A Population-Based Study of the Incidence of Delusional Infestation in Olmsted County, Minnesota, 1976–2010

Summary

BACKGROUND

Delusional Infestation (DI) is a well-recognized clinical entity but there is a paucity of reliable data concerning its epidemiology. Knowledge of the epidemiology of disease is fundamental to an understanding of any disease and its implications. Epidemiology is most accurately assessed using population-based studies, which are most generalizable to the wider population in the US and worldwide. No population-based study of the epidemiology (particularly incidence) of DI, that we are aware of, has been reported to date.

OBJECTIVES

To determine the incidence of delusional infestation (DI) using a population-based study.

METHODS

Medical records of Olmsted County residents were reviewed using the resources of the Rochester Epidemiology Project to confirm the patient’s status as a true incident case of DI and to gather demographic information. Patients with a first-time diagnosis of DI or synonymous conditions between January 1, 1976, and December 31, 2010 were considered incident cases.

RESULTS

Of 470 identified possible diagnoses, 64 were true incident cases of DI in this population-based study. The age- and sex-adjusted incidence was 1.9 (95% CI, 1.5–2.4) per 100,000 person-years. Mean age at diagnosis was 61.4 years (range, 9–92 years). The incidence of DI increased over the 4 decades from 1.6 (95% CI, 0.6–2.6) per 100,000 person-years in 1976–1985 to 2.6 (95% CI, 1.4–3.8) per 100,000 person-years in 2006–2010.

CONCLUSIONS

In this population-based study of the incidence of DI, the age- and sex-adjusted rate was 1.9 per 100,000 person-years.

Introduction

Delusional infestation (DI) is a disorder with dermatologic and psychological components; patients falsely believe that they are infested with parasites, insects, or inanimate objects, despite evidence to the contrary.^1–3 Most frequently, the perceived infestation is in, on or under the skin.² Body orifices may also be affected, however involvement of the entire body is uncommon.² Patients generally experience abnormal skin sensations they believe emanate from parasites or materials on or within the skin, acquired from bedding, clothing, or the environment. They often treat perceived infestations with chemicals such as disinfectants or pesticides. Many excoriate the skin with their fingers, tweezers, or other implements in attempts to remove the offending organisms or materials. Patients often place the materials they are seeing emerging from their skin in containers they bring to their physicians (usually general practitioners, dermatologists, and microbiologists) as proof of infestation. This has been called the “matchbox sign”, and the updated terms “baggies sign” and “specimen sign” have been suggested.^2,4,5 Studies have shown that, even when these samples are examined by pathologists who find no proof of infestation, patients usually remain convinced that they are indeed infested.^6,7 Antipsychotic medications have been described to be effective in the management of DI, however patients are often resistant to the notion of taking antipsychotic treatment²

Delusional infestation is currently the accepted term for this clinical presentation.² Other terms that have been used for this disorder include delusional disorder with parasitosis, delusion(s) of parasitosis, delusional parasitosis, delusion(s) of parasitism, delusion(s) of parasites, parasitosis (delusional), delusory parasitosis, psychogenic parasitosis, neurogenic parasitosis, neurotic parasitosis, Ekbom syndrome, formication and parasites, chronic tactile hallucination(s), dermatophobia, parasitophobia, toxic psychosis, tactile psychosis, monosymptomatic hypochondriacal psychosis, Morgellon(s), psychogenic dermatitis, neurotic dermatitis, neurogenic dermatitis, self-induced excoriations, and psychogenic excoriations.

