PSY1HAE Topic 14: Human Perceptions of Risk and Harm PDF

 Topic 14: Human Perception of Risk and Harm – Which Animals Really Threaten Our Health really pose will be explored and we will consider When you have completed this topic, you will: examples involving different types of...

 Topic 14: Human Perception of Risk and Harm – Which Animals Really Threaten Our Health really pose will be explored and we will consider When you have completed this topic, you will: examples involving different types of animals. We will continue the topic by exploring relatively be realistic in evaluating the threat new diseases like bird flu and mad cow disease, posed to humans by potentially dangerous animals and also reflect on the kind of global threat posed by these diseases. Of course, we all know understand the meaning of the word zoonotic and why many diseases first-hand now what a pandemic feels like, so affecting humans are able to be this topic is particularly timely. Finally, we will transmitted via animals investigate the contentious issue of whether be familiar with a range of zoonotic exposure to animals can have a protective effect diseases, including the route of on human health. transmission, major symptoms and how big a risk they pose to humans be able to describe why vCJD was such a puzzling disease and why it had international ramifications and led to revision of basic food safety rules know how best to avoid major zoonotic diseases and understand that it is not currently possible to eliminate all risk be familiar with the hygiene hypothesis and with the possibility that early exposure to animals and their products may have protective effects on humans In our previous topic we explored the issue of animals as pests and competitors. While many people like to think of animals as being cuddly, or useful, or good to eat, it remains true that humans compete with many animals for resources and regard many as pests. In this topic we will take this negative view of animals to its extreme, examining the risks believed to be posed by animals to human health and to Dangerous animals human lives. We will start by exploring risks To begin this topic, grab a piece of paper and associated with animals capable of harming us list the five animals that you think are most likely directly. We will then focus mainly on zoonoses. to threaten your health or life. Do not spend too What these are and the kinds of risks that they long thinking about this. Just write down which left to eat except human crops. The World animals you should be most afraid of, and why. Wildlife Fund has some programs in place to prevent human-animal conflict. In Australia, the What kinds of animals did you list? Did you native wildlife does not pose the same sort of include humans – the animal most likely to kill threat as African animals. Australia possesses you either violently or by spreading disease? very few large predatory species, although there Most people forget humans but include large have been a few high-profile cases of wild animals that could launch direct attacks against dingoes attacking humans and we do lose a few humans. This is sensible since many of our people each year to sharks and crocodiles. ancestors were killed by animals looking for an easy meal. In reality, however, being attacked and eaten is really only a threat for a very small number of people in developed countries – those living close to big cats perhaps, and those who like to go swimming with sharks or crocodiles. Sharks rate highly on the list of animals people are terrified of, but in Australia, we average only about one fatal shark attack per year. This compares well with the 400 or so people who drown annually while surfing, body More problematic in Australia are a large boarding, scuba diving or swimming. number of venomous species, including snakes, Far more dangerous than sharks, although not spiders, and jellyfish. Injuries from these animals in Australia, are large terrestrial mammals like are relatively rare as they usually harm people hippopotamuses and elephants. Hippos kill only if they perceive a threat to their own well- more people in Africa than any other wild being. If left alone, humans and most deadly animal, primarily because they are extremely species in Australia can coexist quite territorial, very short-sighted, and instinctively comfortably. Did you include snakes and spiders use their large jaws and 60cm (24 inch) long in your list of animals which are likely to harm teeth to crush canoes and people who you? If so, how realistic was your assessment of accidentally get in the wrong place. this risk? Have you, or any of your friends or family members, ever actually been threatened by a snake? For most Australians, the answer to this question is no. Statistics confirm that attacks by animals remain very rare in developed countries, considering how many people spend time hiking, camping, biking, hunting, and fishing in mountain lion, bear, crocodile, or dingo territory. One mountain lion fatality per year in California compares