characterization of acute-phase humoral immunity to monkeypox: use of immunoglobulin M enzyme-linked immunosorbent assay for detection of monkeypox infection during the 2003 North American outbreak

Abstract

A monkeypox outbreak occurred in the United States in 2003. Patient's sera were sent to the Centers for Disease Control and Prevention as a part of outbreak response measures. Clinical and epidemiologic information was abstracted from the case investigation forms. Serum samples from patients were tested by using an immunoglobulin M (IgM)-capture and an IgG enzyme-linked immunosorbent assay ELISA against Orthopoxvirus antigen. The detection of antiviral IgG and IgM antibodies and the kinetics of the antiviral IgG and IgM antibody responses were evaluated. Patients were classified as confirmed, probable, or suspect cases or were excluded as cases based on laboratory test results and epidemiologic and clinical criteria. A total of 37 confirmed case patients with monkeypox were identified, and 116 patients were excluded as case patients based on molecular testing or insufficient epidemiology and clinical data to warrant classification as a suspect or probable case. Of 37 confirmed case patients, 36 had a known history (presence or absence) of smallpox vaccination. Of those, 29 of the 36 either had or developed an IgG response, while 34 of the 36 developed an IgM response, regardless of vaccination status. Serum collected > or =5 days for IgM detection or serum collected > or =8 days after rash onset for IgG detection was most efficient for the detection of monkeypox virus infection. IgM ELISA detects recent infection with orthopoxviruses and, in this case, recent infection with monkeypox virus. In addition, analysis of paired sera for IgG and IgM detected seroconversion, another indicator of recent infection. The ELISA results correlated with the virologic PCR and viral culture results, indicating its diagnostic capabilities for monkeypox and potentially other orthopoxvirus infections due to zoonotic transmission or bioterrorism events.

FIG. 1.

Kinetics of anti-OPXV IgG and IgM responses in confirmed cases with or without a history of smallpox vaccination after rash onset. Results are presented as OD COVs, where values above zero are considered positive. (A) Kinetics of IgG antibody responses in vaccinated and unvaccinated cases within the first 21 days after rash onset; (B) kinetics of IgG antibody responses for all time points in vaccinated and unvaccinated cases; (C) kinetics of IgM antibody responses in vaccinated and unvaccinated cases within the first 21 days after rash onset; (D) kinetics of IgM antibody responses for all time points in vaccinated and unvaccinated cases.

FIG. 2.

ROC plots of serology ELISA data for confirmed cases (positives) and noncases (negatives) with serum drawn ≥14 days after rash onset for IgG and >4 days to ≤78 days after rash onset for IgM. The x axis and the y axis represent the sensitivity (□) and the specificity (▪) of the ELISA, respectively. (A) ROC plot for IgG ELISA; (B) ROC plot for IgM ELISA.