An understanding of the epidemiology of this disorder is essential to an understanding of the disorder and its implications. Epidemiologic data is needed when making decisions regarding the allocation of clinical and financial resources toward the care of patients. Little reliable epidemiologic information has been published on this disorder², and much of what has been published is based on surveys and case series, which do not provide accurate epidemiologic data. A study from the US Centers for Disease Control and Prevention (CDC) estimated the prevalence of Morgellon disease (defined as the self-reported emergence of fibers or materials from the skin accompanied by skin lesions and/or disturbing skin sensations) in a population-based cohort⁷ to be 3.65 (95% CI, 2.98–4.40) cases per 100,000 enrollees among 3.2 million enrollees in Kaiser Permanente Northern California; the study included about 30% of the residents of the 13 counties from which that institution draws its enrollees. Incidence figures were not published.⁷ The CDC study’s Morgellon definition closely parallels the definition of DI we used in this study. Other estimates of the epidemiology of this disorder have been predominantly survey-based (as summarized in Table 1) and report an estimate of prevalence only. The only study regarding the incidence of DI we are aware of (Trabert, 1993), estimates the incidence of the disorder at 0.845 per 100,000 persons.⁸

Table 1.

Summary of Published Epidemiologic Studies of Delusional Infestation

Source	Case Definition	Method	Setting	Incidence Estimate	Prevalence Estimate (Adjusted per 100,000)
Present study	Patients having 1) a conviction of being infested by pathogens (animate or inanimate) with no medical or microbiologic evidence for this belief and 2) abnormal sensations in the skin (criteria adapted from Freudenmann and Lepping2); meeting these criteria for a first-time diagnosis of DI	Population-based	Olmsted County, Minnesota	1.9 per 100,000 person-years	N/A
Pearson et al,7 2012	Self-reported emergence of fibers or materials from the skin accompanied by skin lesions and/or disturbing skin sensations	Population-based	Enrollees in Kaiser Permanente Northern California	N/A	3.65 per 100,000 enrollees
Lepping et al,14 2010	Dependent on individual dermatologist diagnosis criteria; positive cases reported by survey	Survey	UK dermatologists of new and ongoing outpatients with DI over 3 years	N/A	3-year prevalence:0.499 per 100,000 persons; point prevalence:0.148 per 100,000 persons
Trabert,8 1993	Dependent on individual dermatologist diagnosis criteria; positive cases reported by survey	Survey	General practitioners, dermatologists, neurologists	0.845 per 100,000 persons (based on 50.59% response rate)	4.225 per 100,000 persons (based on 50.59% response rate)
Trabert,15 1991	Dependent on individual dermatologist diagnosis criteria; positive cases reported by survey	Survey	Neurologic, psychiatric, dermatologic, and geriatric hospitals and public health	N/A	0.188 per 100,000 persons (based on 33.59% response rate)

Abbreviations: DI, delusional infestation; NA, not available.

Population-based studies are considered the ‘gold standard’ for calculation of the epidemiology of disease. To our knowledge, there has not been a population-based study of the incidence of DI. The objective of this study was to calculate the incidence of DI using population-based data from a single, well-studied, and well-characterized US county. This was done using the resources of the Rochester Epidemiology (REP), which has maintained a comprehensive medical records linkage system for nearly half a century for almost all persons residing in Olmsted County, Minnesota, and can reliably be used to calculate epidemiologic data.⁹ The REP has been used in studies spanning almost every medical specialty and has yielded more than 2000 epidemiologic publications to date.⁹

Methods

Literature Search

A search was performed on PubMed for studies in English regarding the incidence of delusional infestation or delusional parasitosis. The studies found in this search were reviewed for epidemiologic data concerning incidence and prevalence data. In addition, during our review, other studies not indexed in PubMed, or not written in English were found during this article review. In each case, the study’s data and methods were appraised, reviewed, and appropriate data extracted (Table 1).

Study Setting

Diagnoses of DI or its synonyms between January 1, 1976 and December 31, 2010, made in the population of Olmsted County, Minnesota, were reviewed. The start date was chosen because it was the year the institutions involved adopted the International Classification of Diseases, Ninth Revision (ICD-9) medical code system. The end date was selected to include all years that had been updated in the Rochester Epidemiology Project (REP), the source of all records we reviewed.

