The latest issue of Science contains an article describing the worldwide decline of amphibian populations due to the international spread of chytridiomycosis, a lethal fungal disease, largely as a result of the trade in amphibians as pets. The greatest declines are reported for isolated populations in the wet forests of the Americas and Australia. Few of the infected populations are expected to recover.
The global movement of people and goods brings exotic diseases to the most remote ecosystems on Earth, including Ebola and Zika to North America. For the human species, the problem is exacerbated by the concentration of our numbers in dense urban populations. Not since the plague, also known as the Black Death, arrived in Europe in the early 1300s has the human population been more vulnerable to exotic disease. Black Death is estimated to have killed 30 to 60% of the population of Europe.
The most vulnerable populations of animals and humans are those who have not previously encountered the disease agents and thus have no evolutionary resistance to it. The phenomenon is known as the “virgin soil effect”, which explains the vulnerability and devastation of native Americans following the arrival of European diseases. (It is likely that syphilis traveled in the opposite direction with similar impacts in Europe).
Today, our vulnerability to exotic disease is exacerbated by the widespread use of antibiotics and antifungal compounds in agriculture. These agents kill off the vulnerable strains of bacteria and fungi, selecting for the rare resistant strains in nature. Unfortunately, the resistant strains are unaffected when the same antibiotic and antifungal agents are prescribed as drugs for humans. Recent reports describe a fungus, Candida auris, which appears resistant to various antifungal agents and responsible for the death of numerous patients in hospitals worldwide. Some strains of Candida auris are resistant to azole, a fungicide with widespread use to prevent rot on crops.
Just as the world’s frogs have succumbed to exotic diseases newly arriving in their habitats, human populations are increasingly vulnerable to exotic disease, especially when large amounts of antibiotics and antifungal agents are spread as prophylactics during the rearing of chickens, pigs and cattle and in the cultivation of fruits and nuts. The effect is similar to the appearance of DDT-resistant mosquitoes in areas where DDT was broadcast to prevent malaria.
With international transport of people and goods, dense populations in cities, and widespread use of antimicrobial agents to prevent, rather than treat, disease, the human population is more vulnerable to catastrophic pandemics than at any time since the Black Death.
Fear the fever
Fisher, M., et al. 2018. Worldwide emergence of resistance to antifungal drugs challenges human health and food security. Science 360: 739-742.
Scheele, B.C., et al. 2019. Amphibian fungal panzootic causes catastrophic and ongoing loss of biodiversity. Science 363: 1459-1463.