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Small Predator Diversity Plays a Significant Role in the Spread of Infectious Diseases

By Roberta Attanasio

Biodiversity is a term coined to describe the diversity of all living things, from human beings to microorganisms. A New York Times editorial published almost two decades ago aptly describes the importance of the biodiversity concept: “Biodiversity is a hugely important concept that stresses the coherence and interdependence of all forms of life on earth and a new willingness to appraise the meaning of that interdependence, not just for humans but for every one of life’s component parts.”

The editorial goes on to illustrate the alarming effects of biodiversity loss: “Biodiversity is a way of talking about what scientists have long understood and a way of reminding the rest of us of a cardinal fact: that we are standing in the midst of the earth’s sixth great extinction of diverse species, that this extinction is driven by us and that we are not now and will never be immune to its effects.”


Dragonfly. Image credit: whologwhy, licensed under CC BY 2.0

One of these effects is the worldwide spike in infectious diseases, as suggested by a study recently published in the journal Proceedings of the National Academy of Sciences. The study (Predator diversity, intraguild predation, and indirect effects drive parasite transmission) explores how the diversity of small predators shapes the transmission of parasites in wetlands.

Lead author Jason Rohr said in a press release by Penn State: “In the last century, there has been an unprecedented global increase in infectious diseases and a concomitant decline in and homogenization of biodiversity. The controversial ‘dilution effect hypothesis’ suggests that the two phenomena might be linked, or that biodiversity often decreases disease risk.”

The study, which included a series of laboratory experiments, field surveys and mathematical modeling, shows that — in presence of various species of dragonfly larvae — there is a reduction of frog infections caused by trematodes, which are parasitic flatworms also known as flukes. The dragonfly larvae are small predators that eat trematodes.

Val Beasley, senior author of the study, said in the press release that various species of trematodes penetrate tadpoles. The trematodes sometimes kill the tadpoles. In other instances, the trematodes weaken them by causing tissue damage, kidney failure, or severe limb deformities while the tadpoles develop into frogs. He added that other vertebrate species commonly catch trematode infections from bodies of water. These vertebrate species include wildlife, domestic animals and humans — mostly children — who are commonly affected by schistosomiasis in tropical parts of the world.

Schistosomiasis is a parasitic disease carried by freshwater snails infected with one of the five varieties of the parasite Schistosoma, a type of trematode. Although the worms that cause schistosomiasis are not found in the United States, more than 200 million people are infected worldwide.

While most biodiversity research focuses on variations of the parasite hosts, this study highlights the role that the diversity of the predator species (in this case the diversity of dragonflies, which attack the flatworms) play on infectious diseases.

Rohr also said: “In our wetland survey, our microcosms and disease models, we discovered that there were fewer flatworms in frogs where there were more species of flatworm predators. Additionally, the field study indicated that the diversity of these predators was a better predictor of flatworm infections than nutrients, frog immunity or the diversity and abundance of hosts.”

Further work on the diversity of predator species able to decrease parasite populations may be crucial for the management of infectious diseases.

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Author: Roberta Attanasio

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  1. Based on these findings in this article and similar other cases I believe it is difficult to reject the dilution effect hypothesis. It is logical that as we destroy habitats and remove predator from prey while at the same time forcing animals carrying infectious agents into closer contact with ourselves without their habitats that an increase in disease would occur. Ecologist John Orrock performed a study on deer mice (Peromyscus maniculatus) on the Channel Islands in California and the Sin Nombre virus (SNV). This island chain has eight different islands with various levels of predation and so the study only required gathering data from the different islands and assessing the prevalence of SNV compared to various environmental factors on that island, including richness of mouse predators. He found that the islands where SNV was most prevalent were those in which predators were few in number. One interesting piece of information that I found regarding this study was that non-intraguild predators that consumed just the parasite reduced infection but intraguild predators that consume parasites and the tadpole host do not. My proposed reasoning for this was that intraguild predators that consume both will consumer less overall parasites because they have an additional food source. For example a non-intraguild predator may eat 10 parasites in a day for its nutrition but an intraguild predator may only need to eat 2 nematodes containing parasites to reach its daily nutrition. In this case the non-intraguild predators consumer more parasites and therefore have an impact on reducing disease. This is just a theory and I am curious to ask what others believe may be the reasoning for the reduction specifically with non-intraguild predators?

