Lakes and other bodies of surface water host a wide variety of microscopic life, some of which are parasites that can impact human health. While these organisms are a natural part of aquatic environments, understanding them is a component of water safety. Knowing the common parasites, how they are transmitted, and the health concerns they pose is important for anyone interacting with freshwater.
Common Parasites Found in Lakes
Several groups of parasites found in lakes are noteworthy for their interaction with humans. The most common are protozoa, which are single-celled organisms. Two prominent examples are Giardia intestinalis and Cryptosporidium parvum, found in lakes and streams globally. These microscopic parasites are resilient and can survive for extended periods in water contaminated with feces from infected animals or humans.
Another group includes helminths, or parasitic worms, specifically the larval stage of certain flatworms known as cercariae. These organisms are responsible for swimmer’s itch. Their life cycle involves snails as intermediate hosts, which release the cercariae into the water. While their intended hosts are aquatic birds and some mammals, they can mistakenly burrow into the skin of human swimmers.
A rarer but more severe parasite is Naegleria fowleri, the “brain-eating amoeba.” This single-celled organism thrives in warm freshwater, and while infections are rare, they are nearly always fatal.
How Humans Encounter Lake Parasites
The most frequent way people encounter lake parasites is by accidentally ingesting contaminated water. Swallowing even a small amount of water while swimming can be enough to introduce protozoan parasites like Giardia and Cryptosporidium into the digestive system. These parasites are often present in water sources contaminated by animal waste or human sewage.
Direct skin contact is another primary route of exposure, specifically for the cercariae that cause swimmer’s itch. These free-swimming larvae will burrow into the skin of humans wading or swimming in infested waters, particularly in shallow areas where their snail hosts are abundant.
A much rarer mode of transmission involves nasal exposure to water containing Naegleria fowleri. This occurs during activities like diving or waterskiing in warm freshwater, when water is forced up into the nasal passages. This pathway allows the amoeba to travel from the nose to the brain.
Health Concerns from Lake Parasites
The most common illnesses from lake parasites are gastrointestinal, caused by Giardia and Cryptosporidium. Symptoms of giardiasis and cryptosporidiosis appear one to three weeks after exposure and include watery diarrhea, abdominal cramps, bloating, nausea, and fatigue. While most infections resolve in a few weeks, some can cause prolonged intestinal issues.
Swimmer’s itch, or cercarial dermatitis, presents as an itchy, red rash. The symptoms are an allergic reaction to the cercariae larvae burrowing into the skin. Small reddish pimples appear within hours and may develop into small blisters. While the itching can be intense, the condition is not dangerous as the parasites cannot develop further in humans and die within the skin.
The most severe health concern is Primary Amebic Meningoencephalitis (PAM), caused by Naegleria fowleri. This rare brain infection causes headache, fever, and vomiting, progressing rapidly to include stiff neck, confusion, seizures, and hallucinations. PAM is almost universally fatal.
Protecting Yourself from Lake Parasites
You can take several practical steps to reduce your risk of a parasitic infection from a lake.
- Avoid swallowing lake water while swimming. This significantly lowers the risk of ingesting protozoa like Giardia and Cryptosporidium.
- Prevent swimmer’s itch by avoiding marshy areas where snails are visibly present. After swimming, towel off vigorously and shower promptly to remove parasites from the skin.
- Heed all public health advisories about water quality. If a lake is closed or has a warning issued due to contaminants, it should be avoided.
- Never drink untreated water directly from any lake or stream. Water for consumption must be properly filtered or boiled to kill potential pathogens.
The Broader Role of Parasites in Lake Ecosystems
Parasites are a natural and integral part of lake ecosystems, playing a complex role in the food web and population dynamics. They are not simply agents of disease but are involved in transferring energy from smaller intermediate hosts, like snails, to larger hosts like fish or birds that consume them. These organisms also influence the balance of wildlife populations by regulating host numbers, which affects the health and behavior of fish, amphibians, and waterfowl.
The abundance and distribution of parasites are sensitive to environmental conditions. Factors like water temperature, nutrient levels from runoff, and the presence of specific hosts can impact parasite life cycles. Environmental changes, such as warming waters, can alter the geographic range of certain parasites, potentially shifting the risk of exposure for both wildlife and humans.