Chigger bites

INTRODUCTION — Mites are small arachnids that include thousands of species, many of which parasitize animals and plants [1]. In animals and humans, these mites may produce cutaneous lesions, yield allergic reactions, or act as vectors for infectious disease.

The recognition and management of bites from larvae of the Trombiculidae family (also known as "chiggers") will be reviewed here. Synonyms for these organisms include harvest mites, harvest bugs, harvest lice, Mower's mites, and redbugs [2,3]. The term "chigger" should not be confused with "jigger," a term which is typically used to refer to the chigoe flea (Tunga penetrans). Other arthropod bites are reviewed separately. (See "Insect and other arthropod bites".)

LIFE CYCLE — The adult trombiculid mite is typically 1 to 2 mm in length, with a bright red or red-brown color (picture 1A). The life cycle of the mite proceeds through egg, larval, nymphal, and adult stages [4]. Only the larval stage is responsible for chigger bites.

Mite larvae are only 0.15 to 0.3 mm in length (picture 1B). After hatching, the larvae reside on leaves or grass stems waiting for an animal or human to pass by. The larvae attach to the skin of the host organism and remain for up to a few days. Subsequently, the larvae drop to the ground to mature into the harmless nymphal and adult stages [1,4].

The nymphal stage is similar to, but smaller than, the adult form. Adults and nymphs live in the soil and feed on plants, other mites and small insects, and insect eggs.

EPIDEMIOLOGY — Although trombiculid mites are found in most areas of the world, requirements for certain soil conditions and suitable hosts limit common habitats to grassy fields, forests, parks, gardens, and the moist areas along lakes and streams. Because most mites responsible for chigger bites require relative air humidity of at least 80 percent, they usually remain on vegetation less than 20 to 30 cm above the ground surface [5].

In the United States and Europe, skin disease secondary to the bites of trombiculid mites is most common in summer and fall [6,7]. Eutrombicula alfreddugesi is the predominant species in the southeastern and south central areas of the United States; Neotrombicula autumnalis is more prevalent in Europe [2].

PATHOGENESIS — Trombiculid mites reach human skin through areas of easy access, such as the pant cuff or shirt collar. The larvae can migrate widely on the skin in search of an optimal feeding area.

Barriers to migration, such as a belt or elastic waistband, preclude this wandering and might explain the clustering of bites in such areas. Other possible explanations for this finding include preferences of the mite for sites of high local air humidity [8] or thin epidermis [9].

The mites pierce the skin and attach to the skin with jaw-like structures (chelicerae) and secrete digestive enzymes that liquify epidermal cells resulting in the formation of a tube-like opening called a stylostome [10]. After injecting the digestive enzymes, the larvae ingest and feed upon lysed tissue and lymphatic fluid [11,12]. The bites of the larvae induce significant irritation and inflammation of the skin.

CLINICAL PRESENTATION — Cutaneous inflammation and intense pruritus are the classic clinical features. The actual larval bites are painless [13]. Papular and papulovesicular cutaneous reactions are most common; less frequently urticarial, morbilliform, and bullous eruptions occur (picture 2A-B) [10,14]. Itching typically begins within hours after a bite and resolves within a few days; the lesions usually heal within one to two weeks [13]. In individuals with extensive involvement, the intense pruritus may inhibit sleep [10]. Occasionally, the cutaneous eruption and pruritus persist for weeks [10].

The papules or papulovesicles are typically grouped [15]. Most often, the legs and waistline are affected.

In boys, the triad of penile swelling, pruritus, and dysuria has been referred to as "summer penile syndrome," and likely represents a local hypersensitivity response to chigger bites [16]. Symptoms of summer penile syndrome can last from a few days to a few weeks [16].

DIAGNOSIS — The diagnosis of chigger bites is supported by a history of recent outdoor activities in a "chigger-prone" area, as well as by the appearance of pruritic grouped papules, papulovesicles, or bullae in sites of predilection such as the ankles, waistline, or other areas where clothing contacts the skin.

In some cases, mites can be retrieved from infested skin through tape stripping and visualized with light microscopy, confirming the diagnosis. This test is not usually necessary for diagnosis.

