Reverse zoonosis

Sometimes, germs can move from people to animals.

This is called a reverse zoonosis. A bug might bite a sick person. Then that bug bites an animal. Now the animal can get sick too.

Some germs can even live in soil or water. They can stay there for a long time. This helps the germs spread to new places. It is a way germs travel between us and nature.

Most people know that germs can jump from animals to humans. This is called a zoonosis. But germs can also move in the other direction. This is called a reverse zoonosis. It happens when a human passes a germ to an animal.

Many germs use bugs to travel. These bugs are called vectors. For example, a mosquito might bite a person with malaria. The mosquito then carries the germ to another animal.

This can happen with many viruses, like Zika or Yellow Fever. These are called arboviruses, which are germs spread by bugs.

Some germs do not need a living host to stay alive. They can grow in soil or water. These are called sapronotic agents. This includes some types of fungi and bacteria.

Scientists study these paths to keep everyone safe. They look at how germs move between cities and wild forests. They also study how germs move between humans and farm animals like pigs or cows. Understanding these links helps us stop new sicknesses from spreading.

Most people know that germs can jump from animals to humans. This is called a zoonosis. But germs can move in the other direction, too. This is called a reverse zoonosis, or zooanthroponosis. It happens when a germ stays in a human and then moves to an animal.

Scientists also use the term anthroponosis for germs that start in humans. Some germs can even live in things that are not alive. These are called sapronotic agents. They can grow in soil, water, or even decaying plants.

Many germs use small bugs to travel from one host to another. These bugs are called vectors. One way this works is through a blood meal. For example, a mosquito might bite a person who has malaria. The mosquito then picks up the Plasmodium parasite. The parasite starts a cycle inside the mosquito's gut. When that mosquito bites a new animal, the germ moves again.

This can also happen with African trypanosomes. These are tiny germs that move via Tsetse flies. The fly carries the germ between humans and other animals. This can lead to sickness like African sleeping sickness.

Some germs are called arboviruses because they are spread by arthropods. This group includes viruses like Zika, Dengue, and Yellow Fever. These germs can jump between cities and wild forests. This often happens through a bridge-vector, like the Aedes albopictus mosquito. This mosquito can live in cities but also travel to rural areas. It can carry a virus from a person into a wild forest.

This process is sometimes called spillback. It happens when a germ moves from a human back into a wild animal group. This keeps the germ moving in a cycle.

History shows us how these paths change over time. For example, Yellow Fever was likely brought to the Americas about 400 years ago. It may have come from West Africa during the slave trade. Scientists found this by looking at how the virus changed over time.

In 2015, researchers studied the Zika virus in Brazil. They used a strain from a human to infect rhesus macaques. They found that the virus could move from humans to these primates. This proved that reverse zoonosis is a real thing that happens in nature.

Understanding these links helps us see how everything is connected. We can see how a mosquito in a city affects a monkey in a forest. We can see how germs move between humans and farm animals like pigs or cows. For instance, studies show similarities in how pigs react to human serum.

This suggests that germs can move from humans to livestock. Even the way we live near nature changes how germs move. By watching these paths, we can learn more about the world around us.

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.