26 July 2017

Sick after a cool swim?

On hot days, it is tempting to go for a swim in a river or lake. Swimming in a river is never a good idea because currents can be deceptively strong, but there is another hazard lurking in the water: people and dogs can become infected with Leptospira bacteria when swimming in untested surface water.

Field fever

There are various sub-strains of Leptospira that can cause a variety of different symptoms. The umbrella term for these conditions is leptospirosis, also known as field fever. A well-known form of leptospirosis that occurs in people and dogs is Weil’s disease. The disease is carried by rodents, which do not become sick themselves. The bacteria can get into the surface water via rat and mouse urine, and thus contaminate the environment. When swimming in contaminated water, people or dogs can become infected via the mouth, eyes and wounds. 


The majority of patients only experience mild flu-like symptoms such as tiredness, nausea, muscle pain and fever. In many cases, the infection does not cause any symptoms. However, Leptospira infection can cause severe problems such as liver and kidney failure, or bleeding in the lungs. In cases which necessitate hospitalisation, the mortality rate in people can reach 20% and is even higher among dogs. Treatment with antibiotics is possible if administered early enough.

Symptoms in dogs

Early on in infection, the symptoms are mild: general fatigue, poor or lacking appetite, sometimes vomiting. In later stages, the skin, mucous membranes and the whites of the eyes may turn yellow. It is advisable to contact a veterinarian if one or more of these symptoms are present.

The Veterinary Microbiological Diagnostics Centre at the Faculty of Veterinary Medicine can test blood samples upon request by veterinarians. Research into leptospirosis also takes place at the University Clinic for Companion Animal Health (Dutch).


For dogs, there is a vaccine that provides protection from the most common strains of Leptospira in Europe, including the strains that cause Weil’s disease. There is no registered vaccine for people in the Netherlands. That seems problematic, but infection from dog to human is very rare. With proper hygiene measures, the infection risk for humans is practically non-existent. The biggest risk is infection by Leptospira that has been excreted into the water by rodents. For this reason, you should make sure that you always swim in water that has been tested.

Increasing numbers of infections

Leptospirosis is a disease that must be reported: the RIVM or the GGD must be notified of every case. In the months August and September, there are a significantly greater number of infections. In recent years, we have seen a rise in the annual number of infections among both people and dogs. Climate change probably plays a role here. The mild winters allow more rats, mice and Leptospira bacteria to survive, with all the consequences this entails:

In 2014, doctors recorded approximately 40 cases per year in the Netherlands. Since then, this number has more than doubled. In addition, previous cases were mainly people who had been abroad, but we have now seen a rise in local infections.

There is no obligation to report leptospirosis in dogs, but veterinarians estimate that there are approximately 100 to 150 serious cases each year.

One Health

As mentioned above, people and dogs can be infected by the same pathogens. This means that human health is directly linked to animal health.

This idea is also known as the ‘One Health concept’. The Faculty of Veterinary Medicine is playing a leading role in the promotion of this concept. People and animals have direct socio-economic interactions through direct physical contact, the food chain and their living environments. The health of all species can be ensured through improvement of collaboration between doctors, veterinarians, scientists, the business world and other health professionals, so that effective solutions, rules and policies can be developed. One Health is an important area of research in Utrecht University’s strategic theme, Life Sciences.