The threat of zoonoses from live animal markets

Karmele Llano Sanchez Jakarta

An article published last July in Emerging Infectious Diseases reports on the significant impact that the wildlife trade and live animal markets are having on human health worldwide, in particular Asia.

The large population on this continent arguably increases the likelihood of a pandemic occurring. It is in this continent also where the population is at higher risk of contracting animal- borne diseases due to the encroachment of humans into wildlife habitats, and the presence of animal markets where animal products and live animals are on sale.

Presently, an outbreak of H5N1 type A influenza virus (bird flu) has alerted human health authorities in Indonesia. This zoonosis (an animal disease capable of being transmitted to humans) has already cost the lives of dozens of people. However, the most threatening aspect is the possibility that many other zoonotic pathogens could be covertly infecting people basically because they are difficult to diagnose by physicians.

This highlights the question of where these emerging pathogens are hidden and what causes them to emerge. According to Lonnie King et al, three-quarters of emerging human diseases over the past two to three decades have been animal-borne, and the future will probably bring many more such outbreaks.

In addition, the threat of these diseases spreading quickly worldwide is reasonably high due to contemporary global transport patterns: planes, ships, people and animals carry infections in every direction.

Animal health has broad implications, ranging from the health of individual animals, human health and global security. Animal diseases have implications not only for the global food supply but also for human health directly due to the existence of animal borne diseases transmissible to humans (zoonoses).

The pool of infectious pathogens shared between animals in a market will not stay confined to these locations. To the contrary, these animals and their pathogens will be transported to other areas, locally and nationally, and even internationally when animals are smuggled outside of the country.

This not only threatens people but also wildlife, when, for instance, they are eventually released by the owners because they are no longer wanted, or when they escape and become a threat to local wildlife.

The likelihood of disease transmission is also increased when animals are submitted to unnatural conditions causing high stress levels.

Wild animals suffer much stress when they are captured and transported, then forced into a completely unknown environment, surrounded by their most dangerous predator: humans. They are also offered for sale in cramped cages with no space to even stretch their bodies.

At the same time, this stress renders their immune system less effective, making them more prone to infections for which they are not a natural host.

Pathogens for which humans become new hosts are more hazardous while human immunity does not yet posses the ability to fight them.

Some estimations of the numbers of wild animals traded annually worldwide includes approximately 40,000 live primates, 4 million live birds, 640,000 live reptiles, and 350 million live tropical fish. According to one report, in a single market in North Sulawesi up to 90,000 mammals are sold per year.

Over the last 25 years, more than 35 diseases have emerged. In a list of 1,415 human pathogens, 61 percent are known to be zoonotic, and multiple-host pathogens are twice as likely to be associated with emerging infectious diseases in humans. As much as 77 percent of pathogens found in livestock are shared with other host species.

The rash of emerging or reemerging livestock disease outbreaks around the world since the mid 1990s, including bovine spongiform encephalopathy, foot-and-mouth disease, avian influenza, swine fever, and other diseases, has cost the world economy $80 billion.

Efforts to control the spread of avian influenza in Asian countries since 2003 has required the killing of about 140 million chickens. In order to keep up with an increasing population and growth in demand for protein, countries such as Indonesia will have to increase livestock production which will increase the risk of livestock disease outbreaks. Following this tendency, it is very likely that these infections will be linked to wild animals.

In July of this year, studies reported the first case of simian foamy virus (SFV) infection, a non-human primate-borne disease, in a person with known exposure to free-ranging Indonesian macaques in an eco-tourism monkey attraction in Bali.

Alarm warns foreign tourists not to have close contact with macaques in these kinds of premises. However, no risk awareness is raised for local people who keep these monkeys as pets, or who sell them in markets. In all the animal markets of Jakarta and in other large cities in Java and other islands, macaques are commonly seen on sale either as pets or for human consumption. No animal disease control is carried out and no authority warns people of the risk of transmission of diseases.

The poor hygienic conditions in these markets means a risk for sellers, buyers, visitors and the entire environment. The close contact between people and animals and their diseases in the context of any animal market is, arguably, a potential risk for zoonoses to emerge.

All these wild animals harboring pathogens would not be a hazard to human beings at all if they were left alone in their natural habitats far away from humans. It's only when humans cross the barrier and stand in the way of these pathogens that the risk of new disease emerges.

Emerging Infectious Disease reports that some studies have shown that closing down retail poultry markets in Hong Kong for one day per month reduced the rate of H9N2 avian influenza virus in market birds. In the same terms, it seems reasonable to think that closing down animal markets would have an analogous effect in the Indonesian context.

The writer is a veterinarian based in Jakarta.