Measles surveillance: are wastewater tests suitable?
The measles is back with a vengeance. In the United Kingdom, where only 85 percent of school children have received two doses of the so-called MMR (measles, mumps and rubella) vaccine, up to 300 people have been infected with the disease since October. In the USA, nine people in Philadelphia have fallen ill since last month. One case was reported in Atlanta and another in the state of Delaware. In Washington state, an entire family of six is infected. That may not sound like much in itself, but measles is one of the most contagious diseases of all, which without vaccination can lead to serious complications and a fatal long-term effect years later.
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So it’s hardly surprising that the World Health Organization issued a warning on January 23rd. “It is crucial that all countries are prepared to quickly detect and respond to measles outbreaks in a timely manner, as these could jeopardize progress towards measles elimination,” said Hans Kluge, WHO Regional Director for Europe.
Highly contagious
However, detecting measles outbreaks early is not easy. Like many other respiratory viruses, the illness begins with a cough, runny nose, fever and physical discomfort. The tell-tale red-spotted rash only appears after two to four days. At this point, the sick person is already highly contagious. They spread the viruses through tiny droplets emitted by coughing, sneezing, speaking or even just breathing, which remain in the air for hours and can infect others even without direct contact with the sick person.
The extensive wastewater sampling network that was set up during the corona pandemic to detect Covid-19 could now serve as an early warning system in the USA. “I think you could even argue that it’s more important to detect measles than Covid, influenza or other pathogens that we’re looking for,” says Samuel Scarpino, an epidemiologist at Northeastern University in Boston.
Wastewater monitoring relies on standard laboratory tests that look for genetic evidence of pathogens in wastewater – DNA or RNA, depending on the germ. Because people infected with Corona excrete the SARS-CoV-2 viruses in their stool, which then end up in the wastewater. But viruses that are not excreted in the stool can also appear in the sewage system.
Although measles is primarily a respiratory disease, it also enters wastewater through various routes. For example, people also excrete the viruses in their urine or spit them into the sink when brushing their teeth. Some people blow their noses and then throw the tissue into the toilet. “We excrete viruses, bacteria and fungi in so many ways that end up in sewers,” says Marlene Wolfe, an environmental microbiologist and epidemiologist at Emory University and one of the leaders of WastewaterSCAN, a program at Stanford that monitors infectious diseases in municipal wastewater systems .
Measles RNA in wastewater
The literature on the detection of measles in wastewater is sparse but encouraging. In one study, a team of researchers in the Netherlands examined wastewater samples collected during a measles outbreak in an Orthodox Protestant community in 2013 for the virus. They found measles RNA, and the positive samples matched locations where cases had been reported. They even managed to confirm that the virus in one sample was genetically identical to the outbreak strain. But not every case of measles was detected in the sewage system. Some samples surrounding measles cases did not contain measles RNA.
In another study, Nova Scotia researchers developed a tool to simultaneously test wastewater for four pathogens: RSV (respiratory syncytial virus), influenza, Covid-19 and measles. When they tested it in Nova Scotia, they got no positive hits for measles, which didn’t surprise them since no cases had been reported. They showed that their test worked by adding a measles substitute to the wastewater samples and then detecting it in both high and low concentrations.
Wolfe says the real question is whether detecting measles in wastewater would have a public health benefit. Because measles rarely has no symptoms and the rash is so severe, cases are usually noticed. “Some of our other systems work quite well at identifying measles cases when they occur,” she says.
However, if people excrete very large amounts of virus before the signs become visible, the benefit of monitoring is great. “Then there could really be an early warning,” she says. Whether this is the case is not yet known.
Suitable locations for a wastewater monitoring system
What would a wastewater monitoring program for measles look like? “If we were able to find places where the vaccination rate is lower, we could prioritize our resources there,” says Scarpino. “Airports and other ports of entry will also be very important.” In early January, a person infected with measles passed through both Dulles and Ronald Reagan airports near Washington, D.C. The discovery of measles RNA in airport wastewater doesn’t necessarily mean there could be a local outbreak, but “it definitely means that the risk profile is there and we should monitor much more actively,” he says.
Although measles is not yet part of wastewater monitoring, many other pathogens are. Since the late 1980s, health authorities around the world have been testing wastewater for the pathogen that causes infantile paralysis (polio). Because people who have polio shed large amounts of the virus in their feces and many people are asymptomatic, “this is a perfect use case in many ways,” Wolfe says. But wastewater monitoring didn’t come into vogue until 2020, when Covid-19 hit.
The National Wastewater Surveillance System, which the Centers for Disease Control and Prevention (CDC) launched in 2020 to monitor Covid-19, is now also testing for Mpox. WastewaterSCAN is currently testing for ten different pathogens, including Sars-CoV-2, Mpox, RSV, influenza, norovirus and rotavirus. The team publishes this data in a dashboard on its website and shares it with the CDC. Wolfe and her colleagues also recently worked with Miami-Dade County in Florida to examine the feasibility of testing for dengue fever. Although dengue fever is rare in Florida, the team detected a signal in wastewater.
In fact, wastewater surveillance works for most of the pathogens tested, says Wolfe: “It is entirely possible to use this tool to effectively support measles surveillance.”
(jl)
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