CHICAGO, Oct 10 (Reuters) - U.S. health officials have

identified a cluster of cases caused by an mpox variant that are

resistant to Siga Technologies' ( SIGA ) antiviral tecovirimat,

branded as TPOXX, in five U.S. states, federal and state

researchers reported on Thursday.

The cases involved a total of 18 individuals infected with

clade II mpox between Oct. 6, 2023, and Feb. 15, 2024, who had

never taken the treatment before, according to the report

published in the Centers for Disease Control and Prevention's

Morbidity and Mortality Weekly Report.

Clade II mpox cases, which are endemic to West Africa,

sparked a global outbreak in 2022 that prompted the World Health

Organization to declare a public health emergency, and cases

continue to spread at low levels in some countries.

The drug-resistant mpox cases follow an initial, unrelated

cluster reported in California in late 2022-early 2023, and are

the first involving multiple states, according to the report.

Health officials said the latest cases are likely to have

been underestimated because not all cases of mpox undergo

genetic sequencing to determine the variant causing the

infection.

Study authors said routine surveillance is needed to monitor

for the emergence of drug resistance to safeguard the

effectiveness of TPOXX, one of the few treatments approved for

treatment of mpox.

CDC officials advised doctors to closely follow the protocol

for TPOXX use, ensure that patients take the drug properly and

stress that they can still spread the virus while taking the

drug. The findings also underscore the need for additional mpox

treatments, the study authors said.

The clade II strain is separate from the ongoing outbreak in

Africa of a new variant called clade Ib, which prompted the WHO

to declare a second public health emergency in August.

A U.S. government-backed study evaluating the use of TPOXX

in clade Ib cases released in August showed the treatment was

not effective at helping patients recover more quickly than a

dummy treatment.

Mpox, formerly known as monkeypox, is a viral infection that

causes flu-like symptoms and pus-filled lesions. Although

usually mild, the virus can be deadly in some individuals with

weakened immune symptoms. Mpox is transmitted through close

physical contact, including sexual contact.