A B S T R A C T
Some of the most well-known diseases throughout history are known, or speculated, to have an animal source. The COVID-19 pandemic has brought zoonotic disease spillover to the forefront of research and public interest. Zoonotic diseases can have many avenues of transmission and are unpredictable in the manner by which they spillover into the human population. This research project aims to expose the source of the next pandemic and highlight the contributory human interferences. This research blog will include a general overview of zoonotic diseases throughout history, discuss the prevalence of roadkill and the contributing factors, as well as identify and explain the possible avenues through which disease could potentially be spread from roadkill to humans. Though there are extensive preventative efforts already in place, humankind has continually neglected to acknowledge our part in producing pandemics. We must now act to identify and contain sources of future pandemics.
I N T R O D U C T I O N
Most emerging infectious diseases can be traced to a common source: animals. Roughly 2/3 of all human pathogens claim an animal host as a vector or as a reservoir (Lipman, 2015) and more than 75% of emerging infectious diseases can be traced to a zoonotic source (Gonzalez & Macgregor-Skinner, 2014). Some of the deadliest and most well-known epidemics in recent human history are known, or speculated, to have originated in animals including SARS, HIV/AIDS, and Ebola (Lipman, 2015). A zoonotic disease is any disease that exists within an animal population but can infect a human population through various modes of transmission; this ‘jump’ between species is known as a Spillover event ( Lipman, 2015). If the pathogen is successfully able to adapt and thrive within a local human population, the disease is referred to as an emerging disease. Some zoonotic spillover diseases are unable to flourish and die out before they can develop into full-blown epidemics, but some are successful and can wreak havoc on local human populations. Additionally, a zoonotic disease may be successful in adapting to its new human host population, yet lack the physiology to be particularly contagious or infectious. Rebecca Lipman in her writings for the Virginia Environmental Law Journal also maintains that there is a “certain random aspect to zoonoses” (Lipman, 2015). This randomness is demonstrated through evidence that zoonoses -such as Simian Immunodeficiency Virus- can spillover several times into human populations before successfully initiating a pandemic. The COVID-19 pandemic has shifted the global perspective of disease and placed emphasis on eliminating future spillover events and identifying possible avenues of emerging disease.
Government organizations such as the Center for Disease Control and Prevention (CDC) in the U.S. have long pursued the possible avenues of emerging infectious disease. Many governments take drastic action to protect human populations from zoonoses, often killing the animals that harbor the disease, reservoir hosts and amplifier hosts, in great numbers (Lipman, 2015). Laws are often quickly put in place to restrict areas in which the reservoir or amplifier hosts are present, and quarantines are mandated to contain the spread of disease (Lipman, 2015). These actions, however, are only a temporary solution to a problem which has long plagued humanity. So why are zoonotic diseases so prevalent? Can we predict, or at minimum effectively contain, spillover events? And perhaps the most important question to address: what is the next pandemic?
L I T E R A T U R E R E V I E W
A study by Geoghegan found that there exists a set of biological features of viruses that correlate strongly with that virus’s success rate: duration of infection, genome segmentation, how the disease is transmitted, and mortality rate (Geoghegan & Senior, et.al. 2016). The presence of an outer envelope is another biological feature that may play a role in determining the success of an emerging disease (Geoghegan & Senior, et.al., 2016). Genome length, genome type, and the frequency of recombination are factors that were found to have little or no correlation with successfulness of viruses ( Geoghegan & Senior, et.al., 2016). With these factors in mind, as well as the knowledge of increased contact with wildlife in suburban areas, it is not unreasonable to suspect that the next pandemic may begin in our own backyards.
Zoonotic disease is thought to have been a threat to humankind as early as first contact with animal groups by early hominids 5 million years ago (Gonzalez & Macgregor-Skinner, 2014). Remains of Australopithecus, one of the most ancient hominids, has provided evidence of possibly the first microbe exchange between hominid and wildlife (Gonzalez & Macgregor-Skinner, 2014). Historically, mankind adopted hunting large game as a common practice- potentially leading to early spillover events, as pathogens can be passed by ingesting infected animal products (Gonzalez & Macgregor-Skinner, 2014). Then, roughly 1000 years ago, agriculture and the domestication of 22 main species resulted in the emergence of several domestic zoonotic diseases, including flea-or louse-transmitted bacterial zoonoses (Gonzalez & Macgregor-Skinner, 2014). Historical knowledge and modern research have culminated in the basic understanding that the risk of zoonotic disease spillover increases with proximity to animals and with frequency of contact (Lipman, 2015). Wiznia and Christnos found some correlation that exists between the presence of deer and the prevalence of Lyme disease: residential forested areas saw a higher prevalence of Lyme disease (Wiznia, & Christnos, et.al., 2013 ). This finding suggests that modern advancements in infrastructure and expansion of residential areas into animal-inhabited terrain result in increased exposure and proximity to animals, warranting fear of another spillover event.