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  2. Remarkable phenomena in nature is the species diversity. Species that are dominant which grows rapidly may cause decrease in other species. Among nature, it is disheartening news when population of species declines. For example, article of United States Department of Agriculture about Honey Bees suffer from Colony Collapse Disorder. The exact mechanism is not yet known; however, one of the proposed mechanism by CCD is that it could be caused by immunodeficiencies. Just like this situation, there is another modern extinction risk which is decrease in amphibian throughout world. Frogs are one of the classification of amphibians. Past several years, researchers have noticed that there is sudden and serious decrease in these species. According to the amphibian ark researchers, an infectious disease of amphibian’s knowns as Chytridiomycosis is known cause of decline in these species. It is type of fungus or protozoa that is found in wet environment. It is believed that Chytridiomycosis attacks the skin of amphibians. Skin plays a role of secondary respiratory system for amphibians. The researchers stated that this infections among amphibians is very rapid and can occur just in few weeks. This tells us that how rapid decline of amphibians is and needs strong attention to preserve these species. The treatment includes anti-fungal medications. Unluckily, in environment like this there is no any preventive methods to these wild animals.

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    • According to the article, there have been studies that show that the species of amphibians are very hard to quantify as their population is continuously fluctuating. Also there was a decline in the infectious disease, chytridiomycosis, which is caused by the fungus called Batrachochytrium dendrobatidis. There were outbreaks in tropical regions like South America, Australia, and Central America. There was a correlation with the temperature and the amphibian’s immunity to fungal pathogens. The greatest number of outbreaks was in cool and moist environments, which is why the fungus is dependent on temperature and humidity for its survival.

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  3. As humans populations continue to expand and new areas are colonized, we humans tend to overlook the impact that is made to the environment and its biodiversity. Since we humans are accountable for such changes, we should also be responsible to give back to the environment.
    In order to preserve the biodiversity in our environment, biological control measures should be taken into consideration. There are several methods to help maintain biodiversity while decreasing the number of parasites and pathogens. A control method known as augmentation involves the supplemental release of a pest’s natural enemy to boost the naturally occurring population. Thus, in the case of trematodes, we could use this method to introduce additional dragonfly larvae to problem areas. With an increase in natural predators of trematodes in the area, there is a higher possibility to greatly reduce the number of parasites and decrease the risk of frog extinction. Another method to combat pathogens is through the conservation of the already existing natural enemies of the pathogen in the environment. The advantage to this method is that the natural enemies are already adapted to the habitat and the pathogen/pest. This conservation method involves either the reduction of factors that could interfere with natural enemies or the provision of resources that natural enemies need in the environment. Although these methods are usually used in agriculture to control insect pests, the same techniques of biological control can be used in the control of parasites. For example, trichostrongyle nematodes which infect cattle are spread through the dung of the cattle. Dung beetles of the family Scarabaeidae are important for drying up the dung and thereby increasing mortality of the free-living parasites. The dung beetles in this case are involved in the natural biological control of trichostronglyosis of cattle. Methods similar to this type of control could also be applied to other parasites or pathogens. Perhaps in the case of infectious diseases of humans, diseases such as malaria or the West Nile Virus can be biologically controlled with natural predators of mosquitoes.

