Image credit: Muhammad Mahdi Karim, CC BY-SA 4.0, Image Cropped

Invasive snails, parasite spillback, and potential parasite spillover drive parasitic diseases of Hippopotamus amphibius in artificial lakes of Zimbabwe (2021) Schols et al., BMC Biology, https://doi.org/10.1186/s12915-021-01093-2

The Crux

Artificial lakes can be a huge plus for the regions where they are constructed. People come to hang out at them, they can serve as habitat for local or migrating species, and they can also improve water accessibility. In fact, the majority of the research that I did for my PhD took place in artificial, human-made lakes (see here and here). Yet, these artificial lakes can also wreak havoc by destroying local ecosystems and introducing invasive species. Furthermore, because humans build communities around these lakes there is a risk of increased transmission of parasites to livestock and humans alike.

One group of common invasive species in these artificial lakes are snails, which serve as intermediate hosts for many parasites (see Did You Know?). Introduced water plants (like hyacinth) often harbor invasive species like the snails, and dams built to make artificial lakes often block snail predators from accessing the lakes, which means that the snails increase in number due to the release from predation pressure. Today’s authors wanted to understand how invasive snails modified parasite transmission within an artificial lake.

Did You Know: Intermediate Hosts

Many parasites need to infect multiple host species to finish their life-cycle. These are known as trophically-transmitted parasites, because they are transmitted between trophic levels of a food web. A great example are trematode parasites like the ones from today’s study, which infect snails and reproduce asexually within the snail to increase their numbers. They then leave the snail and swim in the water, where they can then infect amphibians or fish, which are then eaten by bird final hosts. The parasites then reproduce sexually within the bird final hosts, laying eggs that the birds defecate into the environment, where the cycle starts over again.

What They Did

The authors wanted to determine if the invasive North American snail (Pseudosuccinea columella) in Lake Kariba in Zimbabe, the world’s largest artificial lake by volume, was increasing parasite transmission within the lake via parasite spillover or spillback. Spillover is when an invasive host brings new, invasive parasites with it to the new environment, while spillback is when invasive hosts become infected with local, native parasites, thereby increasing the number of local parasites in the environment that can then spill back onto the native hosts (or increase parasitism in the invasive host). Knowing which scenario is most likely depends upon figuring out who the intermediate and final hosts are, and that is what today’s authors did.

They first collected data from a hippo that had been culled by the Zimbabwe Parks and Wildlife Management Authority. This involved examining the entire liver and and small piece of the stomach for liver and stomach flukes (the final stage of parasites transmitted by snails). The authors collected data on the morphology and DNA of the parasites. They then collected snails from the lake itself, bringing them back to the lab where they induced shedding, which is when the larval form of the parasite leaves the snail host to look for the next host in the life cycle.

Like the adult specimens collected from the hippo, the authors also collected morphological and DNA data on the larval stages collected from the snails. Using all of the data collected from the hippo and snails, the authors then tried to make connections between these different levels of the food web, and determine if the parasites in the hippo were the same as the ones in the snails.

What They Found

The authors found that there were four distinct trematode species infecting the stomach and liver of the hippo. These four species are transmitted by six snail species known to thrive within Lake Kariba, including the invasive North American snail. While I have only briefly summarized the authors methods, their work confirmed that a series of biological invasions occurred in Lake Kariba. First, the water hyacinth was introduced from South America, which facilitated the invasion of the North American snail. This invasion then resulted in parasite spillback, where the invasive snail was heavily infected by a native trematode species.

Problems?

The authors did an enormous amount of work to find, measure, and classify the parasites examined in this study. The only issue that I have is that the data they collected on the adult parasites came from a single hippo host, and the stomach flukes were only collected from a small part of the entire stomach. Working with large mammals is difficult, and it can be extremely hard to get access to them in the first place, much less sacrifice them in order to dissect them and look for parasites. Unfortunately it means that despite the arduous work involved in today’s paper, there is always the risk that this single hippo (and its parasites) are not representative of the hippo (and parasite) community in Lake Kariba.

So What?

The intense and detailed work that went into this study have shown that the construction of artificial lakes, combined with the introduction of invasive species, has resulted in an alteration of the snail-borne diseases of the local hippo community. This study is only the tip of the iceberg when it comes to understanding how things are changing for hippos, as we lack the historical (and current) info on the parasite burdens of the hippos around Lake Kariba, but this is an important and necessary first step.

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Dr. Adam Hasik is an evolutionary ecologist and Zuckerman Postdoctoral Fellow interested in the ecological and evolutionary dynamics of host-parasite interactions who , having seen hippos last week in South Africa, never wants to have to try and sample them himself. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.