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Ectoparasites


Some arthropods are of medical importance predominantly because of the direct effect of infestation and these are considered in this chapter. Arthropods that cause illness through envenomation (mainly arachnids) are discussed in the following chapter along with other noxious exposures (bites, stings, venoms, toxins). Others, considered elsewhere ( Chapter 1 ), are responsible for disease primarily because they are vectors of viral, bacterial or parasitic pathogens. The few that are both vectors and direct causes of pathology are included in sections describing the vector-borne disease.

Among the ectoparasitic arthropods, the cosmopolitan scabies mites, lice and bedbugs are particularly prevalent. Other cosmopolitan species of mite ( Table 5.1 ) that commonly cause local and/or systemic pathological changes are especially likely to be encountered in a tropical environment and are therefore considered here. Tungiasis is a significant problem in many tropical areas, where it can be a major cause of disability and afflict more than 50% of the indigenous population. However no global estimates of prevalence are available. Although the myiases are rare and can be very serious or even lethal, the larvae of one species of fly are returning to favour as cleansers of contaminated wounds.

Table 5.1
Mites of medical importance
Genus and species Notes
Thryeophagus entomophagus
Dermanyssus gallinae 		Causes ‘pancake syndrome’
Poultry red mite, infests humans
Ornithonyssus bacoti
Ornithonyssus sylviarum
Ornithonyssus bursa 		Tropical rat mite
Northern fowl mite
Tropical fowl mite
Dermatophagoides pteronyssinus
Dermatophagoides farinae
Euroglyphus maynei
Sarcoptes scabiei
Pyemotes ventricosus
Cheyletiella parasitovorax
Cheyletiella blakey
Cheyletiella yasguri
Demodex folliculorum
Demodex brevis 		Main house dust allergen
Common house dust allergen
Common house dust mite
Scabies, Norwegian scabies
Grain, straw mite, causes dermatitis
In dog fur, causes dermatitis
Mainly in dog fur
In dog, cat, rabbit fur
In human hair follicles
In sebaceous glands
Neotrombicula autumnalis
Leptotrombidium akamushi
Leptotrombidium deliense 		‘Harvest mite’
Vector of Orientia tsutsugamushi
(scrub typhus) in Japan
Vector of Orientia tsutsugamushi
(scrub typhus) in rest of South East Asia

Mites (see Table 5.1 )

Aside from trombiculid mites of the genus Leptotrombidium , the larval forms of which are the vectors of scrub typhus in South East Asia, mites only cause pathology as a consequence of direct host contact. Some have a cosmopolitan distribution whilst others have more restricted niches. Globally, the faeces of the house dust mite triggers several allergic conditions and profoundly itchy scabies infestation can occur either sporadically or in outbreaks. Several species, such as the poultry red mite, provoke intense cutaneous irritation in defined epidemiological niches.

Fig. 5.1, Male Thyreophagus entomophagus .

Fig. 5.2, Ornithonyssus bacoti , the tropical rat mite.

Fig. 5.3, Ornithonyssus bursa , the tropical fowl mite.

Fig. 5.4, Varicelliform reaction to bites of Dermanyssus gallinae , the red poultry mite.

Fig. 5.5, Skin test for hypersensitivity to house dust mite.

Fig. 5.6, Dermatophagoides male.

Fig. 5.7, Female scabies mite.

Fig. 5.8, Scabies burrows, papules and vesicopustules on the foot of an infant.

Fig. 5.9, Scabies lesions.

Fig. 5.10, Secondary erythema in scabies.

Fig. 5.11, Chronic eczematous scabies in a Gambian woman.

Fig. 5.12, Hyperkeratotic (‘Norwegian’) scabies in a patient who had suffered a 70% burn injury.

Fig. 5.13, ‘Norwegian’ scabies in an elderly man.

Fig. 5.14, Section of biopsy from patient with 'Norwegian' scabies.

Fig. 5.15, Rash caused by Cheyletiella parasitivorax .

Fig. 5.16, Ventral view of Demodex folliculorum .

Fig. 5.17, Videodermatoscopic observation of a skin-attached and feeding larval Neotrombicula autumnalis mite. (Magnification ×150.)These ‘harvest mites’ are common in grassland in temperate climates. The larvae, the only parasitic form of the life cycle, normally feed on small mammals and birds, but may also attack humans, causing intense irritation. Related trombiculid mites are responsible for transmission of scrub typhus (caused by Orientia tsutsugamushi ) in South East Asia.

Fig. 5.18, Chigger bites to the ankles.

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