A woman and her husband suffer a debilitating skin disease they believe their granddaughter brought home from day care.

A 54-year-old woman presents as a new patient with complaints of skin infections during the past two years. She has no other significant past medical history. The woman and her husband had been healthy until their granddaughter, who lives with them, picked up an “infection” at day care.

Over the next few months, the couple became afflicted with the infection. It caused severe skin infestations that have been refractory to usual hygiene. The woman reports burning all the family linens in a backyard bonfire and now requests referral to a biochemist who can advise her on the proper means to exterminate the “bugs” from the remainder of her property.

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Her unfortunate spouse remains at home, incapacitated by the skin infection, scratching himself constantly. The woman denies any drug or alcohol consumption. Physical examination reveals only superficial excoriations without secondary infection on her forearms, shins, and knees. Routine labs are normal. Stool studies are negative for any sign of infection, and an outside skin biopsy is normal. Wood’s light examination of the patient’s hair is also unrevealing. Skin scrapings do not show any evidence of scabies.

Considering the differentials

Although this vignette sounds unusual, the woman’s complaints mandate further evaluation with an open and inquisitive mind. The differential diagnosis encompasses multiple conditions, but several can be excluded relatively easily.

Pruritus has many causes that require a focused and comprehensive investigation. Hives, or urticaria, is a common etiology, but this patient’s complaints do not fit that picture because there is an aspect of transmission to others in her self-report, suggesting a contagious disease.

Formication, a specific form of visual hallucinations involving “insects” the patient sees on her skin, is often associated with alcoholism, illicit drug abuse (hence the street term, “cocaine bugs”), and some psychoses, but it does not extend to other persons or objects. Infectious etiologies should be examined more closely, as the patient herself has determined that they are a possible cause.

Itch by infection

Several infectious organisms could be the source of a diffuse dermatologic condition; some are quite contagious yet easily treatable. Scabies, which is caused by the mite Sarcoptes scabiei, an obligate human parasite, is a common infection with itching as its key feature. Scabies is easily transmissible within household contacts, but casual contact is not an efficient mode of transmission (for example, there have been no case reports of health-care providers contracting scabies from a patient). The mite lives less than three days in the absence of human contact so that acquisition from fomites is also unusual.

Diagnosis can be made clinically, with confirmation from skin scrapings. The pathognomonic burrow, where the female lays her eggs, measures less than a centimeter in diameter and is often difficult to find, so some clinicians elect to use empiric permethrin. This is a reasonable approach if the clinical suspicion is high, but there are some case reports of permethrin resistance, so definitive diagnosis would be better.

Ivermectin has been used off-label in refractory cases, with good results. In addition, there is some evidence that the itching is a delayed hypersensitivity response to the presence of the organisms, and several weeks of antihistamines may be beneficial even after topical treatment.

Pediculosis is a chronic louse infestation of the skin. Pediculus humanus capitis (“head lice”) is frequently found in the hair of young schoolchildren, with recent estimates that approximately 25% of U.S. elementary schoolchildren are affected. Pediculus humanus corporis (“body lice”) is associated with poverty and poor sanitation. This infection is not commonly seen in the United States except among homeless persons; the chronic manifestations were previously referred to as “vagabond’s disease.”

Phthirus pubis (“crab louse”) is a sexually transmitted louse that lives primarily in the pubic region and can also reside in the axillary and facial areas. All these organisms are obligate human parasites, but unlike the scabies mite, they can survive on fomites for several weeks. Severe itching of the affected area is the major clinical manifestation, and bacterial superinfection may occur. Permethrin is the preferred topical agent; there should be targeted decontamination of all fomites as well. In the case of persistent itching, antihistamines and even a short course of steroids may provide symptomatic relief.

Less common organisms

Additional unusual infections that could cause itching are associated with either water exposure or exotic travel. Cercarial dermatitis (“swimmer’s itch”) occurs when the larval stage of a snail parasite accidentally burrows into a human’s skin (i.e., a person swimming in a body of freshwater) rather than its intended target—typically a duck. The symptoms are mild and resolve after a short time once the larvae expire in the accidental host’s skin, unable to complete their life cycle. Treatment is not required.

Onchocerca volvulus, the long-lived microfilariae that come from the female river black fly of Western and Central Africa, can live up to 15 years in human hosts. Infection causes river blindness and chronic pruritus at sites where the microfilariae migrate throughout subcutaneous tissue. A single dose of ivermectin will kill the organisms.Cutaneous larva migrans is a skin infection by the dog or cat hookworm from the family Ancylostomidae after exposure to affected soil in tropical regions; the larvae burrow several millimeters per day through the host skin to cause the typical appearance. Two or three doses of ivermectin or albendazole are sufficient to treat this.

