June 23 (Reuters) - Smallpox medicine makers Siga Technologies and Chimerix Inc said on Thursday they had received new orders for their antiviral drugs that are expected to be used as monkeypox treatments amid a global rise in infections.

More than 3,000 confirmed cases of monkeypox - which belongs to the family of orthopoxvirus that includes smallpox and cowpox - have been reported from 40 countries where the disease is not endemic.

With the numbers rising, the World Health Organization is set to decide on Thursday whether to declare monkeypox a global health emergency.

Siga received orders worth about $13 million for its oral smallpox drug, Tpoxx, and Chimerix got an order worth $9.3 million for its drug Tembexa.

Shares of Siga rose 4%, while Chimerix's shares were up 5%.

Chimerix, which in May agreed to sell the rights of the drug to Emergent BioSolutions, said on Thursday the contract for new orders is expected to be completed before the sale goes through.

Siga's drug, Tpoxx, is approved in the European Union to treat diseases including smallpox, monkeypox and cowpox, while it is only approved to treat smallpox in the United States and Canada.

"We expect a portion of the courses of oral Tpoxx delivered under these orders will be used for the treatment of active monkeypox cases," Siga Chief Executive Officer Phil Gomez said.

(Reporting by Leroy Leo in Bengaluru; Editing by Amy Caren Daniel and Shinjini Ganguli)