Zoologists first diagnose leprosy in wild chimpanzees

Male western chimpanzee (Pan troglodytes verus) named Woodstock with leprosy plaques on his face.

Fabian H. Leendertz et al. / Nature, 2021

Zoologists first identified leprosy in wild chimpanzees. The sick individuals were found in the national parks of Guinea-Bissau and Côte d’Ivoire from 2015 to 2019, but the first cases of infection occurred at least in 2002. Although new infections usually enter chimpanzee populations from humans, it is highly likely that leprosy was transmitted to them from the small mammals they hunt or from the environment. The research results were published in an article for the journal Nature.

The number of great apes is rapidly declining due to the fault of man. The main problems faced by these primates are tropical deforestation and hunting. In addition, monkeys living near villages and towns or in contact with tourists often become infected with human infections and even die from them. For example, from humans to chimpanzees (Pan troglodytes) antibiotic-resistant bacteria can be transmitted.

In an effort to protect rare primates from human diseases, zoologists regularly monitor their health. To do this, scientists observe monkeys directly and with the help of camera traps, analyze their excrement and perform autopsies on dead individuals. Often, these routine procedures lead to unexpected and disturbing discoveries.

A team of researchers led by Fabian H. Leendertz of the Robert Koch Institute studied health data from Western chimpanzees (P. t. true) from two populations found in protected areas in West Africa. The first population inhabits the Cantanhez National Park in Guinea-Bissau, and the second in the Tai National Park in Côte d’Ivoire.

Chimpanzees from Cantanchez are not used to the presence of people, so they use automatic cameras to observe them. After analyzing 624,194 photographs and videos taken at 211 locations in the park from 2015 to 2019, Leenderz and his colleagues found 241 images of chimpanzees with severe lesions resembling symptoms of multibacillary leprosy. In the monkeys from these images and records, the body was covered with characteristic plaques and nodules, the hair partially fell out, the skin on the face was discolored, and the hands and feet were deformed. The researchers were able to identify four diseased individuals from three groups: two adult males and two adult females. In some of them, symptoms progressed over time, as in people suffering from leprosy.

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Until now, leprosy has only been reported in captive chimpanzees, not in wild ones. To confirm the presumptive diagnosis, the authors collected and examined fecal samples from Cantanche chimpanzees. In the litter of two females, they managed to find the genetic material of the causative agent of leprosy – bacteria Mycobacterium leprae… Thus, this infection is indeed present in the studied population. Most likely, it was she who caused the symptoms in four sick chimpanzees who got into the camera lens.

In Ivorian Tai Park, zoologists have been observing chimpanzees since 1979 – and they have long been accustomed to the presence of scientists. In 2018, experts discovered signs of leprosy in an adult male named Woodstock, belonging to one of three local groups (namely, the southern one). First, small plaques appeared on his ears, lips and under his eyes, which then spread all over his face. The male’s skin lost color, and the nails began to grow abnormally. DNA was found in all fecal samples from this chimpanzee M. leprae.

Since 2000, researchers have dissected the bodies of all chimpanzees that have died in Tai Park and preserved tissue samples. After examining tissue samples from the spleen of 38 individuals, Leenderz et al. Identified genetic traces of the causative agent of leprosy in two individuals. One of them, an adult female named Zora, was killed by a leopard in 2009. Judging by the photographs, already in 2007, her skin began to discolor, and characteristic plaques appeared on her face. Examining preserved skin samples from Zora’s limbs, the authors also found characteristic signs of leprosy. Retrospective analysis of fecal samples showed that the female was infected M. leprae already in 2002 and was ill for at least seven years before her death due to the fault of a predator.

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Leendertz and his colleagues set out to find out how widespread leprosy was in Zora’s group in the year of her death. To do this, scientists examined the feces of 32 chimpanzees collected in 2009. DNA from the causative agent of leprosy was found in samples of three more individuals, including Woodstock. According to the authors, the data collected indicate that, in general, leprosy is rare in Tai Park chimpanzees and is poorly transmitted from individual to individual.

At the last stage, scientists tried to trace the origin of the strains M. lepraethat affected chimpanzees from Guinea-Bissau and Côte d’Ivoire. Bacterial DNA was isolated from samples obtained from infected individuals. The two highest quality genomes were compared to 286 leprosy genomes, including 64 from West Africa. Phylogenetic analysis showed that the strains from the Cantanchez and Tai parks are not closely related to each other. The first belongs to a group that is very rare in humans, but its representatives infect non-human primates living in captivity. It split from other branches in the 6th century AD. The strain from Côte d’Ivoire belongs to a subgroup known from samples from medieval Europe and modern Ethiopia. Its separation from its relatives took place in the second century AD.

Man is considered the main host of the bacterium M. leprae… However, from humans, it penetrated into the populations of some wild animals, for example, ordinary squirrels (Squirrel) and nine-belt battleships (Dasypus novemcinctus). It is logical to assume that West African chimpanzees also contracted leprosy from the inhabitants of the surrounding settlements. However, for Cantanchez Park, this situation is unlikely, since these days they do not allow people to approach the distance necessary to transmit the causative agents of leprosy. Perhaps such contacts took place here in the past. Today, leprosy is already circulating within the chimpanzee population.

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The southern group of chimpanzees from Tai Park interact only with researchers and their assistants. Although its representatives sometimes pick up coronavirus and pneumovirus infections from local residents, for the transmission of which direct contact is not necessary, it is impossible to become infected with leprosy in this way. In addition, only patients with well-expressed symptoms are contagious, and there were no such patients either among scientists or among their assistants. If we add to this the rarity of the strain M. lepraefound in chimpanzees from Tai Park, recent human transmission of leprosy seems particularly unlikely. The version of infection in the distant past is also questionable, since infection is too rare in the local population.

There is a possibility that Ivorian chimpanzees contract leprosy from mammals they hunt, for example, from smaller monkeys and small antelopes (a similar version may be true for their relatives in Guinea-Bissau). This indicates the existence of a previously unknown natural reservoir of this infection. Another possible source of leprosy is the environment. Experiments show that the causative agent of this infection is able to survive in water and organisms of amoebas and arthropods. More research is needed to pinpoint how chimpanzees from Guinea-Bissau and Côte d’Ivoire are infected with leprosy.

After examining the remains of those who died from leprosy in the medieval city of Chichester, the British archaeologist found that the diseased followed a diet based on meat consumption. Basically, it was low-quality meat donated to the leper colony, as well as meat from animals found dead or wounded.

Sergey Kolenov



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