Empiric therapy and follow-up

Although your initial evaluation is unrevealing, you treat the patient with empiric topical permethrin and provide additional prescriptions for her husband. You also give her diphenhydramine for symptomatic relief.

On her return the following month, the patient refuses any physical contact, concerned about contaminating other persons in the clinic. Since her last visit to the office, she says, the germs have consumed all the aluminum pipes in the house. The “bugs” were finally identified via teleconference with a foreign university that specializes in identifying unusual organisms and was found on the Internet. She has the infectious creatures, known as “no see-ums,” in a carefully sealed jar but states that they can only be visualized using a special microscope she bought online. She is unwilling to open the jar, for fear she may contaminate the clinic.

After much reassurance, the patient agrees to closer examination. She points to some of the organisms on her left wrist, but you can see only bits of fabric and excoriations. During your evaluation, she becomes paranoid about the pipes in the office and seems more tangential in her history. The remainder of your assessment is unrevealing. Skin scrapings and Wood’s light examination of her hair show no evidence of scabies or lice.

A diagnosis too bizarre for analysis

This unfortunate woman suffers from delusions of parasitosis, an unusual disorder of unknown prevalence that typically ends up being evaluated by either a dermatologist or infectious diseases specialist following referral by a frustrated primary-care clinician. The patient will insist that her body is directly invaded by some skin pathogen, but there will be no objective evidence to support this. First described in 1946, the condition seems to be more common among women than men, usually of middle age. Often there is a history of an-tecedent “infection” that seems to precipitate the elaborate delusion, but there is no association with any specific infectious agent. The inciting event (the infection) is typically self-described and not a common condition that would have prompted medical attention.

The disorder has yet to be well studied because of its unusual and at times bizarre presentation. Case series over the past few decades have several elements in common, however. Dermatologists point out that the patients have excoriative lesions from excessive scratching in easily reached areas of the body, such as the forearms, ankles, and face.

Many patients demonstrate proof of the infection with the “matchbox sign.” (The original description came from the patient who kept the delusional pathogen in a matchbox, which was empty to the clinician.) Numerous variations are possible, including jars, bags, or other containers that may hold dead skin, insects, or other nonpathogenic material. Patients can even present with a delusion shared by a spouse, a situation known as folie à deux, as in this case presentation. There is also frequent changing of doctors and expressions of frustration with previous clinicians.

The ever-elusive cure

Although this appears to be a psychiatric disorder, most patients do not usually have a concomitant psychiatric diagnosis. Some experts have suggested that the delusion may be associated with major depressive disorder. Psychiatric authorities tend to classify it as part of the group of paranoid delusions.

There does not seem to be any effective treatment. Atypical antipsychotics have been tried, with anecdotal success. Haloperidol as well as pimozide has been used, but there has been little follow-up as these patients often do not comply with therapy. Some reports point out that even when there is no intervention, the delusions fade within three months and that additional measures, such as referral or medications, should be deferred until that time.

If offered, psychiatric referral is often refused, and patients may be hostile toward doctors who state that there was no objective evidence of infection. In fact, there are Web sites (for example, www.skinparasites.com) that reach out to individuals who suffer from this condition. Believers insist that clinicians are to blame for a lack of effort in diagnosing the as-yet-unidentified skin parasite and that they have failed to investigate adequately ways to treat the “millions of affected individuals.” There is much on the Internet about how these persons are not delusional and that there are alternative treatments available.

Options in this situation are few. It is obviously essential to exclude actual infection with comprehensive evaluation. Once the findings have been assessed and there is no evidence of pathologic infection, reassurance is the only real intervention. The patient should also be discouraged from more aggressive decontamination that could cause harm.

Dr. Spak is an infectious diseases staff physician at Baylor University Medical Center in Dallas.

Read on

  • Zomer SF, De Wit RF, Van Bronswijk JE, et al. Delusions of parasitosis. A psychiatric disorder to be treated by dermatologists? An analysis of 33 patients. Br J Dermatol. 1998;138:1030-1032.
  • Habif TP. Clinical Dermatology: A Color Guide to Diagnosis and Therapy. 4th ed. Philadelphia, Pa: Mosby; 2004: chap 3.
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  • Wilson FC, Uslan DZ. Delusional parasitosis. Mayo Clin Proc. 2004;79:1470.