Sierra Leone has lost a fourth doctor to Ebola after a failed effort to transfer her abroad for medical treatment, a government official said Sunday, a huge setback to the impoverished country that is battling the virulent disease amid a shortage of health care workers.

Dr. Olivet Buck died late Saturday, hours after the World Health Organization said it could not help medically transport her to Germany, chief medical officer Dr. Brima Kargbo confirmed to The Associated Press.

Sierra Leone had requested funds from WHO to transport Buck to Europe, saying the country could not afford to lose another doctor.

WHO had said that it could not meet the request but instead would work to give Buck "the best care possible" in Sierra Leone, including possible access to experimental drugs.

Half of Ebola-infected doctors have died

Ebola is spread through direct contact with the bodily fluids of sick patients, making doctors and nurses especially vulnerable to contracting the virus, which has no vaccine or approved treatment.

More than 300 health workers have become infected with Ebola in Guinea, Liberia and Sierra Leone. Nearly half of them have died, according to WHO.

The infections have exacerbated shortages of doctors and nurses in West African countries that were already low on skilled health personnel.

Sierra Leone's efforts to fight the Ebola epidemic suffered a blow when Sheik Umar Khan, the country's leading Ebola doctor, died of the virus in July. (Umaru Fofana/Reuters)

So far, only foreign health and aid workers have been evacuated abroad from Sierra Leone and Liberia for treatment.

Dr. Sheik Humarr Khan, Sierra Leone's top Ebola doctor, was being considered for evacuation to a European country when he died of the disease in late July.

Cuba's health ministry announced Friday it will send more than 160 health workers to fight Ebola in Sierra Leone, a move that WHO Director-General Dr. Margaret Chan said would "make a significant difference."

Liberian president pleads for U.S. assistance

West African countries hardest hit by the Ebola outbreak are falling short on treatment and infection control measures because of a lack of resources and medical staff.

In Liberia, the country that has recorded the highest number of Ebola cases and deaths, officials on Saturday released a letter that President Ellen Johnson Sirleaf sent this week to U.S. President Barack Obama appealing for a dramatic increase in support to address an emergency she said "threatens civil order."

Liberia President Ellen Johnson Sirleaf, right, demonstrates how to wash hands properly in order to prevent the spread of the Ebola virus earlier this summer. Sirleaf appealed to the U.S. for help fighting the epidemic. (Abbas Dulleh/Associated Press)

Sirleaf described how her country's health system had been swiftly overwhelmed by the outbreak, and how doctors were afraid to go to work after seeing their colleagues die. "Diseases that were treated with relative ease pre-Ebola now take lives because of the pall that Ebola has cast over our health system," she said.

In the letter dated Tuesday, Sirleaf asked for the U.S. to set up and operate at least one Ebola treatment unit in the capital, Monrovia, and to help restore services at 10 non-Ebola hospitals. She also requested help in maintaining "air bridges" to get supplies and personnel into the country, noting that many airlines have canceled flights in and out of Monrovia.

"I am being honest with you when I say that at this rate, we will never break the transmission chain and the virus will overwhelm us," Sirleaf said.