by Sanjana Kulkarni

SARS-CoV-2 may have spread to humans from an animal host, but it is not the only disease-causing agent (i.e. pathogen) to have done so. Lyme disease, Ebola virus, influenza, HIV, the plague, and rabies virus are just some examples of zoonotic diseases, meaning that they originated in animals and spread (i.e spilled over) to humans. 

Many human activities, such as deforestation and hunting, raise the risk of zoonotic spillover by increasing contact between humans and wildlife and their associated pathogens. Another byproduct of human activity correlated with zoonotic spillover risk is biodiversity loss. Biodiversity is a measure of the variety of organisms that live in a habitat. When animal species are lost from a habitat, disease transmission in that habitat tends to increase. This subsequently increases the risk of a zoonotic disease spilling over to humans.

Just like the viruses that cause colds and measles circulate among humans, other diseases circulate in animal populations as well. These reservoir host animals are good at propagating pathogens and spreading them to other animals, including to humans. Reservoir hosts tend to be animals with large, dense populations like birds, bats, and rodents. They are adaptable and thrive in urban and semi-urban areas where many people live. 

Reservoir animals are often the least affected by human activity, but the biological causes of this adaptability are not fully known. Climate change, agricultural and urban expansion, and hunting and fishing affect which animals can live adjacent to human environments. The proportion of reservoir animals in a habitat generally increases with human activity because non-reservoir species, including many predators and competitors of reservoir hosts, are lost from a habitat first. With fewer other animals to keep the reservoir animal population in check, reservoir animals proliferate and increase the risk of zoonotic spillover. 

What makes a good reservoir host?

Birds, bats, and rodents are some of the best reservoir hosts because they are abundant and can live near humans. Bats and rodents make up around 60% of the 6,000 known mammalian species, and there are more than 10,000 known species of birds. More species lead to greater pathogen diversity. Because bats and birds can fly, they can travel quickly and spread an infection to many people and other animals. 

Not all reservoir animals directly spread diseases to humans, however (Figure 1). Other animals, or even inanimate objects, can be intermediate transmitters. Fleas, ticks, and mosquitoes are notorious disease transmitters. They can bite a diseased animal and become infected, then bite a human who subsequently falls sick. Disease can also be spread via inanimate objects. For example, infected body fluids from animals can contaminate objects or food, which are then touched or consumed by people. 

The dilution effect

After years of coevolution with their reservoir hosts, pathogens have come to rely on reservoir animals to replicate and spread. Other animals in a habitat (non-reservoirs) can transmit these diseases, but to a lesser extent than reservoir animals. Because animals spread zoonotic pathogens unequally, changes in the balance of reservoirs vs. non-reservoirs in a habitat affect the risk of spillover to a human. Compared to reservoir animals, non-reservoir animals make fewer copies of a pathogen and do not spread it as efficiently. When biodiversity is high, a disease gets diluted among non-reservoirs. Overall disease spread within the habitat and to humans is lower when there are other animals besides reservoirs present. This principle is called the dilution effect. 

The impacts of biodiversity loss

The dilution effect is well-characterized in two diseases: Lyme disease and West Nile virus. Lyme disease is a tick-borne illness that causes nearly half a million infections annually in the United States and can cause serious health complications if not treated. Even with antibiotics, some people have persistent symptoms like fatigue and brain fog, so it is best to avoid infection altogether. However, this is becoming more difficult as Lyme disease is spreading rapidly across North America.

The reservoir animal of Lyme disease is the white-footed mouse. In North American forests, ticks feed on mice, deer, opossums, and other mammals, but the bacteria are best adapted to grow in mice. When ticks bite animals, the bacteria that cause Lyme disease are shared between tick saliva and the blood of the host animal. Ticks that bite mice get more bacteria than ticks that bite other animals. If mice make up a larger proportion of animals in a habitat, then the biting ticks will carry more bacteria on average. Mice can live in places with both high (i.e. forests) and low (i.e. suburbs) biodiversity, but they are diluted when there are other animal hosts present (Figure 2).

A similar dilution effect has been observed in West Nile virus. This virus infects both birds and mammals and can cause serious brain infections. It originated in Uganda and was imported to New York City in 1999. Since then, it has become established in birds and mosquitoes in North America. West Nile virus infections tend to be high where bird diversity is low. Bird species in which West Nile virus grows best – crows, jays, finches, sparrows, and thrushes – can be found in low biodiversity communities like cities. Birds that don’t grow the virus as well – coots, quail, pheasants, woodpeckers, and parakeets – are generally only found in communities with high biodiversity like forests. In high biodiversity communities, West Nile virus gets diluted, and the overall amount of virus per bird is lower. Mosquitoes that bite these birds have less virus in them, so humans are less likely to get sick when they are bitten by these mosquitoes.

Looking to the future

The dilution effect does not mean that more biodiversity equates to less disease. Rather, it predicts how the amount of disease changes in a habitat when biodiversity is lost from that habitat. Zoonotic spillover risk is driven by the proliferation of reservoir hosts, which tends to happen when biodiversity decreases from a baseline level, but overall biodiversity (all animals) is not strongly associated with zoonotic spillover risk. 

There are many open questions in dilution effect theory and disease ecology. How frequently does the dilution effect occur, and can we predict it from ecosystem features? What are the biological reasons behind certain animals harboring so many zoonotic diseases? How does the dilution effect play out when there are multiple zoonotic pathogens across multiple species? 

Dr. Felicia Keesing at Bard College in Annandale, New York has extensively studied dilution effect theory: “In intact natural ecosystems, with naturally high levels of biodiversity, predators and competitors often keep superspreader species at lower abundance. So protecting naturally high levels of biodiversity is critical for mitigating pathogen transmission and spillover.” In addition to habitat conservation, scientists are studying whether restoring biodiversity to areas from which it has been lost can reduce disease spread. 

As we have seen with the COVID-19 pandemic, novel zoonotic diseases can have catastrophic worldwide consequences. It is therefore critical that we know where and how diseases originate so that we can anticipate new disease threats and respond rapidly when they emerge. 

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Sanjana Kulkarni is a second-year PhD student studying infectious diseases at Harvard Medical School.

Figures created with BioRender.com

For More Information:

• More examples of the dilution effect in malaria and parasitic diseases.
• The dilution effect is also relevant for non-human diseases in amphibians and corals.
• Check out an overview of the dilution effect here.  
• For a more detailed look at biodiversity loss and zoonotic diseases, see this review article.

This article is part of our special edition on diversity. To read more, check out our special edition homepage!