Mpox is a disease caused by the highly infectious monkeypox virus. It's quite easily passed on from one person to another. But it originally came from infected animals.
The virus was discovered in 1958 when two outbreaks of pox-like infection were seen in monkeys shipped from Singapore to Denmark for research. Studies since then, however, has shown that monkeys are not the natural host (reservoir) of the virus. This idea was dropped in the 1970s when cases in humans were seen in west and central Africa. In 1979 a large-scale survey of 43 species of animals revealed further evidence of exposure to the virus.
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It's not yet certain which animals are the natural carriers. But, based on the available evidence, it is thought to be rodents and small mammals mostly found in west and central Africa. Rodents first came into the mix when the 1979 evidence implicated at least one species of terrestrial rodent, and more prominently squirrels. Further data have shown that about 12% of people presumed to have been infected by contact with animals had recent contact with squirrels.
I'm a veterinarian and virologist with research in this field. I keep a close watch on new developments and studies that fill in missing gaps in our knowledge about virus transmission from animals to humans.
In recent research that I was involved in we found evidence of orthopoxvirus infections in a genus of mice (Praomys and Mus) and the black rat. Orthopoxvirus is a genus of viruses that cause smallpox, cowpox, horsepox, camelpox and mpox.
A brief history
The first reported human case of mpox was in 1970 in a nine-year-old boy in Zaire (today the Democratic Republic of Congo or DRC). This was during the drive to vaccinate people against smallpox. Once smallpox had been eradicated in 1980, people were no longer vaccinated - and the number of mpox cases increased. But people vaccinated against smallpox were protected from mpox.
Major outbreaks of mpox occurred between 1996 and 1997 in the DRC and sporadic cases occurred in other African countries in the following decades.
Cases have been reported in Benin, Cameroon, the Central African Republic, Gabon, Côte d'Ivoire, Liberia, Nigeria, the Republic of the Congo, Sierra Leone and South Sudan.
Mpox epicentre
Mpox occurring in nature is mostly confined to the humid forest regions of west and central Africa. Sustained reporting of cases has been confined largely to the DRC in central Africa and recently Nigeria in west Africa.
There are two clades (natural groups) of the mpox virus and a subdivision of clade II (IIa). Clade I, the Congo Basin clade, is more deadly and transmissible than clade II, the west African clade. Both can be found in Cameroon, which is geographically a division between the central and west African countries.
In Cameroon, the Sanaga river appears to form a natural barrier between the two clades.
Tracing the animals involved
Evidence about the virus and the spread of disease indicates animals that may be a reservoir of the mpox virus. In the DRC, 43 species of animals, including primates, have shown positive evidence of exposure to orthopoxvirus. At least one species of terrestrial rodent, and more prominently squirrels, have shown antibodies specific to the virus that causes mpox - in other words they have been exposed to the virus.
In another investigation about 12% of people presumed to have been infected had also had contact with animals, particularly squirrels. Evidence of antibody to orthopoxvirus was also detected in vervet monkeys in Cote d'Ivoire.
Early association of mpox virus with illness in primates like the genus Macaca led to the suspicion it might be a primate pathogen of east or south Asian origin. Later identification of several human infections in remote African villages, however, changed this idea. This ruled out east or south Asia as the origin.
Because many of the first people to be infected with mpox had been exposed to primates, it was initially thought that primates were the main source and were largely responsible for transmitting the infection to humans. But now more kinds of animals seem to be the route for transmission to humans.
It has also been suggested that agriculture creates habitats suitable for squirrels and for African pouched rats, common during harvest times. These rodents likely had contact with humans, leading to virus transmission.
Three genera of African rodent, Graphiurus (African dormice), Cricetomys (giant pouched rat) and Funisciurus (African striped squirrel), have been named as likely reservoirs of the mpox virus.
The first time the virus was found in a wild animal was in 1985 when a squirrel (Funiscirurs anerythrus) with mpox symptoms was captured in then-Zaire.
In a recent study we conducted in Nigeria, we found evidence of orthopox in mice and black rats. This means other rodent species may be reservoirs for the mpox virus.
Transmission
The potential of the mpox virus to spread among household members is lower than the potential of smallpox.
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We assume, therefore, that mpox is being sustained in endemic areas through the re-introduction of the virus from animals to humans.
Transmission to humans is mainly associated with rain forest habitat. This supports the idea that it comes from forest dwelling rodents.
One form of contact could be hunting for bushmeat. The suspected reservoir rodents like Cricetomys and Funisciurus are hunted extensively for food by people in rural communities.
People could also become infected if they are bitten or scratched by animals. Or by handling an infected animal or animal products. Eating inadequately cooked meat and other products of infected animals is another possible risk factor.
Protective measures
Mpox prevention requires strict adherence to biosecurity and personal hygiene. Biosecurity means preventing disease-causing agents from entering or leaving any place where they can pose a risk to animals, humans, or the safety and quality of a food product.
Creating awareness of the risk factors such as contact with and handling of wildlife and bushmeat can reduce exposure, especially in forested rural areas.
Clement Meseko, Veterinarian & Virologist, National Veterinary Research Institute, Vom, Jos
