Ebola and other tropical viruses

What is Ebola and what are the other tropical viruses?

There are five types of Ebola and while infection with some (such as Ebola Reston, found in the Western Pacific) are so mild that people rarely get symptoms, other types such as Ebola Zaire (found in Africa) may be rapidly deadly.

Ebola was identified for the first time in 1976 in the Democratic Republic of Congo, and in Southern Sudan. It is one of about 30 new diseases to affect humans over the past few decades.

Theories on the origins of these diseases are widespread but many of the most frightening appear to have emerged from sub-Saharan Africa. HIV, which leads to Aids, has been linked to a similar virus common in West African monkeys, and the first ever recorded HIV sample was taken from a man in what is now the Democratic Republic of Congo in 1959.

Ebola and a few other haemorrhagic fevers have been responsible for a tiny number of deaths compared to Aids, and the number of symptomatic cases reported outside Africa has been miniscule. But the devastating speed at which they strike and the far higher possibility of transmission from human to human have made the thought of a major outbreak a terrifying prospect.

However, Ebola is not the only viral haemorrhagic fever which claims lives in Africa, and beyond. Marburg fever gets its name from the town in Germany in which it broke out in 1967 and shares its symptoms with Ebola. It claimed seven lives from the 25 people infected in Marburg and Frankfurt. Those initially infected were laboratory workers exposed to African green monkeys which had been imported for research. But the majority of cases occur in Africa.

Other well-known haemorrhagic fevers are:

• Lassa fever - first noticed in the 1960s after an outbreak in Nigeria
• Rift Valley fever - mainly found in sub-Saharan Africa
• Congo-Crimean haemorrhagic fever - found in many parts of Africa, the Middle East and even warmer parts of the former Soviet Union, in which an outbreak is ongoing.

Causes and risk factors

Scientists first became aware of the potential of Ebola to destroy whole communities in the mid 1970s, when severe outbreaks in Sudan and the former Zaire killed a total of approximately 440 people. The Zaire strain of the virus is the most deadly to date, proving fatal in just under 90 per cent of those who contract it.

The virus is passed on through contact with blood, secretion or bodily fluids of an infected person – those with the disease start to haemorrhage and cough up or vomit blood, so in outbreaks the disease often spreads from patients to the health care workers looking after them.

Symptoms start to appear anytime from two to 21 days later.

However, how and why each outbreak starts is completely unknown. One theory is that there is a reservoir of the virus in bats, which are unaffected by it, and the virus passes from here to non-human primates such as chimpanzees who in turn pass it on to humans who come into contact with them.

By the time symptoms appear, the virus will have reproduced itself many times and spread through the blood to many organs. The major organs it affects are the liver, kidneys, spleen and reproductive organs.

Of the other haemorrhagic fevers, Lassa fever is spread from rodents which are the natural host. Rift Valley fever is spread by mosquitoes, whilst Congo-Crimean haemorrhagic fever is spread by ticks

Symptoms

Very often, flu-like symptoms such as a sore throat, headache and high temperature are the first sign of infection. This is followed by nausea, vomiting and diarrhoea.

The person affected may start to become delirious and dehydrated, with impaired kidney or liver function. They can begin to bleed internally, either from the major organs themselves or from tiny blood vessels, the digestive tract and gums. Eventually, this can cause enough blood loss to cause shock and respiratory problems, leading quickly in many cases to death.

Treatment

There is still no specific treatment for Ebola - no standard anti-viral therapies such as interferon have any effect. A vaccine has been produced that was 100 per cent effective in protecting a group of monkeys from the disease, but attempts to replicate the success in humans have so far proved unsuccessful. At present if someone beats Ebola, they do it by themselves, albeit with intensive medical support with intravenous fluids, and/or blood transfusions, or oral rehydration with electrolyte solutions. Survivors can be expected to make a full recovery, although occasionally reversible personality changes have been noted in such lucky patients.

Careful barrier nursing and avoidance of contamination with infected body fluids is still the best way to limit an outbreak.

Treatments in development

Scientists have developed vaccinations against both Ebola and Marburg which work on laboratory animals, and there are promising signs of some therapies that can be used on people affected. Some experiments use antibodies from the marrow of Ebola survivors. Much of the scientific work underway is focused on finding the original source of the disease - the reservoir. One project examined thousands of animals in the rainforests of West Africa in a bid to isolate those hosting the virus.

Some scientists say that the growing numbers of so-called emerging diseases are due to increasing forays by humans into the tropical forests. This brings them into contact with new creatures - and new infections - making it possible there could be even more powerful viruses waiting to play havoc in the human body.