The REP Resource

Population-based studies in Olmsted County are possible because of the REP, a centralized, computerized system that indexes medical diagnoses and surgical procedures from Mayo Clinic, Olmsted Medical Center, and other health care facilities in Olmsted County. The REP links this information to each person, regardless of where the patient went for care.¹⁰ Virtually all medical care in the county is provided by either Olmsted Medical Center or Mayo Clinic, which makes the REP’s population-based studies possible. During the dates of our study, approximately 95% of Olmsted County residents gave permission to use their medical record for research purposes.¹¹

Located in southeastern Minnesota, 90 miles southeast of Minneapolis and St. Paul, Olmsted County is relatively geographically isolated. According to the 2010 US census, Olmsted County had a population of 144,000 people. More than 70% of the county’s population resided in the city of Rochester. The county’s residents are largely white, not of Hispanic origin (83% in 2010), with a greater percentage of college graduates than Minnesota’s general population (39% of Olmsted County residents report having a bachelor’s degree or higher, compared with 31% for the state of Minnesota). Comparisons have shown that the population, despite these educational differences, is socioeconomically similar to the general white population in the United States.^10,12 It is also important to note that Olmsted County residents’ age and sex distributions resemble those of the entire US population.¹²

Approval

This study was approved by the institutional review boards of both Mayo Clinic and Olmsted Medical Center.

Case Definition

We defined the criteria for DI as:

• A conviction of being infested by pathogens (animate [eg, insects or worms] or inanimate [eg, fibers]) without any medical or microbiologic evidence for this belief

and

• Abnormal sensations in the skin that are explained by the first criterion (criteria adapted from Freudenmann and Lepping²).

Charts for patients meeting these criteria while residents of Olmsted County were reviewed. Those with an initial diagnosis of DI during the defined period were considered incident cases and included in the study.

A search was performed using the REP medical records linkage system, which uses ICD-9 billing codes for diagnoses during the dates specified. The diagnoses of “delusional parasitosis” and “other simple phobia” were extracted from the database, and each patient’s medical chart(s) was reviewed for inclusion as per the above criteria. In addition, pathology reports, as well as earlier and subsequent patient documentation were reviewed for evidence of actual infestation, doubtful diagnoses, or previous initial diagnoses outside of the inclusion criteria. Medical records were reviewed and socioeconomic data including age at diagnosis, sex, race and ethnicity were abstracted from the records meeting inclusion criteria. In this retrospective study, there were no withdrawals. Where the diagnosis was in question or data were uninterpretable, these cases were excluded from the study.

A sample of data from our institution during 2001–2007 was reviewed against a more broad search performed using the following terms: delusional disorder with parasitosis, delusion(s) of parasitosis, delusional parasitosis, delusion(s) of parasitism, delusion(s) of parasites, parasitosis (delusional), delusional infestation, delusory parasitosis, psychogenic parasitosis, neurogenic parasitosis, neurotic parasitosis, Ekbom’s syndrome, formication and parasites, chronic tactile hallucination(s), dermatophobia, parasitophobia, toxic psychosis, tactile psychosis, monosymptomatic hypochondriacal psychosis, Morgellon(s), psychogenic dermatitis, neurotic dermatitis, neurogenic dermatitis, self-induced excoriations, and psychogenic excoriations. No additional cases suitable for inclusion were found in this review.

To determine if any diagnoses of delusional infestation were missed by narrowing our focus to the above mentioned two diagnosis codes, patient charts with other diagnoses (delusional disorder, paranoia, restless leg syndrome, disturbance of skin sensation, unspecified transient mental disorder in conditions classified elsewhere, other specified transient mental disorders due to conditions classified elsewhere, lichenification and lichen simplex chronicus, unspecified paranoid state, psychogenic pruritus, hallucinations, dermatitis factitia, infestation NOS, psychic factors associated with diseases classified elsewhere) were retrieved from the REP, and a randomized sample of these patient charts were reviewed. We found no cases of delusional infestation in this additional chart review.

