Many infectious diseases cross the species barrier, and most infectious diseases in humans are zoonotic. Once a zoonotic pathogen infects humans, its success in the human population depends on whether it becomes capable of efficient human-to-human (H2H) transmission.
In a review article
published in Current Opinion in Virology
, Mathilde Richard, PhD, from Erasmus Medical Centre, Rotterdam, the Netherlands, and colleagues discuss some key factors involved in H2H transmission of contact-transmitted zoonotic pathogens. They focus on four modes of transmission, using examples of zoonotic diseases to highlight these factors.
Skin Contact of Zoonotic Pathogens
Yaws is an infection caused by Treponema pallidum subspecies pertenue (TPE). This neglected tropical disease
is endemic in West Africa, South East Asia, and the Pacific. It is a chronic, relapsing disease that spreads by skin-to-skin contact, and is characterized by lesions that predominantly affect skin, bones, and cartilage.
However, in recent decades, increasing evidence has also shown high levels of TPE infection of monkey species in Africa. Studies have also indicated that, genetically, the organisms infecting monkeys closely resemble the agent that causes yaws in humans. This suggests the potential for zoonotic transmission, with monkey species in Africa serving as a reservoir for human infection.
According to the authors, several factors have contributed to efficient H2H transmission of TPE. For example, the organism uses antigenic variation, altering its antigens to evade immune surveillance and allow it to chronically infect the individual. Skin ulcers may contain high concentrations of organisms, and their infectious dose is low. In addition to infecting an individual via cuts or abrasions, TPE can also penetrate healthy mucous membranes. Host factors such as crowded living conditions also facilitate transmission of the organism. And environmental factors such as high humidity and high temperature allow TPE to better survive outside the host. In addition, “lack of surveillance and inadequate health care favor the persistence and spread of human yaws in affected countries,” the authors say.