really well with over 4,000 motor vehicle fatalities. For some reason, though, people are It has also been estimated that 200 people have particularly frightened by the thought of animal been killed by elephants in the last decade. attacks. This is probably because of their Often these unfortunate incidents are directly evolutionary significance. We talked about attributable to the kinds of competition we biophobia previously. Humans are much more talked about in the last topic. The animals are likely to be frightened of things that WERE likely forced to live close to people because most of to kill us during our evolution than we are to be their habitat is destroyed and they have nothing frightened of things like cars and guns that ARE more likely to kill us now, but that are so new in Established zoonotic diseases evolutionary terms that we have had no time to In addition to causing direct physical harm and adapt. We might learn to be frightened of these injury, either intentionally or accidentally, things, but it is not something that happens animals can adversely affect our health indirectly almost automatically like our fear of animals. through the transmission of diseases called Even within the animal domain, we tend to be zoonoses. This is a term used to refer to any scared of the wrong species. What species of infectious disease that can be transmitted from animal do you think causes the most deaths in animals to humans or vice versa. Realistically, the USA each year? Surprisingly enough, it turns zoonotic diseases pose a much greater risk to out to be deer. People do not generally get us than predation or direct attack. This is attacked by deer, but thousands of deer die because of how these diseases evolved. each year when they are hit by cars and, in some of these accidents, the humans involved also fare poorly. About 100 people are killed every year. This compares to about 5 humans killed annually in the USA by sharks and 1-2 by alligators and crocodiles. Wolves do not even rate a mention, although dogs do not fare particularly well. From 1979 to 1996 dog bites killed 380 people in the USA, or 20 per year, mostly children. Remember, though, that this is fewer than the number of people killed by lightning. There were another 40 deaths due to As we have discussed previously, most of our bee stings, about 3 per year due to rattlesnake history was spent as small bands of hunter bites and 3 due to bites from black widow gatherers. Because these bands of humans spiders. Animals also cause deaths in workplace rarely came in contact with each other, diseases accidents. Between the years of 1992 and 1997 that are transmitted only between humans 350 workplace deaths in the USA involved tended to die out fairly fast – epidemics only animals. That’s about 70 per year. Cattle and really became possible when our populations horses were the main animals involved. grew and we started interacting with each other Statistics in Australia are not so easy to come more frequently. To survive, therefore, a by, but one might hazard a guess that biological pathogen had to either cause some kangaroos probably kill more people than most kind of chronic infection that could stay alive in animals, simply because of their propensity to the human host for long periods of time without jump in front of those fast-moving capsules we causing death, or it had to be able to live in both call cars! non-humans and humans. Many disease organisms can do this because humans and other animals are so similar biologically. It is not surprising, then, that many modern human diseases started out as zoonotic diseases. It is difficult to be certain which diseases jumped from animals to humans rather than vice versa, but there is good evidence that this list includes measles, smallpox, influenza, and diphtheria. It might also include HIV, the common cold, and tuberculosis, although this is less certain. And, of course, investigations into the origins of COVID-19 are continuing. There greatest artists, politicians, and scientists of all are a number of well-established zoonoses time. Information like this makes you think twice including rabies, brucellosis, anthrax, about not using pesticides to kill mosquitoes salmonella, and the Bubonic plague. These and fleas doesn’t it? diseases continue to cause havoc in many Other ways of contracting zoonotic diseases countries, especially in the developing world include direct contact with infected animals, and especially among the poorest people. usually by way of bites and scratches, eating of Millions of people are affected annually. contaminated animal products or water, There are various ways for humans to contract droplets of saliva or blood coming into contact zoonotic diseases from animals. What do you with your own bodily fluids, ingestion of think is the most common form of transmission? contaminated products from your hands when they have not been washed well enough and, unfortunately, some diseases can travel straight through your skin. The best way to avoid zoonotic