Chapter 69: Ectoparasites That Cause Human Disease

Ectoparasites are organisms that are found either on the skin or only in the superficial layers of the skin. Ecto is a prefix meaning “outer.” Virtually all ectoparasites are arthropods; that is, they are invertebrates with a chitinous exoskeleton.

The ectoparasites that cause human disease fall into two main categories: insects (six-legged arthropods) and arachnids (eight-legged arthropods). The ectoparasites discussed in this chapter include insects such as lice, flies, and bedbugs and arachnids such as mites, ticks, and spiders.

Many arthropods are vectors that transmit the organisms that cause important infectious diseases. A well-known example is the Ixodes tick that transmits Borrelia burgdorferi, the cause of Lyme disease. Table XII–3 describes the medically important vectors. However, in this chapter, the arthropods are discussed not as vectors, but as the cause of the disease itself. Table 69–1 summarizes the common features of diseases caused by the medically important ectoparasites that are described in this chapter. Ectoparasites of minor medical importance are briefly described at the end of the chapter.

1. Lice

Disease

Pediculosis is caused by two species of lice: Pediculus humanus and Phthirus pubis. P. humanus has two subspecies: P. humanus capitus (head louse), which primarily affects the scalp, and P. humanus corporis (body louse), which primarily affects the trunk. P. pubis (pubic louse) primarily affects the genital area, but the axilla and eyebrows can be involved as well.

Note that the body louse is the vector for several human pathogens, notably Rickettsia prowazekii, the cause of epidemic typhus, whereas the head louse and the pubic louse are not vectors of human disease.

Important Properties

Lice are easily visible, being roughly 2 to 4 mm long. They have six legs armed with claws by which they attach to the hair and skin (Figure 69–1). Pediculus has an elongated body, whereas Phthirus has a short body and resembles a crab, and hence its nickname, the crab louse (Figure 69–2). People infected with Phthirus are said to have “crabs.”

Nits are the eggs of the louse and are typically found attached to the hair shaft (Figure 69–3). They are white and can be seen with the naked eye. Nits of the body louse are often attached to the fibers of clothing.

Transmission

Head lice are transmitted primarily by fomites such as hats, combs, and towels. These are especially common in school children. Body lice live primarily on clothing and are transmitted either by clothing or by personal contact. Body lice leave the clothing when they require a blood meal. Pubic lice are transmitted primarily by sexual contact.

Widespread infestations of body lice occur when personal hygiene is poor (e.g., during wartime or in crowded refugee camps).

Pathogenesis

Adult lice feed on blood and, in the process, inject saliva into the skin, which induces a hypersensitivity reaction and, as a consequence, pruritus.

Clinical Findings

Pruritus is the main symptom. Excoriations may result from scratching, and secondary bacterial infections may occur. In pediculosis capitis, the adult lice are often difficult to see, but the nits are easily visualized. In pediculosis corporis, the adult lice are primarily in the clothing rather than on the body. In pediculosis pubis, the adult lice and nits can be seen attached to the pubic hair.

Laboratory Diagnosis

The laboratory is not involved in diagnosis. Nits fluoresce under ultraviolet light of a Wood’s lamp, which can be used to screen the hair of large numbers of people.

Treatment

Permethrin (Nix, RID) is the treatment of choice, as it is both pediculicidal and ovicidal. However, resistance to permethrin is increasing. Ivermectin (Sklice) is also effective, and resistance has not been reported. Nits are removed using a fine-toothed (nit) comb. Patients with body lice often do not need to be treated, but the clothing should be either discarded or treated.

Prevention

Children should not share articles of clothing. Many schools have a policy that children cannot attend school until they are nit-free, but the need for this exclusionary approach is under review. The personal items of affected individuals, such as towels, combs, hair brushes, clothing, and bedding, should be treated. Sexual partners of those infested with pubic lice should be treated and tested for other sexually transmitted diseases.

2. Flies

Disease

Myiasis is caused by the larva of many species of flies, but the one best known is the botfly, Dermatobia hominis. Fly larvae are also known as maggots. Note that maggots are occasionally used to debride nonhealing wounds, but these maggots do not cause myiasis.

Important Properties

The flies that cause myiasis are found worldwide and infest many animals as well as humans. Human infestation occurs most often in tropical areas. Dermatobia is common in Central and South America.

Transmission

The precise route of transmission varies depending on the species of fly. In one scenario, the adult fly deposits its egg in a wound and the egg hatches to produce the larva. In another, the fly deposits the egg in the nostrils, in the conjunctiva, or on the lips. In yet another, the fly deposits the egg on unbroken skin and the larva invades the skin.