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  4. There are a myriad of factors that are contributing to biodiversity loss, therefore it’s extremely difficult to pick which are the most significant and which approaches to consider. I completely agree with Peter M. that pollution reduction should be one of the highest priorities. The levels of pollution in the world are ridiculously high, contributing to an ever-rising loss of species and an unhealthy planet. While I do agree that destruction of already-established urban areas may be drastic, I believe that there should be a greater effort to minimize the destruction of wildlife habitats for unnecessary development. There are many acres of land, in just my county alone, which went through the initial stages of development for a neighborhood or store complex just for the project to be abandoned. All the trees and foliage in these areas have been removed, destroying countless homes and resources of the indigenous wildlife. Another approach to consider, which I am sure many would disagree with, is the implementation of population control policies. The hard truth is that this planet has a maximum population which it can realistically sustain. While it may be chilling to think about, this is the reality. If population levels continue to rise, the world will witness even higher loss of biodiversity, as well as extreme decreases in natural resources and ever-increasing levels of pollution. These detrimental events could contribute to an even higher rise in infectious diseases. There is much controversy over whether population control violates basic human rights; many state that parents should have the right to choose how large a family they want. While I do believe everyone is entitled to their rights as a person, I often find myself questioning this argument when I see families with more children than they could possibly afford or support. Limiting the number of children parents can have would lower the strain being placed on the Earth; this would decrease housing needs, causing lower rates of habitat destruction, less consumption of natural resources, and therefore less pollution. These improvements would allow more species to thrive, bringing biodiversity back to a healthy level. These factors, combined with a sustainable number of people in the world, could also make it easier to manage infectious diseases.

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  5. Trematodes in the environment are around for one reason, like every other organism in the world, survival. I do not believe that in order to fight off these trematodes that an increase in dragonfly populations would be the answer. Humans never seem to learn from the causes and effects of certain actions that we do to our environment. By increasing one population of an organism, or anything really, runs a risk of causing an imbalance of the environment that we place it in. Instead, we should take responsibility and clean up the environment by reducing the amount of pollution and waste that we throw into it, especially into bodies of water. Maybe filtration methods should be acted upon, in areas that are endemic with these species of trematodes, to help decrease the risk of infectious disease. Another method, that could be done, as stated in an article from the CDC, is building a wall and preventing surface runoff from entering ponds and lakes. One last option, to combat this infectious disease, is by controlling host populations of these areas that certain species of trematodes have part of their life cycle in.

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    • Biodiversity is important because it allows for survival. All living organisms have the innate need for survival. It is intriguing to know the length to which trematodes go to survive and eventually infect their hosts. This is also similar to how Mycobacterium tuberculosis tries to survive in a phagosome by preventing the fusion of the phagosome and the lysosome. In order to survive, these organisms develop techniques that help them adapt in their desired environment. Ribeiroia ondatrae is an example of a trematode that infects tadpoles. Trematodes usually have intermediate(s) host where they develop and mature partially or completely before moving to their final host. These organisms are adapted to go through different environmental conditions just to survive and reproduce. One of their pathways of infection is that they are released from feces/urine from an already infected organism into water bodies, and they get into snails where they mature and later leave to infect the tadpoles. They are highly adaptive to be able to survive in different species of organisms. This knowledge can be useful in reducing their population in the environment. These organisms might eventually develop ways to evade their predator but proper sewage disposal will be one way to curtail them. Sewage disposal might not be an issue in developed countries, but in developing countries, there is a great need for proper sewage disposal. Also, there should be no buildup of stagnant water in the environment; this allows the thriving of other disease-causing microbes in the environment. Proper sewage disposal is one way to interrupt the life circle of the trematodes and reduce the infections they cause.

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  6. There are many factors including a loss of biodiversity that lead to a spread of infectious disease. Other such causes, as stated in this article, include climate variation, ecological uneasiness, and host switching. All of these factors play a role in parasite diversification which leads to a broader range of infectious disease in animals and humans. “Parasites are resource specialists with restricted host ranges, yet shifts onto relatively unrelated hosts are common during phylogenetic diversification of parasite lineages and directly observable in real time”. The ever increasing climate warming threatens the sustainability of ecosystems and biodiversity. As the range of hosts shift for parasites (as well as other agents of infectious disease), they must diversify to search for a new source of resources. “From an epidemiological standpoint, episodes of global climate change should be expected to be associated with the origins of new host–parasite associations”. What this means for humans and other newer hosts is less of an immunological response as a new threat arises that the body has not been previously prepared for. In addition, the diversity of the parasites’ predators decrease, as mentioned with the dragon-flys above.

