The recent declaration of the zika virus as a “global emergency” by the WHO as a consequence of the epidemic in Latin America highlights once more the threat that emerging diseases represent for public health. It is not easy to predict when and where new infectious diseases will appear, but what we do know is that, with climate change and globalization, new infectious threats such as zika will continue to emerge and spread.
All of them are virus of which relatively little is known and against which there are no approved drugs or vaccines
This is why, ends of last year, the WHO brought together scientists and public health experts in order to establish a list of the emerging pathogens most likely to cause the next pandemia. What do they have in common? All of them are virus of which relatively little is known and against which there are no approved drugs or vaccines. They are called emerging because, even if they have existed for thousands of years in animal reservoirs, they were recently identified (over the last decades) after causing disease in humans. They are feared for their high mortality rate and because, even if they are only moderately contagious, certain hospital-associated settings or mutations could increase their human to human transmission.
1 & 2. Ebola and Marburg disease
The Ebola virus was first identified in 1976 and the Marburg virus in 1967. Both can jump from their natural reservoir (bats) to human and non-human primates. The virus can then spread to other humans through direct contact with blood and bodily fluids. Both diseases are characterized by fever, diarrhoea, vomiting and bleeding, and have a case fatality rate that varies between 25 and 90%. There are 5 species of Ebola virus, of which the Zaire species has caused the most outbreaks and deaths.
There is no approved treatment for Ebola, although two candidate vaccines are being currently evaluated
There is no approved treatment for Ebola, although two candidate vaccines are being currently evaluated. The first Ebola outbreaks occurred in rural areas of Central Africa, but the last outbreak, the largest ever, spread explosively in urban zones of West Africa. The Marburg virus is also endemic to Africa and has caused outbreaks in Uganda, Democratic Republic of Congo, Kenya and South Africa.
3. Lassa Fever
The Lassa fever virus was first identified in 1950 and its reservoir is the multimammate rat. It is transmitted to humans through the inhalation of aerosols or the ingestion of food contaminated by rat urine or droppings. Human to human transmission may also occur after exposure to blood or bodily fluids and it was estimated that up to 20% of Lassa fever cases may be due to this kind of transmission, with some individuals acting as “super spreaders”. Even though 80% of infected people do not develop symptoms, one out of five infections may be severe: 15 to 20% of patients hospitalized with Lassa fever die from the illness. There is no vaccine, although an antiviral treatment has proved to be relatively efficient. The disease is endemic in West Africa.
4. Crimean-Congo Haemorrhagic Fever (CCHF)
The CCHF virus, first identified in Crimea in 1944, is transmitted to humans by ticks on domestic animals such as cattle. Human to human transmission can also occur through exposure with bodily fluids. The virus causes a hemorragic fever with a case fatality rate of up to 50%. The disease is endemic in countries below 5º north latitude, in Africa, the Balkans, Middle East and Asia.
5. Rift Valley Fever Virus
The RVF virus was identified for the first time in Kenya in 1931 and, although it mainly affects animals, it can also infect humans. Its lethality rate is very low (1%) but the virus can occassionally cause a severe form of the disease characterized by ocular disorders, encephalitis and/or haemorrhagic fever, in which case the mortality is of 50%. The vast majority of human infections occur through the handling or ingestion of blood, organs or milk of infected animals.
Although the RVF virus mainly affects animals, it can also infect humans. Its lethality rate is very low
Additionally, infections can also result from the bites of infected Aedes mosquitoes or flies. To date, no human to human transmission has been reported. A series of candidate vaccines against RVF are currently being tested. Outbreaks have been reported in Africa, Yemen and Saudi Arabia.
The coronavirus that causes Middle East Respiratory Syndrome was first identified in Saudi Arabia in 2012. Camels seem to be an important virus reservoir, although most cases reported are due to human to human transmission, almost exclusively in hospital settings. It is a mildly contagious virus that is transmitted upon close exposure to respiratory secretions, such as coughing. Its mortality rate is around 40% and it affects mainly older elderly people with underlying medical conditions. So far, all reported cases are linked to countries in the Arabian Peninsula, although a recent large outbreak in South Korea, triggered by an imported case, confirms the pandemic potential pandemic of the virus.
The Severe Acute Respiratory Syndrome is caused by another coronavirus, similar to MERS. Its natural reservoir is not known with certainty but is probably bats and cat-like mammals called civets. It was reported for the first time in 2003 in Asia, from where it rapidly spread to more than 12 countries in America, Europe and Asia causing 8,000 cases and more than 8,000 deaths. In contrast with MERS, SARS was transmitted outside hospital settings and affected mainly healthy, young people. Since 2004, no other SARS case has been reported.
8 & 9. Nipah and Hendra Virus
Both viruses belong to the same family and share the same reservoir: bats. Nipah virus was first isolated in 1999 after an encephalitis outbreak among pig farmers in Malaysia and Singapur. Since then, periodic outbreaks have been reported in Bangladesh. It causes mild disease in pigs, but severe disease in humans, killing 40% of infected people.
Nipah causes mild disease in pigs, but severe disease in humans, killing 40% of infected people
It is transmitted to humans by direct exposure to pigs, bats and/or date palm sap contaminated by infected bats. Transmission between humans, probably via saliva, has also been reported for the Bangladesh outbreaks. Concerning Hendra virus, one single outbreak of the disease was reported in Australia, in 1994, where horses and seven persons were affected, with a lethality rate of 60%. Australia is currently investigating the therapeutic efficacy of neutralizing antibodies against the virus.
Chikungunya and Zika
The chikungunya and zika viruses were not included in the list when it was established, but were classified as serious threats that require more R&D investment. In fact, due to the high association observed between foetal malformations and Zika infections in pregnant women, the WHO recently declared Zika a public health emergency.
This priority list of “dangerous suspects” should provide the basis for accelerating research and development in diagnosis, vaccines and treatments
Other diseases with epidemic potential such as tuberculosis, HIV/AIDS, malaria, avian influenza and dengue were not included because there are major ongoing control and research networks for these pathogens, as well as approved or pipeline treatments.
This priority list of “dangerous suspects”, that will be reviewed periodically, should provide the basis for accelerating research and development in diagnosis, vaccines and treatments against these pathogens, and hopefully avoid another sanitary and humanitarian crisis like the recent Ebola outbreak in West Africa.