diseases is not by avoiding contact with animals, but by practicing basic hygiene. Wash your hands after touching animals or animal products, wear protective clothing and face masks, avoid bites and scratches, make sure your pets are vaccinated and treated if they become ill, and so forth, all precautions that are pretty much common sense these days, at least in developed The humble mosquito turns out to be the countries. Or are they? As we develop closer deadliest beast known to humanity, killing about and closer relationships with our pet animals, 2 million people each year worldwide by we may forget that they can harbour disease. transporting diseases like malaria, encephalitis, Many of us are quite comfortable kissing and West Nile virus, and Dengue fever from host hugging our pets and sharing our food with animals to humans. Not all diseases carried by them. Is this putting our health at risk? To mosquitoes are zoonotic but many are. The evaluate this critically we need to look at how tsetse fly is also fairly nasty, killing up to 70,000 some of the most common zoonotic diseases people annually, as are a variety of ticks, fleas, are spread. and other insect parasites that act as vectors, spreading diseases that they do not actually catch from one organism to another. Even the Black Death is attributed by many people to a type of flea carried by rats. This disease caused one of the most devastating pandemics in human history, wiping out nearly 75 million people in the late 1300s and undoubtedly changing the course of human history. Have a second look at this figure - 75 million people in the late 1300s was a significant proportion of the entire population. Imagine all the potential Rabies that was lost when these people died - the children and grandchildren that never had an This is a deadly disease caused by a virus that opportunity to be born potentially included the is carried in saliva and that can affect all mammals, including dogs, cats, and farmed animals, as well as wild animals like raccoons, Anthrax skunks, bats, and foxes and, of course, humans. This is another zoonosis that was once quite People usually contract rabies after being bitten common, affecting thousands of grazing animals by an infected animal and it is almost always like sheep and cattle. It is a bacterial disease and fatal once it is established. The rabies virus forms spores that can infect humans who come attacks the brain and nervous system, causing in contact with infected animal skins, fur, wool, acute inflammation or encephalitis. This leads to horns, leather, or contaminated soil. It used to cerebral dysfunction including anxiety, be a particular risk for people who spent their insomnia, confusion, agitation, paranoia, and days sawing up horns to make buttons. We see other abnormal behaviours. it only infrequently now but occasionally there are anthrax outbreaks. These are likely to occur forever because the bacterial spores survive in the ground indefinitely. We also occasionally see anthrax used in bioterrorism attacks, like when it was mailed through the USA postal service in 2001. Anthrax can enter the human body through the intestines, lungs, or skin, and produces distinct symptoms based on its site of entry. Pulmonary anthrax causes respiratory infection followed by respiratory collapse, and is fatal unless treated Rabies is not an issue in Australia, due mostly before symptoms appear. Gastrointestinal to our tough quarantine restrictions, and it is not anthrax causes severe gastro symptoms, a great risk in most developed countries today, including vomiting, diarrhoea, weight loss, and as most pet animals are vaccinated against the inflammation, with fatality rates of 25-60%. disease. Also, there are fewer wild animals in Cutaneous anthrax causes nasty looking skin most developed countries than was previously lesions that are itchy but less often fatal (20%), the case and those that are infected tend to probably because people seek treatment avoid people. Rabies is a very real threat in sooner. Thousands of people and animals used developing countries, however, so if you are to die of anthrax each year in Europe, Asia, and ever in one of these countries and come into North America until Louis Pasteur developed the contact with an animal that looks like it might first vaccine in 1881. The fact that we do not have rabies, remember that it probably is not have to worry about it so much now probably able to think normally, so get well away from it. helps explain our relaxed attitudes towards some animal species. People in countries where rabies is still common often have quite different attitudes towards Toxoplasmosis animals than is the case in Australia, and This is a disease caused by a parasite. It can be governments in these countries often take a contracted through contact with cat faeces, but much harder line against roaming dogs and cats is more commonly associated with drinking than we do. There are often reports of local infected water, eating contaminated meat that is councils shooting