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Specific sexual behaviors were linked to where monkeypox symptoms showed up on the body of infected individuals, researchers in Spain reported.

Proctitis, an inflammation in the lining of the rectum, was more likely in individuals who reported anal receptive sex, while nearly all of those who presented with ulcerative tonsillitis reported oral receptive sex, found Oriol Mitjà, PhD, of University Hospital Germans Trias i Pujol in Badalona, Spain, and colleagues in a study published in .

"Lesion swabs showed the highest viral loads, which, combined with the history of sexual exposure and the distribution of lesions, suggests close contact is probably the dominant transmission route in the current outbreak," the group wrote.

The findings were consistent with a viral load more than three orders of magnitude higher in lesion samples than in respiratory samples.

"Strikingly higher viral loads in lesion swabs than in pharyngeal swabs should be further investigated to guide the decision on whether respiratory transmission is relevant and respiratory isolation at home is necessary," the researchers added.

Their study enrolled 181 human monkeypox virus-infected patients from three sexual health clinics in Madrid and Barcelona. Of these patients, 166 identified as gay, bisexual, or men who had sex with men (MSM), and 15 identified as heterosexual men or heterosexual women.

Investigators collected lesion, anal, and oropharynx swabs for PCR testing and surveyed patients on specific sexual activities, asking them to report on number of sexual partners and sexual practices, including whether they practiced vaginal insertive sex, anal insertive or receptive sex, and oral insertive or receptive sex.

Proctitis was an absolute 31% more likely among MSM who reported anal receptive sex than among other MSM (38% vs 7%, P<0.0001). These patients also had a higher frequency of viral prodrome than the group that did not report anal receptive sex (62% vs 28%, P<0.0001).

Of the 18 out of 19 patients presenting with ulcerative tonsillitis who reported oral receptive sex, higher viral load on the skin was found compared to throat swabs (mean cycle threshold value 23 vs 32, P<0.0001).

"Our study strengthens the evidence for skin-to-skin contact during sex as the dominant mechanism of transmission of monkeypox, with important implications for disease control," the authors wrote.

"Anorectal and genital epithelium routes exhibit the highest probability of sexually transmitted infection acquisition because they have a lower degree of keratinization and a higher frequency of antigen-presenting cells such as macrophages and dendritic cells," they noted.

In , Dimie Ogoina, MD, of the Niger Delta Teaching Hospital in Bayelsa, Nigeria, and president of the Nigerian Infectious Diseases Society, noted the findings were similar to studies outside countries where human monkeypox is endemic.

"There have been no previous reports of proctitis from countries where monkeypox is endemic, but atypical presentations, such as genital rash as the first symptom and lack of distinct febrile prodrome, have been reported from Nigeria," Ogoina wrote.

Ogoina also pointed to a on monkeypox in a cohort of 16 countries that found the virus in the seminal fluid in 29 of 32 tested samples. That study reported complications such as conjunctival lesions, acute kidney injury, and a self-limiting myocarditis, which were not observed by the researchers in Spain.

Mitjà and co-authors noted that 32 individuals in their cohort (18%) acquired monkeypox infection despite a smallpox vaccination history, which "warrants further investigation to better understand the protection provided by vaccination in the context of the current outbreak."

Limitations of the study included no testing of other bodily fluids such as blood and semen, "although it is still unclear whether these bodily fluids contribute to monkeypox virus transmission," Ogoina pointed out.

"The authors also did not evaluate longitudinal changes in cycle threshold values that could have differential peaks at the different lesion sites during the natural history of human monkeypox," he added.

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