Cases were reviewed for evidence of actual infestation. No patients with infestation such as scabies or creeping eruption were found; those cases had been excluded per our case definition.

Although a broad range of diagnosis codes were reviewed to find miscategorized or misdiagnosed cases of DI, another concomitant diagnosis such as psychosis or schizophrenia was not a reason for exclusion in this study.

Statistical Methods

Incidence rates per 100,000 person-years were calculated using incident cases of DI as the numerator and age- and sex-specific estimates of the population of Olmsted County as the denominator. The populations at risk for the years 1976–2000 were estimated using census data from 1970, 1980, 1990, and 2000, with linear interpolation for intercensal years. The populations at risk for the years 2001–2010 were obtained from US Intercensal Estimates (www.census.gov). Because the population of Olmsted County is nearly all white, incidence rates were directly age-and sex-adjusted to the structure of the 2000 US white population. Incident cases were grouped into intervals based on age at diagnosis (0–19, 20–39, 40–59, 60–79, ≥80 years) and on year of diagnosis (1976–1985, 1986–1995, 1996–2005, 2006–2010). The relationships of age at diagnosis, sex, and year of diagnosis with incidence of DI were assessed by fitting Poisson regression models using the SAS procedure GENMOD (SAS Institute Inc, Cary, North Carolina). P values less than .05 were considered statistically significant.

Results

All medical records for patients who resided in Olmsted County at some point in their lives and received a diagnosis of “delusional parasitosis” (a narrower, but more common term for DI², used in ICD-9 diagnosis coding) or “other simple phobia” during the defined period and were retrieved using the resources of the REP. Each record and the diagnosis code(s) affiliated with it from our search criteria, was then reviewed to determine whether it met the case definition set by the study. From 470 medical records of Olmsted County residents flagged by the REP as containing a diagnosis of either “delusions of parasitosis” or “other simple phobia,” 64 patients met inclusion criteria.

The demographic characteristics of the 64 patients who met study inclusion criteria are summarized in Table 2. The mean age at diagnosis for the 64 incident cases was 61.4 years (median, 63 years; range, 9–92 years).

Table 2.

Summary of 64 Incident Cases of Delusional Infestation

Characteristic	No. (%)
Age at diagnosis, y
0–19	2 (3)
20–39	5(8)
40–59	22(34)
60–79	23(36)
≥80	12(19)
Sex
Female	41 (64)
Male	23 (36)
Race
White	47 (73)
Black	3 (5)
Other	1 (2)
Unknown	13 (20)
Year of diagnosis
1976–1985	10 (16)
1986–1995	3 (20)
1996–2005	23 (36)
2006–2010	18 (28)

Table 3 summarizes age- and sex-adjusted incidence. Overall incidence of DI was 1.9 (95% CI, 1.5–2.4) per 100,000 person-years. The age-adjusted incidence was higher in females: 2.3 (95% CI, 1.6–3.0) per 100,000 person-years for females compared with 1.7 (95% CI, 1.0–2.4) per 100,000 person-years for males, although this difference was not statistically significant (P=.16).

Table 3.

Incidence of Delusional Infestation in Olmsted County, Minnesota, 1976–2010

Age and Sex Incidence
	Females		Males		Total
Age, y	No.	Ratea	No.	Ratea	No.	Ratea
0–19	1	0.2	1	0.2	2	0.2
20–39	3	0.5	2	0.3	5	0.4
40–59	16	3.3	6	1.3	22	2.3
60–79	15	6.1	8	4.0	23	5.2
≥80	6	7.2	6	16.2	12	10.0
Total	41	2.3b	23	1.7b	64	1.9c

Incidence by Decade
	1976–1985		1986–1995		1996–2005		2006–2010
Sex	No.	Rateb	No.	Rateb	No.	Rateb	No.	Rateb
Female	7	1.9	8	1.7	14	2.4	12	3.3
Male	3	1.3	5	1.4	9	1.8	6	2.2
Total	10	1.6c	13	1.5c	23	2.1c	18	2.6c

^aIncidence per 100,000 person-years.