stray dogs and cats or laying not well cooked, or receiving an infected organ poisons. This horrifies many Westerners but transplant or blood transfusion. Most people makes perfect sense if you consider the level of who have the toxoplasmosis parasite do not get risk posed by the rabies virus. Would you want sick. In fact, it is estimated that about 60 million your child to be put at risk by infected animals? Americans carry the parasite. Those who do develop symptoms of the infection may experience swollen glands, fatigue, muscle pain, fever, headache, cough, sore throat, and rash. Toxoplasmosis is important to know about because it can be serious in people with weak immune systems and it can cause serious damage to the brain, eyes and other organs of foetuses whose mothers become infected during pregnancy. Toxoplasmosis is a threat in our community and occasionally influences how we perceive and react to animals. To avoid infection, some people choose to rehome pet cats when they (the person not the cat) become pregnant. You might know somebody who has done this or might have done it yourself. Most experts assert that this reaction is unnecessary and attention to basic hygiene is just as effective. Pregnant humans should avoid cleaning cat litter boxes, especially if the cat spends time outdoors and in contact with stray animals, but getting rid of a cat is generally not warranted. What do you think about this issue? Would you dispose of a pet that you believed posed a risk to your unborn baby – even if the experts said the risk was minimal? Would it depend on how long you had lived with the cat, or how much you cared Newer zoonoses about it? Once you know about a risk, even a In addition to older diseases that are well tiny one, what is the appropriate way to react? established, most new diseases, about 75% of Salmonella those that have affected humans over the past 10 years, have been caused by pathogens Dogs and cats are not the only pet animals that originating from an animal or from products of can infect humans with diseases and parasites. animal origin. One major factor contributing to Reptiles often carry Salmonella on their skin. the appearance of new zoonotic pathogens in People who contract Salmonella after touching human populations appears to be increased reptiles or through other means can become contact between humans and wildlife, caused seriously ill. Reptiles also shed Salmonella in either by encroachment of human activity into their faeces, so people can become infected by wilderness areas or by movement of wild touching a reptile's cage or other contaminated animals into areas of human activity due to surfaces. Symptoms include severe diarrhoea, environmental disturbances. Another is our vomiting, fever, and chills. Antibiotics can be modern animal-keeping practices, where we used, however, they are not always necessary in contain thousands of animals in close proximity. mild cases. If you listed snakes as one of the New diseases are particularly scary because animals most likely to threaten human lives, was humans lack immunity to them. They also have it because they potentially carry disease? Most the potential to spread over long distances people are afraid of snakes because they might before being detected, because so many more get bitten, but this may be less likely than of us, and the animals that might carry the disease transmission, especially for people who diseases, travel. Below are a few examples of keep reptiles as pets. recent zoonotic diseases you might have already heard about. Bird flu easily change, or mutate, to more easily infect humans. In fact, early in 2009 it was reported Probably one of the best-known examples of the for the first time that human to human newer zoonotic diseases is bird flu. Birds, just transmission of bird flu occurred in a family in like people, get the flu. This is not normally a Indonesia. Bird flu can make people very sick or problem for us because the bird flu virus only even cause death, and there is currently no affects birds. In birds it is highly contagious, like vaccine. How have you reacted to reports of bird flu in humans, and can range from being mildly flu in the past? Were you concerned at all about inconvenient to devastating. This pattern of the potential for widespread devastation, or did effects is always a problem with diseases you just brush it off as something happening because it means they can be spread easily by elsewhere? Australia is quite well protected those who are not catastrophically affected, and because of our geographical location, but when this problem is magnified in birds because they international travel is common, the potential for can travel long distances so easily. When diseases to affect the entire globe is very real. domestic birds contract a new strain of flu from a passing wild bird an entire flock can die within Diseases associated with the bushmeat trade a couple of days. Bushmeat is a term used to refer to meat obtained from wild animals through hunting, particularly meat