The monkeypox virus responsible for outbreaks in multiple countries over the past two months shows an excessive number of genetic mutations, indicating “accelerated evolution” and possible adaptation to humans, a team of scientists said on Friday.

The scientists have detected what they said were “the first signs of microevolution” of the monkeypox virus during human-to-human transmission and a key genetic mutation that could explain the increased transmission of this strain.

Monkeypox is a rare disease of animal origin caused by a virus whose natural reservoir remains unknown. But animals such as non-human primates and rodents might harbour the virus and have caused occasional spillovers in humans, mainly in Africa.

But since early May, over 4,000 monkeypox cases have turned up across Europe, North America and South America and a World Health Organisation expert committee met on Thursday to assess whether the outbreak constitutes a public health emergency.

India has not documented any monkeypox case so far, but the Union health ministry had last month released guidelines for surveillance, rapid detection and isolation of cases.

Now, a genomic study by scientists in Portugal has indicated that the monkeypox virus that has caused the 2022 outbreak most likely has a single origin, a 2017-18 outbreak in one endemic country, possibly Nigeria.

The monkeypox virus detected in 2022 potentially represents “the continuous circulation and evolution of the virus that caused the 2017-18 Nigeria outbreak,” the researchers said, describing their findings on Friday in the journal Nature Medicine.

The 2022 monkeypox virus also diverges from the related 2018-19 viruses by around 50 genetic variations — or far more than expected for this class of viruses — implying ongoing accelerated evolution. Their analysis has also revealed the first signs of what they said was ongoing microevolution — 15 minor variants or gene deletions during the ongoing human-to-human transmission.

“These are interesting observations but anticipated,” said Sudanshu Vrati, a virologist and executive director of the Regional Centre for Biotechnology in Faridabad (Haryana), who was not associated with the study.

The pace at which the 2022 monkeypox strain has moved and spread raises the question whether it has acquired certain mutations that have enhanced its capacity to spread from human to human, Vrati said.

But Vrati and others say monkeypox infection, which spreads mainly through close contact, would be easier to control than respiratory infections such as Covid-19.

The analysis from Portugal also points to possible human adaptation, potentially driven by APOBEC3, a set of human enzymes that modifies the viral genome to prevent viral infection.

“What we observed is that the mutations generated by APOBEC3 in the 2022 monkeypox virus affect viral proteins known to interact with the human immune system,” said Joao Paulo Gomes, the biologist who led the study.

Krishanpal Karmodiya, an infectious disease biologist at the Indian Institute of Science Education and Research, Pune, said the study would evoke questions whether the mutations in the virus might in some way give it some advantage against human immune defences.