Supermarket pre-washed vegetables test positive for Cryptosporidium

In a recent study published in Parasitology Research, researchers used molecular techniques to determine the presence of the food- and waterborne parasite Cryptosporidium parvum in samples of ready-to-eat, pre-washed vegetables collected from various supermarkets in the United Kingdom (U.K.) in the wake of a recent increase in Cryptosporidium cases in the country.

Study: Presence of Cryptosporidium parvum in pre-washed vegetables from different supermarkets in South East England: A pilot study. Image Credit: Khomulo Anna/Shutterstock.com

Background

Cryptosporidium is a protozoan parasite belonging to the phylum Apicomplexa and is the etiological agent of cryptosporidiosis. The two most common Cryptosporidium species that are responsible for infections in humans and animals are C. hominis and C. parvum.

Infections from these intracellular extra-cytoplasmic parasites have symptoms ranging from abdominal cramps and nausea to even life-threatening conditions and death among immunocompromised individuals and children younger than five years.

In infants younger than 12 months, Cryptosporidium is responsible for the second-highest number of diarrhea cases. At the same time, it is also the second most common reason for deaths associated with diarrhea among children between one and two years of age.

About the study

In the present study, the researchers assessed samples of pre-washed vegetables obtained from various supermarkets in Kent County, U.K., to determine if they were contaminated with Cryptosporidium.

While the parasite is typically transmitted through contaminated water or food carrying Cryptosporidium oocytes, some studies have reported that the oocytes can also spread through inhalation. However, Cryptosporidium outbreaks in industrialized nations such as the U.K. and the United States have been either waterborne or foodborne.

A 2015 study reported that the largest outbreak of Cryptosporidium to date in Scotland and England had 74 confirmed cases, and it was through contaminated pre-cut vegetables sold in supermarkets.

The ability of Cryptosporidium to form oocytes helps the protozoan parasite survive adverse environmental conditions and persist outside of the host for months. Furthermore, chlorine-based disinfectants cannot remove Cryptosporidium oocytes, and the only method to effectively eliminate the parasite is by filtering or boiling the water.

A recent outbreak of C. hominis in 2023 in the U.K., consisting of over 2,400 confirmed cases, was attributed to exposure to the pathogen through farm animals, swimming pools, foreign travel, and food.

The recent availability of ready-to-eat vegetables, especially pre-washed salad greens, and other vegetables that need not be cooked, has also increased the risk of Cryptosporidium contamination.

For this pilot study, the researchers obtained 36 samples of pre-washed vegetables from four supermarkets in Canterbury. The vegetable samples were washed in 0.9% saline and incubated. The fluid obtained from the washing was centrifuged, and the remaining pellet was processed for deoxyribonucleic acid (DNA) extraction.

The genomic DNA obtained was then used for a nested polymerase chain reaction (PCR), and the small subunit ribosomal ribonucleic acid (SSU rRNA) was amplified. For the samples that were positive for SSU rRNA, the gene coding for the 60 kDa glycoprotein (gp60) was also amplified.

The amplified genes were then sequenced and used as queries for the Basic Local Alignment Search Tool to determine the sequences in the GenBank database that were the closest match.

Results

The results showed that 58% of the samples were positive for Cryptosporidium, and four samples out of the 24 that yielded sufficient genomic DNA were positive for C. parvum with high percentage sequence identities.

The pre-washed and cut vegetable samples positive for Cryptosporidium were largely composed of red cabbage, iceberg lettuce, carrots, lamb’s lettuce, spinach, chard, and red and green butterhead lettuce.

Since pre-cut and pre-washed vegetables are meant to be eaten as salads without being cooked, they are subjected to more thorough chlorine-based washes than unpackaged and uncut vegetables.

However, Cryptosporidium oocytes’ ability to resist chlorine-based disinfectants results in them persisting on the vegetables and causing parasitic infections.

Vegetable contamination could occur at any stage of production, through the water used for irrigation or from handlers during the processing, packaging, and transport of vegetables. Furthermore, these vegetables can come in contact with other unprocessed vegetables during packaging or transport, leading to cross-contamination.

These results highlight the importance of stringent hygiene measures during the packaging and transport process and the need to explore other sanitization methods such as ozone washes, boiling, and filtration.

Conclusions

Overall, this pilot study found that a significant percentage of pre-washed and ready-to-eat vegetables sold in supermarkets in the U.K. were contaminated with the protozoan parasite Cryptosporidium.

Given the ability of Cryptosporidium oocytes to resist chlorine-based disinfection, the researchers believe that other more effective disinfection methods need to be explored.