Formication, essentially a tactile hallucination, is an abnormal skin sensation similar to that of insects crawling over or within the skin. From the Latin formicare meaning to creep like an ant, (not to be confused with fornication!), formication was described in 1890: "A variety of itching, often encountered in the eczema of elderly people. It is described as exactly like the crawling of myriads of animals over the skin."
Formication is a specific form of the general set of abnormal skin sensations known as paresthesias and tactile hallucinations. It is associated with several disorders: a physical or psychological side effect of substance abuse, diabetic neuropathy, diseases of the spinal cord and peripheral nerves, and extreme alcohol withdrawal with delirium tremens.
This illusion or hallucination that ants or other insects are creeping on or under the skin is a common side effect of large-scale use of cocaine, amphetamines ("amphetamites"), or crystal meth (also known by street names such as "ice," "glass," "chalk," "crank," "meth mites," or "coke bugs.") Formication is also a side effect of many prescription drugs (see below). People suffering from formication, particularly if they are delirious or intoxicated, may pick and scratch themselves to such an extent that they trigger self-induced bleeding and excoriations of the skin on areas easy to reach: the face, scalp, neck, thighs, and forearms. This-self-mutilation often leads to gouged out pits and ulcers.
Patients often seek numerous opinions from medical doctors, exterminators, and entomologists, and then often complain about the incompetence of the advice received while treating the specialist with hostility and suspicion.
The false sensation of bugs crawling on or within the skin was first reported in chronic cocaine users in 1889. Patients with drug-induced formication often have an associated fixed delusion of parasitosis: a form of psychosis in which sufferers hold a delusional belief they are infested with parasites and claim to have seen "bugs" or "worms" crawling under or out of their skin. Some have even used common household scissors to cut out the imagined worms.
Delusions of parasitosis that are not related to drug abuse are more common among middle-aged and elderly women. Patients with this disorder may be fully functional, but nevertheless have a false belief that they are infested with parasites. They may profess to have the parasites in a collection of material (typically pieces of skin, scabs, hair, lint, dried blood, living organisms such as flies or ants, and other debris) and request that it be studied under a microscope or sent to a laboratory for identification. This clinical presentation is called the "matchbox sign" because patients have often used a matchbox to collect the material. Now small plastic bags are ommonly used for this purpose.
Delusional parasitosis may be a shared illness, in which some of the patient's close contacts also believe they too are infested. When two family members are involved, usually husband and wife or parent and child, the delusion is known as folie à deux ("a madness shared by two"). The psychological film Bug (2007) portrays a folie à deux involving a man and woman who believe they are infested with government-implanted, nano-technological insects.
Reassuring patients that no parasitosis exists can be difficult because they typically refuse to believe that their disease is functional rather than organic. Although results of objective laboratory tests may satisfy the doctor, patients frequently deny negative test results. Referral for psychiatric therapy must be approached with delicacy in patients with delusional parasitosis. Management of drug-induced formication obviously involves abandoning the offending drug.