Ticks are responsible for the transmission of many zoonoses, infectious diseases transmitted from animals to humans. A study published in Nature Communications has revealed new bacterial species, previously unknown, transmitted by ticks to wildlife in the tropical forests of Guyana. These highly endemic bacteria, belonging to the genera Anaplasma et Ehrlichiahave evolved unique transmission cycles, specific to ticks, mammals and birds of Guyana, but transmissions to humans exist.
Major vectors of pathogens, ticks are particularly well known in Europe for their role in the spread of zoonoses such as Lyme disease. By feeding on wildlife, ticks can then transmit zoonotic pathogens to humans. Guyana is a biodiversity hotspot due to the ancient Amazonian forests which, in addition to significant wildlife, are home to many species of ticks. Monitoring of humans, wildlife, and ticks living in these tropical rainforests has revealed the existence of a great diversity of bacteria transmitted by ticks, previously unknown.
Anaplasmosis and ehrlichiosis are the most frequently reported tick-borne diseases in humans after Lyme disease. They are also the most common tick-borne diseases in livestock, causing major economic losses. These diseases are caused by infection with bacteria of the genera Anaplasma et Ehrlichia. Several species of these bacteria are already known in the northern hemisphere to be the cause of worrying emerging diseases such as human granulocytic anaplasmosis and human monocytic ehrlichiosis. This work, carried out by researchers from the CNRS in Montpellier, highlights a majority of new strains and species unknown elsewhere. They highlight that the tropical rainforests of Guyana constitute one of the regions in the world with the highest biodiversity. d’Anaplasma and D’Ehrlichia. Paradoxically, species that can infect humans, such as Anaplasma phagocytophilumresponsible for human granulocytic anaplasmosis, and Ehrlichia chaffeensisresponsible for human monocytic ehrlichiosis, have not been observed in Guyana.
Metagenomic and phylogenomic analyzes show that most of these infectious agents are highly endemic, with a majority of new species specific to this region. While ruminants and rodents are the primary animal hosts of most Anaplasma and Ehrlichia in the northern hemisphere, Guyana species infect other types of animals (opossums, sloths, armadillos, passerines, etc.), indicating that they have specific transmission cycles adapted to Amazonian ecosystems. Genome sequencing of new species, such as Anaplasma sparouinense infecting humans, Anaplasma amazonensis infecting sloths and Ehrlichia cajennense detected only once in a tick, indicates that these bacteria all possess genes homologous to virulence factors identified in species known to be pathogenic to humans and domestic animals in the northern hemisphere.
It is still too early to estimate the health risk that these new tick-borne diseases could present for the human and animal populations of Guyana. Their simple existence, however, reminds us that our knowledge of the diversity of pathogens circulating in little-explored natural areas still remains very partial. The expansion of human activities in these regions will inevitably lead populations to be exposed to these infectious agents.