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  7. I have always been alarmed at the decrease in biodiversity in the world. An excellent article that explores how humans have impacted biodiversity is ”Species Extinction Happening 1,000 Times Faster Because of Humans?” by Christine Dell’Amore . In this article, Dell’Amore investigates how many extinctions occur and how scientists and researchers can quantify and reduce the rate of extinction. With regards to the decrease in biodiversity and its effects on infectious disease, it comes to no surprise to me that ill effects would be seen by this horrific trend in wildlife. But what are we as humans supposed to do to counter this problem? Without going into drastic measures such as the stopping of all new building projects or the destruction of urban areas which once housed many species of animals, I believe that a good start would be to reduce the amount of pollution in the world. Pollution in the air and water not only has direct effects on the health of humans, but also indirectly through its reduction of biodiversity. Economic ramifications and corporations will fight the idea, but I believe that technology must advance to a point which would allow humans, wildlife, and industry to coexist. There is much more information about the effects of pollution on biodiversity at the WWF.

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  8. There are numerous common diseases that have had effects on a worldwide scale. The diminishing effects on biodiversity and its distinct species in terms of biological diversification are being equally affected, whether it is the predator/parasite relationship in animals, insects, and humans.
    Think about infectious diseases that occur worldwide and take into consideration the modes of transmission. It begs to question whether the emphasis should be placed on the predator/parasite scenario explained by Rohr which leaves out the greatest influencers of the increasing infectious diseases, the human species. In future years to come, mankind is daily creating for itself the potential of becoming extinct species in the same ways as our following predecessors. This does not warrant questioning because the day to day activities of humans greatly impacts the shifts in the ecosystem and its biodiversity of species therein. Therefore, within a geographic location how can one create for a similar mediation of infectious disease, as seen in the dragonfly larvae, fluke worm (trematode), and the frogs?

    There are so many communal infectious diseases that exist on a worldwide basis such as the one mentioned by Dr. Attanasio, schistosomiasis which is parasitic. Individuals become infected by the fluke worm causing infectious disease in them and extreme infection in snails. There are many common worldwide infections such as Malaria where mosquitoes are the vector, Leishmaniasis which is spread by a sand-fly, and water infected animal and human feces that cause Cryptosporidiosis. My point here is this, the parasite/predator population is a large one and focuses on this particular relationship creates imbalances in effectively managing infectious diseases, because of the vastness of which infections are spread on the basis of vectors. There are numerous ways in which infectious diseases spread such as animals, insects, bacteria, viruses, and humans. How do we control for one aspect of biodiversity without creating imbalances in others areas?
    Human activity alone can account for the vast increase of infectious diseases. Consider various habitats being shifted due to deforestation, landscaping and architectural development of infrastructure and establishment of roadways. One aspect of a risk factor that has led to infectious disease transmission is “baiting and supplemental feeding” of wildlife according to Sorensen research study,”Impacts of wildlife baiting and supplemental feeding on infectious disease transmission risk: A synthesis of knowledge.” To answer my question about simulating a means for which there exist mediators for the reduction of infectious disease is what researchers have found in “baiting” in animals. Thus by proving the food animals in their habitat causes a high concentration and increased transmission of diseases. However, they found that feeding herbivores can alleviate vector transmission by improving physical condition of the animal so as to better combat disease, thus reducing transmission risks. How can one create shifts in the ecosystem that will facilitate the right predators for parasites in order to circumvent increases in infectious diseases? Seemingly enough, these events may just likely be observed coincidences.

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  9. Increasing the pathogen’s predator might not be a way to control pathogen’s population in all situations. For instance, there is a fungus parasite called cordyceps of the genus ascomycetes and it works by controlling the brain of its predator. It works mostly in insects and arthropods. If an infection is caused by cordyceps, insects that prey on it as a way of controlling the infection get controlled instead. This fungus makes the insect go all the way up to the top of the plant and then causes the insect to attach it’s mandible to the plant to hold fast. The insect then dries up and the fungus grows and burst out of the dead insect spreading its spores. This increases the chances that more cordyceps will grow not helping the situation. Cordyceps makes the insect go to the top of the plant so that the spores that get released are able to spread.

