Conservation by the Numbers: Mathematical Models Link Illegal Wildlife Trade & Disease Risk

February 16, 2016
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Above image: Wall of cages: wild and domestic birds for sale at a Peruvian animal market, courtesy of Elizabeth Daut.

Communications Coordinator

A new paper recently published in the peer-reviewed journal PLOS ONE by conservation medic Elizabeth Daut and coauthors is among the first to investigate the influence of illegal wildlife trade on the introduction and spread of infectious diseases.

The researchers developed two mathematical models to evaluate the hypothetical transmission of the highly infectious and fatal Newcastle disease among white-winged parakeets, Peru’s most trafficked parrot. Their results suggest that an outbreak of the disease, combined with a conservative illegal harvest rate, would lead to a nearly 25 percent population decline in two years, and up to 44 percent with higher—albeit still realistic—poaching rates.

“Introducing just one infected individual could provoke an outbreak of Newcastle disease in susceptible populations of white-winged parakeets. The conservation concern is that Newcastle-related deaths, combined with illegal harvest for the wildlife pet trade for domestic consumers, could result in overwhelming losses of these birds,” said Daut, a postdoctoral fellow at the National Socio-Environmental Synthesis Center (SESYNC).

The illegal wildlife pet trade is thriving in Peru—and it is also often a gateway for disease outbreaks. Newcastle disease is fairly common in Peru among birds that aren’t vaccinated, such as with backyard poultry flocks and fighting cocks. In animal markets, where chickens are housed in wire cages alongside illegally caught parrots, pathogens can easily spread from infectious to healthy birds.

After being exposed to sick birds at markets or along the transport chain, infected white-winged parakeets often make their way back to the wild, either by escape or deliberate release. In many cases, the birds are released—without any health surveillance—after being confiscated by authorities. Lacking funds to maintain the animals in captivity or to pay for diagnostic testing to rule out any worrisome infectious diseases, the authorities frequently release animals back to the wild shortly after confiscation.

When infected parakeets are released, wild populations are put at risk. The paper’s authors point out that introduced infectious diseases have previously been linked to major declines of wildlife populations (as in the case of white-nose syndrome and little brown bats), and even species extinctions (as in the case of the parasite Trypanosoma lewisi and Christmas Island rats).

But disease outbreaks in wildlife populations can be difficult to identify, especially in the dense Amazon jungle of Peru, where live animals are often difficult to observe and carcasses disappear quickly.

“That’s why mathematical models are such a valuable tool for the conservation community: they help us evaluate the interaction between illegal wildlife trade and risk of diseases that might otherwise go unseen—until it’s too late,” Daut said.

The paper, “Interacting Effects of Newcastle Disease Transmission and Illegal Trade on a Wild Population of White-Winged Parakeets in Peru: A Modeling Approach,” Elizabeth F. Daut, Glenn Lahodny Jr., Markus J. Peterson, and Renata Ivanek, was published online January 27, 2016, in the journal PLOS ONE.

Mathematical codes for the epidemic model of Newcastle disease transmission are accessible here.

The National Socio-Environmental Synthesis Center, funded through an award to the University of Maryland from the National Science Foundation, is a research center dedicated to accelerating data-driven scientific discovery at the interface of human and ecological systems. Visit us online at and follow us on Twitter @SESYNC.

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