Delusional Infestation

Delusional Infestation (DI) is a rare psychotic syndrome. The syndrome was previously known as delusional parasitosis.

Sufferers of DI believe, against all medical evidence, that their skin and/or body is infested by small, vivid (or less frequently ‘inanimate’) pathogens.

The two core symptoms that characterise DI patients are:

  • the rigid belief that they are infested by ‘pathogens’ (small, vivid, inanimate [rare], often ‘new to science’) or
  • abnormal sensations (itching, biting, crawling) in/on the skin, which can be explained by the above.

Characteristics of patients

DI can occur in any age group. It is most common in older people aged 40 to 60 years. It is more prominent in females with few social contacts.

Average female to male ratio is 2:1.

The educational background of sufferers is varied, ranging from having completed high school education to professional degrees.

DI may be caused by:

  • emotional trauma (such as job loss, divorce or social isolation)
  • substance abuse (such as cocaine, amphetamines or cannabis)
  • medication or certain other medical conditions.

Symptoms of delusional infestation

Patients with delusional infestation frequently describe continuous itching, crawling and biting sensations, especially on the skin. 

Some can provide extensive and elaborate description of the pest, their ‘life cycle’ and behaviour.

Most DI patients provide pieces of skin, dust or other material as evidence. This behaviour is termed specimen sign, match box sign, or baggies sign as the samples are often presented in match boxes or bags.

Specimen samples for DI examination

Typical specimens from DI patients sent for examination - A:  Skin scrapings from patient’s skin containing dust and lint often collected onto sticky tape. B: Urine collection container containing dust and plant matter

Patients often strongly reject the possibility of psychological or other explanations. This results in ‘doctor hopping’ for evaluation by many different physicians, microbiologists and tropical disease specialists in order to obtain a diagnosis.

Most DI patients attempt to extract or eradicate these ‘pathogens’. Extraction methods can include mechanical removal of skin and tissue, application of insecticides, gasoline or kerosene. These practices often lead to self-mutilation. In extreme cases, DI patients are known to resort to suicide.

These approaches are dangerous and patients should allow GPs to collect samples for diagnosis.

Characteristics of imaginary pathogens

Imaginary pathogens can vary greatly in:

  • size
  • colour
  • behaviour
  • source of infection.

Description of pathogens could be:

  • specific (such as mites, scabies)
  • non-specific (such as vermin or insects
  • ‘too small to see’.

Colour could be:

  • black
  • white
  • grey
  • skin-coloured
  • change colour, usually from red to green.

These imaginary pathogens can also ‘hide, change shape, jump or fly’ to evade detection, especially when a medical practitioner is examining the patient.


All patients suffering from delusional infestation should be referred to a GP for assessment and possible treatment.

Samples should only be investigated if collected and requested by a GP.

The Medical Entomology team within the Environmental Health Hazards Unit of the Department of Health can provide identification services for any invertebrates found during sample collection and will only identify material collected by GPs.

Samples should be collected into screw-top vials, labelled with an identification code (patient surname or other; date of collection and any other relevant details from the patient’s history) and double bagged before being sent to the Medical Entomology team.

All findings of identifications, or lack thereof, will be reported back to the GP, who can then make an informed diagnosis. The GP can then support the patient in obtaining psychiatric treatment if required.

More information

Medical Entomology
Postal address: PO BOX 8172, Perth Business Centre, WA 6849
Street Address: 37 Kensington Street, East Perth, WA 6004
Phone: (08) 9285 5500