Wildlife markets and disease transmission

The problem is, pigs and other animals do fly

A consortium of scientists from the New York-based Wildlife Conservation Society announced this week that one way to reduce the risks of future SARS-like diseases is to control wildlife markets. Specifically, markets selling wild animals for their meat not only threaten wildlife populations, but also present a grave threat to humans. A recent example of the problem is the suspected link between wildlife markets in China and the outbreak of SARS in humans. Other possible cases include bushmeat and Ebola outbreaks in Africa, West Nile virus and monkeypox. Even the primate origins of HIV point to a link between wildlife and human disease.

Since humans first walked upright they have eaten wildlife. But human population densities were far lower than today – well under one person per square mile in most tropical forests, for example. Animals were only hunted on a scale to support the subsistence needs of local human populations, and international trade in wildlife was negligible or absent.

In many areas around the world, traditional hunting is little changed today, where wildlife is carried for a maximum of one or two day’s walk back to the community. Consumers and animals live in similar ecosystems and have been co-existing for many generations. Cross-species diseases do still occur in these remote rural towns, but some resistance to local diseases has developed over the ages and many local, religious, and cultural rules on the handling and consumption of animals developed to protect people from these illnesses.

But in today’s global marketplace, wildlife is just another commodity. Wildlife for food markets and the pet trade are often transported over enormous distances. For example, animals found in markets in Guangzhou, Guangdong Province, China include soft-shelled turtles captured in Sumatra (1,900 miles away), pangolins from Vietnam (930 miles) and Thailand (1,100 miles), pythons from the Mandalay area, Myanmar (1,950 miles?), and red-eared sliders from Florida, USA (9,000 miles). Even “local” wildlife might include animals from forested southern China around Kunming, 800 miles distant.

Daily Possibility of Disease Spillover

The result is a dangerous integration of circumstances, with animals and consumers from different ecosystems coming into contact. The lack of resistance to new pathogens makes humans and animals fertile, uncontrolled laboratories for viruses and bacteria to adapt and rapidly mutate. Further, the staggering numbers of animals and people in contact change one-in-a-million odds of a disease spillover into almost a daily possibility. Even under the most hygienic conditions, this pool of viruses, bacteria, and other pathogens creates optimal conditions for diseases to multiply rapidly and jump between species to exploit new potential hosts — something the most “successful” diseases do all too well. Under this scenario, two problems are created. First is the high risk of new diseases spreading into human populations. Second is that this can create a “fear factor” amongst people – their concern that wildlife is unhealthy might cause them to try to remove the threat by killing the wildlife. Shooting flying foxes was proposed in Southeast Asia when they were thought to be carrying nipa virus, even though the link has not been definitively proven and the disease is rarely found in flying foxes. Large-scale killing of sparrows and crows during the Great Leap Forward in China in the late 1950’s because they were thought to be pests led to failed rice crops and massive famine because the birds had really been helping to control actual insect pests.

In almost all cases, eradication schemes are not cost efficient or effective means to reduce disease spread when compared to health education, sanitation, and controlling animal movement. Moreover, eradication schemes do not address the fundamental problem of our creating conditions which maximize opportunities for disease build-up and cross-species transmission. Much research is still needed on the links between viruses in different species and human disease, and means of transmission between the two. But we already know enough to minimize the risks to humans – if we reduce or stop live animals being transported over long distances into markets for food, medicinal uses or for the pet trade, we are not only helping to conserve those species in the wild, but we are also protecting ourselves from the risks of new, virulent and potentially fatal diseases.