Usually, humans get infectious diseases from each other. But not always. Some pathogens (disease-causing viruses, bacteria and parasites) aren't too fussy about their hosts.
Humans, other animals, and passing the pathogen
A zoonosis (plural zoonoses) is a disease that humans share with other vertebrates (mammals, birds and even fish). Zoonoses include rabies (from bats as well as dogs); Variant Creutzfeldt-Jakob disease (the human manifestation of "mad cow"); parrot fever; bird flu and swine flu.
We humans tend to forget that it can also work the other way round: we can infect other species too. The term sometimes used for this is “reverse zoonosis”. Some interesting examples of what we can give to animals: TB to elephants; mumps to dogs; hepatitis, measles and respiratory illnesses to chimpanzees and other primates; herpes to marmosets; flu to ferrets; diarrhoea to cats.
Sometimes infections can go from humans to animals and then to other species and/or back to humans again. And sometimes, a disease that has only been transmitted previously among, for example, pigs, crosses over to humans.
If these infections are viral, they can mutate fairly rapidly and even mix together in complicated ways: the current strain of swine flu making international news is thought to contain elements of human influenza, bird flu and swine flu.
The meat market: breeding ground for trouble
It’s not easy to trace the cause and origin of such outbreaks, and often scientists have to make a good guess. Many, even most, infectious human diseases are thought to have emerged from animal populations, many times throughout history, and we can’t blame ourselves for that.
But we can blame ourselves for making the problem many times worse. It doesn’t take much to see that breeding huge numbers of animals, keeping them penned up together in stressful and often unsanitary conditions, transporting them or bits of them all over the countryside and the globe, and in some cases, feeding them chopped-up bits of each other, is just asking for trouble.
And trouble has found us - in the form of avian flu, SARS and Creutzfeldt-Jakob disease, to name some of the most infamous zoonotic suspects.
Variant Creutzfeldt-Jakob disease probably had its origins in bovine spongiform encephalopathy (BSE), which spread among cattle because their feed contained, grotesquely, brains of infected members of their own species. Severe acute respiratory syndrome (SARS) is thought to have most likely developed in the "wet" markets in China, where different species of live animals are kept in unhygienic and often cruel conditions, coming into close contact with the people who buy and slaughter them.
Prof David Benatar, Head of the Department of Philosophy at the University of Cape Town, who has written and argued extensively on the topic, says: "The same principle – of meat production being a likely contributing factor to these disease outbreaks – applies in the case of swine flu."
"Where you have large numbers of animals in close proximity to humans, you create a breeding ground for zoonotic diseases and you make it easier for these to jump the species barrier."
It’s also asking for trouble eating those animals that are closest to us, the primates: moral issues aside, because they are genetically and physiologically similar to us, we can easily share their pathogens and vice versa. HIV/Aids is thought to have had its origins in the human consumption of other primates i.e. bushmeat hunting, a practice that still continues.
Tackling the root of the problem
Diseases like swine flu are fought on many fronts – destroying infected animals, establishing quarantine, educating the public about basic precautionary hygiene and seeking a medical cure. But we also need to reduce the chances that they’ll start in the first place – and one of the most basic ways to do this is to reduce our exploitation of animals for the fleeting satisfactions of the dinner-table.
"We've heard it's safe to eat pork if it's cooked properly," says Benatar, "but we're not hearing enough about the upstream causal factors, in other words, that it's the demand for pork, and the industry that produces it, that is helping to create the problem in the first place."
Pigs are charming, intelligent, also physiologically similar to humans and – given half a chance, in decent conditions – clean. But if these attributes, and the other less-emotive pro-vegetarian arguments don't sway you, to give up meat for good, then at least consider cutting down on how much of it you consume. You could be doing your own species a huge favour.
- Olivia Rose-Innes, EnviroHealth Editor, Health24, April 2009
Benatar, D. The Chickens Come Home to Roost. The American Journal of Public Health, September 2007, Vol. 97, Issue 9
Shimshony, A. Tuberculosis in elephants: a reverse zoonosisInfectious Disease News, December 2008