The first state-specific analysis of syphilis among men who have sex with men (MSM) shows they have dramatically higher incidence than men whose only sexual partners are female, the CDC is reporting.
Data from 2015, analyzed with a new methodology, show that the incidence of primary and secondary syphilis among MSM was 309.0 cases per 100,000 people, compared with 2.9 per 100,000 among men who reported sex with women only, according to Alex de Voux, PhD, of the CDC's epidemic intelligence service, and colleagues at the CDC and Emory University in Atlanta.
The disparity was even more marked when the rate among MSM was compared with the 1.8 cases per 100,000 population seen among women, the researchers reported in the April 7 issue of
The findings are consistent withbut are more precise, De Voux and colleagues said, because they were able to use a new methodology to estimate the size of the MSM population. She had first reported on the new results late last year at 2016 STD Prevention Conference.
County-by-county data from the U.S. Census Bureau's included the number of households with a male head-of-household and a male partner, De Voux and colleagues reported, and that information could be used to estimate the MSM population per county.
For the syphilis analysis, the researchers used data from the 44 states that had information on the sex of partners in at least 70% of reported cases. Those states accounted for 83.4% of all 23,872 reported cases in 2015, De Voux and colleagues reported.
State-specific incidence rates among MSM ranged from 73.1 per 100,000 population in Alaska to 748.3 in North Carolina, the investigators found. Syphilis incidence among MSM was highest in the South and West and four of the five states with the highest rates among MSM -- Louisiana, Mississippi, North Carolina, South Carolina, and New Mexico -- were in the South.
Overall, incidence among MSM was 106 times what was seen among men who have sex with women only, the researchers found. State-specific rate ratios ranged from a low of 39.2 in Minnesota to a high of 342.1 in Hawaii, they reported, and there were no states in which the rate among MSM was lower than the rate among men who have sex with women only.
The overall syphilis among MSM was 167.5 times the rate among women, with state-specific rate ratios ranging from 63.7 in Louisiana to 2,140.3 in Hawaii, De Voux and colleagues reported.
Interestingly, the highest overall syphilis rate in the U.S., seen in 1946, was 70.9 cases per 100,000 population -- a rate exceeded by the lowest state-specific rate among MSM in 2015: the 73.1 cases per 100,000 observed in Alaska.
The researchers cautioned that the data are based on 44 states and might not reflect the nation as a whole. Similarly, the estimates rely on the American Community Survey data; under-reporting of same-sex households would skew the outcome.
As well, they noted, the analysis did not include cases in which the sex of partners was unknown and if MSM are less likely than other men to report the sex of their partners, the findings might under-estimate the rate of disease among MSM.
Finally, De Voux and colleagues cautioned, not all cases of syphilis are diagnosed and reported.