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Ectoparasites
Richard J. Pollack, ... Scott A. Norton, in International Encyclopedia of Public Health (Second Edition), 2017
Abstract
Ectoparasites are a taxonomically diverse group of organisms that infest the skin of human beings and other animals. Ectoparasitic arthropods and nematodes are similar in that an individual organism can produce skin lesions that are large enough to see with the unaided eye. Ectoparasitic infestations are often intensely itchy, causing considerable annoyance and discomfort. These conditions are often focally hyperendemic in impoverished communities, with a particularly high prevalence in vulnerable families, households, and neighborhoods.
Pediculosis (infestation by head and body lice) and scabies are found to some degree in all human populations, but myiasis (fly larva infestation), tungiasis (sand flea disease), and cutaneous larva migrans occur mainly in tropical and subtropical environments. Except for body lice, the organisms discussed in this article are not vectors of pathogenic microorganisms. In other words, most ectoparasites do not carry disease-causing agents; they are, instead, the direct cause of disease. Mortality is low, but the cumulative morbidity from the direct discomfort, secondary bacterial infections, and sequelae of those infestations and infections is considerable.
Despite the abundant presence of ectoparasitic infestations across human populations, biomedical science lacks firm evidence-based practices to reliably control these organisms. In addition, head lice and scabies are developing resistance to some chemical compounds employed to treat infested individuals, prevent spread, and control outbreaks.
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Introduction to Ectoparasitic Diseases
James H. Diaz, in Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases (Eighth Edition), 2015
Mechanisms of Ectoparasite-Borne Diseases and Injuries
The arthropod ectoparasites can threaten human health directly by burrowing into, feeding, dwelling, and reproducing in human skin and orifices (mites, fleas, and flies) or by blood or tissue juice sucking (fleas, lice, mites, and ticks). The arthropod ectoparasites can also threaten human health indirectly by infectious disease transmission (fleas, mites, and ticks). Ticks are the most versatile ectoparasitic arthropods and can transmit a variety of infectious diseases (viral, bacterial, and protozoan) and even inject paralytic toxins (tick paralysis) during their prolonged blood meals. Unlike other ectoparasites, ticks can be infective as males and females at birth (by transovarial pathogen transmission) and throughout all stages of their development (by trans-stadial pathogen transmission). The most commonly encountered arthropod ectoparasites, excluding ticks, and the major clinical manifestations of their infestations are featured in Table 293-2.
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Mites and birds: diversity, parasitism and coevolution
Heather Proctor, Ian Owens, in Trends in Ecology & Evolution, 2000
Ectoparasites play important roles in the lives of birds. Among these parasites, mites offer unique potential because of their extraordinary ecological and evolutionary diversity. However, the basic biology of most mites is poorly understood, and misleading extrapolations are sometimes made from better studied systems involving lice and fleas. Most importantly, not all bird-associated mites are parasitic; indeed, recent research suggests that some might even be beneficial. Here, we summarize what is known about the diversity of bird–mite relationships, and highlight how mites provide an ideal tool for the study of host life histories, sexual selection, immunocompetence and cospeciation.
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Potential impacts of climate change on veterinary medicinal residues in livestock produce: An island of Ireland perspective†
Kevin M. Cooper, ... Christopher T. Elliott, in Trends in Food Science & Technology, 2015
Ectoparasiticides
Livestock are also susceptible to infestation by ectoparasites. Sheep are particularly vulnerable to external parasitic conditions such as blowfly strike, sheep scab (Psoroptic mange, a form of allergic dermatitis caused by the mite Psoroptes ovis), and ticks, lice, mites and keds. Treatments generally involve pour-on products or, to a lesser degree, dipping in synthetic pyrethroid or organophosphorus compounds, although concerns over toxicity to the farmer and the environment persist. Ectoparasite infestations are also treated with injectable macrocyclic lactones.
Treatment with pour-on and dipping products can be considered food safety issues since pyrethroid residues can be detected in edible tissues, and meat is monitored routinely to ensure compliance with maximum residue limits, as is also the case with macrocyclic lactones.
Climate warming is predicted to have profound effects on the incidence of blowfly strike in Great Britain (Rose & Wall, 2011) through faster blowfly development, increased numbers of generations and prolonged periods of favourable conditions for fly survival. A warmer climate may also affect strike incidence indirectly through changes to the seasonal pattern of sheep susceptibility and the timing of seasonal farm management practices (Wall et al., 2011). Conversely, lice infestations tend to be worse in cooler seasons, so climate change is unlikely to make veterinary treatment more common. By contrast, climate change is thought to have already affected the epidemiology and geographical distribution of tick infestations making them more prevalent (Taylor, 2012b). However, precise prediction of climate effects on the incidence of ectoparasite infestations is uncertain due to subtle and conflicting interactions of humidity and temperature, free-living and host-bound life stages, and indirect effects on the host species and husbandry practices (Morgan & Wall, 2009; Wall et al., 2011). Indeed, one recent Europe-wide model predicts less favourable conditions for tick survival on IoI by 2050, but improving conditions by 2080 (Porretta et al., 2013). Nevertheless, climate change raises the possibility of changing patterns of veterinary drug administration to combat ectoparasites with consequent potential for impacts on residues in edible tissues.
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Volume 2
Marc J. Klowden, in Encyclopedia of Animal Behavior (Second Edition), 2019
Abstract
Ectoparasites that do not live permanently on their hosts need to engage in behaviors that allow them to identify and locate them when necessary. This is particularly important for blood-feeding arthropods that transmit agents of disease because their host-seeking behavior is the basis for their ability to serve as vectors. Strategies for host-location by blood-feeding arthropods differ according to their means of locomotion. Mosquitoes and ticks represent two distinct approaches to behaviors that bring them to their vertebrate hosts.
