#92b: CoVid-19: How Animals and Humans Interface With Infectious Diseases
“What is known, however, is that the interface between humans and animals is of paramount importance in the process. As we increase our interactions with animals through hunting, the trading of animal foods, animal husbandry practices, wet markets, and the domestication of animals/exotic pets, the probability of cross-species transmission dramatically increases.”
It’s April 1, and here in the United States, that’s known as April Fools’ Day. Traditionally it’s a day where people prank each other, and there’s lots of laughter and fun. But instead of being lighthearted, we are in a somber time. We fear for our lives, our loved ones, our livelihoods, and our economy. I haven’t said much about the coronavirus because I feel like there are plenty of other experts taking care of the facts and making sure that everybody is informed. I want to talk a little bit about the origins of this disease so that perhaps we can start to change our behaviors and decrease the risk of this happening again.
First of all, like all things, this is more complex than it seems. Second of all, I am NOT an expert on this topic, and I realize that there are many things I do not know or understand about how diseases and infections spread from animals to humans and how much we can control. However, the more I learned about this topic, the more I discovered just how relevant it is to our health.
Experts believe that the coronavirus originated in a wet market in Wuhan, China. A wet market is a place that sells meat, fruits, and vegetables kind of like a farmers market. It is called a wet market because the floors are always wet because of the cleaning of vegetables and the hosing down of the meat stalls. On the other hand, WILDLIFE markets are places that sell live and dead animals from familiar to exotic for consumption. In China, 54 animal species can be legally traded live for human consumption, including koalas, snakes, minks, snapping turtles, crocodiles, dogs, cats, and rats amongst birds, chickens, and pigs, and other common domesticated farm animals. It is theorized that the COVID-19 originated in from bats, and the intermediary animal was a pangolin. The Pangolin is an endangered wild animal, also known as a scaly anteater. Lookup a picture of them; they are super cute.
You may be wondering how something like this is possible, so I want to break down all the different ways that humans can acquire infections from animals. I will also offer a few ways that we can decrease our risk of contracting diseases from animals.
There are three main ways that infections can be spread from animals to humans:
1. Vector Diseases - Vectors are living organisms that transmit infections between animals to humans or humans to humans, such as mosquitoes, flies, fleas, lice, ticks. Basically, these organisms carry the germs back and forth. Vector-borne infections are caused by parasites, bacteria, or viruses, and they account for 17% of all infectious diseases causing more than 700,000 deaths annually. Do you know what the world’s deadliest creature is? The mosquito! Mosquitos are a powerful vector in many parts of the world, and they can very quickly spread diseases between humans. Commonly known vector-borne diseases include malaria, dengue, Chagas disease, leishmaniasis, plague, Lyme disease, tick-borne encephalitis, and West Nile virus. Because these diseases are transmitted via a vector, you don’t have to come in direct contact with the animal. Still, they often originate from animals, and many occur near animal habitats.
The best way to prevent these infections is to be aware of areas that these infections are endemic and protect yourself from the actual vector. However, climate change and overcrowding may increase vector diseases. The increase in vector-borne diseases occurs because changes in weather such as increased rainy seasons, for example, can lead to more mosquitoes. Deforestation can also lead to changes in habitats, which can create more opportunities for vectors to thrive.
2. Zoonotic Diseases are infections that occur through direct and indirect contact with animals and are caused by viruses, bacteria, parasites, and fungi. These are very common and may account for 60% of known infectious diseases and 75% of new or emerging infectious diseases. Direct contact means that you come into contact with an infected animal’s bodily secretions or are bitten or scratched by an animal. Indirect contact means that you come into contact with surfaces where the animal has roamed, such as chicken coops, barns, plants, and soil.
The coronavirus is a zoonotic disease. The theory is that this virus first originated in bats, was spread to Pangolins, which then spread it to humans. Although it is illegal to eat Pangolins in China, they are valued for their medicinal properties in traditional Chinese medicine, and they are also a highly prized exotic dish. Experts believe that it was through contact with these animals that humans became infected with the coronavirus.