from wild animals killed in West and Central Africa. Humans have been eating bushmeat for centuries, but this has recently attracted international attention because logging (and the development of logging tracks) in these countries has resulted in much more of the country being accessible to hunters. In addition to providing food for local populations, there is a sizeable black market in bushmeat products, which are transported illegally to countries like the USA and China, to be served in very expensive settings – usually without undergoing the rigorous tests used to control the quality of domestic products. How much would you pay to eat a zebra steak? How about gorilla soup? Most people are horrified by the idea, so why do you think there is demand for such products among Westerners? What sort of person would pay hundreds or thousands of dollars to eat an endangered animal? The big problem with bird flu is that lately it has The bushmeat trade is devastating populations posed health risks to people. The first case of a of rodents, birds, and ungulates (hoofed bird flu virus infecting a person directly, H5N1, mammals), as well as primates such as gorillas, was in Hong Kong in 1997. Since then, this bird chimpanzees, and bonobos, many of whom have flu virus has spread to birds in countries in Asia, never before been exposed to hunting on the Africa, and Europe. People who have contact kind of scale that modern humans are capable with infected birds can become sick, as can of. In addition to causing species loss, however, people who eat infected birds that are not well the bushmeat trade is of concern because it cooked. Human infection is still very rare, but potentially exposes humans to zoonotic the virus that causes the infection in birds could diseases we have never been close to before. This includes those carried by primates which, symptoms of HIV even though they are testing because we are so closely related to these negative for both HIV and SIV. This suggests species, we are often particularly vulnerable to. that new strains of a HIV-like virus might exist in wild animals, infecting people who eat them. Experts warn that these new strains could fuel an already disastrous global HIV pandemic. Also, it’s important to note that many zoonotic diseases can travel both ways. When Europeans colonised America, and then Australia, they carried with them diseases against which native human populations had no immunity. Might one of our diseases wipe out primates living in remote areas? Is this something we should be thinking about as we try to conserve This interesting TED talk shows how new viruses endangered species? What if the very people are discovered and how they transmit from who are trying to save these animals are also animals to humans. Diseases carried by putting them at risk? There have been a few ‘bushmeat’ that we should be worried about reports of COVID-19 affecting animals in homes include a number of what are called Viral and zoos and a particular frenzy about mink, Hemorrhagic Fevers or VHFs. The Ebola virus is who appear to be highly susceptible. In probably the most well-known of these but November 2020, Denmark, the world’s largest there are several others that exist in chimps and exporter of mink fur, killed up to 15 million gorillas and possibly spread to humans during animals after finding that some were infected the butchering and hunting of these animals, with a mutated variant of COVID-19. This was a although the fruit bat is another likely candidate. controversial move, but the virus has since been VHFs initially cause fever and fatigue, but this found in over 400 mink farms in at eight develops into bleeding from internal organs and different European countries. What if COVID-19 bodily orifices, vomiting, diarrhoea, and muscle started affecting our companion animals? How pain. This can progress to shock and seizures would you react? and is fatal in up to 90% of cases. No vaccine Risks associated with domesticated animals or treatment is currently available. Domesticated animals are generally considered Some people also think that HIV crossed over to to be safer to eat than wild animals, probably humans during the butchering of apes infected because we are familiar with most diseases that with SIV, the Simian or monkey virus from which affect these animals and have them under some HIV is believed to have evolved. The World form of control. In the late 1980s, however, 165 Health Organization estimates that AIDS has people died after developing some unusual killed more than 25 million people since it was neurological symptoms. These symptoms recognised in 1981, with another 30 million resembled a disease already known to affect people currently living with the disease. Two humans, called Creutzfeldt-Jakob disease or thirds of these are in sub-Saharan Africa, CJD, so the affected people were said to be including nearly 2 million children. suffering from variant-CJD or vCJD. SIV infection has now been reported in 26 CJD is what is known as a prion disease. Prions