As Cyclospora Cases Rise, a BU Expert Explains the Parasite Outbreak
Lettuce or salad greens have been flagged as a potential outbreak source, but raspberries have fueled previous cyclosporiasis events. Photo via iStock/5PH
As Cyclospora Cases Rise, a BU Expert Explains the Parasite Outbreak
Boston University infectious diseases expert David Hamer on how cases started, how it spreads, and how to protect yourself
Think about a summer lunch, and you probably crave a crisp salad or berry-packed parfait. But a recent multistate outbreak of cyclosporiasis—an intestinal illness caused by the parasite cyclospora—has many people thinking twice about fresh produce. The parasite spreads through food or water contaminated with feces and can cause watery diarrhea, abdominal pain, nausea, fatigue, and a low-grade fever.
“Current results point to lettuce or salad greens as a potential source for this outbreak,” health officials in Michigan said via a news release issued Monday, though they cautioned the investigation is ongoing and other foods have not been ruled out. They also did not identify a grower or supplier.
As of July 14, cases have been reported in 39 states, the majority of the country, says David Hamer, a Boston University School of Public Health professor of global health and a core director at the Center on Emerging Infectious Diseases. “This has become a big outbreak.”
Hamer studies tropical infectious diseases, including malaria and diarrheal diseases, and has also led global health research on maternal and child survival, neonatal sepsis, and pneumonia. As a faculty member affiliated with BU’s National Emerging Infectious Diseases Laboratories and the Precision Diagnostics Center, he also helped develop mitigation strategies to keep the University’s campus healthy and safe during the COVID-19 pandemic.
The Brink spoke with Hamer, who also holds an appointment in infectious diseases at the BU Chobanian & Avedisian School of Medicine, about what’s behind the outbreak, why it’s difficult to track, and how people can protect themselves.
Q&A
With David Hamer
The Brink: How do people become infected with cyclospora?
Hamer: They consume food that’s contaminated with, basically, the cysts of the parasite. So it’s a protozoan parasite. It’s in the same family as cryptosporidium, which was first identified in both Nepal and Peru.
It [comes from] contaminated food. The most common [transmission] vehicle—the food that it can be carried on—has been imported raspberries, both black and red. In particular, there was a big outbreak tied to Guatemala several years ago.
The Brink: Why are these outbreaks so often linked to fresh produce?
Hamer: I’m going to speculate a little bit, but people are shedding the cysts and they end up contaminating the environment. If there are inadequate options for sanitation—places for people to go and defecate, and instead they defecate in the fields—then it can lead to contamination. And then if there’s inadequate washing of the food and produce afterward, the stage is set for transmission of the parasite.
The Brink: Why isn’t rinsing fresh produce always enough to eliminate the spores?
Hamer: The surface of a berry like a raspberry, for instance, can be very irregular, and [the cysts] may be adherent to the surface and may be hard to adequately wash. Cilantro, salad mix, basil, and snow peas—those are the sorts of vehicles in past outbreaks.
The Brink: Why is this current outbreak elusive and hard to track?
Hamer: To diagnose cyclospora, you have to look for it [with] a stool test or a special PCR assay. And unless people think of it, and order it, or order a multiplex panel, they might not make the diagnosis.
The Brink: Is it important to try to make the diagnosis?
Hamer: Well, it’s treatable. It can cause symptoms and can persist for a couple of weeks. And not necessarily diarrhea that whole time, but nausea and pretty profound fatigue.
This is a treatable infection, and the primary treatment is a drug called trimethoprim-sulfamethoxazole. Bactrim is the trade name. [It can be] a problem for people with sulfa allergies, but otherwise, it works quite well. Because cyclospora can cause persistent symptoms, it’s definitely worth making a diagnosis and treating.
The Brink: Are there any promising new diagnostic tools or surveillance methods emerging this time around? You said there was an outbreak a few years ago. Is there anything new that we have to track in this outbreak?
Hamer: Not that I’m aware of, other than the fact that there are multiplex pathogen panels that can be used for diagnosis. They have been around for a number of years, but really it’s the last decade that these have become much more widely used. And they really are a great tool where they exist, although they’re a little bit more expensive.
The Brink: Do you think scientists are close to identifying the source of this outbreak?
Hamer: I think the Centers for Disease Control and Prevention is trying hard to investigate this, [suggesting] food diaries and assessing what people have consumed to see where there is overlap and what the common vehicle might be. I’m surprised they haven’t figured it out yet, honestly.
The Brink: Do you want to speculate why they haven’t done that yet?
Hamer: It could be something different from the usual ways it’s transmitted.
And say it’s lettuce instead of cilantro or raspberries. If both controls and cases might have consumed the same thing, it becomes harder to tease out the source. Multistate suggests it’s something that’s pretty common across the country and is distributed widely.
The Brink: Is climate change affecting the spread or seasonality of cyclospora?
Hamer: I don’t know, but it has been for other gastrointestinal pathogens. Many of the cyclospora outbreaks have been in the summer in particular, and if there’s a longer, warmer period, then theoretically it might change the epidemiology somewhat.
The Brink: What steps can people take to reduce their risk of infection?
Hamer: That’s tricky, because we don’t really know how this is being transmitted.
I would say with all produce, whether it be lettuce, cilantro, or other sort of leafy vegetables, they should be washed well. I don’t even trust prewashed—I wash.
The Brink: Really?
Hamer: I think it’s better to be safe. When you wash berries, they have all these little spaces where things can hide, so it’s trickier.
My hope is that the CDC’s enterics division will figure out what the source is pretty soon. Then once they do, there will be alerts to the public about which foods to be careful with. They may try to trace it back to the company that produced [this produce]. Is it one farm where things were being picked? Then try to figure out whether there was some contamination there.