DIFFERENTIAL DIAGNOSIS — The differential diagnosis for chigger bites includes:

●Other arthropod/insect bites (see "Insect and other arthropod bites")

●Allergic contact dermatitis (see "Clinical features and diagnosis of allergic contact dermatitis")

●Bullous pemphigoid (in patients with pronounced bullae) (see "Clinical features and diagnosis of bullous pemphigoid and mucous membrane pemphigoid", section on 'Clinical features of bullous pemphigoid')

TREATMENT — Vigorous cleansing with soap and water may help to remove the mites. Treatment is aimed at managing symptoms.

Itching can be treated with topical antipruritics such as menthol or calamine lotion. Potent topical corticosteroids may help to improve both itching and inflammation [15]. Application of the topical corticosteroid with occlusion (eg, under plastic wrap) may improve the efficacy of topical corticosteroid treatment [3]. Oral sedating antihistamines (eg, hydroxyzine, diphenhydramine) may help to improve pruritus. (See "Pruritus: Therapies for generalized pruritus", section on 'Role of antihistamines'.)

Intralesional corticosteroid injections (eg, 2.5 to 5 mg/mL of triamcinolone acetonide) may be beneficial for reducing inflammation in individual papules that fail to resolve with topical therapy [3]. However, treatment with intralesional corticosteroid injections typically is not necessary.

Other interventions — Since the mites do not burrow into the skin and the pruritic eruption usually starts after the mites have already detached from the skin, interventions to eradicate the mites from the skin (eg, acaricides or home remedies aimed at suffocating the mites, such as applying nail polish) are not indicated.

Clothing worn during the period of outdoor exposure to the mites should be washed in hot water. Alternatively, clothing can be treated with pyrethroid insecticides to kill mites.

PREVENTION — Preventive measures include behavioral changes and insect repellants. Heavily infested areas should be avoided, and trousers should be tucked inside socks.

Insect repellents, including DEET (N, N-diethyl-meta-toluamide or N, N-diethyl-3-methylbenzamide) and dimethyl phthalate, applied to skin and clothing are effective in repelling mites [17]. Repellents containing naturally occurring aromatic oils, such as citronella oil, tea tree oil, jojoba oil, geranium oil, and lemon grass oil, also may be effective [18]. Clothing may also be treated with a pyrethroid insecticide, such as permethrin [19]. Insect repellent options are reviewed in detail separately. (See "Prevention of arthropod and insect bites: Repellents and other measures".)

MITES AS DISEASE VECTORS

Scrub typhus — Scrub typhus, an infection caused by the organism Orientia tsutsugamushi, is transmitted via the bite of the harvest mite in rural areas of Asia and Australia [20]. (See "Scrub typhus".)

Hantavirus, Borrelia, and Ehrlichia — Although hantavirus-specific ribonucleic acid (RNA), Borrelia-specific deoxyribonucleic acid (DNA), and Ehrlichia-specific DNA have been detected in trombiculid mites in Texas, the Czech Republic, and Spain, respectively [21-24], transmission from trombiculid mites to humans has not been documented. (See "Epidemiology and diagnosis of hantavirus infections".)

SUMMARY AND RECOMMENDATIONS

●Epidemiology – Chigger bites are due to infestation with the larval form of trombiculid mites (picture 1A-B). Trombiculid larvae are encountered in areas such as grasslands, forests, and around lakes and streams. (See 'Introduction' above and 'Epidemiology' above.)

●Clinical presentation – Chigger bites are initially painless but typically become intensely pruritic within a few hours. The most common manifestation is papules or papulovesicles at the sites of bites (picture 2A-B). Urticarial, macular, or bullous eruptions also may occur. Pruritus usually resolves over a few days, and the cutaneous eruption usually resolves over one to two weeks. Occasionally, symptoms persist for longer. (See 'Clinical presentation' above.)

●Diagnosis – The diagnosis of chigger bites is supported by a history of recent outdoor activities in a "chigger-prone" area, as well as by the appearance of pruritic grouped papules or papulovesicles on the ankles, waistline, or other areas where clothing contacts the skin. (See 'Diagnosis' above.)

●Treatment – Vigorous washing with soap and water will easily displace any attached larvae. Chiggers do not burrow into the skin.

We suggest treating lesions symptomatically with topical antipruritics or potent topical corticosteroids (Grade 2C). Sedating antihistamines may help with pruritus when needed. Clothing worn during the period of exposure should be washed in hot water or treated with pyrethroid insecticides. (See 'Treatment' above.)

●Prevention – Preventive measures include avoidance of chigger-prone areas, tucking trousers inside of socks, and applying repellent to skin and clothes. (See 'Prevention' above.)