Understanding Swimmers Itch in Platte Lake

Your children just spent the day swimming on your beach. They now are covered in dozens of red, raised bumps on their legs and feet. You and your family were the unintended target of a small parasite called a schistosome. It wasn’t looking for you. It was trying to find a key host in its life cycle, a bird and most likely a merganser. The rash is called a cercarial dermatitis, commonly known as Swimmers Itch.

Although the medical problem caused by Swimmers Itch in humans is only skin deep, it can be a real nuisance and even affect your property values. That is why the PLIA contracted with Freshwater Solutions to study the prevalence of the culprit, a schistosome, in Platte Lake. The schistosome that causes swimmers itch has two hosts that it cycles between, birds and snails. Parasite eggs are released from the birds in their feces. The eggs hatch within an hour in the water and liberate a miracedium. That miracedium has about 24 hours to find and attach to the proper snail host before it runs out of energy and dies.

Once inside the snail, it develops further into a cercariae. Cercariae are released, often by multiple snails at the same time and are designed like a microscopic torpedo, 1/80th of an inch long, with a singular objective: Find a host bird and penetrate their skin so it can continue its life cycle. Because cercariae do not have any way to feed, they rarely live longer than a day. Most of them are released in the early morning and the number of them actively swimming drops as the day progresses. They are not very strong swimmers and are quite fragile. Their head can be easily disrupted from their forked tail. In spite of these limitations, they still manage to get the job done.

Understanding this lifecycle is critical to developing effective strategies to eliminate Swimmers Itch from lakes.

Numerous species of schistosome are capable of causing swimmer's itch in Michigan. qPCR is a method used to look for DNA and it is so sensitive that it can even distinguish between different species of schistosomes that cause Swimmers Itch. Freshwater Solutions has studied Walloon Lake, Lake Charlevoix, Elk Lake, Skegemog Lake, Long Lake, Lake Leelanau, Lime Lake, and Glen Lake since 2018 using qPCR analysis to measure schistosome cercariae levels in the water.

Using this tool, nearby lake associations, such as Glen Lake, were able to apply for a permit to remove mergansers and interrupt the life cycle of the parasite. The Michigan Department of Natural Resources began providing nuisance control permits in 2018 to trap and relocate summer resident common mergansers if a lake association can demonstrate there is a swimmer's itch problem caused, at least in part, by common mergansers. This strategy sounds like a simple solution to the problem. The problem?  These parasites are very adaptable and are not limited to just mergansers. They can infect other bird species such as Canadian geese, Mallards, Red winged blackbirds, even canaries.

Mergansers are almost 100% infected and have the highest parasite load in their feces than any other bird. These birds are extremely mobile and can cover several miles of shoreline in a single day. Another problem? Not all the mergansers are summer residents. It is possible that migratory mergansers can infect a lake in the spring and be on their way leaving behind infected snails who may not release the cercarea for months. That may be one reason that despite extensive control efforts on a number of lakes in Northern Michigan, some relocation programs have not been effective at reducing the snail infection prevalence or the number of schistosome cercariae in the water.

The 2019 Freshwater Solutions Study confirmed Swimmer’s Itch is a part of the Platte Lake ecosystem. It has found DNA evidence that the Lake has the “Itch”. Most residents already knew that and at the Annual Meeting in  2019 an informal study showed that the north shore had the most complaints of cases.

What did we learn about the extent of Swimmer Itch in Platte Lake from this ongoing study?

First, a little background on what Swimmer’s Itch is. Swimmer’s Itch in humans is only skin deep, but it can be a real nuisance and can last for days in some people. If you understand the cycle of the parasite that causes the problem it will help to establish the strategies you can use to avoid seeing your family and friends covered in dozens of red, raised bumps on their legs and feet.

That rash has a medical name “cercarial dermatitis” and is the consequence of being the unintended target of a small parasite called a schistosome. It wasn’t looking for you. It was trying to find a key host in its life cycle, a bird. Historically the Merganser has been incriminated as the major culprit as they have the highest rate of infectivity and our analysis showed that of the 13 Mergansers tested on Platte Lake, 12 were infected.

But that’s not the entire story. These parasites are very adaptable and are not limited to just mergansers. They can infect other bird species such as Canadian geese, Mallards, Red winged blackbirds, even canaries. In fact, the most common waterfowl on our lake is the Mallard and unfortunately 4 of the 28 tested were positive. The Mallard population also increases during the summer season as the Mergansers and other birds migrate increasing from about 61% of the bird population in early June to over 90% by late July. In summary, from the Freshwater Solutions report “we can confidently conclude that both summer resident mallards and common mergansers are the definitive hosts for at least some of the schistosomes causing Swimmer’s Itch on Platte Lake.”

Understanding that fact is important because it undermines one of the most popular strategies for controlling Swimmer’s Itch recently, the relocation of the Mergansers. That may be one reason that despite extensive control efforts on a number of lakes in Northern Michigan, some relocation programs have not been effective at reducing the snail infection prevalence or the number of schistosome cercariae in the water.

The schistosome that causes swimmers itch has two hosts that it cycles between, birds and snails. Parasite eggs are released from the birds in their feces. The eggs hatch within an hour in the water and liberate a miracedium. That miracedium has about 24 hours to find and attach to the proper snail host before it runs out of energy and dies. Once inside the snail, it develops further into a cercariae. Cercariae are released, often by multiple snails at the same time and are designed like a microscopic torpedo, 1/80th of an inch long, with a singular objective: Find a host bird and penetrate its skin so it can continue its life cycle. Because cercariae do not have any way to feed, they rarely live longer than a day. Most of them are released in the early morning and the number of them actively swimming drops as the day progresses. By 4 pm, only 20% of the cercaria are still present. They are not very strong swimmers and are pretty fragile. Their head can be easily disrupted from their forked tail.

So what did our analysis show about the other primary host, the snail population in Platte Lake?

First, the most common snail that resides in our Lake, the Pleurocera sp. Snail, is immune to the parasite. Platte Lake does not have a diverse community of snails, but rather is dominated by one species (one that does not harbor avian schistosomes). Of the over 250 snails collected, only one of the snails was infected.

So now, let’s look at the parasite itself for possible solutions. We know that the parasite is a lousy swimmer, swims at the surface, gets blown around in the water, prefers a morning swim and it has a tendency for its head to fall off in turbulent water. Freshwater Solutions has studied these vulnerabilities and has found that creating long swim baffles that extend about a foot into the water creates an effective barrier and can reduce infection if you swim on the leeward side. They created and studied a device called the “Smasher” that causes water turbulence and decapitates the schistosome as well as a surface rake that can remove the parasite from the surface water. All of these worked to decrease the parasitic DNA in the swim area.

Combating Swimmer’s Itch is a work in progress. If a lake has swimmers itch, it does not mean that lake is polluted. In fact, the opposite is true. A healthy lake promotes a high diversity of species, including the birds and snails that are the hosts for the causative agents of swimmer's itch. There is no cure for Swimmers Itch, yet. For now, there are only strategies you can employ to reduce your risk. However, because this is such an important issue, with new methodologies like qPCR DNA testing, etc. being employed, the PLIA will continue its partnership with Freshwater Solutions to help our members stay on the cutting edge on the status of our lake and Swimmer Itch.