Transmission electron micrographs of SARS-CoV-2, monkeypox, Ebola and Marburg virus particles. Images: NIAID/Flickr, CC BY 2.0
- Why are we seeing so many viral disease outbreaks? The more optimistic reason could be testing bias in a world already sensitised by COVID-19.
- A majority of monkeypox cases worldwide have been identified among those seeking medical care and among their contacts.
- There is also a more worrying, possibly more realistic scenario. All these viruses (except poliovirus) have reservoirs in wild animals and have ‘jumped’ to humans.
- In their new niche, the viruses continue to evolve as we are seeing with waves of emerging novel coronavirus variants.
- In the past 50 years, researchers have identified about 1,500 new pathogens. Of them, about half are viruses, and over 75% of these are zoonoses
The WHO maintains a running list of health emergencies and disease outbreak news. A cursory look will tell the discerning viewer that human-made situations are responsible for most of them. Conflicts have precipitated refugee crises, and most recently a global energy and food crisis. As human activities take global warming to a tipping point, climate change creates further opportunities for disease emergence and spread.
The ongoing COVID-19 pandemic has also made matters worse in multiple ways beyond healthcare.
On Saturday, the WHO declared monkeypox disease a ‘public health emergency of international concern’, its highest level of alert. As of July 20, more than 16,500 cases of monkeypox had been reported from 74 countries.
The world is now confronting two viral diseases as global health emergencies: COVID-19 and monkeypox. Further, sporadic outbreaks of Ebola virus have persisted since 2013, when the virus re-emerged in West and Central Africa, and two cases of Marburg virus were reported recently from Ghana in West Africa.
Several recent detections of wild type poliovirus in the southeast region of Africa is a grim reminder that the world is not free from polio and there remains high risk of its international spread. Reduced immunisation during COVID-19 has further exacerbated this risk.
‘Monkeypox’ is a misnomer. The virus was discovered in 1958 in research monkeys, but its natural hosts are rodents and other small mammals. It belongs to the same family of viruses as the one that causes smallpox – the only human infectious disease eradicated thus far, following intense global vaccination and public health campaigns in the 1960s and 1970s.
The virus is endemic in West and Central Africa, transmitting to humans from either rodents or from infected humans through close contact and respiratory droplets.
Prior to this year, the few cases of monkeypox seen outside Africa were all associated with travel or contact with rodents imported from Africa. The largest such outbreak was in the US in 2003, when 47 people were infected by pet prairie dogs, and traced to rodents imported from Ghana.
The 2022 outbreak is different. The virus appears to have found a niche for human-to-human transmission in gay, bisexual and MSM (men who have sex with men) populations, with most cases reported among men aged 20-45-years old. Wider community spread is already being observed in the affected countries.
Unlike COVID-19, monkeypox is a known disease with tools available to control it. The age distribution of cases shows that immune memory from smallpox vaccination, which was discontinued globally in 1980, but in the early- to mid-1970s in most Western countries, protects from symptomatic disease.
Stockpiles of first, second and third generation smallpox vaccines, with increasingly better safety profiles, are available with the WHO and several countries. An FDA-approved drug called Tecovirimat is also available to treat smallpox and other non-variola poxviruses, including monkeypox. Both will have to be scaled up and deployed to control this outbreak.
The Ebola and Marburg viruses belong to the family of filoviruses that have fruit bats as their natural hosts, or reservoirs, cause haemorrhagic disease and transmit efficiently between humans through infected body fluids.
Why are we seeing so many viral disease outbreaks? The more optimistic reason could be testing bias in a world already sensitised by COVID-19, in which people seek medical care at the first sign of any symptoms. A majority of monkeypox cases worldwide have been identified among those seeking medical care and among their contacts.
However, there is also a more worrying and possibly more realistic scenario. All these viruses (except poliovirus) have reservoirs in wild animals and have jumped to humans either directly or via domestic animals. In their new niche, the viruses continue to evolve as we are seeing with waves of emerging novel coronavirus variants.
In the past 50 years, researchers have identified about 1,500 new pathogens. Of them, about half are viruses, and over 75% of these are zoonoses – i.e. they have animal reservoirs from which they spread in humans. Increasing contact between wild animals on one hand and domestic animals and humans on the other makes this jump possible.
Deforestation causes loss of habitat, increasing the frequency with which wild animals come into contact with domestic animals and humans. The emergence of the Nipah virus in Malaysia, Bangladesh and India has been traced to fruit bats passing the viruses on through half-eaten fruits, tree sap and surface contamination to pigs and humans on farms in the vicinity of forests.
A study published on April 28 this year predicted that climate change could lead to over 15,000 instances of diseases crossing into new species in the next 50 years. It estimated that even if the average global temperature rise is kept to less than 2º C from pre-industrial times, more than 300,000 mammalian species will meet for the first time, making it possible for the viruses they carry to mingle and make the jumps.
The Democratic Republic of Congo (DRC) has reported a majority of the world’s monkeypox cases in past years. High biodiversity, a deforestation rate of about 0.25% to 0.4% per year, and increasing pressure on land caused by refugees from war-affected neighbouring South Sudan and Central African Republic, have all increased the risk of zoonoses.
The DRC also holds about 50% of the world’s known cobalt deposits and accounts for about 70% of its production. As the world shifts to renewable energy and electric cars to mitigate the climate crisis, it will require more batteries, which means more cobalt mining, more deforestation and increased risk of zoonotic transfers.
Have we caused enough deforestation to reach a tipping point? Have temperatures gone up and climate patterns changed enough to speed up the spread?
The link between climate change and its mitigation on the one hand and the emergence of new diseases is a complex web. It has no quick solutions, but will require long-term changes to the way we live – based on sustainability, equity and fairness. This challenge will only get bigger and more complex with time.
Note: This article was edited at 3:37 pm on July 26, 2022, to clarify that there two Ebola vaccines.
Shahid Jameel is an Indian virologist. He is currently a fellow at OCIS and Green Templeton College, University of Oxford, UK.