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Examining Human-to-Human Transmission of Contact-Transmitted Zoonotic Pathogens


Respiratory Contact of Zoonotic Pathogens

Four human coronaviruses (HCoV-229E, -NL63, -OC43, -HKU1) are responsible for causing the common cold. Of these, HCoV-229E originated in bats, and HCoV-OC43 in cattle. The zoonotic origins of the other two remain unknown.
Although these coronaviruses have low pathogenicity, they are mainly transmitted via airborne droplets to the nasal mucosa, and thus efficiently transmit between humans. Virus replicates locally in the upper respiratory tract. In the case of HCoV-229E, this is facilitated by an abundant supply of its receptor on non-ciliated bronchial epithelial cells.

Multiroute Transmission of Zoonotic Pathogens

Several routes have been identified for transmission of Ebola virus, which causes Ebola virus disease (EVD)—a hemorrhagic fever with a high fatality rate.
Direct contact with infected patients or their bodily fluids, as well as with contaminated surfaces or materials, is the main mode of transmission of the virus. Indeed, Ebola virus has been isolated from blood, breast milk, and semen of infected patients. Studies have also detected Ebola virus RNA in sweat, tears, and stool, as well as in samples from the skin, vagina, and rectum. In particular, the 2014 EVD outbreak emphasized the potential for sexual transmission of the virus.
Although some researchers hypothesized about the possibility of airborne transmission of Ebola virus, this mode of transmission is now considered unlikely. However, Ebola virus may be spread to humans through handling infected bushmeat and via contact with infected bats.
The authors discuss several pathogen-associated factors that promote H2H transmission of Ebola virus, including the high concentration of virus in bodily fluids, as well as its very low infectious dose.
Human behaviors and societal factors also contribute to efficient transmission of the virus. For example, cultural traditions often involve mourners bathing in water from the washing of corpses. During the 2014 EVD outbreak in West Africa, inadequate health care systems in Guinea, Sierra Leone, and Liberia also contributed to disease spread, as did ease of travel between these countries. International travel-associated cases of EVD were also documented.
In their concluding remarks, the authors summarize that, while some factors were specific for a particular pathogen or mode of contact transmission, other factors (such as immune evasion, high viral load, and low infectious dose) were common to several modes of transmission.
“The identification of such factors will lead to a better understanding of the requirements for human-to-human spread of pathogens, as well as improving risk assessment of newly emerging pathogens,” they conclude.
Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.
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