Two cases of swine flu resistant to the drug Tamiflu were diagnosed over the weekend, the U.S. Centers for Disease Control and Prevention said on Saturday. Tamiflu and Relenza were the only two medications that had thusfar proven effective in treating the virus (also known as H1N1), which already proved resistant to other seasonal flu remedies.

Tamiflu had been particularly popular, as it is ingested orally. Relenza, which is inhaled, is therefore not recommended for patients with respiratory conditions. Both drugs work by disabling an enzyme the virus needs in order to grow. Many governments in the world, including Israel's, have stockpiled Tamiflu in large quantities since the swine-flu pandemic began.

Reports of Tamiflu-resistant strains of swine flu began in late June. In patients with weakened immune systems the virus might stay active for longer periods, acquiring resistance to Tamiflu and potentially spreading to others.

At the end of June, a representative of the Swiss pharmaceutical company Roche, which manufactures Tamiflu, said a Danish patient infected with a strain of the virus was not responding to the medicine. Earlier studies on Tamiflu indicated that 0.4 percent of adults and 4 percent of children afflicted with normal seasonal flu were likely not to respond to the drug.

Ten more such patients were identified in the world since June, with three in Japan, one in Singapore, one in China, one in Denmark, one in Canada, and two in the United States. However, no patient-to-patient transmission of the Tamiflu-resistant strain has yet been documented.

The two U.S. cases reported over the weekend were documented in Seattle. The swine flu patients were said to be suffering from leukemia and a depleted immune system. One patient is a young man who received a marrow transplant in May, and was said to be recovering at home; the other is a woman in her forties, who is still hospitalized. The two were said to have not been in contact, nor has it been documented that they have infected family members or medical staff with the Tamiflu-resistant strain.

The World Health Organization said it was following the reports, but no changes were yet made to its guidelines on treating the virus.

No Tamiflu-resistant strains have been recorded in Israel, where five more labs for testing for the disease are soon to be allocated by the health ministry. "The spread of a drug-resistant strain is usually slow, and by the time it spreads significantly vaccines against the disease will, in all likelihood, be developed. However, Tamiflu is one of the only medicines that can slow or weaken the disease, so it's crucial that swine flu react to it," said Dr. Amnon Kiro, chief of influenza research at the Israeli Pediatric Association research network.

At the moment, checks for swine flu are only carried out at the Health Ministry laboratory in Tel Hashomer, and in a laboratory at Hadassah University Hospital, Ein Kerem in Jerusalem. The checks are only meant to identify the virus, rather than test its response to Tamiflu.

Thirteen Israelis are presently in intensive care due to swine flu, and seven fatalities have so far been reported.