July 01, 2019
Outbreaks of a parasitic infection linked to swimming pools, farm animals, and childcare settings have risen steadily over the past decade, according to the Centers for Disease Control and Prevention (CDC).
From 2009 to 2017, outbreaks of cryptosporidiosis, a diarrheal disease caused by the microscopic parasite Cryptosporidium, increased an average of 13% per year, the researchers write. More than one third of the reported outbreaks were linked to waterborne exposure via swimming pools, wading pools, water parks, and other sources of treated recreational water.
The study, by Radhika Gharpure, DVM, of the CDC's Division of Foodborne, Waterborne, and Environmental Diseases, and colleagues, was published online June 27 in Morbidity and Mortality Weekly Report.
Spread through fecal-oral transmission via water, food, contact with infected people or animals, or contact with surfaces that have been contaminated with the parasite, cryptosporidiosis produces profuse, watery diarrhea and other gastrointestinal symptoms that can last for weeks in otherwise healthy people and can be life threatening to those with compromised immune systems.
Because Cryptosporidium is chlorine resistant and can survive for more than a week in treated water, it is most commonly spread through water. Of 444 cryptosporidiosis outbreaks (7465 cases) voluntarily reported to the CDC during the 8-year period of this investigation, 156 (35.1%) were associated with exposure to treated recreational water, resulting in 4232 (56.7%) cases of the illness, the authors report.
Outbreaks associated with spread through treated recreational water "can result in hundreds or thousands of cases," the authors write. This is because one diarrheal incident can cause the excretion of hundreds of oocysts, which can survive for days, potentially infecting anyone who swallows the contaminated water over the course of a week.
Outbreaks were also commonly associated with contact with cattle (14.6%, 549 cases) and contact with infected individuals in childcare settings (12.8%, 418 cases).
Of the 444 reported outbreaks, 13 (3%, 88 cases) were associated with unpasteurized milk or apple cider during the 8-year period. The outbreaks were possibly linked to contaminated udders, apples, or processing equipment, the authors write.
When considered by transmission source, the largest annual increase in outbreaks was associated with cattle, increasing an average of 20.3% (95% confidence interval [CI], 9.2% – 31.4%) per year from 2009 to 2017, followed by outbreaks associated with childcare settings, which increased an average of 19.7% (95% CI, 8.8% – 30.5%) per year.
From 2009 to 2016, the annual number of reported treated recreational water-associated outbreaks increased an average of 14.3% per year (95% CI, 3.4% – 25.2%). "[H]owever, because of a decline in reported outbreaks in 2017, no trend was found for the annual number of treated recreational water-associated outbreaks during 2009–2017 (P = 0.293)," the authors note.
Cryptosporidiosis outbreaks overall peaked during July and August, largely as a result of recreational water exposure, although some variation was observed, depending on the transmission source. "[T]he number associated with treated recreational water peaked in June–August, the number associated with cattle contact peaked during March–May, and those associated with contact with infected persons in child care settings peaked during July–September," the authors explain.
It is likely the study underestimates the true magnitude of cryptosporidiosis outbreaks, the authors write. They also note that the observed trend toward increased prevalence may partially reflect the implementation of more advanced molecular testing methods in recent years.
The development of effective cryptosporidiosis prevention strategies should take into account that Cryptosporidium oocysts are infectious immediately upon excretion, that the volume of excreted oocysts is an order of magnitude higher than the quantity needed for human infection, and that the oocysts are highly resistant to chlorine.
Because a single site of recreational water-based exposure can trigger a community-wide outbreak, primary prevention of contamination is important, the authors note. To this end, the CDC recommends that individuals with diarrheal illness not swim or attend childcare until 2 weeks after diarrhea has resolved.
In the event a cryptosporidiosis outbreak occurs, "substantial decontamination measures are needed," the authors write. These include hyperchlorinating public treated recreational water venues and disinfecting all potentially contaminated surfaces in childcare settings with hydrogen peroxide.
The risk for human infection from livestock transmission of Cryptosporidium peaks during the spring calving season. The CDC recommends hand washing following direct or indirect contact with cattle, goats, and sheep or their living environments. "Additional preventive measures include, but are not limited to, removing clothing and shoes worn in the animals' living environment before entering other environments (eg, a home) to reduce risk for cross-contamination," the authors write.
"Reversing the increasing trends in annual numbers of reported cryptosporidiosis outbreaks overall and those associated with treated recreational water, contact with cattle, or contact with infected persons in child care settings will require implementing effective prevention measures," they add.
The authors say continued advances in molecular testing technologies should help optimize parasite identification and prevention strategies.
The authors have disclosed no relevant financial relationships.