The last 30 years have seen a rise in emerging infectious diseases in humans, of which more than 70% are zoonotic. Zoonoses are diseases that normally exist in animals but have the potential to transmit to humans. They can be caused by many different infectious agents including bacteria, fungi and viruses.
Zoonotic infections have always been a part of the human disease landscape and most have come from domestic animals. The long lists includes anthrax, tuberculosis, plague, yellow fever and influenza. But with changes in environment, human behaviour and habitat destruction, these biosecurity threats are increasingly emerging from wildlife species.
Although it was established over a century ago that rabies was linked to bats, the research community was surprised to find that the SARS virus – which claimed more than 800 lives and cost more than $80bn globally – emerged from bats to civets and ultimately infected humans in the wet markets of southern China.
The World Health Organization (WHO) and most infectious disease experts agree that the source of the next human pandemic is likely to be zoonotic and wildlife is likely the prime suspect. While much effort has understandably gone into preparations for avian influenza, the next deadly pandemic may be the result of a currently unknown zoonotic agent.
Since the identification of bats as the probable source of the SARS epidemic, the global focus on emerging infectious disease has turned to them to understand and ultimately predict the source of the next human pandemic.
In the last 20 years, a significant number of highly lethal viral diseases have emerged from bat species across the world. These include Hendra virus and Australian bat lyssavirus in Australia and Nipah virus in Malaysia and Bangladesh, where regular outbreaks reach mortality levels of 100%. Haemorrhagic fever viruses, including the feared and lethal Ebola and Marburg viruses, have also emerged from bats in Africa and Asia.
After crippling the globe in 2003 and 2004, SARS appeared to have vanished until last year when there was a deadly human SARS-like outbreak in Saudi Arabia. It killed six out of 12 infected patients and cases of infection continue to emerge. Known as SARI (severe acute respiratory infection), this infection has now shown the critical capacity to transmit from one person to another and, like its precursor, initial evidence supports its emergence from bats.
Zoonotic diseases in humans can take several different courses. For some, like rabies and West Nile virus, humans are “dead-end” hosts. That is, they transmit (spill over) from their animal reservoir (host) into humans but as there’s no subsequent human-to-human transmission, the disease is restricted from spreading.
Others, such as SARS and avian influenza, spill over to humans, cause disease and are able to transmit from person to person before being eradicated or “burning out” from the human population, leaving no residual infection except in its animal host.
The third are diseases such as HIV AIDS, which spilled out of primates decades ago and has persisted in the human population ever since. And measles and mumps, which probably entered the human population thousands of years ago and are somewhat controlled but still circulating.
It’s impossible to completely safeguard against zoonotic diseases but steps can and are being taken to limit the opportunity for spill-over events through monitoring and rapid response when and where they do occur.
Controlling zoonotic diseases and protecting our animals, people and environment from increasing biosecurity threats will not only take a global effort but a multidisciplinary one. It cannot be addressed adequately with traditional human medical strategies where disease is fought in the human population only.
If we are to prepare and respond adequately to the next zoonotic attack, the approach needs to be diverse, taking in medical, veterinary, ecological and environmental factors. The transition will be complex, but necessary if we are to protect the global community from zoonotic disease as best as we can. After all, the stakes are high.