Dermatobia is especially interesting in that it deposits its egg on a mosquito. When the mosquito bites a human, the warmth of the skin induces the egg to hatch and the larva enters the skin at the site of the mosquito bite.

Pathogenesis

The presence of the larva in tissue induces an inflammatory response.

Clinical Findings

The characteristic lesion is a painful, erythematous papule resembling a furuncle (Figure 69–4). The lesion may also be pruritic. The larva can often be seen within a central pore. Some patients report a sense of movement within the lesion. A history of travel to tropical regions is commonly elicited. Cutaneous myiasis is the most common form, but ocular, intestinal, genitourinary, and cerebral forms occur.

Laboratory Diagnosis

The laboratory is not involved in diagnosis except when identification of the larva is needed.

Treatment

Surgical removal of the larva is the most common mode of treatment. If the larva is visible, manual extraction can be performed. If the larva is not visible, the central pore can be covered with petroleum jelly, thus causing anoxia in the larva. This induces the larva to migrate to the surface.

Prevention

Prevention involves limiting exposure to flies, especially in tropical areas. General measures, such as wearing clothing that covers the extremities, mosquito netting, and insect repellant, are recommended.

3. Bedbugs

Cimex lectularius is the most common bedbug found in the United States. It has an oval, brownish body and is about 5 mm long (Figure 69–5). Bedbugs reside in mattresses and in the crevices of wooden beds. At night, they emerge to take a blood meal from sleeping humans. The main symptom of a bedbug bite is a pruritic wheal caused by a histamine-related hypersensitivity reaction to proteins in the bug saliva (Figure 69–6). Some individuals show little reaction. The bite of a bedbug is not known to transmit any human disease. Calamine lotion can be used to relieve the itching. Malathion or lindane can be used to treat mattresses and beds.

1. Mites

Disease

Scabies is caused by the “itch” mite, Sarcoptes scabiei.

Important Properties

The adult female Sarcoptes mite is approximately 0.4 mm in length, with a rounded body and eight short legs (Figure 69–7). It is found worldwide, and it is estimated that several hundred million people are affected around the globe.

Transmission

It is transmitted by personal contact or by fomites such as clothing, especially under unhygienic conditions (e.g., in the homeless and during wartime). It is not a vector for other human pathogens.

Pathogenesis

The pruritic lesions result from a delayed hypersensitivity reaction to the feces of the mite. The mite is located within the stratum corneum of the epidermis.

Clinical Findings

The typical lesions in immunocompetent people are either tracks or papules that are very pruritic (Figure 69–8). The most common sites are the hands, wrists, axillary folds, and genitals. Areas of the body where clothing is tight, such as along the belt line, are often involved. The itching is typically worse at night.

In immunocompromised individuals, an extensive crusted dermatitis (Norwegian scabies) can occur. These patients may be infested with thousands of mites. Excoriations may become infected with Staphylococcus aureus or Streptococcus pyogenes, resulting in pyoderma.

Laboratory Diagnosis

Microscopic examination of skin scrapings reveals the mites, their eggs, or fecal pellets.

Treatment

Permethrin (Elimite) is the drug of choice. Ivermectin is also effective. Topical steroids are used to relieve the itching.

Prevention

Prevention involves treatment of close contacts of the patient and treating or discarding fomites such as clothing and towels. Mass administration of ivermectin reduced the prevalence of scabies.

2. Ticks

Disease

Tick paralysis is caused by many species of ticks, the most common of which in the United States are Dermacentor species. Ticks are vectors for several human diseases, including Lyme disease and Rocky Mountain spotted fever, but in this chapter, we discuss only tick paralysis, which is caused by a toxin produced by the tick itself.

Important Properties and Transmission

Female ticks require a blood meal for maturation of their eggs, and hence it is the female that causes tick paralysis as well as serves as the vector of diseases. Ticks are commonly found in grassy woodland areas and are attracted by carbon dioxide and warmth from humans. A tick attaches to human skin by means of its proboscis.

Dermacentor andersoni, the wood tick, is more common in the western United States, whereas Dermacentor variabilis, the dog tick, is more common in the eastern states (Figure 69–9). Both species can cause tick paralysis. Presently in the United States, there are no cases of paralysis caused by Ixodes ticks; however, such cases are reported in other countries, especially Australia.

Pathogenesis

Paralysis is mediated by a neurotoxin that blocks acetylcholine release at the neuromuscular junction—an action similar to that of botulinum toxin. The toxin is made in the salivary gland of the tick. The tick must remain attached for at least 4 days prior to the onset of symptoms.

