99 Kenyans had Mpox antibodies in blood collected since 2013

Several months before Kenya announced its first Mpox case in July, a team of researchers visited a sex workers programme in Nairobi seeking stored blood.

This was not a surprise visit.

Sex Workers Outreach Programme (SWOP) clinics regularly host people coming to support health projects that help sex workers.

This time, the researchers were seeking evidence of Mpox virus in Kenya long before the current outbreak.

They tested 656 blood samples obtained from male and female sex workers enrolled at Swop clinics in Nairobi between 2013 and 2018.

They screened for antibodies to orthopoxvirus, the virus family that includes Mpox. At least 99 had these antibodies.

This is a sign the individuals had been exposed to an infection.

The researchers have narrowed down that infection to Mpox. They are confident the disease has been quietly spreading in Kenya for more than 10 years, long before the Ministry of Health announced the first case last month.

Of particular concern is the high positive cases among female sex workers (63) and men who have sex with men (16).

“Our data suggests that Mpox introduction among sex workers in Kenya probably occurred before identification of MPXV (monkeypox virus) reemergence in Nigeria [in 2017], the 2022 epidemic, and the ongoing outbreak in DRC,” they reported last week.

One blood sample from 2013 had antibodies. The others are from 2014 (five positive samples), 2015 and 2016 (three each), 2017 (61 positive samples) and 2018 (26).

The study was led by graduate students from the University of Manitoba, which supports the Swop clinics in Nairobi.

They reported their findings last week in the journal Emerging Infectious Diseases.

“We identified strong seropositivity among sex workers and gay, bisexual and other men who have sex with men,” they said in their paper, titled ‘Retrospective Seroprevalence of Orthopoxvirus Antibodies among Key Populations, Kenya’.

Mpox infection causes rashes all over the body. Most patients recover within two weeks without treatment.

“This is a wake-up call for public health officials,” one of the researchers said.

“The data suggests that there has been undetected Mpox transmission within Kenya’s key populations, which may have gone unrecognised due to the stigmatisation and fear of persecution that these groups often face.”

One of the challenges in identifying Mpox exposures lies in the antigenic similarity among human orthopoxviruses such as smallpox.

The blood samples tested are for individuals aged 19-69 years.

The researchers said that given that smallpox vaccination ended more than 50 years ago, the antibodies detected in younger individuals cannot be attributed to prior vaccination.

They also ruled out the possibility that the antibodies could be attributed to other orthopoxviruses like camelpox.

Although camelpox virus has been reported among camels in northern Kenya, few human orthopoxvirus infections have been reported in the region and zoonosis is rare.

Also, the clustering of seropositivity among key populations such as sex workers and gay people in Nairobi suggested the source of infection could not have been a camel.

“No other orthopoxviruses are known to infect humans in Kenya; thus, our serologic data suggest potential Mpox exposure,” the authors said.

The findings raise significant concerns about the potential for undiagnosed Mpox cases in Kenya and other regions of East Africa.

Kenya has announced four cases from the current wave of outbreak in East Africa.

The researchers are calling for urgent action to expand Mpox surveillance in Kenya, particularly among high-risk populations.

Kenya reported its first case on July 31. The patient was a 42-year-old long-distance truck driver, who lives in Kinoo, Kiambu county.

“Since the declaration of the first case of Mpox, 28 contacts of the said case have completed a 21-day follow-up period without developing symptoms. They have been discharged from active follow-up,” Health CS Deborah Mulongo said in a statement.

The Ministry of Health confirmed a second case of Mpox on August 23.

The second patient is also a truck driver. He was screened at the Malaba One Stop Border Post in Busia county, the CS said in a statement.

The patient, who has a history of travel to the Democratic Republic of Congo, an area currently battling a significant Mpox outbreak, is now in isolation and receiving treatment at a health facility in Busia.

The third and fourth cases were reported last week.