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New Money, Old Parasite
By Carol A. Kemper, MD, FACP
Dr. Kemper is Clinical Associate Professor of Medicine, Stanford University, Division of Infectious Diseases, Santa Clara Valley Medicine Center.
Dr. Kemper does research for GSK Pharmaceuticals, Abbott Laboratories, and Merck . Editor Stan Deresinski, MD, FACP, Clinical Professor of Medicine, Stanford, Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, serves on the speaker's bureau for Merck, Pfizer, Wyeth, Ortho-McNeil (J&J), Schering-Plough, and Cubist; does research for the National Institutes of Health, and is an advisory board member for Schering-Plough, Ortho-McNeil (J&J), and Cepheid. Peer reviewer Timothy Jenkins, MD, Assistant Professor of Medicine, University of Colorado, Denver, Denver Health Medical Center, reports no financial relationships relevant to this field of study.
This article originally appeared in the May 2010 issue of Infectious Disease Alert.
Every year about this time, I see a couple of unhappy local residents who present with an intensely pruritic, erythematous papular mystery rash. In contrast to flea bites, which are simple raised papules, the lesions seem umbilicated or have a central bite mark. The patient typically is floored when I explain they have rat mite bites, and they need to go home and set traps I mean, this is Palo Alto! It is estimated that half the homes in this area have resident roof rats, or Rattus norvegicus, especially at this time of year, when they are looking for a warm, dry place to nest. The spiny rat mite, Laelaps echidnina, is the most common ectoparasite found in large rodents.
Chiggers and mites are frequent causes of dermatosis in patients referred to the ID clinic, especially in travelers,1 in whom the rash must be distinguished from those of sand flies and other parasites. Chiggers and non-human mites typically cause an intensely pruritic, red, bumpy rash. Non-human mite dermatosis can result from animal or plant infestations, including animal habitats, dens, bird nests, fruits, trees, and furniture. Mites cannot jump, but they can crawl about 1 inch per hour on warm dry skin. Just like scabies, female mites burrow under the skin, where they lay their eggs. A sampler of different mites is as follows:
Chiggers are free-living ectoparasites, meaning the larvae feed for a few hours and then drop to the ground, before maturing into nymphs (including the trombiculid chigger species Leptotrombidium, which can transmit scrub typhus). Chigger bites are initially painless but, within hours, become intensely pruritic, followed by an erythematous papular eruption, called prurigo, which lasts a day or two. The bites are commonly found in areas where clothing is tighter, such as around waistbands, underwear, thighs, and ankles.
Zoonotic (or non-human) scabies from any number of mammalian and bird species can infect humans, who are essentially dead-end hosts; the larvae never develop in humans, although symptomatic infection from mites burrowing under the skin may still respond to 5% pyrethrine or ivermectin. Common mites in this category include the poultry mite (seen in poultry handlers, typically on the hands); bird mites (in bird fanciers, breeders, and pet shop owners), various rodent mites, such as the rat mite and the common house mouse mite (the latter can transmit Rickettsialpox [R. akari], resulting in a typical eschar). Pigeon mites have been known to cause infestations in apartment buildings, where they roost outside of windows and one hospital experienced a nosocomial outbreak of pigeon mites in patients and nurses.2 Bird-mite bites are usually self-limited and can be managed with antihistamines and topical corticosteroids.
Plant mites are more unusual, but include the North American and European straw itch mites, which can cause infestations in caned furniture, straw baskets, and straw rugs; they are most typically seen after hay rides in the fall. A characteristic "comet tail" has been described, which is literally the track of the mite moving away from the bite site. Other Pyemotes (plant mites) relatives can occasionally cause outbreaks, such as a large outbreak of North American oak leaf gall mite in residents in Pittsburg, KS. Plant insect mites can also occur in back packers, campers, and resort-goers, especially during the summer months, when mites breed and feed.
In travelers, chigger and mite bites must be distinguished from sand flea bites, such as those from Tunga penetrans (found in sub-Saharan Africa and South America) and Tunga trimamillata (found in Ecuador and Peru), which infect both animals and humans. The pregnant sand flea females burrow under the skin, causing inflammation and ulceration the legs and feet can become so that walking becomes painful. A recent consult was just this a young woman who took a brief Easter jaunt to Machu Pichu and presented with severe tungiasis with dramatic lower-extremity swelling and multiple small black eschars just above her sock line quite different from mite bites.
Patients never seem as excited or intrigued as I am when they present with one of these dermatoses, but at least they are relieved to have an answer, even it means going home and setting rat traps.