Deer pose a particular threat to modern suburbia. Odocoileus virginianus (White-tailed Deer) frequently populate residential areas. Neighborhoods, such as Elm Heights in Bloomington, IN, have expanded far into areas long-occupied by wildlife. As a result, a variety of wildlife and deer frequent the area but seem to lack basic self-preservation instincts. Pictured in Image 1, a deer stands on the sidewalk of Elm Heights, staring down a small dog. Notice how close the deer is to the dog and the person accompanying the dog, as well as the deer’s proximity to the road. A group of deer are seen congregating in a parking lot in Image 2, clearly unbothered by the proximity to cars and people. Deer are host to many zoonotic diseases including Bartonellosis and Leptospirosis plus tick-borne disease secondary to deer populations including Lyme disease and human granulocytotropic anaplasmosis (Sherrill & Snider, et.al., 2012). The risk of spillover of one of the aforementioned diseases undoubtedly increases in a populated area that has a significant deer population.
The threat is no less prevalent with the added context of roads. Deer, who are seemingly unafraid of the roadways and the cars that populate them, often put themselves in harms way by walking in or crossing the road. Even in residential areas, automobile speeds are often high enough to either seriously injure or kill a deer. This is observed most readily on the curbs of busy intersections or on the sides of backroads where it is not uncommon to see deer, or other wildlife, remains. The remains are either eaten by predators and scavengers or cleaned up by a city maintenance service, often animal control. A Bloomington squirrel who met an unfortunate end while attempting to cross the road is pictured in Image 3. The commonality of roadkill is not only a demonstration of the frequency and proximity of our interaction with wildlife such as deer, but also a possible form of transmission of zoonotic diseases. Additionally, residential development and the building of roadways fragments forested areas. These areas cannot support larger predators, but smaller species, such as rabbits, mice, and deer, thrive (Lipman, 2015). This leads to increased animal populations in these areas resulting in an increase in secondary disease carriers such as ticks: forest fragments that are smaller than 3 acres have been found to have roughly 7 times the amount of Lyme-disease infected ticks than larger forested areas (Lipman, 2015). This serves to provide evidence that road construction may actually increase the zoonotic spillover likelihood while decreasing the land area.
Every day we encroach further upon lands inhabited by wildlife. By 2050, more than 25 million kilometers of roadway will be built globally- a 60% increase in total road length from 2010 (Laurance, Clements, et.al., 2014). Roughly nine-tenths of this projected new infrastructure is likely to be built in developing countries, especially tropical regions (Laurance & Arrea, 2017). A global map of roadways is included as Image 4 for reference of the density of roadways, shown in black. Previously, I had planned to focus my research efforts on suburban and urban areas, as this data would be most applicable to Bloomington residents; However, due to these startling projections, I have decided to expand the focus of this research blog to include tropical and subtropical regions, in addition to the region of primary interest: The United States. The effects of roadways on local animal populations is also more thoroughly researched in Africa and Asia. Nonetheless, I believe that the research of these areas will ultimately be applicable and beneficial to suburbia.
With the massive expansion of infrastructure and roadways into forested areas, human-animal contact will undoubtedly increase. During the peak of the Coronavirus lockdown, when drivers opted to stay home, the roadkill rate among large species declined by 58% (Katz, 2020). For animals living close to roadways, roadkill is the leading cause of death (Forman & Alexander, 1998). In the US, an estimated 1-2 million animals are killed via motor vehicle collisions, amounting to an animal being killed every 26 seconds (Gaskill, 2013). From this information, we can reasonably derive that many more animals will die as the expansion of roadways continues. Other than contributing to the increasing roadkill rates, roadway expansion can contribute to many other ecological detriments for animals including fragmented habitats, limited gene flow, and restricted access to food and water (van der Ree, Jaeger, et al., 2011). Increasing roadkill rates may be worrisome for humans as well: increased exposure to dead animals as we encroach further into their habitats is a classic recipe for a spillover event.
In a simulated study designed to study disease within deer populations, a higher prevalence of disease was found in the population hit by cars, compared to the general population (Nusser, Clark, et. al., 2008). Another study designed to review the sampling bias of roadkill found that animals that are un-health or weak are more likely to live close to roads or attempt to cross roads (Conner, McCarty, et.al., 2000). Therefore, the animals hit by cars are likely to be sick. This increases the chance of spillover if we consider that roadkill is biased and maintained to an already sick population of animals. It is unclear if the proximity to roads is the cause of this group’s general lack of health or if sickness drives populations to roads. The aforementioned environmental detriments of roads may play a role in the sickness of these animals, but sound pollution, vehicle exhaust, and road-related stress may play a role as well.
