A spillover or cross-species transmission is the ability of a foreign virus to jump from one species to a new host species, and spread within the population of that new host species. Note that diseases caused by pathogens that infect both animal and human populations are generally called zoonosis. Aside from viruses, several bacteria, fungi, and parasites are zoonotic.
However, in some incidents, after a pathogen had jumped from one species to another species, it mutates and becomes a new strain capable of efficiently and exclusively replicating, spreading, and evolving within the new host population. This is the primary definition or a spillover of cross-species transmission.
Several notable viruses had jumped from animals and became exclusive pathogens of the human population. Further, some of them have become major threats to public health, thus causing epidemics and pandemics. These include HIV-AIDS, diseases due to human coronaviruses such as SARS and MERS, Ebola fever, avian influenza or bird flu, swine influenza, and COVID-2019.
Explaining How Viruses Jump from Animals to Humans
The exact mechanism that facilitates the transfer remains unknown, especially how a virus can cross the species barrier. However, factoring in how viruses jump from animals to humans is critical in examining the origin of an outbreak and the virulence of the new viral pathogen,
Several factors play a role in the jump. Furthermore, these factors also serve as the general causes of zoonoses, which, on the other hand, have two modes of transmission. Take note of the following details:
• Direct Zoonosis: The infectious disease is directly transmitted from animals to humans through media such as air or direct contact such as consumption or through bites.
• Indirect Zoonosis: The infectious disease is transmitted via an intermediate species or vector that carry the pathogen without getting infected.
Specific Factors of Spillover or Cross-Species Transmission
There are also specific factors behind spillover. For several pathogenic human diseases and epidemics, the origin of the involved pathogen can be traced from one of these factors. Below are further details:
1. Farming and Animal Husbandry
Different strains of the influenza virus have infected numerous animal and human species. Note that human flu is similar to avian flu, bird flu, as well as dog flue and horse flu. Close interactions between animal and human species can foster transmissions, such as in the case of farming and animal husbandry.
The 1918 influenza pandemic or Spanish flu caused by the H1N1 strain was hypothetically traced from a live piggery and poultry near a military camp and military hospital facility in Étaples, France. In an agricultural fair in Maryland in 2017, flu-stricken hogs were exhibited and infected a group of fairgoers. The responsible virus was a strain of H3N2v.
2. Contamination of Food and Water Supply
Several species are natural carriers or reservoirs of viruses and other pathogens. For example, bats have notoriously been known as origins of some notable viral epidemics in the human population, to include Ebola disease, the Nipah virus infection, and the SARS-CoV-2 virus that caused the 2019-2020 Global COVID-19 Pandemic.
In the case of the Nipah outbreak in Bangladesh, the epidemic was traced to excretions from fruit bats that contaminate date palm trees. The affected communities have the tradition of consuming the sap or juice from trees, thus facilitating the spillover.
3. Uncontrolled Human and Wildlife Interactions
There are also numerous viral diseases associated with traditional human practices, particularly the interactions between communities and wildlife. Note that hunting and bushmeat have been pinpointed as the precursor to HIV and the origin of Ebola outbreaks in Africa. Remember that viruses are abundant in the environment, particularly in wild animals.
An increasing interaction between humans and wildlife provide a high probability of encounters with novel viruses that have human pathogenic potentials. Furthermore, interactions between wild animals and domestic animals such as wild boar and farm-raised hogs serve as a link between novel viruses and human communities.
Hypotheses on How Viruses Jump from Animals to Humans
Note that the exact mechanism of viral spillover remains unknown, there are some working hypotheses. These include geographic proximities and intraspecies behaviors, as enumerated above. Other hypotheses have something to do with natural selection and phylogenetic relatedness of different species.
For starters, it is believed that viruses with a rapid mutation rate are more likely to overcome the species barrier or the host-specific immunological defenses. Also, it is essential to remember that viruses would attempt to infect new species.
The rapid mutation increases the probability of a particular virus to overcome the species barrier. Mutation allows them to gain new traits. Of course, most mutations will have no effect. Some can be detrimental. However, there is still a probability that the newly acquired traits would increase its cross-species pathogenic potential.
Viruses are constantly encountering and trying to infect new species. Most of the time, these attempts fail because they are unable to cross the species barrier, particularly due to genetic dissimilarities between the original host and the new target species, as well as the different immunological differences of different species.
However, in some cases, some similarities in genetic makeup can facilitate cross-species transmission. Take note that this has been observed in human and nonhuman primates. In fact, the origins of HIV and human adenoviruses have been traced back to the interactions between humans and primates such as apes, chimps, and monkeys.
Understanding how viruses jump from animals to humans necessarily requires understanding the fact that these organic pathogens continuously mutate and evolve not only to adapt to their host but also to increase their rate of survival. Hence, a spillover or cross-species transmission is a demonstration of evolution via natural selection.
FURTHER READINGS AND REFERENCES
- Gurley, E. S., Hegde, S. T., Hossain, K., Sazzad, H. M. S., Hossain, M. J., Rahman, M., Sharker, M. A. Y., Salje, H., Islam, M. S., Epstein, J. H., Khan, S. U., Kilpatrick, A. M., Daszak, P., and Luby, S. P. 2017. “Convergence of Humans, Bats, Trees, and Culture in Nipah Virus Transmission, Bangladesh.” Emerging Infectious Diseases. 23(9): 1446-1453. DOI: 3201/eid2309.161922
- Miller, R. S., Sweeney, S. J., Slootmaker, C., Grear, D. A., Di Salvo, P. A., Kiser, D., & Shwiff, S. A. 2017. “Cross-Species Transmission Potential Between Pigs, Livestock, Poultry, Wildlife, and Humans: Implications for Disease Risk Management in North America.” Scientific Reports. 7(7821). DOI: 1038/s41598-017-07336-z
- Parrish, C. R., Holmes, E. C., Morens, D. M., Park, E-C., Burke, D. S., Calisher, C. H., Laughlin, C. A., Saif, L. J., and Dazak, P. 2008. “Cross-Species Virus Transmission and the Emergence of New Epidemic Diseases.” Microbiology and Molecular Biology Reviews. 72(3): 457-470. DOI: 1128/MMBR.00004-08