Unusual conditions: delusional infestation with insects or spiders

Insects often scare or disgust us. But a small proportion of people don’t just experience normal fear. They live with the terrifying and unshakeable belief that insects have invaded their body despite medical evidence suggesting otherwise. This is called a delusion of infestation, or delusional infestation.

Delusions of infestation feature in popular films like Bug and A Scanner Darkly. These detail the extreme emotional distress, preoccupation and conviction with which the characters believe insects have infested them.

Although delusional infestations of insects are most commonly reported, some people report infestations of parasites, larvae, worms, fibres and even small animals.

Many people believe these symptoms are a side-effect of substance use, where they are known as “coke bugs” or “meth mites”. Government public health messages about the dangers of “ice” promote this view.

But delusions of infestation can occur in the absence of other conditions (known as the primary form and given the name delusional disorder, somatic subtype) or secondary to a range of other conditions such as schizophrenia, mood disorders, dementia and medical ailments.

People with the primary disorder do not have other delusional ideas or thought disorder like people with schizophrenia. If they experience hallucinations (seeing, hearing or feeling things others can’t) then these are only related to their belief of infestation, for instance seeing bugs on their skin.

There’s little research into delusional infestation, so it is difficult to estimate how common it is. Also, the limited detail provided in published case studies means we are not confident the diagnoses are always correct.

Why do people develop these delusions?

Prominent entomologist Jeffrey Lockwood says a normal disgust or fear of insects has an evolutionary basis. This is because insects can harm our health and spread disease; so, being frightened of insects is to our advantage and helps us survive.

Lockwood has also argued we find insects threatening because they have minds of their own, reproduce quickly, move unpredictably and can live on and in us.

But with delusional infestations, additional factors are at play. Researchers suggest the involvement of dopamine activity (a neurochemical released by the brain with a range of functions), pre-existing skin conditions or skin sensitivity, specific brain regions and psychological factors.

One psychological approach proposes a “two hit” model. This model suggests an itch or tactile hallucination is followed by the belief insects caused it.

This belief is caused by cognitive biases such as “jumping to conclusions” and occurs when people form a belief with little evidence. This cognitive bias is also common in the development of delusions in other psychotic disorders.

Where do people go for help?

People with delusions of infestation rarely seek help from mental health services. Instead, they visit pharmacists, general practitioners, dermatologists, emergency departments and vets. They sometimes also approach non-medical professionals, like pest exterminators and removalists.

People will often bring evidence of their infestation for testing. These are called “matchbox signs” or “specimen signs” and are usually particles of dust or skin rather than insects.

People often have a series of investigations and procedures to test for infestation. When no evidence is found, they may then be referred to psychiatrists or psychologists. But they often refuse this referral.

Some people take matters into their own hands to “remove” or “scratch out” insects, often needing medical attention.

How is it treated?

Psychiatrists tend to prescribe antipsychotic medications to treat the condition. Antipsychotic medications are used to treat a variety of psychotic disorders, including schizophrenia, and aim to reduce the strength of the delusion and associated distress. For a time, doctors recommended the antipsychotic pimozide but this came with significant side effects and safety issues.

Now, doctors prescribe a range of different antipsychotics, depending on how acute the symptoms are, the individual person and other medical difficulties.

While some research finds antipsychotic medications improve or stop symptoms in 60-100% of cases, some report less success. As there have been no randomised-controlled trials, which would provide stronger evidence, we still don’t know how effective these drugs really are.

Of the case studies that have been published, researchers don’t always report how long patients stayed on their medications, if they had side-effects, and if they remained symptom free over time. Very few also said whether people’s quality of life improved or if they returned to their previous level of functioning after taking medication.

What about psychological treatments?

Published research rarely refers to psychological treatments for delusional infestations. This could be due to the nature of the disorder, as patients often refuse to accept the problem may be psychological, so refuse psychological treatment.

But interventions such as cognitive behavioural therapy plus medication has been a popular and effective treatment for other psychotic disorders.

Cognitive behavioural therapy addresses thoughts and behaviours. The aim is to reduce the level of conviction people have about their delusions, their preoccupation with the delusion and the distress and change to functioning that can result from the delusion. This may prove a promising area for future research and intervention with delusions of infestation.

Jessica O'Connell works for Orygen Youth Health. This is a service for young people experiencing mental health difficulties including psychotic disorders.

Henry James Jackson currently receives research funding from NH&MRC. In the past he has received research funding from NH&MRC, ARC, Ian Potter Foundation, Australian Rotary Health Foundation, Victorian Health Promotion Foundation, Beyond Blue, University of Melbourne and Janssen-Cilag. In the past he has received book royalties from Cambridge University Press and McGraw-Hill.