Natural selection in Africa: how sickle cell disease can be beneficial

Have you heard of Malaria or Sickle Cell Anaemia? You may not have, as they aren’t very common in Australia. Majority of people with these diseases’ all live in Africa. Why is this the case?

Malaria

Malaria is extremely common in Africa accounting for 94% of the world’s cases and deaths in 2019! This disease is caused by a parasite that gets passed from person to person through certain breeds of mosquitos. It causes a fever in all ages and can progressively get worse, leading to death.

How does this work?

Malaria infection is a cycle. The mosquito lays eggs, and feed them nutrients, which normally comes from our blood. But as soon as they bite someone who has Malaria, the mosquito is infected and acts as a vector for the disease. This means that every person the mosquito bites after infected will get the parasite and get malaria.

What happens after you’ve been bitten?

The parasite stays in the bloodstream for around an hour before making its way to the liver, which it will then move to the liver to multiply. It stays here for anywhere between 6 and 16 days, before moving back into the bloodstream and infecting red blood cells. The parasite divides in the blood cells, causing them to burst. During this time, you might not even have symptoms! But if a healthy mosquito bites you, you can spread the disease to them restarting the cycle.

Sickle Cell Anaemia

 

Sickle Cell Anaemia is a genetic condition, that occurs when you have two faulty copies of the HBB gene, which forms haemoglobin, the thing that carries oxygen around our bodies. Normally our red blood cells are nice and circle and are really good at delivering oxygen. When these faulty copies come, the red blood cells squish into a moon shape and make them unable to do their job. These cells die quickly as they can’t function properly, and get filtered through the spleen for removal.

Again, sickle cell anaemia is really common with 80% of global cases coming from Africa or from people who have migrated from Africa.

Why are these two diseases so common in Africa

It’s pretty bizarre that two life threatening diseases are so common in the same region, it makes you wonder if they’re linked at all.

The answer: yes.

Individuals who only have one faulty HBB gene are more immune to Malaria. Although it doesn’t stop the parasite from entering your body after being bitten by the mosquito, it does stop the parasite from taking over your whole body. This is because when the sickled red blood cells are getting cleared from the body, it takes the parasite with it, potentially removing the parasite within the first hour of infection!

This relationship between malaria and sickle cell anaemia is called natural selection, as the individuals with one ‘faulty’ HBB gene have a selective advantage to malaria as they are less likely to get sick.

However, it does have its downfall. As when two people who both have one faulty gene start a family, they have a 25% chance of their children having sickle cell anaemia, which can cause its own deadly health problems, separate from malaria.