H Y P O T H E S I S
Increased proximity and contact with animal populations has increased the likelihood of a zoonotic spillover event. Contact with dead animals has been a source of spillover in the past and so the increasing rates of roadkill, especially in heavily populated areas, could pose a great threat to humankind. If roadkill was to be a future source of a zoonotic infectious disease, I propose that it would likely occur as a result of one, or a combination of several, of the following avenues:
- Direct contact with infected bodily fluids during roadkill removal
- Flies or ticks acting as an amplifier host
- Direct consumption of infected animals
A V E N U E S O F S P I L L O V E R
- DIRECT CONTACT
Many zoonotic parasites, bacteria, and viruses are spread via direct contact with infected bodily fluids or tissues (Wright, Jung, et.al., 2008). Exposure can also occur via contact with a contaminated surface or via an indirect fecal-oral route (Wright, Jung, et.al., 2008). This avenue of possible transmission would be of highest concern for highway cleanup crews, animal control, and unlucky concerned citizens. This mode of transmission is the least likely to occur simply because it is unlikely that a pathogen of any kind would be able to survive long enough in a dead animal host to be transmittable to humans. Additionally, it is common practice to take safety precautions, such as wearing gloves, when handling dead animals.
- AMPLIFIER HOSTS
A study conducted by Gogarten hypothesizes and provides convincing evidence that flies in tropical rainforests can transmit communicable diseases from deceased mangabey primates to other members of a sample group (Gogarten, 2019). The flies can carry Bacillus cereus biovar anthracis (referred to as Bcbva) which causes sylvatic anthrax (Gogarten, 2019). DNA was collected from the flies around the sample group, mangabey fecal matter, and dead mangabeys to compare the Bcbva isolates. The isolates were similar enough to conclude that flies are able to carry infectious bacterial pathogens from dead mangabeys/ fecal matter to other members of the group. The implications of this discovery suggest that flies are able to transmit infectious pathogens from infected dead organisms to surrounding local populations. In the context of roadkill, a spillover event may occur as a result of flies acting as an amplifier host for a vertebral mammal sourced disease; if an infected animal is killed on a major roadway, flies, acting as decomposers, will soon thereafter be present and will spread whatever pathogens the animal was previously carrying to other animals or to human hosts.
Ticks transmit a greater variety of diseases, more than any other arthropod (Omodior). There are currently more than 35 known tick-borne diseases, however, with this track record, ticks have the potential to become a vector or amplifier host for the next great pandemic (Omodior). Ticks and flies pose a particularly dangerous threat as a potential avenue of transmission of a zoonotic infectious disease: they are small, can transmit a disease via bite, and can travel great distances in short amounts of time. A study by Eisen found that, though unlikely, ticks are able to transmit Lyme disease to humans from dead animals (Eisen, 2018).
- INGESTION
Though to many Americans, the idea of eating roadkill or dead animals is an uncivilized or unclean practice, many people in West African countries rely on bushmeat and dead animals for an affordable source of protein. Even more surprising, is that many Americans make regular practice of eating roadkill as well. See Image 5 for a map of roadkill laws in the United States. I distinctly remember the day my grandparents drove from Michigan to Kentucky for a visit: after a brief greeting, my grandfather proudly pulled open his trunk to show us all the turkey that he had hit during the trip. Eating fresh roadkill is frequently practiced in the United States, among many other countries, prompting worry that direct ingestion may be a possible avenue of transmission for an emerging zoonotic disease. As previously discussed, direct contact with infected bodily fluids or bodily tissues can result in disease transmission. Therefore eating roadkill, such as deer, turkey, duck, and yes, even opossum may be a possible mode of transmission and a source of the next spillover event.
C O N C L U S I O N S
Zoonotic spillover events have always been a looming threat, but COVID-19 has brought this lurking danger to the forefront of science, research, and public interest. The increasing roadway and infrastructure construction have put us in closer proximity to animals. Additionally, roadways pose a threat to the health and well-being of animals in the form of sound pollution, car exhaust, fragmented forests, limited access to resources, and of course, motor vehicle collisions. For animals living close to roadways, cars are the leading cause of death (Forman & Alexander, 1998) and the ‘roadkill bias’ suggests that animals killed on roadways are likely to be diseased or unwell (Conner, McCarty, et.al., 2000). There are multiple possible avenues by which disease can be transmitted from roadkill to human populations including direct contact, amplifier hosts like flies and ticks, and ingestion. All of these factors combine to arouse suspicion of roadkill being the source of the next pandemic.
