Human Study Participants

The protocol for this study (part of the multi-objective study) was reviewed and approved by the institutional review board for human subjects research at the Centers for Disease Control and Prevention (CDC) (Atlanta, GA, USA). Approval was received (by deferral) from partner institutions in Wisconsin (Wisconsin Department of Health and Family Services), Illinois (Illinois Department of Public Health), Indiana (Indiana State Department of Health), Kansas (Department of Health and Environment), and Georgia (Emory University). No monetary incentives or other forms of compensation were provided to study participants.

Definitions and Participant Enrollment

The 47 persons who met criteria for confirmed or probable MPXV infection on the basis of a combination of clinical symptoms, exposure information, and laboratory criteria (12–14) (www.cdc.gov/ncidod/monkeypox/casedefinition.htm), and who had exposure to an infected prairie dog, were considered eligible for enrollment as study case-patients. Standardized case definitions were used to assign confirmed and probable case status (Table 1). Persons defined as having probable cases were those who had illness onset <21 days of exposure to MPXV who experienced fever (>37.4°C) and vesicular pustular rash, or rash (potentially uncharacterized) plus had immunoglobulin (IgM) to orthopoxvirus. MPX cases were confirmed on the basis of any of the following laboratory findings from clinically derived specimens: MPXV isolation, detection of MPXV-specific nucleic acid signatures, positive electron microscopy findings, or positive immunohistochemical findings (the last 2 in the absence of other orthopoxvirus virus exposures).

Persons eligible for enrollment as study controls were those who 1) had exposure to an infected prairie dog either in a household or workplace setting, 2) did not acquire MPX disease (i.e., did not meet the clinical criteria for MPX disease as outlined in the case definition), and 3) did not show signs of infection in the absence of disease (i.e., were negative for IgM to orthopoxvirus).

Local health department personnel contacted potential study case-patients by telephone to ascertain their willingness to participate in the study and to request the names of potential family or social contacts. Persons who volunteered were then enrolled in person at the time of interview, and written consent was obtained. Thirty case-patients and 35 potential controls ultimately consented to participate in the study.

Three persons enrolled as potential controls were found to have elevated levels of IgM to orthopoxvirus. These persons were excluded as controls and were classified as MPXV infected without clinical disease. These 3 infected but not diseased case-patients were retained in a separate category to allow assessment of MPXV infection, as well as MPX disease, as an outcome measure.

MPXV-infected Prairie Dogs

A prairie dog was defined as being MPXV infected if tissues obtained postmortem were positive for MPXV DNA by culture or PCR (15). Alternatively, in the absence of laboratory testing, the animal was defined as being infected if it had signs and symptoms of MPXV infection and was housed at a facility where other animals known to have been MPXV infected were also kept (5,16).

Data Collection

Beginning in late July 2003 (≈40 days after illness onset of the last known human MPX case), face-to-face interviews were conducted with case-patients and controls. A standardized questionnaire was used to obtain information regarding demographic profile, exposure to potentially infected animals and infected humans, signs and symptoms of illness, and factors that might affect susceptibility, including smallpox vaccination history. The data used in this study pertaining to type of animal exposure have been summarized (10).

Blood Collection and Serologic Analysis

At the time of interview, a blood sample was requested from all study participants for detection of antibodies to orthopoxvirus and investigation of cellular immunity. Donation of a blood specimen was not required for enrollment in this study, but those specimens that were obtained were assayed for IgG and IgM reactive with orthopoxvirus antigen (vaccinia) according to the methods of Karem et al. (12).

Data Analysis

Demographic and medical history data were analyzed as dichotomous categorical variables (i.e., present vs. absent). Two classification schemes were used for age groups; one grouped participants according to whether they were adults (>18 years of age) or children (<18 years of age) and the other grouped participants according to whether they would have had an opportunity to have received smallpox vaccine as a routine childhood immunization (>33 and <33 years of age, respectively). Routine smallpox vaccinations stopped in the United States in 1972. Regarding the setting in which the exposure occurred, if a participant indicated he or she was exposed at home and an additional setting, home was noted as the principle setting, otherwise the response was left as recorded. Occupation was stratified into nonanimal-related versus animal-related. Animal-related occupations were veterinarian, veterinarian technician or assistant, or pet store employee. Responses from participants who indicated exposure to >1 infected prairie dog were combined into 1 response; affirmative responses were selected over negative responses when exposure to different animals was not uniform.