different species of African non-human primates, are unusual infectious agents made completely many of whom are hunted and sold as food. It of proteins. The proteins they are made of are is believed that the HIV virus has jumped from found throughout healthy people and animals, primates to humans at least seven times and but with a prion disease, they somehow get people in Cameroon are showing up with folded into abnormal structures that aggregate extra-cellularly within the central nervous no way to detect the disease in living cattle. This system to form plaques known as amyloids. makes it impossible to contain the disease with These disrupt the normal tissue structure targeted culling (killing). causing irreversible damage to the brain and It took a long time for somebody to figure out eventual dementia. All known prion diseases are that BSE might be a prion disease, because untreatable and fatal. these diseases, as you might recall, are not contagious in the normal sense of the word but are typically spread among animals who eat each other’s brains. This is not something we normally associate with cows who, like most other food animals, are herbivores, eating grass or grains. In modern industrial cattle-farming, however, commercial feeds are often used, which contain ingredients including antibiotics, hormones, pesticides, fertilisers, and protein supplements. Worldwide, soybean meal is the primary plant- The puzzling issue with vCJD is that, while based protein supplement fed to cattle. several prion diseases are known, these are all Soybeans do not grow well in Europe, however, usually transmitted by direct infection – that is so cattle producers tend to use less expensive by eating infected tissue. For example, there is animal by-product feeds as a protein alternative. one called kuru in humans, which is a disease These can be made up of a whole range of that used to affect cannibals who ate the brains ingredients, including things like meat and bone of affected persons. The people affected by vCJD meal – produced from the minced-up left-overs certainly were not cannibals and prion diseases of the slaughtering process, including sick and do not usually jump species, so it was very injured cattle, sheep, or chickens. difficult to figure out what was going on. Somebody eventually made a link between vCJD and another disease that had been affecting cattle in England, called bovine spongiform encephalopathy (BSE) or mad cow disease. Just like prion diseases, BSE affects the brain, turning it into something resembling a sponge and leading to a range of neurological problems and then death. Cattle affected by BSE may be nervous or aggressive, show abnormal posture and difficulty with coordination and standing up, decreased milk production, and reduced body This practice was widespread in Europe prior to weight despite a good appetite. about 1987 but was not considered dangerous Although BSE never caused the type of because, before being included in food stuffs, devastation in cattle that we associate with animal products are rendered. This means they other livestock diseases, it is fatal and all are heated to a temperature which kills most infected cattle eventually die. There is no disease organisms. Unfortunately, however, vaccine or treatment. It is also particularly nasty standard rendering processes do not because, although death usually occurs within 2 necessarily destroy the BSE agent and a slight weeks to 6 months of visible symptoms, the change to the rendering process in the early incubation period can be 4-6 years and there is 1980s is believed to have resulted in a large increase in the infectious agents in cattle feed. In addition, people are still being diagnosed A contributing factor seems to have been a with vCJD. This is expected to continue for years change in British laws that allowed a lower because the disease has a long incubation temperature sterilisation of the protein meal. period and is resistant to normal infection While other European countries like Germany control procedures. Initially it appeared that required animal byproducts to undergo a high only people with a specific genetic makeup were temperature steam boiling process, this susceptible to vCJD, but it in 2017 it was requirement had been eased in Britain as a reported that the disease had developed in measure to keep prices competitive. someone with a different genotype, many years after this person was likely to have been The end result of all these seemingly unrelated infected. This could mean that a second wave of events is that it now seems likely that over the disorder is still coming. So far, only people 400,000 herbivorous cows and steers, infected born before 1989 have been affected, but the with BSE by the non-herbivorous protein full extent of the problem is not yet known. supplements they were fed, entered the food chain in England in the 1980s. Only meat-eating humans were infected with vCJD. For some of these people, there is direct evidence that they had consumed beef from cattle infected