Clinical Findings

An ascending paralysis resembling Guillain-Barré syndrome occurs. Ataxia is an early presenting symptom. The paralysis is symmetrical and can ascend from the legs to the head within several hours. Respiratory failure and death can occur. Recovery typically occurs within 24 hours of removal of the tick.

The tick is often found at the hairline at the back of the neck or near the ear. Children younger than 8 years are most often affected.

Laboratory Diagnosis

The laboratory is not involved in diagnosis.

Treatment

Treatment involves removal of the tick.

Prevention

Tick bites can be prevented by application of insect repellant and wearing clothes that cover the extremities. Searching for and removing ticks promptly is an important preventive measure.

3. Spiders

Two species of spiders cause most of the significant disease in the United States, namely, the black widow spider (Latrodectus mactans) and the brown recluse spider (Loxosceles reclusa). The black widow spider is about 1 cm in length with a characteristic orange-red hourglass on its ventral surface (Figure 69–10). The brown recluse spider is also about 1 cm in length, but has a characteristic violin-shaped pattern on its dorsal surface (Figure 69–11). It is also called the “fiddleback” spider.

Neurotoxic Disease

The bite of the black widow spider causes neurologic symptoms primarily. Within an hour after the bite, pain and numbness spread from the site. Severe pain and spasms in the extremities and abdominal pain occur. Fever, chills, sweats, vomiting, and other constitutional symptoms can occur. In contrast to the bite of the brown recluse spider, tissue necrosis does not occur. Most patients recover in several days, but some, mainly children, die. Antiserum, if available, to the venom of the black widow should be given in severe cases. The antiserum is made in horses, so testing for hypersensitivity to horse serum should be performed.

It is interesting that the neurotoxin is encoded by WO virus, a bacteriophage that infects Wolbachia bacteria. Wolbachia bacteria infect many insects, including black widow spiders.

Dermonecrotic Disease

The bite of the brown recluse spider causes tissue necrosis symptoms primarily. The necrosis is due to proteolytic enzymes in the venom. Pain and pruritus at the site of the bite occur early, followed by vesicles and then hemorrhagic bullae (Figure 69–12). The lesion ulcerates, becomes necrotic, and may not heal for weeks to months. Skin grafting may be required. Antiserum to the venom of the brown recluse spider is not available in the United States.

1. Demodex

Demodex mites are also known as hair follicle or eyelash mites. They cause folliculitis, especially on the eyelashes (blepharitis) and on the face. They block the follicles, causing an inflammatory response and loss of eyelashes. Dry eyes and chalazions can occur. They are implicated as a cause of rosacea-like lesions on the face.

These mites are very small. As many as 25 mites have been found in one hair follicle. The diagnosis is made by observing the mite with a slit-lamp biomicroscope. Treatment involves careful debridement of the affected areas plus application of tea tree oil ointment.

2. Trombicula

Trombicula mites are also known as harvest mites or chiggers. The bite of the larva causes papules accompanied by intense itching, most often where tight-fitting clothing meets the skin. The pruritic papules result from an allergic response to proteins in the saliva that are injected into the skin at the time of the bite. The larvae are not blood sucking but obtain nutrients from dissolved skin cells. They are found in vegetation in hot, humid regions, such as southeastern states of the United States. Treatment involves the use of oral antihistamines or topical steroids to control itching. The chiggers can be killed with permethrin.

3. Dermatophagoides

Dermatophagoides mites are also known as house dust mites. They feed on exfoliated human skin cells. They do not cause disease in humans directly, but proteins in their feces are powerful allergens for some people. Small particles in house dust can become airborne, be inhaled, and induce asthma and atopic dermatitis.

4. Tunga

The flea, Tunga penetrans, causes tungiasis. The small female flea penetrates the skin, usually of the feet, and lays eggs there. An erythematous papule develops at the site that evolves into a whitish nodule with a dark center (the flea). (See Figure 69–13.) The lesion causes itching at the outset, then pain.

The fleas are commonly found in the sand of tropical beaches and walking in bare feet is a risk factor. It is widely found in the tropical areas of Latin America, Africa, and Asia. Tungiasis is a disease typically seen in returned travelers.

No drug is effective. Treatment involves removing the flea with a needle. If not removed, the flea will die in 4 to 6 weeks and the lesion will resolve. Cases of tetanus are associated with tungiasis, so a tetanus booster is recommended.

There are many local names for the Tunga flea, such as chigoe flea, jigger flea, and bicho de pie (foot bug). Note that, despite the similarity in name, these fleas are not chiggers. Chiggers are mites in the genus Trombicula (see above).