Prevalence of Delusional Infestation—A Population-Based Study

Delusional infestation (DI) (formerly known as delusions of parasitosis or Morgellons disease) is a disorder in which patients have false fixed beliefs of infestion with animate or inanimate pathogens. Patients are often resistant to medical evidence and reluctant to pursue psychiatric evaluation. Therefore, dermatologists must be aware of this challenging disorder.

Knowledge of DI epidemiology is critical. Disease incidence describes the rate new cases are diagnosed. Disease prevalence reflects the proportion of cases in a population at a given time. Our group has previously reported the age- and sex- adjusted incidence of DI as 1.9 (95% CI, 1.5-2.4) per 100 000 person-years.¹

To our knowledge, a population-based study of DI prevalence does not exist. To date, the only DI prevalence data include 2 limited surveys of outpatient practices and a retrospective review of a single dermatology clinic.^2-4 Our objective was to determine the population-based DI point prevalence on December 31, 2010, in Olmsted County, Minnesota.

After institutional review board approval, the Rochester Epidemiology Project (REP), a linkage system of medical records, was used to identify all patients.^5,6 Delusional infestation was defined as the perception of a pathogen infestation with abnormal skin sensations but without reasonable evidence of an infestation.¹ After a search of the REP with DI-related terms as described in Bailey et al,¹ no additional patients with DI were identified.

In prevalence calculations, the numerator was patients with DI who were residents of Olmsted County, Minnesota, on December 31, 2010. The denominator was determined from age- and sex-specific estimates of the population of the county in 2010, which were calculated from an enumeration of Olmsted County provided by the REP.^5,6 Prevalence was age- and sex-adjusted to the US white population in 2010.⁵ Poisson regression models were used to fit age and sex with prevalence. Patient data were deidentified.

Of the 35 patients identified, 34 received a new diagnosis of DI while a resident of Olmsted County, Minnesota, from 1976 through 2010; the 1 patient who was not considered to have an incident case moved into the county and was still a resident on December 31, 2010.

The cohort included 13 men (37.1%) and 22 women (62.9%). The mean age at DI diagnosis was 64.5 years (median, 69 years; range, 12-93 years). Prevalence by age and sex is summarized in the Table and Figure. Age- and sex-adjusted DI prevalence was 27.3 per 100 000 person-years. Age-adjusted prevalence increased significantly with age for both sexes (P < .001). The difference in DI prevalence between sexes was not significant (P = .33).

To our knowledge, this study is the first to report population-based DI prevalence. The overall prevalence is considerably higher than other prevalence rates.^2-4 Our findings indicate that DI prevalence may be underreported. The higher rates reported in our study may reflect our use of the REP, which enables population-based statistics and includes patients presenting to any general practitioner and specialist in Olmsted County; previous studies have reported rates among patients presenting to a particular outpatient practice or to a subset of practices.^2-4

Our study has limitations. The study population represents one predominantly white Minnesota county, which may not represent DI prevalence in diverse populations elsewhere in the world. Also, this study did not differentiate between primary DI and secondary DI from formication induced by comorbid diseases or other factors, potentially overestimating prevalence.

This study reports the first population-based DI prevalence. Although rare, DI seems to be more common than previously suspected and occurs most frequently in older men and women. Future studies may explore the primary burden of this challenging and often refractory disorder to better estimate disease prevalence.

Corresponding Author: Mark D. P. Davis, MD, Department of Dermatology, Mayo Clinic, 200 First St SW, Rochester, MN 55905 (davis.mark2@mayo.edu).

Accepted for Publication: January 3, 2018.

Published Online: April 4, 2018. doi:10.1001/jamadermatol.2018.0004

Author Contributions: Drs Kohorst and Davis had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Kohorst, Pittlekow, Davis.

Acquisition, analysis, or interpretation of data: Bailey, Andersen, Davis.

Drafting of the manuscript: Kohorst, Bailey, Pittlekow.

Critical revision of the manuscript for important intellectual content: Bailey, Andersen, Davis.

Statistical analysis: Kohorst, Davis.

Administrative, technical, or material support: Pittlekow, Davis.

Study supervision: Andersen, Pittlekow, Davis.

Conflict of Interest Disclosures: None reported.

Funding/Support: This study was supported in part by the resources of the Rochester Epidemiology Project, which is supported by the National Institute on Aging of the National Institutes of Health under Award Number R01AG034676; principal investigators: Walter A. Rocca, MD, and Barbara P. Yawn, MD.

Role of the Funder/Sponsor: The funding sources had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Disclaimer: The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

Additional Contributions: We are indebted to Barbara A. Abbott and Christine M. Lohse, MS, for their assistance in preparing this study for publication. They were not reimbursed beyond their regular salaries.