Nipah Virus – How Pig Farming Provided a Pathway for Disease
The 1998-1999 Malaysian outbreak of Nipah virus exemplifies how factory farming provides a pathway for disease to spread and develop into a lethal threat to human health. Researchers from Johns Hopkins University say that, in the outbreak, domesticated pigs being farmed for meat served as a biological vector between wild fruit bats (the host species) and humans.1Leibler JH, Otte J, Roland-Holst D et al. 2009. Industrial food animal production and global health risks: exploring the ecosystems and economics of avian influenza. Ecohealth. 6 (1) 58-70.
The Nipah virus is highly contagious in pigs and an infected pig may exhibit no symptoms, while others develop acute feverish illness, laboured breathing and neurological symptoms such as trembling, twitching and muscle spasms. In humans, symptoms also range from hardly any to severe acute respiratory infection and fatal encephalitis.
The Malaysian outbreak caused respiratory disease in pigs and a high death rate in humans killing 105 of the 265 patients. The World Health Organisation says that the case fatality rate is estimated at 40-75 per cent.2WHO. 2018. Nipah virus. Available: https://www.who.int/news-room/fact-sheets/detail/nipah-virus [Accessed 1 June 2022].
Many of the humans infected were pig farmers and a respiratory illness was observed among pigs on some of their farms. An investigation revealed that in 1997, both pig and human cases had occurred on a single large intensive pig farm in northern Malaysia. At this farm, Nipah virus-infected fruit bats had been attracted to fruit trees planted around the pig farm. The virus spillover event occurred when pigs ate fruit contaminated with bat saliva or urine.
“In intensive systems, genetic selection and management of livestock creates frequent contact opportunities, high animal numbers, and low genetic diversity, providing opportunities for ‘wild’ microorganisms to invade and amplify or for existing pathogens to evolve to new and more pathogenic forms.”
The spread of disease among the pigs was made easier by the high density of over 30,000 animals and by transporting them to other farms located in what later became the main outbreak area in south Malaysia. Agricultural intensification thereby provided the pathway for a virus circulating in fruit bats to infect pigs and then people. Researchers from the University of London describe how: “Pigs then acted as amplifier hosts for human infection.”5Jones BA, Grace D, Kock R et al. 2013. Zoonosis emergence linked to agricultural intensification and environmental change. Proceedings of the National Academy of Sciences USA.110 (21) 8399-8404.
Summing up the role of factory farms in disease, they said: “In intensive systems, genetic selection and management of livestock creates frequent contact opportunities, high animal numbers, and low genetic diversity, providing opportunities for ‘wild’ microorganisms to invade and amplify or for existing pathogens to evolve to new and more pathogenic forms.”3Jones BA, Grace D, Kock R et al. 2013. Zoonosis emergence linked to agricultural intensification and environmental change. Proceedings of the National Academy of Sciences USA.110 (21) 8399-8404.
In a paper published in Interface, a journal of the Royal Society, scientists describe two different stages of a deadly disease outbreak and a missed opportunity for early detection and prevention. They suggested that it was the Nipah virus jumping repeatedly from fruit bats to pigs that had deadly consequences and say that it was this repeated introduction that changed the infection dynamics in pigs and subsequently in humans. The initial introduction of the virus created a “priming” effect that allowed a secondary introduction to persist in what was then a partially immune population. As pigs born after the initial event gradually lost their maternal antibodies, they became susceptible and allowed Nipah virus persistence.6Pulliam JR, Epstein JH, Dushoff J et al. 2012. Agricultural intensification, priming for persistence and the emergence of Nipah virus: a lethal bat-borne zoonosis. Journal of the Royal Society. Interface. 9 (66) 89-101.
The emergence of Nipah virus in Malaysia was thus the product of two drivers. First, agricultural intensification, in the form of commercial pig production and fruit orchards, creating the pathway for the repeated transmission of the virus from fruit bats to pigs. Second, the initial spillover primed the pig population for persistence of the pathogen on reintroduction, leading to the increased transmission among pigs and to humans. Once infected pigs were sold outside the region, the opportunities for greater human exposure, infection and disease followed.7Daszak P, Zambrana-Torrelio C, Bogich TL et al. 2013. Interdisciplinary approaches to understanding disease emergence: the past, present, and future drivers of Nipah virus emergence. Proceedings of the National Academy of Sciences USA. 110 (Suppl 1) 3681-368.
According to this study, the discovery of historical cases after the identification of an emerging pathogen is common and has been seen before with Nipah virus, SARS, HIV and H5N1 influenza.4Pulliam JR, Epstein JH, Dushoff J et al. 2012. Agricultural intensification, priming for persistence and the emergence of Nipah virus: a lethal bat-borne zoonosis. Journal of the Royal Society. Interface. 9 (66) 89-101.
The authors of this study support previous calls for increasing targeted global surveillance of livestock in regions of high wildlife biodiversity to improve the chances that “viral chatter” of wildlife origin is detected prior to a widespread epidemic in livestock or people. A more effective solution would be to stop factory farming altogether.