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  10. As humans, we often overlook the fundamental role we play in the Earth’s ecosystem and the organisms which inhabit it along with us. With the recent issue of human perpetuated global warming and its profound impact on the nature of several environments as we know it today, it has become more important than ever that we recognize and acknowledge our responsibility in the decimation of many species that are rapidly nearing extinction. For instance, the rise in CO2 characteristic of global warming that has led to a dramatic increase in overall temperatures, excessive deforestation, and black market demands for goods obtained by poaching and/or illegal hunting have all contributed to a decline in the total number of extant diverse species. Consequently, the detrimental fate humans have placed upon the Earth’s environment and non-human populations have not gone unpunished, and repercussions have begun to be manifested in the form of an increase in the contraction of infectious diseases. As a result, disruptions in the predator-prey dynamics of the biological food chain can lead to increases in prey species that infect a broader range of hosts/niches, gives rise to increased number of cases of pathogenic diseases, and/or induces the formation of new pathologies. For example, Roossinck and Garcia-Arenal contend in the article Ecosystem simplification, biodiversity loss and plant virus emergence , that the loss in biodiversity in agro-ecosystems due to an increased demand for food in sufficient amounts to satisfy an ever increasing human population has led to the emergence of new and increased incidences of virus diseases in both wild and domestic plant crops. Overall, one can see how it would important to address the issue of biodiversity loss and the overarching effect that humans have on the fate of agricultural, vector, and their own populations before it is too late to reverse any fatal outcomes.

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  11. It is very important that the population of key predator species of life threatening diseases remain constant or continually increase. I read an article titled Deer, predators, and the emergence of Lyme disease that discussed the importance of Red Foxes on the spread of Lyme disease in North America. For a while people thought that the disease was being spread because deer populations were increasing and deer are important for the development of ticks but this was quickly proven wrong. Red Foxes are eaten by Coyote and when their populations decrease Lyme disease spreads fast. A good way to fix this problem would be to introduce a predator of the coyote so that the Red Fox population doesn’t decrease so rapidly and the spread of Lyme disease can decline.

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  12. Many people are not aware of the declining amphibian population in recent years. Like many others, I would think the main issue regarding amphibian endangerment would be environmental factors, however infectious diseases have been a big contributor to the declining of these animals in recent years. For example, Chytrid is an infectious fungus that has causes skin damage in frogs. The skin is essential to the survival of frogs because of drinking water and nutrient uptake. This disease is also one of the main contributors to the almost 40 percent decline in the amphibian population. About 200 species have also gone extinct due to infectious diseases since the 1970s. Even though many of the frogs do not die from chytrid; they are still carriers and can infect other environments. Many scientists believe that the next global extinction could occur due to the number of amphibians infected by infectious diseases.

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  13. The concept of increased predator diversity controlling a parasitic population should be considered in human infectious agent control. The World Health Organization states that land-usage changes, rapid urbanization, international travel and trade, and human introduction to pathogens all have a significant effect on patterns of infectious diseases. Rapid urbanization, for example, includes destroying the habitats of many organisms in order to create a more urban environment for humans. This means that pathogens and their natural hosts or predators now have to find a new habitat or live amongst the humans who have taken over their habitats. Thus, these pathogens infect humans and we have an increase of old diseases or formations of new diseases. We could use the concept of increasing the pathogen’s predator as a way to control the pathogen’s population. For example, if we destroy a habitat that included Mammal A which is the predator of Platyhelminthes A then Platyhelminthes A would not have a predator thus there would be an increase in its population. It would then find a host, which would be the humans who now inhabit that location. A solution would include bringing in an influx of Mammal A to control Platyhelminthes A’s population and reduce human infection.

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