with BSE. So, this is now assumed to be the mechanism by which all patients contracted the disease. In a nutshell, humans created a new problem by feeding cattle something they were never supposed to eat. As you can imagine, this led to a public uproar in England and the rest of the world. BSE was In the USA, soybean meal is fairly cheap, so the controlled by killing any cattle or other animals use of animal byproducts in cattle feed was that were even remotely likely to have been never very common. A single case of BSE was exposed to contaminated food stuffs and by identified in Washington in December 2003, banning the inclusion of mammalian or ruminant however, and this had huge ramifications. Japan, protein in ruminant feed. This was financially the top importer of USA beef, stopped all devastating for the British beef industry. Even imports. This cost beef producers about $1.4 worse, many people were forced to kill their best billion annually. The bans were lifted in 2005 breeding stock, animals that might have been but were reintroduced when somebody left a developed over many generations, as well as spinal column in a crate of veal headed for family pets, even though there was very little Tokyo. All nervous system tissue, likely to carry chance of these animals having the disease and BSE, was supposed to be removed prior to absolutely no chance of them passing it on shipping. Although the prevalence of BSE was unless they were eaten. Only about 220 people always extremely low in the USA, sixty-five were eventually diagnosed with vCJD nations had full or partial restrictions on beef worldwide, with most of these being based in from the USA because of concerns about BSE. the United Kingdom, but entire expanses of the As a result, beef exports dropped from $3.8 English countryside were rendered virtually billion pre-BSE to $1.4 billion in 2005. animal-free in a short period of time, with piles of burning animals all over the place. This One of the countries to benefit from this was affected jobs and the economy, stopped the Australia. Most of our agricultural animals are tourist trade, and had terrible effects on mental (or were until relatively recently) grass fed and, health that will last for ages. so far, there have been no reported cases of BSE. Thus, Australia picked up many of the beef bodies. Many of us think we would be happy to customers who deserted Britain and the USA. wipe out the mosquito if we could, although there are so many of them that we would This may make eating beef seem really risky, but probably all end up with pesticide poisoning. even at the height of this event, the chance of Also, keep in mind that mosquitos are what is contracting vCJD was less than 1 in 400 billion. called a keystone species. If we wipe out In comparison, your odds of being killed in a car mosquitoes, entire ecosystems are likely to crash are 1 in 242, and your odds of death from collapse. And what about the animal we are a fall in the bathtub 1 in 10,760. Thousands of MOST likely to catch a disease from? This is the people die annually because their doctors human animal sitting close by to you, in the next prescribe incorrect medications for them, a far room or even in the next house. Human to greater number than that affected by vCJD. Still, human disease transmission is far more this story does serve as a warning for what can common than animal to human disease happen when we mess with the food chain. transmission. Despite our best efforts to keep things under control, we can’t always foresee problematic outcomes. This is one of the arguments against genetic engineering of plants and animals, as we will explore in later topics. How to avoid zoonotic diseases One of the things you should know about zoonotic diseases is that we are unlikely to eliminate them completely while we have animals in our society, particularly if we have close relationships with those animals. So, does this mean that we should simply have NO Since it is not an option to just remove all animals in society? How would you feel about potential disease carriers from our society, we this as a solution? need to find ways to manage the threat associated with zoonotic diseases, while ensuring that all the good things brought to us by animals are retained. Pessimists predict that it is only a matter of time before we face another pandemic of plague proportions – with even more devastating consequences than previous ones given our larger population and our tendency to travel and spread our germs all over the place. This may be true, and could make the COVID-19 pandemic look like a picnic, but does Before you answer this question, think again it really justify how we currently seem to over- about all the different ways that animals are react to new threats? At present, when we are useful to us, and about their inherent worth. threatened by a new zoonotic disease, we tend Would you really want to live in a society with to respond by killing thousands of animals. This no animals in it? was certainly the case in Denmark, when