What is it? Why is it important?
The Dilution Effect theory has been studied and discussed for several years, now, and if you’re involved in conservation, ecology, One Health, human or animal disease, or other related areas, it’s probable you’ve heard the term.
So what is the Dilution Effect?
The term was popularized in a study performed by Richard Ostfeld and Felicia Keesing, which they published in the Canadian Journal of Zoology in 2000[1]. They studied a deforested habitat where the prevalence of Lyme disease among the people living nearby was far greater than elsewhere in Lyme-endemic areas. What they found was that it had a lot to do with the wild animal population. Specifically, the white-footed mouse.
Lyme is a disease caused by a bacteria that resides in the blood of an infected host. Ixodes ticks are responsible for spreading Lyme disease to people when they bite and feed off them. When these ticks are newly hatched from their eggs as larvae, they find a small animal to feed on so that they can grow and molt. That small animal can be a raccoon, bird, mouse, etc. If that animal has Lyme inside them, the tick is infected when it sucks their blood. Once it molts, it can then infect a person if it decides to attach and feed on him.
But the thing that made this particular Lyme-prevalent area unique was its population of white-footed mice. These mice, for unknown reasons, are what’s known as an “amplifier host”. This means that, when they are infected with Lyme disease, something about their biology and physiology makes them really good at multiplying the disease inside themselves and infecting the ticks that feed off their blood. You see, transmission and infection by diseases—especially ones spread by ticks and insects—is very complex. Lyme disease does not replicate and survive the same in every species. Some animals, once infected, are such poor hosts that they don’t spread it, even if a tick sucks their blood.
So this area was swarming with infected ticks and amplifier hosts, and the people living nearby were constantly getting infected. Judging by the situation, Ostfeld and Keesing determined that the way to rectify the problem would be to increase diversity of the animal population. This is the Dilution Effect. Basically, it means that if there is a wide range of animals in an area, many of them would not be good hosts and spreaders of Lyme disease. That way, when the larval tick is looking around for an animal to feed on, the odds of it feeding on an amplifier host and getting infected will be far less, and thus its chances of also infecting any human being it may happen to bite.
Ostfeld and Keesing determined four basic conditions that must be present for the Dilution Effect to apply:
- Vector has generalized feeding habits. Basically, the tick isn’t picky; it feeds off a lot of different animals.
- The vector acquires the pathogen from hosts. So, the tick is infected by feeding off an animal that already has the disease.
- Reservoir competence among hosts varies. This means that some of the animals are really good at transmitting the disease to the ticks, while other are not so great at it.
- The most competent reservoir host tends to be a community dominant. The animal that is really good at infecting the ticks (in this case, the white-footed mouse) tends to be a very popular host for the ticks to feed on.
So the idea is, if there is a wide range of hosts with a wide range of competencies, incidence of the disease in the ticks will decrease. Most other groups that found support for the theory generally claimed that large, forested areas supported more diverse populations than smaller, isolated patches surrounded by human cities.[2]
However, there has been some dissent with this theory. Linske et. al.[3] made a good point when they warned against broadly applying the concept everywhere; even Ostfeld and Keesing admitted in their paper that this rule was not universal. For example, Linske’s group studied Lyme disease prevalence in both a large, undisturbed forest and smaller woodland fragments in residential areas in Connecticut. But they found that the number of Lyme-infected ticks was far higher in the huge forest than in the residential areas. Why was this the opposite of what Ostfeld and Keesing found? The reason is that the forest in Connecticut was mature and had mostly upper canopies, which actually resulted in a wildlife population that was not very diverse. The fragmented residential areas, on the other hand, had a lot of local diversity and competition, which beautifully fulfilled the conditions for the dilution effect.
While the dilution effect is an important theory, there’s
still a lot of investigating to do on this subject—not only in terms of how it
works on the whole, but also how it works in specific areas. It’s important to
take the entire ecosystem, its environment and animal population, into account
when studying the effects of disease on human populations.
[1] http://www.nrcresearchpress.com/doi/abs/10.1139/z00-172#.XDleyExFxPY
[2] https://www.caryinstitute.org/science-program/research-projects/biodiversity-community-ecology-and-dilution-effect/dilution-effect
Allan, B. F., F. Keesing, and R. S. Ostfeld. 2003. Effect of forest fragmentation on Lyme disease risk. Conserv. Biol. 17: 267–272.
LoGiudice, K., R. S. Ostfeld, K. A. Schmidt, and F. Keesing. 2003. The ecology of infectious disease: effects of host diversity and community composition on Lyme disease risk. Proc. Natl. Acad. Sci. U. S. A. 100: 567–571.
Vet Med and One Health 