The mystery of the missing disease
No one knows why yellow fever doesn't occur in India. And it's both puzzling and mildly disconcerting
Dear Friends!
It’s been a long time, though in my defense I was traveling nonstop for the past few weeks. Let me just say that it feels weird to be waking up in my own bed this weekend after subsequent weekends in Jaipur, Delhi, Medinipur, and Chicago. And I’ll say this - if you live in Jaipur, then I envy you. If you’ve never been, then you should visit. It really is one of the most remarkable places (not just in India) but in the world. Next time I’m in India visiting family or for work, I’ll be sure to make a side trip to Rajasthan again - perhaps this time to Udaipur or Jodhpur.
Now on to the sciencey-weincey stuff.
This one is a headscratcher. Let me first set the stage.
This is the distribution of the Aedes aegypti mosquito. Take a look at it for a moment. The red areas are those that where people have the highest chance of encountering this kind of mosquito.
This parasite is found across Africa and most of South America. It’s also recently been introduced to the southern parts of the United States. In Asia, the mosquito is prevalent across South and South East Asia.
India is a hotspot for this mosquito where it is notorious for spreading dengue. This mosquito also transmits yellow fever, another viral disease caused by a virus from the same family. Fortunately, there is a vaccine for yellow fever, but it is available in limited stocks.
Now, let’s look at the global distribution of yellow fever across the world. Notice anything different?
Where is yellow fever in India? In fact, where is it Asia?
Not that I’m complaining, of course, but yellow fever is missing. And the absence is an unsolved medical mystery. This is the focus of my most recent science column for Hindustan Times.
Yellow fever is a serious public health concern in sub-Saharan Africa, which epidemics occur from time to time. Thirty-two countries in Africa, with a collective population of 610 million people are at risk. It is also endemic to South and Central America, where urban areas in the tropical regions are at elevated risk. The World Health Organization is concerned that “Latin America is now at greater risk of urban epidemics than at any time in the past 50 years.”
The virus, which is related to dengue, was not always a scourge in the western hemisphere. Spread of yellow fever to new regions of the world has historically been linked to migration and urbanization.
In fact, yellow fever was introduced in large numbers by the mass migration of humans from Africa to the Americas through slavery. The Atlantic slave trade led to spread of the disease and devastating epidemics in the Americas. From there, it spread and caused epidemics in Spain, France, England, and Italy.
Such migration on large scale has not occurred from Africa to Asia, but there is still an unsolved medical mystery waiting to be solved. Despite years of travel from regions of the world where yellow fever is prevalent, there has never been a major outbreak of the disease in India or anywhere else in Asia. This despite the fact that yellow fever is related to dengue and the mosquito that spreads both diseases is rampant in India. No one really knows why yellow fever never caught on.
All the factors of climate and environment that are present in tropical and subtropical regions of Africa and South America where yellow fever is a scourge are also present in India. In fact, dengue, which is caused by a related virus in a similar manner is endemic to India. And in 2016, the first documented cases of yellow fever in Asia were reported from travelers. Further spread has not been documented, but it may only be a matter of time before new cases are reported.
In Asia, there are around 2 billion people who live in areas where the mosquito vector for yellow fever is prevalent. The WHO cautions that there is potential for outbreaks because of the density of the mosquito vector.
We only need to look at the recent past to see how two other viruses spread by the Aedes aegypti mosquito, Zika and chikungunya, can gain a foothold in parts of the world where there are people whose immune systems have not encountered these viruses.
As we observed with COVID-19, unprecedented air travel has blurred international boundaries and led to the rapid spread of infectious diseases. And with the current outbreak of monkeypox, a disease endemic to parts of Africa that we had not seen elsewhere, we have real-time spread of a disease beyond its recent boundaries.
We cannot ignore the risk of yellow fever, especially when all the ingredients for an epidemic are in place. We must be vigilant.
That’s it for this week. I’m dipping my toes back into the water now so I’ll keep the newsletter short for a while.
Until next time, here’s a photo from my travels during the past five weeks. Fatehpur Sikri is gorgeous and bereft of tourists right now (but you’ll have to navigate the scammers and touts to get to the sights).
Take care,
Anirban
Is it possible Asians are naturally immune? Or the mosquitoes in Asia don't carry the virus? Both appear odd, but just to check if it has been ruled out or not.