There does not appear to be any one underlying cause for delusional parasitosis, although it probably has to do with fundamental aspects of brain chemistry. This syndrome is often classified into three types according to the underlying condition as below. However, there is increasing literature on itch and chronic pruritus (itch) that indicate that delusional parasitosis may not be a psychiatric syndrome per se and instead may have a fundamental origin in the itch nerve pathways between the skin and the brain. There are many physiological syndromes that cause the symptoms ascribed to delusional parasitosis.

Delusional Infestation and Chronic Pruritus: A Review

Primary Psychotic Delusional Parasitosis 
Delusional parasitosis is the sole psychological disturbance

  • Patients otherwise behave normally
  • Reason in a rational manor

Secondary Functional Delusional Parasitosis (see Table 1)
Where there is an underlying psychiatric condition, including: 

  • Schizophrenia
  • Paranoia
  • Depression
  • Anxiety disorders
  • Obsessional states

Secondary Organic Delusional Parasitosis 
Where there is an underlying physical illness, including: 

  • Drug abuse
  • Hypothyroidism
  • Cancer
  • Cerebrovascular disease
  • Tuberculosis
  • Neurologic disorders
  • Vitamin B12 deficiency
  • Diabetes mellitus
  • Prescription drug interactions