^bIncidence per 100,000 person-years age-adjusted to 2000 US white population.

^cIncidence per 100,000 person-years age- and sex-adjusted to the 2000 US white population.

Incident cases were grouped into 20-year intervals on the basis of patient age at diagnosis. The incidence of DI varied significantly by age group (P<.001), increasing in stepwise fashion from a low of 0.2 (95% CI, 0.0–0.9) per 100,000 person-years in the 0–19-year age group to a high of 16.2 (95% CI, 5.9–35.2) per 100,000 person-years for males aged 80 years or older. Females aged 80 and older were found to have an incidence of 7.2 (95% CI, 2.6–15.7) per 100,000 person-years. Incidence thus increased with age, especially after the age of 40 years (Figure 1).

Figure 1.

Incidence of delusional infestation by age at diagnosis in Olmsted County, Minnesota, 1976–2010.

Table 3 also summarizes incidence by decade. The DI incidence increased over the 4 decades studied, from 1.6 (95% CI, 0.6–2.6) per 100,000 person-years over the decade between 1976 and 1985 to 2.6 (95% CI, 1.4–3.8) per 100,000 person-years in the 5 years between 2006 and 2010; however, this increase was not statistically significant (P=.07) (Figure 2).

Figure 2.

Age-adjusted incidence of delusional infestation by year of diagnosis in Olmsted County, Minnesota, 1976–2010.

Discussion

To our knowledge, this study is the first to address DI incidence in a population-based cohort, finding an overall age and sex-adjusted incidence of 1.9 per 100,000 person-years from 1976 through 2010. There appeared to be a trend for a difference in incidence by sex, with incidence increasing for both men and women with age as well as a trend toward an increase in incidence over the past 4 decades.

A strength of this study is that it drew its data from the REP, a reliable linkage system used for the study and estimation of population-based incidence rates. However, our data have several potential limitations. One such limitation is that our study did not differentiate between primary and secondary forms of DI. Concomitant psychiatric diagnoses such as depression, psychosis or schizophrenia were addressed in a separate study.¹³ Additionally, cases of DI may have been missed if the patients were coded as having a disorder other than delusions of parasitosis or other simple phobia. To address this, additional diagnosis codes of possible miscategorization were retrieved, and a randomized sample of these charts (14,272) was reviewed to determine if any cases of DI had been missed. We found no cases of delusional infestation in this additional chart review. Thus, although our calculation may underestimate the incidence, it seems that almost all cases in the county were identified by searching these ICD-9 codes. The population studied was limited to southeast Minnesota, and its demography was primarily white. As such, our study’s data may not be representative of DI in other parts of the United States or the world. It is also noteworthy that inclusion depended on accurate diagnosis and coding by clinicians and staff.

The trend of increased incidence of DI may in part be due to a greater awareness of this entity, leading to greater frequency of diagnosis, and also the more recent publicity surrounding the use of the term Morgellons disease since 2002.

These limitations notwithstanding, our data indicate that DI is a rare disease. The incidence increases across the life span, especially after the age of 40 years. There is a trend for an increased incidence in the general population over the past 4 decades. The condition was more common in women than men, although this difference was not statistically significant.

• Population-based studies of the incidence of delusional infestation (DI), a disorder with dermatologic and psychological components, have not been performed.

• While still rare, DI is more common than earlier estimated. The annual incidence was 1.9 (95% CI, 1.5–2.4) per 100,000 person-years. Mean age at diagnosis was 61.4 years (range, 9–92), and incidence increased with age and time.

Abbreviations

DI

delusional infestation

ICD-9

International Classification of Diseases, Ninth Revision

REP

Rochester Epidemiology Project