Odds ratio (OR) with 95% confidence interval (CI) was calculated to examine associations between disease status and exposure variables (e.g., age and smallpox vaccination status). Multiple logistic regression analysis was used to obtain ORs adjusted for the effect of smallpox vaccination (17) and to examine the risk for disease or infection associated with given exposures. The level of statistical significance established for the analysis was p<0.05. The small size of the study population precluded further assessment of independent risk factors. All statistical analyses were performed by using SAS version 9.1 (SAS Institute, Cary, NC, USA).

Characteristics of Case-Patients and Controls

Sixty-one persons from 4 states affected during the MPX outbreak were enrolled in the study (Table 2). The 2 categories of MPX case-patients evaluated were persons who met the epidemiologic case definition for confirmed or probable MPX (case-patients, n = 30) and persons who did not have illness meeting the case inclusion criteria but who had elevated levels of IgM to orthopoxvirus at the time of enrollment and sampling (infected but not diseased [IBND] patients, n = 3). Twenty-eight study controls were identified from the pool of enrollees. All case-patients and controls had exposure to ill or infected prairie dogs; 4 persons reported exposure to other potentially infected animals (i.e., African rodents).

Figure 1

Figure 1. Age distribution of monkeypox virus–infected case-patients (A) and controls (B) and smallpox vaccination status. No study participants reported having received a smallpox vaccination within 25 years of August 2003.

Most persons enrolled were >18 years of age at the time of study initiation (Table 2). The mean age of case-patients at study enrollment (25.0 years) was significantly lower than that of controls (33.3 years; 2-sided p = 0.013, by Student t test); the median ages of the 2 groups were 27.0 and 36.5 years, respectively. Only 6 case-patients (20%), 3 IBND case-patients (100%), and 15 controls (53.6%) reported having had >1 smallpox vaccinations (Figure 1). All but one of the smallpox-vaccinated participants was >31 years of age at the time of the study, which is consistent with their having received routine vaccination before the 1972 abandonment of this practice in the United States (18). The only vaccinated study participant <31 years of age, a control, had been vaccinated as a child as a precaution for international travel.

Characteristics of IBND Patients

Three persons classified as MPX IBND patients were adults (mean age 40.7 years, median age 38.0 years), and all reported having had >1 smallpox vaccination before 1972; 2 had visible vaccination scars. Symptoms of systemic illness (chills, sweats, myalgias) developed in all 3 patients in the 21 days after last known exposure to an MPX-infected prairie dog (Table 2), but classic MPX disease (1 reported papular rash without fever and 2 had fever without lesions or rash) did not develop. Two persons reported having touched infected prairie dogs (handled >1 infected animal), and the third had only indirect exposure to an infected animal (touched soiled bedding near an infected animal); none reported a bite or scratch from an ill prairie dog. The settings in which these persons were exposed differed for each person.

Risk Factors for Acquiring MPX

Evaluation of associations between exposure variables and MPX disease outcomes with inclusion of unadjusted OR, adjusted OR (aOR, adjusted for history of smallpox vaccination), and 95% CI is shown in Table 3. Results relating to MPX disease as an outcome measure (i.e., that excluded the 3 IBND patients from the case group) are shown, unless results were qualitatively different when the 3 IBND patients were included. In those instances, results are reported for the diseased plus the 3 IBND patients.

When not considering participant age or exposure history, a history of remote smallpox vaccination (>25 years previous) was protective against acquisition of MPX illness (OR 0.2, 95% CI 0.1–0.7). However, smallpox vaccination status was highly correlated with age (Spearman rank-order correlation r² = 0.809, p<0.001), and its effect independent of age could not be assessed. Age group and sex did not differ between case-patients and controls.