it was found that mink are susceptible to COVID-19, Think also about those animals that we are most but I’m sure you’ve seen photos of thousands of likely to catch a disease from. This would include chickens or sheep being killed in other events, our pets and the animals we eat and wear. It often just in case they may be affected. would also include the mosquito, able to inject contaminated animal products directly into our Is this acceptable? What if it were cats or dogs those interested in human-animal relationships who were potential carriers? Would you be should know about. A systematic review of the prepared to have your much-loved family pet effect of exposure to cats and dogs on the risk killed to avoid a pandemic? This approach is of asthma and allergic rhinitis, published in common in some countries but would be very 2020 by Gao and colleagues, concluded that unpopular in countries where pets are valued as there was a potential protective effect of family members. Why is it that we are reasonably exposure to these animals. The evidence is comfortable with the idea of killing thousands of messy, however, and not entirely convincing. production animals but not thousands of pets? Moreover, an entirely different picture emerges This may say more about human psychology in developing countries, where the animals that than it does about the other animals involved. children come into contact with are likely to be carrying all manner of diseases and parasites. In an earlier topic we spoke about the importance of veterinary medicine in influencing modern relationships between humans and animals. This is one situation where the general good health of our pets may be of great importance in determining their impact on our own health. Summary In this topic we considered animals that threaten human health or even human lives. We began by considering animals that kill us, perhaps as a Can exposure to animals protect humans from source of food or because they perceive us as a medical disorders? threat. Such animals are virtually non-existent in In this topic, the focus has been on how developed countries but can pose a threat exposure to animals can be bad for our health. elsewhere. A far greater risk exists in the While there are risks involved in human-animal modern world due to our reliance on animals in relationships, there is also emerging evidence to all aspects of our lives, and due to the tendency suggest that exposure to animals can be good of disease organisms to be able to take for us, particularly when we are young. In 1989, advantage of multiple biological hosts. Many epidemiologist David Strachan found that zoonotic diseases exist for which we lack both allergic diseases such as hay fever and eczema immunity and effective means of control. It may were less common in children from large be simply a matter of time before a major families. He proposed that this may be because pandemic occurs that affects both humans and such children are exposed to more infectious animals in ways far more serious than COVID- agents during childhood, and therefore build up 19 has to date. How we manage outbreaks their immunity to these agents in a gradual presents a number of challenges, since it is manner (Strachan, 1989). This hypothesis, impossible to entirely eliminate the threat posed subsequently called ‘The Hygiene Hypothesis’, by zoonotic diseases. Moreover, emerging has since been investigated worldwide. There is evidence indicates that living closely with a good summary of The Hygiene Hypothesis on animals may have health benefits for humans, Wikipedia if you need a quick overview, but this rather than health costs, in terms of mediating isn’t necessary for the purposes of this subject. the prevalence of several important medical Interestingly, while the link between family size disorders. There is still much work to be done and infectious disease is controversial, there is to fully reveal the importance of exposure to fairly strong evidence to suggest that children animals. However, removing all animals from our who grow up with animals are protected from a urban environments is likely to be counter- host of medical disorders. This is something productive in terms of protecting our health.  References and/or supplementary resources Cripps, J. P. (2000). Veterinary education, zoonoses and public health: A personal perspective. Acta Tropica, 76(1), 77–80. Gao, X., Yin, M., Yang, P., Li, X., Di, L., Wang, W.,... & Liu, J. (2020). Effect of exposure to cats and dogs on the risk of asthma and allergic rhinitis: a systematic review and meta-analysis. American Journal of Rhinology & Allergy, 34(5), 703-714. Langley, R. L. and Morrow, W. E. (1997). Deaths resulting from animal attacks in the United States. Wilderness and Environmental Medicine, 8, 8-16. Strachan, D. P. (1989). Hay fever, hygiene and household size. British Medical Journal, 299, 1259- 1260. Von Mutius, E., & Vercelli, D. (2010). Farm living: Effects on childhood asthma and allergy. Nature Reviews Immunology, 10(12), 861-868.