ear tags [13].

Humans are as susceptible to ectoparasite infestation as animals are, and are often affected by the same pests; for example, close contact with pets can result in infestation with fleas, ticks, lice, and mites and, although more common in animals, humans can also suffer from myiasis, especially in tropical regions. Scabies and head lice [14], as well as being socially embarrassing, can cause significant health problems. Resistance is a major issue, with multiple resistance mechanisms identified in different populations of head lice, including kdr (knockdown resistance) mutations of the sodium channel and oxidative metabolism resistance mechanisms...