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Introduction to Ectoparasitic Diseases
James H. Diaz, in Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases (Eighth Edition), 2015
Conclusions
Recent epidemiologic evidence now supports the endemicity of several ectoparasitic diseases and their arthropod vectors (Table 293-3) and human and animal reservoir hosts throughout the developing world and in many parts of the developed world, including Europe and the United States. Ectoparasitic diseases have also reemerged in regions where they were once effectively controlled. Ectoparasitic diseases will continue to reemerge in the developed world for several reasons, including the following: (1) the globalization of trade and commerce with ectoparasites and their human and animal hosts traveling worldwide on airplanes and container ships; (2) mass movements of populations from rural to urban areas and from developing to developed nations; (3) the worldwide legitimate and illegal trade of exotic animals and animal hides and skins; (4) the accidental and intentional introduction of exotic animal species into new regions with welcoming ecosystems; (5) the increasing frequency of pyrethroid-resistant strains of ectoparasites, especially head lice and scabies mites; and (6) the growing populations of susceptible, and often immunocompromised, human hosts living in long-term care facilities and in crowded and impoverished periurban communities.11,12
The isolation of the trench fever pathogen, Bartonella quintana, in head lice from homeless persons in the United States illustrates how well socioeconomic factors, human behavioral trends, and vector adaptations can support ectoparasite persistence with significant public health consequences.13 Formerly thought to be susceptible to the safest pyrethroid pesticides and incapable of transmitting infectious diseases, head lice have acquired the capability to harbor B. quintana like body lice and to potentially transmit trench fever to new naïve host populations. The eradication of pyrethroid-resistant head lice infestations in homeless persons in crowded shelters and children in schools will require the use of more powerful pesticides with potential for adverse effects, such as carbaryl, lindane, and malathion, or the use of new, safer alternatives, such as oral and topical ivermectin-containing pediculicides.11
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Foods, Materials, Technologies and Risks
T. de Waal, M. Danaher, in Encyclopedia of Food Safety, 2014
Ectoparasiticide Resistance to Drugs and Impact on Public Health
Resistance of arthropods to the lethal effects of parasiticides has been known for more than a century. To date more than 500 species of resistant insects, mites, and ticks have been documented of which 40% are parasites of humans or animals.
The mechanism of resistance can be classified as either physiological or behavioral. Physiological mechanisms include increased metabolism of the insecticide (detoxification), decreased sensitivity of the target site, or decreased penetration into the target organism. The resistance mechanism generally involves one gene, although a combination of genes have also been implicated in certain cases.
Tolerance or resistance to all of the organophosphates currently in the market has been reported in specific ectoparasites (e.g., Rhipicephalus (Boophilus) microplus and adult blowflies) although largely restricted to certain parts of the world. The development of resistance to the organochlorines as well as environmental concerns has led to their withdrawal from the market.
Pyrethroid resistant strains of the cattle tick (R. (B.) microplus), horn flies (Haematobia irritans), houseflies, and lice have been reported. Resistance against amitraz has also been documented in several strains of R. (B.) microplus in Australia, but no resistance in multihost ticks has thus far been reported. To date no data on the development of ectoparasite resistance is available for the MLs. In animal health resistance of ectoparasites against insect growth regulators are still rare.
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Parasitism
Klaus Rohde, in Encyclopedia of Biodiversity, 2001
II. Types of Parasites
There are many types of parasites, distinguished by the site of infection, kinds of hosts, state of development, etc. Ectoparasites are parasites that live on the external surface of hosts, for example fleas and lice of various terrestrial vertebrates, and Monogenea and Copepoda of freshwater and marine fishes. Endoparasites are parasites that live in the tissues and organs of their hosts, such as tapeworms, flukes, and protozoans of vertebrates. An obligatory parasite is a parasite that cannot survive without a host, such as the malaria parasite, and a facultative parasite is a parasite that can also live without a host, such as maggots, which normally are saprophagous but can infect living hosts as well. A permanent parasite is associated with a host for long periods, whereas a temporary parasite is found in or on a host only for short periods: examples of the former are human helminths and blood protozoans, and examples of the latter are mosquitoes and leeches that visit hosts for blood-sucking only for short periods. Larval parasites, like the praniza larva of isopods, are parasitic only at a larval stage, and adult parasites, to which most metazoan parasites belong, are associated with a host during part or the whole of their mature phase. Periodic parasites (leeches, mosquitoes) visit a host at intervals. Intraspecific parasites parasitize individuals of the same species; for example, males of certain deep-sea fish are permanently attached to females of the species and derive food from them. Hyperparasites (of the first, second, etc. degrees) are parasites living on or in other parasites. An example of a hyperparasite of the first degree is Udonella, a monogenean parasitic on copepods, which themselves parasitize marine fishes. Microparasites, i.e., protozoans, bacteria, viruses, and some helminths, are small, with short generation times; they reproduce on or in a host usually at high rates, the duration of infection is usually much shorter than the life span of the host, and they induce immune responses in vertebrates. Macroparasites, i.e., most helminths and arthropods, do not reproduce on or in the host, they have longer generation times than microparasites, immune responses are lacking or weak and depend on infection intensities, and infections are often chronic and lead to morbidity rather than mortality. Parasitoids, many species of Hymenoptera, lay their eggs into insect hosts which may survive for some time but are invariably killed by the growing larvae of the parasitoids. Hence, parasitoids are predators rather than genuine parasites.
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