Delusional parasitosis mimicking cutaneous infestation in elderly patients

Patient 1

An 88-year-old man gave a 12-month history of seeing insects attacking his legs and crawling along the floor of his house. He described these insects as 4 cm long, black and white bugs with beaks, which pecked at his legs, causing wounds. He often felt a sharp stinging sensation heralding their presence. He also had burning pain in both legs below the knees. He had had his house fumigated twice in the previous year and put various chemicals across his doorways and bed to ward off the bugs. He described no other hallucinations or delusions.

He was not using any regular medications and had never been a consumer of alcohol. He lived alone and managed all activities of daily living independently. His home was clean, and he had no pets.

Score on Folstein Mini Mental State Examination was 28/30. Neurological examination revealed signs of peripheral neuropathy of the lower limbs. His visual acuity was poor. During the examination, he pointed to several “bugs” on his legs, which were actually pieces of skin peeling from superficial ulcers.

Nerve conduction studies confirmed peripheral neuropathy. Computed tomography and magnetic resonance imaging of the brain showed ischaemic changes. Magnetic resonance angiography showed severe stenosis of the left internal carotid artery. Septic screen and serological tests for syphilis gave negative results.

The patient’s symptoms were thought to be due to neuropathic pain. He was prescribed carbamazepine (100 mg twice daily) to alleviate the sensory stimuli, and risperidone (0.5 mg in the morning, 1 mg at night). After 2 weeks, the pain and delusions had decreased substantially.

Diagnosis: Delusional parasitosis associated with medical conditions — neuropathy and poor visual acuity.

Patient 2

A 72-year-old woman had a 20-year history of the delusion of worms crawling throughout her body, especially around a scar on her hip. She was convinced that the scar should be surgically explored.

Past medical history included type 2 diabetes mellitus, normal pressure hydrocephalus with shunt insertion, ischaemic heart disease and fractured right neck of femur. She lived in a hostel. Her cognition was normal, and clinical examination found no abnormalities.

She was treated briefly with thioridazine (40 mg daily), which provided some benefit but caused drowsiness. She was then treated with pimozide (2 mg twice daily), which was changed to olanzapine (2.5 mg in the morning, 5 mg at night) because of continuing agitation, paranoid ideation about her neighbours and “intruders”, and aggression to neighbours and hostel staff. Her symptoms decreased but did not resolve, and compliance was poor.

Diagnosis: Paranoid schizophrenia with major psychotic symptom related to infestation.

Patient 3

An 81-year-old woman was referred with a persistent belief that she had scabies and lice infestation of her eyes, nose, arms and anus. This resulted in her persistently washing her clothes and herself and reporting the retirement village where she lived to the Health Department. She had received anti-scabies treatment empirically.

She had a long history of severe depression after the death of her husband, for which she took doxepin. She had paranoid ideation about her neighbours and saw things crawling down the walls, and had moved residences several times to avoid these problems.

Other medical problems included treated hyperthyroidism, oesophageal stricture and partial pneumonectomy. She did not drink alcohol. Score on Folstein Mini Mental State Examination was 28/30.

She was prescribed haloperidol (0.5 mg twice daily) and continued taking doxepin. The delusions of scabies subsided.

Four years later, she developed new thoughts that dirt was being deposited in her unit by builders working nearby. She had stopped taking haloperidol in the interim. She became agitated, covering her furniture and closing all gaps around doors and windows. She was prescribed olanzapine (2.5 mg at night).

Diagnosis: Delusional parasitosis in conjunction with depressive illness.

Patient 4

A 74-year-old woman was referred for assessment of tactile hallucinations. She had a 2-month history of presumed worm infestation and persistent complaints of anal and vaginal pruritus despite multiple courses of an anthelminthic. She had complained previously of feeling head and body lice.

Her past medical history included chronic airways limitation, trigeminal neuralgia, which was treated with sodium valproate, and temporal arteritis, which had responded previously to corticosteroid therapy. She lived alone and was previously active, but had became depressed and isolated because of these delusions.

No abnormalities were found on physical examination. Examination of a skin specimen, which the patient thought contained worms, showed cotton threads. Full blood count was normal, with no eosinophilia and normal erythrocyte sedimentation rate. Sigmoidoscopy found no abnormalities, while computed tomography of the head showed a previous left parietal infarct.

She was prescribed pimozide (2 mg daily), with complete resolution of the worm sensation.

Diagnosis: Isolated delusional parasitosis.