Several types of direct exposure (touching or receiving a bite or scratch sufficient to break the skin) and indirect exposure (nontactile) with infected animals were associated with risk for MPX. Regarding direct exposures, having touched a sick animal was significantly associated with development of MPX (OR 3.8, 95% CI 1.2–11.7) regardless of smallpox vaccination status (aOR 4.0, 95% CI 1.2–13.4). Having received a scratch from an infected animal was significantly associated with disease but only when not adjusted for smallpox vaccination (OR 5.6, 95% CI 1.1–28.6; aOR 3.9, 95% CI 0.7–21.1). Among 9 persons who were scratched by an infected prairie dog, 1 had had prior smallpox vaccination. Only 1 MPX case-patient (a previously vaccinated person) reported a prairie dog bite; no controls reported having been bitten.

Among indirect exposures, having been near an infected animal (defined as having come within 6 feet of the animal without touching it) was not associated with development of MPX. Having cleaned the cage or touched used bedding of an infected animal was significantly associated with MPX development, both with and without adjustment for prior smallpox vaccination (OR 5.3, 95% CI 1.5–18.9; aOR 5.3, 95% CI 1.4–20.7). Also, regardless whether exposure was direct or indirect, having had daily exposure to the animal while it was ill was significantly associated with MPX developing (OR 3.8, 95% CI 1.2–11.7; aOR 4.0, 95% CI 1.2–13.4).

When MPX infection (n = 33, 30 patients and 3 IBND patients), rather than MPX disease, was chosen as the outcome measure, only direct (touch) exposure (OR 5.0, 95% CI 1.4–17.6; aOR 4.9, 95% CI 1.3–17.9) and having cleaned the cage or touched used bedding of an infected animal (OR 3.6, 95% CI 1.2–10.7; aOR 3.6, 95% CI 1.2–11.4) were significantly associated with infection. Again, regardless of age or exposure characteristics, reported history of prior, remote smallpox vaccination was protective against MPX infection (OR 0.3, 95% CI 0.1–0.9).

Exposure Characteristics of Vaccinated and Unvaccinated Adults and Children

Because the study population was too small to support multivariate analysis, we were unable to evaluate associations between disease outcomes and individual exposures while controlling for other potentially contributing factors. Therefore, we evaluated types of exposures to prairie dogs among different study subgroups (smallpox-vaccinated persons vs. unvaccinated persons, and adults vs. children) to examine whether differences in proportions exposed could have biased observed associations between disease acquisition and previous smallpox vaccination or age.

Figure 2

Figure 2. Characteristics of exposure to infected prairie dogs of A) vaccinated and unvaccinated monkeypox case-patients and controls and B) case-patients (adults and children) and controls. *Denotes statistically significant differences in exposure between...

The proportion of smallpox-vaccinated and unvaccinated study participants who reported various exposures to infected prairie dogs is shown in Figure 2, panel A. For all but 1 category of exposure, a smaller proportion of smallpox-vaccinated persons reported exposure than unvaccinated persons, which potentially introduced bias in interpreting lack of prior vaccination as a risk factor for disease. The exception, having been bitten by an infected animal, was reported by only 1 person, a smallpox-vaccinated person in whom relatively benign MPX developed. This person was not hospitalized and had <25 lesions. Unvaccinated persons had a higher frequency of scratches and breaks in the skin (10 [27.0%]) than did smallpox-vaccinated persons (1 [4.8%]; p = 0.044, by 2-tailed Fisher exact test). A similar evaluation of exposure characteristics in children and adults indicated that while certain exposures were more common in children than in adults (e.g., having had an infected animal as a pet, having had daily or direct exposure to an infected animal) (Figure 2, panel B), neither age category uniformly had higher proportionate exposure.

Risk Factors for Vaccinated and Unvaccinated Participants

To evaluate potential differences in risks for MPX disease acquisition between smallpox-vaccinated and unvaccinated persons, case-patients and controls were stratified by vaccination status and exposures were examined. No exposure or demographic variables were associated with infection or disease status when participants were stratified by smallpox vaccination status. However, power to detect statistically significant associations was limited because of small numbers.