Symptoms appear from two days to three weeks after infection, meaning an infected person who doesn't feel ill could board a plane, said Ben Neuman, a virologist at the University of Reading in Britain. Since Ebola in its earliest stages can resemble nothing more than flu, no one else would know either, he said.
"One person per plane load would have something that you would possibly call flu-like symptoms," Neuman said in a telephone interview. "Do you want to detain all those people coming into your airport? Do you have the manpower to do that, and send them all for testing? It's expensive, and difficult."
The question of trans-continent infection became more key this week after Patrick Sawyer, a Liberian government worker who flew to Lagos, Nigeria, on July 20, died from Ebola five days later. His death gave Africa's most populous nation its first case, a frightening hint at how global air travel may enable the spread of disease.
"We've known it's a risk ever since the beginning of the Ebola outbreak, and now it's finally happened," Neuman said. "He could have gone anywhere on this flight."
No special screening is being done at U.S. airports, Martin Cetron, director of global migration and quarantine at the U.S. Centers for Disease Control and Prevention in Atlanta, said Monday.
Ebolavirus first appeared in 1976 in two simultaneous outbreaks: in Nzara, Sudan, and in Yambuku, Democratic Republic of Congo, according to the World Health Organization. The latter was in a village near the Ebola River, from which the disease takes its name.
There are five species of ebolavirus, three of which are associated with causing large outbreaks in Africa: Bundibugyo, Sudan and Zaire, which is responsible for the current outbreak. Tai Forest ebolavirus is known to have caused only one non- lethal human case. That was in Ivory Coast in 1994, according to a log of outbreaks by the WHO.
The fifth species, Reston ebolavirus, was first identified in 1990 in the Washington suburb of Reston, Virginia, and has been found in the Philippines and China. While it can infect humans, no illness or death in people have been reported.
People who may have been exposed to the virus are generally monitored for fever for 21 days, the CDC said. Only about 50 percent of patients show extreme symptoms that can include bleeding from the eyes, ears and nose. Others have only a fever as an early symptom.
Historically, Ebola virus disease in Africa has killed up to 90 percent of those infected. In this outbreak, the fatality rate is nearer 60 percent, said Stephen Monroe, deputy director at the CDC's National Center for Emerging & Zoonotic Infectious Diseases.
Early treatment may lead to higher survival rates, the agency said. With no cure for the disease, treatment focuses on replenishing fluids, maintaining proper blood pressure, replacing lost blood, and treating related infections.
The agency is sending out a health alert to U.S. doctors and health-care providers that will focus of the importance of asking patients about their travel history and outlining the symptoms of Ebola virus disease.
Ebolavirus isn't carried in the air like tuberculosis or flu, said David Heymann, a professor at the London School of Hygiene and Tropical Medicine. It is spread by direct contact with an infected person's blood or other bodily secretions, including vomit, saliva or feces.
While an infected person who sneezes or coughs directly in another person's face could infect that person, Ebola primarily enters the body through tiny cuts or abrasions, or through mucus membranes of the eyes, nose, ears and mouth. The physical effect of the virus makes it less likely to spread through air travel, Neuman said.
"If you look at the numbers, there are probably about 300 people currently infected with Ebola virus, and most of those would be too ill to sit up or walk," Neuman said. "So the odds are small, but it is something that needs to be watched."
Health workers are more at risk than fellow travelers, doctors say, along with family members who touch or kiss the body of a victim during burial rites.
Two U.S. citizens have been infected: Kent Brantly, medical director of the Ebola center run by Samaritan's Purse in Liberia's capital of Monrovia, and Nancy Writebol, a worker at the center, the Boone, North Carolina-based charity said in a statement.
Both are both being treated at the center in Monrovia, and Brantly's family, now in the states, is under fever watch there in an "overabundance of caution," the CDC's Monroe during the briefing.
The U.S. is well prepared to handle infected patients with 20 CDC quarantine stations at U.S. airports that are designed to deal with anyone with symptoms of a wide range of infectious illnesses, including Ebola, the agency has said.
The State Department is closely tracking Ebola, is aware of reports about the two U.S. infected citizens and is updating travel warnings as information becomes available, said Jennifer Psaki, a department spokeswoman.
"In terms of what we're considering, I don't have anything to predict," Psaki said. "We're taking every precaution."
Lagos-based airline Arik Air said Monday it has temporarily suspended all flights to Liberia's capital, Monrovia, and Sierra Leone's capital, Freetown. Togo-based Asky Airlines, the carrier Sawyer took, said Tuesday it also suspended flights to the two cities.
Other airlines may take similar measures, said John Rose, chief operating officer at iJET International, which advises businesses on risk management in Africa.
"Airlines are asking the questions, 'At what point do we consider not flying to those countries? What happens if European countries shut off air space?' " Rose said by phone. "Those are realistic scenarios."
Government-level restrictions may also increase, including more detailed screening of arriving flight passengers and closing of borders and air space, he said.
Among global security situations iJET is monitoring, including Gaza and Ukraine, Ebola is at the forefront of the company's concerns as "Ebola is the one we have the least control over," Rose said.
The current outbreak has killed 672 people of 1,201 reported infections in Guinea, Sierra Leone and Liberia, the WHO said in a statement. Sawyer's death hasn't been officially confirmed as an Ebola case by the Geneva-based agency but is being regarded as probable.
Most of the cases have been reported in the region where the three countries' porous borders meet and people frequently cross without border checks by foot, vehicle or bicycle.
Fruit bats are thought to be Ebola's natural host, according to the WHO. Monkeys, chimpanzees, gorillas, porcupines and forest antelope can also be infected, and human outbreaks can begin when people eat the meat of infected animals.
The only case in which an Ebola victim was known to be taken out of Africa by aircraft came in 1994, when a 34-year-old Swiss zoologist became infected while performing an autopsy on a chimpanzee in Ivory Coast.
Eight days later, she developed chills, then diarrhea, an all-body rash and temporary memory loss. She was flown to Switzerland by air ambulance for treatment, accompanied by health workers wearing masks, gloves and gowns. She recovered without infecting anyone else.
The Ebola outbreak will be difficult to stop as it can easily "re-ignite" from even one case, the CDC's Monroe said. Even with no new cases, it would take 42 days for health workers to be sure the outbreak was over.
"The response to these outbreaks will be more of a marathon than a sprint," Monroe said.
_ With assistance from Makiko Kitamura in London, Nicole Gaouette in Washington and Jason Gale in Melbourne.
