Reverse zoonosis

Reverse zoonosis, also called zooanthroponosis, describes a specific type of disease transmission. It occurs when a pathogen is stored in humans and then moves into non-human animals. While many people are familiar with zoonosis, which is the transfer of disease from animals to humans, reverse zoonosis moves in the opposite direction. The term anthroponosis is also used to describe pathogens that originate in humans. This term can cover transmission between humans or from humans to animals. Understanding these directions is vital for studying how infectious diseases move through different ecosystems.

Scientists use various terms to categorize how these pathogens behave. A zoonosis is technically any disease transferred between animals and humans or between different animals. However, the term is often used to describe diseases moving from animals to humans due to medical biases. In 1967, a joint committee from the World Health Organization and the Food and Agriculture Organization helped clarify these definitions. They recommended using "zoonosis" to describe the bidirectional exchange of pathogens between humans and animals. This helped resolve confusion between terms like anthropozoonosis and zooanthroponosis.

Some pathogens do not require a living host to survive or grow. These are known as sapronotic agents, a term derived from the Greek word for "decaying." These agents can grow and replicate in abiotic environments, which are non-living things. Examples of these environments include soil, water, decaying plants, animal corpses, and excreta. Fungal agents like coccidioidomycosis and histoplasmosis are common sapronotic agents. Some bacteria, such as those in the genera Clostridium and Bacillus, also fall into this category. However, obligate intracellular parasites cannot be sapronotic. These include viruses and Rickettsia because they must enter a living cell to replicate.

Many reverse zoonoses rely on arthropod vectors to move between hosts. An arthropod vector is a small creature, like a mosquito or a fly, that carries a pathogen. For example, the Plasmodium parasite causes malaria. In a reverse zoonosis scenario, a mosquito feeds on an infected human and picks up the parasite. The parasite then undergoes a sporogenic cycle inside the mosquito's gut. When the mosquito bites a different animal, it transmits the infection. This cycle can involve both humans and non-human animals, such as primates.

Arboviruses represent another significant group of pathogens that move through vectors. Arboviruses are viruses transmitted by arthropods, such as the Flavivirus and Alphavirus genera. This group includes well-known diseases like Zika, Dengue, and Yellow Fever. These viruses can move between urban human environments and sylvatic cycles in wild forests. This movement often involves a "bridge-vector," such as the Aedes albopictus mosquito. This species can survive in cities but also travel into rural or forested areas. This allows the virus to jump from humans into wild animal populations, a process called spillback.

Historical data provides evidence of how these transmission cycles are established. Yellow fever is a notable example of a pathogen that created a new sylvatic cycle in the Americas. A 2007 study used nucleotide substitution rates to show that yellow fever was introduced from West Africa approximately 400 years ago. This likely occurred during the era of the American slave trade. The virus may have been carried by infected humans, mosquitoes, or even mosquito eggs on sailing vessels. Once introduced, the virus moved from humans into wild primate populations, establishing a permanent cycle in the new environment.

Modern research continues to confirm the potential for reverse zoonosis in various species. In 2015, researchers in Brazil studied the Zika virus by infecting rhesus macaques. They found that Zika-positive tissue was recorded in both the mothers and the placentas for up to 105 days. This confirmed that the virus can move from humans to non-human primates. Similarly, studies on African trypanosomes show that the Tsetse fly can transmit diseases like sleeping sickness between humans and livestock. Molecular studies have even shown similarities in how pigs, goats, and cows react to human serum, suggesting a human-to-animal path.

These complex cycles show how closely human health is linked to animal and environmental health. When urban and wild ecosystems overlap, the risk of spillover and spillback increases. For instance, the Chikungunya virus has no preference for specific primate species, making host switching very easy. As humans continue to interact with different environments, the movement of these pathogens becomes more dynamic. By studying these pathways, scientists can better understand how diseases like Dengue and Zika persist in the world.