Arthropod Bites--Real and Imagined Fire Ant Stings Scabies Formication

November 1, 2006

Red imported fire ants (Solenopsis invicta) were introduced into the United States in the 1930s by ships from South America that docked in Mobile, Ala. Fire ants are now active throughout the southeast Sunbelt states. Colonies are also found in California; one of the largest is in Orange County.

Red imported fire ants (Solenopsis invicta) were introduced into the United States in the 1930s by ships from South America that docked in Mobile, Ala. Fire ants are now active throughout the southeast Sunbelt states. Colonies are also found in California; one of the largest is in Orange County.

The 2- to 5-mm, reddish brown ants often build their mounds in grassy areas. Fire ants swarm over humans, as well as animals, that are perceived as a threat to the colony (eg, when a lawn mower passes over a mound or when a mound is intentionally disturbed by a child). Photo A shows the foot of a man who was swarmed by fire ants as he mowed his lawn. Photos B and C show the upper leg and torso and close-up of the arm of an intoxicated woman who was stung repeatedly after she fell asleep on a fire ant mound in a public park in Houston.

The bite of a fire ant is no worse than that of other similarly sized ants. The fire ant grabs hold with its mouthparts so that it can readily wheel its body around inflicting many stings. The stings from a single ant appear as a dotted circle. Patients stung by fire ants instantly feel intense burning pain at the site, hence the name "fire" ants; the small pustular lesions typically develop about a half hour afterward and evolve over several days.

The pustules that result from fire ant stings are commonly mistaken for an infectious process. In fact, these lesions are induced by venom alkaloids and are sterile. Antibiotic therapy is indicated only for secondary infection of the lesions, which may be related to intense scratching--indeed, broken pustules increase the likelihood of infection.

The lesions eventually resolve without intervention. In some cases, large local reactions with edema and erythema may develop; these can be easily mistaken for a bacterial infection. The early onset of such a reaction confirms that the cause is noninfectious; in the absence of fever or other systemic signs, antibiotic therapy is unwarranted. Pruritus and pain can be treated with antihistamines (oral or topical), topical corticosteroids, and analgesics; over-the-counter preparations are usually sufficient. Anaphylactic reactions in sensitized persons with fire ant stings require the standard treatment.1,2 *

These intensely pruritic lesions in the web spaces of the hands and on the naval of a young girl were the result of a scabies infestation.

Patients with scabies present with itchy, erythematous papules or vesicles often in curvilinear patterns in the web spaces of the hands and feet or in the flexor surfaces of wrists and elbows, the belt line, buttocks, and genital areas.1 The lesions are caused by pregnant mites (Sarcoptes scabiei)that burrow into the skin and lay eggs. Because the mite infestation is transmissible to close contacts, a history of multiple family members with a common itchy rash is a red flag for scabies.

Standard treatment in the United States is 5% permethrin cream, which is applied to the entire body, with the exception of the head and neck, as 2 applications, 1 week apart. Ivermectin, 200 µg/kg, is an effective single-dose oral treatment for scabies; however, it has not been studied in small children.1,2

Advise patients that the pruritus may last up to 6 weeks after therapy, despite mite eradication, and provide adjunctive antipruritic therapy.1 Otherwise, patients may return with complaints of recurrent scabies infestation because of continued itching. Persistent pruritus results from an immune reaction to mite proteins, eggs, and feces in the skin. *

The false sensation of bugs crawling on or within the skin is a sensory hallucination commonly associated with psychostimulant drugs. It was first reported in chronic cocaine users in 1889.1 Drug-induced formication has been referred to as "coke bugs," "meth mites," and "amphetamites," depending on which drug caused the hallucination. Patients with this disorder often have self-induced dermatosis caused by intense picking and scratching of the skin. The lesions may appear as multiple well-circumscribed, erythematous papules and partially healed scabs in easy-to-reach areas, such as the face, scalp, neck, anterior thighs, and arms (lesions on the dorsal forearms are often worse on the side opposite the patient's dominant hand).

Patients with drug-induced formication often have an associated fixed delusion of parasitosis and claim to have seen "bugs" or "worms" crawling under or out of their skin. The woman in Photo A complained of bugs in her skin. In addition to her face, she had lesions on her arms, shoulders, and neck; no other areas were affected. She reported a history of methamphetamine abuse concurrent with her alleged infestation. The man in Photo B thought he had a "worm infestation" in his neck and used a pair of household scissors to cut out the imagined worms; his toxicology screening was positive for cocaine, opiates, and benzodiazepines.

Delusions of parasitosis that are not related to drug abuse are more common among middle-aged and elderly women.1,2 Patients with this disorder may be fully functional, but have a fixed false belief that they are infested with parasites. They may claim to have the parasites in a collection of material (typically pieces of skin, scabs, hair, lint, and other debris) and demand that it be observed under a microscope or sent to the laboratory for identification. This clinical presentation is called the "matchbox sign" because, historically, patients used a matchbox to collect the material, although now small plastic bags are most frequently used for this purpose.3 The matchbox sign suggests that the patient has seen other clinicians who have discounted a parasitic infection as a cause of their lesions.

Delusional parasitosis may be a shared illness, a folie à deux, in which the patient's accompanying close contact also believes in the infestation. Family members and friends who do not share the delusion can be of help in the patient's evaluation and treatment. Reassuring patients that no parasitosis exists can be quite difficult because they typically refuse to believe that their disease is functional rather than organic. Although results of objective tests (such as complete blood cell count, skin biopsy, and stool evaluation for ova and parasites) may satisfy the clinician, patients frequently deny negative test results and may even have their delusion reinforced by the clinician's willingness to order such tests.

Referral for psychiatric therapy must be approached with tact in patients with delusional parasitosis. Management of drug-induced formication obviously involves discontinuation of the offending drug. Pimozide is recommended for non-drug-induced delusions of parasitosis.1-3 *

Dr Benoit is a resident physician in the department of emergency medicine and Dr Suchard is associate professor of clinical emergency medicine and director of medical toxicology in the department of emergency medicine at University of California, Irvine Medical Center in Orange.

References:

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Johnston G, Sladden M. Scabies: diagnoses and treatment.

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de Leon J, Antelo RE, Simpson G. Delusion of parasitosis or chronic tactile hallucinosis: hypothesis about their brain physiopathology.

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Anonymous. The matchbox sign.

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