Water from 100 treatment plants

During the COVID-19 pandemic, RIVM researched levels of the coronavirus SARS-CoV-2 found in wastewater. It is also possible to detect the presence of the measles virus in wastewater.  RIVM is now doing a trial run to see whether the measles virus can be found in wastewater in the Netherlands. At this point, RIVM is testing wastewater samples from about 100 wastewater treatment plants in the Netherlands on a weekly basis. These treatment plants were selected for various reasons, for example because they are in an area with low vaccination coverage, in a large city, or in an area where measles is currently circulating.  If necessary, RIVM can also check samples from the over 200 other wastewater treatment plants in the Netherlands to see if they contain the measles virus.

24-hour wastewater collection

Over a 24-hour period, a small amount of water from the wastewater treatment plant is sampled a number of times. If large volumes of water flow in at once, for example due to heavy rainfall, then the water boards and water laboratories collect more water. This is how they account for possible dilution due to the greater water intake. The method makes it possible to ensure that the water collected over 24 hours offers an accurate impression of the entire day.

Virus particles per 100,000 inhabitants

The collected water samples are sent to RIVM, where the water is investigated for the measles virus. If the virus is present, then the researchers calculate the viral load: the number of virus particles per millilitres of water. The amount of water collected on that day is also taken into consideration in the calculations.

The researchers then divide the viral load by the number of inhabitants connected to that wastewater treatment plant. That yields the number of virus particles per 100,000 inhabitants for each wastewater treatment plant. This does not necessarily offer any indication of the exact number of people with measles in a specific area. However, the researchers can compare the current level to the levels in previous weeks. This makes it possible to see how much the measles virus is spreading in each region, and compare it with trends in other regions.

Advantages of wastewater surveillance

Wastewater surveillance can reveal the spread of the measles virus in a specific area. If many people in an area contract measles at the same time, the Municipal Public Health Service (GGD) cannot report each individual case and conduct source and contact tracing for each patient separately. In such situations, wastewater surveillance can offer an accurate impression of the situation and how it is developing over time.

Wastewater surveillance has a number of additional advantages:

• People do not need to take any direct action. For example, there is no need for people to be tested. 
• Wastewater surveillance also detects virus particles from people who may not even know that they are infected. 
• Wastewater surveillance is anonymous. Each wastewater treatment plant serves so many people that it is impossible to trace which specific individuals were the source of the virus particles. 

Not directly comparable, not a replacement

Wastewater surveillance supplements our knowledge, but the results are not directly comparable to the number of cases reported by doctors and laboratories. Similarly, it cannot replace source and contact tracing. There are a number of reasons for this:

• It is not known how many virus particles are excreted by one infected person. This could vary, depending on such factors as the person’s age or the severity of the infection.
• It is not known when an infected person is excreting virus particles. 
• It is not known how long an infected person will continue excreting virus particles. The virus may still be detectable for weeks, even after a person is no longer contagious. 
• Moreover, wastewater surveillance does not detect people with measles who do not use the toilet, such as children still in nappies. 
• Wastewater surveillance measures levels of virus particles at the point in time when the water sample was taken. Cases reported by doctors and laboratories involve new patients. 
• People may use a toilet in a different region than where they normally live – at the office, while visiting friends or family, or while on holiday. 
• If very few people are infected, it may not be possible to detect the presence of the measles virus in wastewater. The same applies if the water is heavily diluted by major rainfall.

RIVM is currently only sharing the results of measles monitoring from wastewater research with the Municipal Public Health Services (GGDs). By comparing wastewater surveillance trends with measles cases reported by doctors and laboratories over the coming months, we will learn to understand the results of wastewater surveillance more effectively. This will enable us to make the best possible use of this information in the future.