Are predators good for your health? Evaluating evidence for top-down regulation of zoonotic disease reservoirs

Severe Acute Respiratory Syndrome (SARS) shows how vulnerable we are to disease agents that typically reside, unnoticed, in animal populations. In a sense, the SARS outbreak is nothing new – pathogens have been jumping from wildlife or livestock populations to humans since antiquity, often with devastating results. Recent analyses estimate that about 60% of all infectious disease agents affecting humans are zoonotic in origin, meaning that the pathogen typically resides in non-human vertebrate reservoirs; about three-quarters of emerging infectious diseases of humans are zoonotic (Taylor et al. 2001). Transmission to humans can come from swallowing or inhaling waste products from the animal reservoir, from eating them or being bitten, or from ectoparasites such as mosquitoes, fleas, or ticks that deliver pathogens to human hosts. In some cases, notably SARS and AIDS, once the pathogen has jumped to our species, humans become a self-sufficient arena of transmission – but these are exceptions to the more typical zoonotic pattern, in which humans can only acquire an infection from nonhuman reservoirs. The vast majority of zoonotic reservoir species are mammals, and most are rodents (Mills and Childs 1998). Monkeypox, hantavirus pulmonary syndrome (HPS), Lassa fever, Argentine and Bolivian hemorrhagic fevers, Lyme disease, granulocytic ehrlichiosis, leishmaniasis, bubonic plague, scrub typhus, tick-borne encephalitis, Crimean-Congo hemorrhagic fever – for these diseases and many more, we are the unwitting victims of pathogens that cycle, often cryptically, within rodent populations. Most of these diseases are characterized in humans by unpredictable outbreaks followed by lulls. For example, HPS was first discovered when an epidemic struck the US Southwest in 1993 (Yates et al. 2002). Other zoonoses appear to be a more constant threat, but with some years worse than others. For instance, Lyme disease strikes, on average, about 15 000 people in the US each year, but annual case loads vary by up to 40% (Ostfeld 1997; Wilson 1998). What causes outbreaks and bad disease years? Have zoonotic diseases become more common or severe in recent years, and if so, why? Are human-caused alterations of the environment playing a role in generating patterns of disease risk or incidence?