COVID-19 unfortunately limited my data collection and local and national authorities in animal control and sanitation could not speak with me as I had hoped. Additionally, I do not have the funds to conduct a comprehensive survey of Bloomington or another region’s roadkill rates and prevalence. A more in-depth study of the likelihood of each avenue of transmission and additional studies pertaining to the prevalence of roadkill within specific regions would be required to ascertain the true chance of zoonotic spillover. While there are currently no studies suggesting that roadkill can be a source of zoonotic spillover, it is entirely possible and should not be overlooked. As COVID-19 has demonstrated, a pandemic unleashed on an unprepared and unsuspecting population can have devastating and deadly consequences. Every possible avenue of zoonotic spillover must be thoroughly researched and monitored to prevent what could be the next pandemic.
R E F E R E N C E S
Conner, M. M., Mccarty, C. W. & Miller, M. W. (2000). Detection of bias in harvest-based estimates of chronic wasting disease prevalence in mule deer. Journal of wildlife diseases, 36 (4), s. 691–699. doi:10.7589/0090-3558-36.4.691
Eisen, L. (2018). Pathogen transmission in relation to duration of attachment by Ixodes scapularis ticks. Ticks and tick-borne diseases, 9 (3), s. 535–542. doi:10.1016/j.ttbdis.2018.01.002
Gaskill, M. (2013). Rise in Roadkill Requires New Solutions. From https://www.scientificamerican.com/article/roadkill-endangers-endangered-wildlife/
Geoghegan, J., Senior, A., Di Giallonardo, F., & Holmes, E. (2016). Virological factors that increase the transmissibility of emerging human viruses. Proceedings of the National Academy of Sciences of the United States of America, 113(15), 4170-4175. https://www.jstor.org/stable/26469275
Gogarten, J. F., Düx, A., Mubemba, B., Pléh, K., Hoffmann, C., Mielke, A., Müller-Tiburtius, J., Sachse, A., Wittig, R. M., Calvignac-Spencer, S., & Leendertz, F. H. (2019). Tropical rainforest flies carrying pathogens form stable associations with social nonhuman primates. Molecular ecology, 28(18), 4242–4258. https://doi.org/10.1111/mec.15145
Gonzalez, J. P., & Macgregor-Skinner, G. (2014). Dangerous viral pathogens of animal origin: Risk and biosecurity: zoonotic select agents. Zoonoses – Infections Affecting Humans and Animals: Focus on Public Health Aspects, 1015–1062. https://doi.org/10.1007/978-94-017-9457-2_41
Katz, C. (2020). Roadkill rates fall dramatically as lockdown keeps drivers at home. from https://www.nationalgeographic.com/animals/2020/06/decline-road-kill-pandemic-lockdown-traffic/
Lipman, R. (2015). Zoonotic diseases: Using environmental law to reduce the odds of a future epidemic. Virginia Environmental Law Journal, 33(1), 153-171. http://www.jstor.org/stable/24789372
Laurance, W., Clements, G., Sloan, S. et al. (2014). A global strategy for road building. Nature 513, 229–232 https://doi.org/10.1038/nature13717
Laurance, W., Arrea, I. (2017) Roads to riches or ruin? Science, 358 (6362), 442-444. DOI: 10.1126/science.aao0312
Nusser, S. M., Clark, W. R., Otis, D. L. & Huang, L. (2008). Sampling Considerations for Disease Surveillance in Wildlife Populations. The journal of wildlife management, 72 (1), s. 52–60. doi:10.2193/2007-317
Omodior, O., Saeedpour-Parizi, M. R., Rahman, K., et. al. (2020). Tick species classification using image-level convolutional neural network and transfer learning.
Richard T. T. Forman, & Alexander, L. (1998). Roads and Their Major Ecological Effects. Annual Review of Ecology and Systematics, 29, 207-C2. http://www.jstor.org/stable/221707
Sherrill, B., Snider, A., Kennedy-Stoskopf, S., & DePerno, C. (2012). Survey of zoonotic pathogens in white-tailed deer on Bald Head Island, North Carolina. Southeastern Naturalist, 11(3), 529-533. http://www.jstor.org/stable/41679682
Van der Ree, R., J. A. G. Jaeger, E. A. van der Grift, and A. P. Clevenger. 2011. Effects of roads and traffic on wildlife populations and landscape function: road ecology is moving towards larger scales. Ecology and Society 16(1): 48. [online] URL: http://www.ecologyandsociety.org/vol16/iss1/art48/
Wiznia, D., Christos, P., & LaBonte, A. (2013). The Use of deer vehicle accidents as a proxy for measuring the degree of interaction between human and deer populations and Its correlation with the incidence rate of Lyme Disease. Journal of Environmental Health, 75(8), 32-39. http://www.jstor.org/stable/26329603
Wolf, G. (2019). Roadkill laws in all 50 states. Stateline.
Wright, J., Jung, S., et.al. (2008). Infection control practices and zoonotic disease risks among veterinarians in the United States. Journal of the American Veterinary Medical Association. 232 (12). 1863-1872. https://doi.org/10.2460/javma.232.12.1863
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