#3: Decades of change

This week: How to eradicate a disease without a vaccine, earnings over a lifetime, and some fun podcasts.

This is my third post of Scientific Discovery, a weekly newsletter where I’ll share great new scientific research that you may have missed. Check out the About page if you’re interested in why I’m writing this.

Highlights

#1: Guinea worm disease is close to being eradicated worldwide

Post: Guinea worm disease is close to being eradicated – how was this progress achieved? (Our World in Data, 2022)
Content warning: you might want to avoid reading this if you’re eating or feel squeamish about worms.

You often hear that smallpox is the only human disease¹ that’s been eradicated. That will soon no longer be true, and I’ve written a post about it here for Our World in Data.

Here’s the lowdown.

In the late 1980s, when the eradication campaign began, there were nearly a million cases of guinea worm disease reported worldwide. Last year, there were only 15.

The disease has been recorded in history for thousands of years. It’s spread by drinking contaminated water that contains guinea worm larvae. Their larvae tend to be found in water fleas in stagnant water – such as ponds, cisterns, open wells and lakes – that people might drink from if they’re unaware of the risk and don’t have access to clean drinking water.

Note that this chart shows the number of reported cases on a log scale. Our World in Data (2022)

It’s a terrible and debilitating disease. When people drink contaminated water, the worm’s larvae can invade their stomachs and intestines, and they slowly grow into mature adult worms. The worms crawl through their joints and bones, which causes arthritic conditions, and intense pain when it emerges – around a year later – through a blister in the skin.

People try to find relief from the pain by going into water, but unfortunately this allows the worm to release its larvae into the water – which restarts its lifecycle!

There are some ways to treat the symptoms of the disease. This is mainly by removing the worm with a stick carefully as it emerges, treating the pain with pain medication and treating secondary infections with antibiotics.

But there isn’t a vaccine against the disease.² Instead, this much progress has been made against the disease because we know how it spreads – through contaminated water.

In the 1980s, as part of the ‘international drinking water decade’, organisations including the US CDC, the Carter Center, the WHO and UNICEF began a global eradication campaign. Cases declined rapidly in many countries due to improvements in access to clean drinking water, provision of water filters, larvicides, and education about where to drink water safely.

Now, there are only a handful of countries with any cases of the disease. In 2021, there were 15 cases reported in total, and 8 of them were in Chad.

Countries in blue have been certified free of guinea worm disease. This happens after they’ve reported zero cases through active surveillance for three consecutive years. Our World in Data (2022)

You can find out more in my post, but here’s some exclusive content that didn’t fit into it!

• The eradication campaign was a massive effort by many people and many organisations. But one person who played a big role was former US president Jimmy Carter, who at one point negotiated a 4 month long ceasefire during the Sudanese civil war specifically to help healthcare workers get eradication efforts underway.³

• Many sources say that this is an unusually easy disease to eliminate. Reportedly, it was eliminated in some villages in Pakistan during seasons of drought, which meant larvae in water just kind of… died out. Unfortunately, it was later reintroduced in some of these areas when there was mass migration during Partition.

• Occasionally, people say that the Rod of Asclepius (a symbol which is used in logos by many health organisations such as the WHO) actually shows a guinea worm on a stick, rather than a snake on a rod.⁴ I looked into this, and I’m pretty sure that’s not true – but there’s lots of detail on the history of the symbol here. The symbol is also often confused with the caduceus symbol, but it’s not that either!

• It will take an absolute minimum of 3 years for global eradication, but it’s more likely to happen some years after that.⁵ The remaining barriers are: containing outbreaks in dogs in some regions, violence and conflict, and insufficient monitoring in the Democratic Republic of Congo.

More links

#2: Men and women have wide differences in their lifetime earnings

Paper: Gender Inequality in Lifetime Earnings (de Castro Galvão, 2022)

This is a fascinating study about gender differences in earnings over a lifetime.

Many people report that women in the US earned a median of 58 cents for every dollar that men earned, in the 1960s. In 2020, the figure was estimated to be 83 cents.

But these estimates can be misinterpreted – gender differences in earnings overall are much wider than people might assume.

This is because those figures are based on the differences in earnings over a single year, among people who worked full-time. But women are less likely to work full-time than men, less likely to work extra hours, and more likely to drop out of the formal workforce.⁶

So how much do earnings diverge over a lifetime?

This great study, which uses a large sample followed up over time, makes estimates. Quoting the results: “Women born in the 1930s earned 25% of men’s average lifetime earnings. By the 1970s cohort, women earned roughly 56% of men’s lifetime earnings.”

de Castro Galvão (2022)

Overall, gender differences in lifetime earnings are getting smaller, but the pace of this narrowing has slowed down dramatically.

The study also explores specific changes. For example, for people born in the 1930s, getting to a later career stage meant smaller gender differences between their earnings. But the opposite was true for people born in the 1960s and 1970s: gender gaps in earnings widened later in life.

Some more findings: Women are working more hours over their lifetimes than in the past. The ‘motherhood penalty’ – the disadvantage mothers face in earnings – has lessened over time. Dropping out of the formal workforce made a much bigger impact in the past than it does for women born in the 1970s.

And there are many more interesting details in the paper.

Even more links

• How democratic is each country? How has this changed over time? And how do you measure that? My awesome colleague Bastian Herre at Our World in Data has created a new democracy explorer where you can compare many different measurements of democracy, and he has also written an explainer on how each measure works.

• I’ve been binging on an amazing podcast called Stuff You Missed in History Class; it’s really fun and informative. My favourite episodes are filled with drama: on the beheaded Doge of Venice and the life of Charles Ponzi. But there are many other great episodes, like on the history of rabies and the creation of timezones.

• Birds used to be a lot bigger than they are today, and many more were flightless. One example is the Terror bird, which actually looks terrifying and could run very fast. This episode of The Science of Birds podcast is all about giant birds, why they’re much smaller now, and why flying evolved.

And that’s all for this week!

As before, please let me know if you spot any mistakes or think I’ve missed anything important⁷ and I hope you’ll subscribe if you haven’t already :)

See you next time.

– Saloni

1. I specified human disease here because it’s not the only disease that’s been eradicated: rinderpest, a disease that infected cattle, buffalo and other animals, was also declared eradicated in 2011. Some people also argue that SARS1 was eradicated but, while I’m sympathetic to the argument, there was no global certification process to confirm it, so I won’t count that here.

2. Being infected by the guinea worm doesn’t seem to provide much immunity against reinfections, as some people were infected year after year. Many sources say that infections provide no immunity, but I haven’t found direct evidence for that. Regardless, a lack of acquired immunity from infection makes it more difficult to design an effective vaccine against the disease.

3. This is reported briefly in many places, but I’d like to read a deep-dive into how it all happened. If you know about it or know someone else who might, let me know!

4. I also shared this factoid unknowingly with one or two people before I looked into it. I’m sorry! I was wrong!

5. I’ll cautiously make a forecast about this, but you should know there’s still lots of uncertainty, and people on the ground will have a much better understanding than I do. Still, here are my estimates of the chances of eradication – I’ll assume eradication efforts continue at the same pace.
Eradication by 2025: <5% (This could happen if there were 0 reported cases in all countries for 3 consecutive years starting 2022. It’s unlikely because there have been small outbreaks in dogs in Chad and because the DRC has not had sufficient monitoring yet). Eradication between 2025–2030: 40%, between 2030–2035: 40%, after 2035: ~15%.

6. This represents the formal workforce – jobs where pay is recorded, rather than, for example, household work that goes unpaid.

7. I have open DMs on Twitter here, but you can also get in touch with me by commenting on this Substack post.