Few diseases cause as much fear as Ebola, which was first discovered nearly four decades ago and has been occurring ever more frequently.

Five strains of the haemorrhagic fever are found in humans and primates; though fruit bats are believed to be the reservoir for the disease, since primates die if they get it.

Much of the information about Ebola is bleak, but, more positively, it has been shown to be genetically stable – unlike the flu virus which frequently mutates.

However, the latest outbreak is particularly worrying on a number of fronts, says Prof Tom Solomon of the University of Liverpool. “Firstly, it is larger than previous outbreaks. More than 670 people have died. Secondly, it appears to be spreading between different geographical places more easily than other outbreaks. So far it has spread from Guinea to Sierra Leone, Liberia, and now Nigeria, ” he says.

Symptoms

Doctors have documented cases of infection passed on from the handling of infected chimpanzees, gorillas, fruit bats, monkeys, forest antelope and porcupines.

Up to this week, there have been nearly 3,700 known cases since 1976, says Prof Andrew Easton of the University of Warwick. Of that number, 2,262 have died: “That’s an overall fatality rate of 62 per cent. The current outbreak with over 1,200 cases is the largest seen,” says Prof Easton.

Varying death rate

So far, the mortality rate in the current outbreak stands at 67 per cent. Early symptoms following an incubation of between two and 21 days include fever, headache, muscle pain, or a sore throat. Such symptoms are common so recognising infection early is difficult. Diagnostic tests are possible if suspicions do exist, though not often in west Africa.

The later symptoms – by which time it is often too late to do much to help – include diarrhoea, vomiting, rash, failing kidneys and livers, stomach pain, internal and external bleeding. Death usually occurs 10 days after the patient has convulsions and enters a coma. The cause of death is multiple organ failure.

The disease is transmitted only by direct contact with the blood, secretions, organs or other bodily fluids of infected persons or animals, or from needles and clothing. Those most at risk – and this has been confirmed in the latest outbreak – are doctors and nurses, or family members trying to care for the ailing patient.

Fruit bats

Unusually, perhaps, Ebola can be passed on if a man who has recovered from the disease has sex within seven weeks because the virus stays alive in semen.

There are no treatments or vaccines available at present although a number of vaccines and drugs are in clinical trials on animals. Some are in the early round of testing on humans.

Testing on humans is particularly difficult since Ebola outbreaks generally occur and disappear quickly. Quarantine and early burials remain the best deterrents.

“While we know a great deal about the virus we have a long way to go in providing effective and accessible prevention and treatment for those at risk,” Prof Easton says.