In zoonotic infections, are five stages through which animal-derived pathogens evolve to cause disease in humans. Stage 1 is transmitted only from animal to animal and cannot infect humans. Stage 2 is like rabies, it only originates in animals and can be transmitted to humans, but humans cannot infect other humans. Stage 3 pathogens can be transmitted human to human, but the spread between humans is limited to a few cycles, and so the outbreak is limited. Examples of stage 3 pathogens include Ebola or monkeypox. Monkeypox is transmitted to humans via infected bats or other wild animals and the eating of infected bushmeat. Stage 4 pathogens are like stage 3 but lead to more extended outbreaks like dengue or pandemic influenza and the plague. The plague, also known as the black death, is also a vector disease. It originated in infected rats and was transmitted between humans by fleas. I assume that the COVID-19 is stage 4 pathogen because it quickly became a pandemic. Stage 5 pathogens originate from an animal, but then it becomes exclusively an infection transmitted between humans. An example of this type of pathogen is HIV. HIV was thought to originate from eating chimp meat and may have evolved from the Simian Immunodeficiency Virus (SIV).
3. Food-borne and waterborne pathogens. Food-borne pathogens are infections that occur in raw foods and can cause illness in humans. Each year 1 in 6 Americans will get ill from eating contaminated food!
Two germs, in particular, cause many infections in the United States, E. coli, and Salmonella.
E. coli 0157:H7 is a bacteria that can cause a severe and sometimes deadly infection in humans. It lives in the guts of animals, including grass-fed cows, and it contaminates meat directly during the processing of meat. E. Coli infects 265.000 people each year and leads to 100 deaths annually.
Salmonella is another bacterial infection that causes 1.3 million infections, 26,500 hospitalizations, and 420 deaths each year in the US. Samonella originates from chickens and chicken eggs, lizards, turtles, cats, and dogs. Salmonella can also infect plants as they are growing but originates from animal feces in the irrigation water.
These bacteria can also be ingested indirectly through the contamination of water sources because of runoff from nearby farms.
Factory farming increases exposure to infections because of the nature of raising animals in this way. Hundred and thousands of animals are kept in crowded conditions. These conditions are breeding grounds for diseases. To decrease illness in these animals, they are often fed antibiotics. A significant percent of antibiotics in the United States are utilized in animal agriculture. Because of this, antibiotic resistance is becoming more common in animals.
Overpopulation and deforestation also put us in closer contact with wild animals. Deforestation increases with our consumption of factory-farmed animals. Forests are cleared to provide a place to grow their food or to provide a place for them to graze.
What can YOU do to decrease the risk of contracting infectious diseases from animals?
1. Decrease your meat consumption and stop supporting factory farming
2. Do not support the eating of wild and endangered animals
3. Watch the documentaries Eating Animals and Cowspiracy for more information on the impact of eating animals on our earth and the link to infectious diseases.
4. Listen to all my episodes this month and learn how we can all live more sustainably on this earth.
Change is hard, but if we don’t change willingly, the change will be forced upon us.
Remember, we are all connected, and we share this planet with many other creatures. What can we do to tread lightly and enjoy our lives, make it better for animals and nature and decrease our risk for disease? Are you willing to make changes to do that?
References:
Campbell-Lendrum D, Manga L, Bagayoko M, Sommerfeld J. Climate change and vector-borne diseases: what are the implications for public health research and policy?. Philos Trans R Soc Lond B Biol Sci. 2015;370(1665):20130552. doi:10.1098/rstb.2013.0552
Lloyd-Smith JO, George D, Pepin KM, et al. Epidemic dynamics at the human-animal interface. Science. 2009;326(5958):1362–1367. doi:10.1126/science.1177345
Pike BL, Saylors KE, Fair JN, et al. The origin and prevention of pandemics. Clin Infect Dis. 2010;50(12):1636–1640. doi:10.1086/652860
American Meat Institute Fact Sheet E. coli O157:H7 July 2010 https://www.meatinstitute.org/index.php?ht=a/GetDocumentAction/i/61119
Washington State Department of Health Animal Transmitted Diseases www.doh.wa.gov/youandyourfamily/illnessesanddisease/animaltransmitteddiseases
World Health Organization Vector-Borne Diseases March 2, 2020 www.who.int/news-room/fact-sheets/detail/vector-borne-diseases
Center for Disease Control and Prevention Zoonotic Diseases www.cdc.gov/onhealth/basics/zoonotic-diseases.html
Origins of Major Human Infectious Diseases, National Academy of Sciences www.ncbi.nlm.nih.gov/books/nbk114494/figure/appa.f49/?report=objectonly
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