Environmental Dermatology

What’s Eating You? Tropical Rat Mite (Ornithonyssus bacoti)

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The tropical rat mite (Ornithonyssus bacoti) commonly infests wild and pet rodents, but they are not host specific. Bodily contact with wild or domesticated rodents is the most common source of infestation. Mites can live off many mammal hosts for a long period of time; therefore, living in quarters infested by mice can lead to mite exposure. Human infestation presents as urticarial, pruritic, cutaneous lesions that may be misdiagnosed as an arthropod bite, an infection, or contact dermatitis. Symptomatic relief of pruritus can be provided with a topical corticosteroid or antihistamine. The most effective treatment is an antiparasitic, such as permethrin cream, as well as extermination of rodents, mites, and any other pests in the patient’s living space.

Practice Points

  • The tropical rat mite (Ornithonyssus bacoti) can infest humans who make bodily contact with a rodent, reside in living spaces infested with rodents, or own any pets.
  • Patients infested with rat mites may present with pruritic, erythematous, cutaneous lesions with secondary excoriations that can be mistaken for an infection or dermatitis.
  • The recommended treatment of rate mite infestation includes antiparasitic medications such as permethrin or pyriproxyfen. Preventive measures include proper disinfestation of living spaces.



The tropical rat mite (Ornithonyssus bacoti) belongs to the family Macronyssidae. Theses mites are commonly mistaken for red bird mites or Nordic bird mites because they belong to the same family and have similar characteristics.1 Although O bacoti is called the tropical rat mite, it also can be found in moderate climates.2,3


The life cycle of a tropical rat mite lasts 11 to 13 days and includes 5 stages: egg, larva, protonymph, deutonymph, and adult.1,2 The length of the mite (0.3–0.7 mm) varies with the stage of development.1 Adults can reach 0.75 to 1.40 mm, with females larger than males and possibly visible with the naked eye.1,2

Two or 3 days after a blood meal, the female mite lays approximately 100 eggs in its nest but not on the surface of a host. The eggs hatch into larvae after 1 to 4 days and go on to complete their life cyle.1 During developmental stages, mites occupy their hosts for blood meals. Mites search for their hosts at night and prefer wild or pet rodents for blood meals but are not host specific and can be found on many mammals including birds, cats, racoons, and squirrels.4

Although tropical rat mites prefer rodent hosts, they can infest humans when their preferred host is unavailable. In the United States, the first case of human dermatitis due to a tropical rate mite occurred in 1923. In Europe, rat mite dermatitis was first reported in a human in 1931, possibly due to contamination of sailing vessels.4

Infestation and Transmission

Tropical rat mites prefer wild and pet rodents as hosts because the mites are able to feed on their blood over long periods.4 During the day, the mite spends most of its time hiding in dark dry spaces; it is most active during the night, traveling to find a host for meals.3-5 If a preferred host is not present, the mite may choose to infest a human.5

Human infestation occurs most often upon close bodily contact with an infected animal or pet rodent that was sold without parasites having been eliminated.3-5 Mites are able to survive without a host for as long as 6 months; they may travel after a meal.1,2 Therefore, individuals who do not have a pet rodent can be infested if an infected wild rodent has infested their living space.1,3-5

Clinical Presentation of Infestation

Patients infested with tropical rat mites present with pruritic cutaneous lesions, most often on unclothed parts of the body that are easily exposed to mites; lesions rarely occur on the scalp.5 People of any age or gender can be infested. Rat mite bites can present as single or grouped, pruritic, erythematous papules ranging in size from 4 to 10 mm in diameter.5-7 Excoriations may be present due to excessive scratching. Although rare, vesicles or nodules have been reported.5,7

Diagnosis of the underlying cause of the cutaneous manifestations often is difficult because mites are not visible during the day, as they are less active then.2 Lesions often are misdiagnosed as an allergic response, a bacterial infection, or various forms of dermatitis.1 A parasitic cause often is not considered unless the physician or patient detects a mite or many trials of therapies fail to provide relief.1,3-5 Eliciting a thorough history may disclose that the patient has had close contact with rodents or lives in a community center, shelter, or shared space. If any of the patient’s close contacts have a similar presentation, infestation with mites should be considered.


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