Leprosy in Wild Chimpanzees- Should we be worried?

On 11th November, 2020 an article in science reported the first confirmed cases of Leprosy in wild chimpanzees ( https://www.sciencemag.org/news/2020/11/leprosy-ancient-scourge-humans-found-assail-wild-chimpanzees ).

Leprosy is one of the oldest infectious diseases reported in history and is on the WHO list of neglected tropical diseases. It is highly infectious that in ancient times when there was no cure, quarantine was the only way of preventing the spread of the infection within communities. Leprosy colonies were established where any person infected with the disease was sent to spend the rest of their life and were restricted to contact with non infected individuals. Today, the picture of leprosy has changed, the disease is rare and those infected are treated with antibiotics. With treatment the infected individual is expected not to be infectious and can not spread the disease to others. According to WHO official figures from 159 countries, there were 208 619 new leprosy cases registered globally in 2018.

Leprosy (Hansen’s disease), is caused by Mycobacterium leprae. It is a chronic disease which affects the mucosal surfaces of the upper respiratory tract, skin, peripheral nerves, and the eyes (https://www.who.int/health-topics/leprosy#tab=tab_1).

Leprosy affects people of all ages, even though in ancient times the disease was incurable, today leprosy is curable, with early treatment believed to prevent disability. Transmission of leprosy is likely through close and frequent contact with an infected individual via droplets from the nose and mouth.

In 2018, there were 184212 cases of leprosy giving a prevalence of 0.2/10 000. Over the last two decades over 16 million people have been treated for leprosy. In 2015, the distribution of new leprosy cases is shown in Figure 1.

WHO | Epidemiology
Figure 1: Global distribution of leprosy, 2015

Why should we be concerned with leprosy being found in Chimpanzees?

In 2005 a comprehensive comparison of the human and chimpanzee genome showed that these genomes were 99% identical with 96% perfect identity (https://www.nature.com/articles/nature04072). This close identity between the genomes implies that genetically there are things that are shared between humans and chimpanzees, including diseases. One example is Monkeypox disease which has been reported in wild chimpanzees in Taï National Park, Ivory Coast (https://www.nature.com/articles/s41564-020-0706-0), but also has caused several human outbreaks in several parts of Africa including Ivory Coast (Figure 2).

Figure 2; Distribution of Monkeypox disease

Could the discovery of Leprosy in Chimpanzees indicate a new turn in human leprosy? After an individual is infected by M. leprae, symptoms may occur within 1 year or may take up to 20 years or or over in others.

  1. Is it possible that some of these strains are already circulating in humans?
  2. Is the Chimpanzee leprosy strain going to jump to humans?
  3. If it does jump to humans will it be able to cause disease?
  4. If yes, how severe would the disease be?

Different genotypes of M.leprae have been sequenced, the 3I genotype from humans, nine banded armadillos and red squirels; the 2F genotype from Chimpanzees (Ivory Coast) and the 4N/O strain from Chimpanzees (Guinea Bissau) (Figure 3).

Figure 3; Genotypes of M. leprae in humans and Chimpanzee

Categories infectious disease

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