Travelers and Human–Animal Pathogen Transmission: A One Health Review of Zoonotic and Reverse Zoonotic Diseases

Background: International travel can accelerate the geographic spread of zoonotic pathogens and, under certain circumstances, contribute to epidemic or pandemic dissemination. Travelers may also transmit human pathogens to wildlife at tourism interfaces, acting as intermediaries in zoonotic and reverse zoonotic transmission. Methods: Archived WHO and ECDC reports were screened for major zoonotic outbreaks in which international travel contributed to cross-border spread. PubMed and Scopus were searched for studies of human-to-wildlife transmission involving tourists, visitors, or travelers and traveler behaviors that could facilitate transmission. Results: Among 14 major zoonotic outbreaks, travelers commonly acted as spreaders following initial animal-to-human spillover by importing infections into new regions, sometimes resulting in onward transmission. This pattern was prominent with respiratory infections such as SARS, MERS, influenza, and COVID-19 and vector-borne infections such as Oropouche and Zika. Twenty-one studies reported possible or confirmed reverse zoonotic transmission at human–wildlife tourism interfaces. Pathogens included respiratory viruses, Mycobacterium tuberculosis, enteric pathogens, and antimicrobial-resistant pathogens. Affected wildlife included great apes, primates, elephants, felines, carnivorans, ungulates, avians, reptiles, and urban-adapted wildlife in national parks, zoological parks, rehabilitation centers, and semi-urban parks. Five additional studies documented risky tourist behaviors, including close approach to, physical contact with, and feeding and visiting wildlife while having respiratory symptoms, and inadequate vaccination against measles, influenza, and polio. Conclusions: Travelers can influence transmission bidirectionally by disseminating zoonotic pathogens and exposing wildlife to human pathogens. Travel medicine should incorporate wildlife-protective counseling on distancing, hygiene, waste disposal, and immunization alongside conventional traveler protection.