Credit CDC PHIL
# 7016
Like murders, not every disease outbreak ends up being solved, and sometimes success only comes after months of dogged detective work, when the identity, source, and route of an outbreak are at last revealed.
Last summer the UK reported a sharp spike in Cryptosporidium (aka `Crypto’) cases during the month of May, which I covered in a pair of blogs (see Tales From The Crypto & More Tales From The Crypto).
At the time, no source of infection was immediately apparent, and so general precautions were offered to the public as the `usual suspects’ were investigated.
In June, the HPA released a statement:
Update 8 June: Increase in cases of cryptosporidiosis
(EXCERPT)
The Health Protection Agency (HPA) continues to lead a multi-agency investigation to determine whether recent cases of cryptosporidiosis are linked to a common source.
<SNIP>
So far investigations have not identified a likely source of infection. The Drinking Water Inspectorate has confirmed that there is currently no evidence that public water supplies are implicated.
Cryptosporidiosis is caused by an organism called Cryptosporidium, which is found in soil, food, water or surfaces that have been contaminated with infected human or animal droppings. People can become infected by consuming contaminated water or food, by swimming in contaminated water, for example in lakes or rivers, or through contact with infected animals. The most common symptom is diarrhoea, which can range from mild to severe.
In January of 2013, in Eurosurveillance: More Tales From The Crypto, we saw a report indicating that in addition to the UK, the Netherlands and Germany also saw unusual spikes in Cryptosporidium infections last summer.
Again, no immediate cause, or causes, was identified. This from the Eurosurveillance report:
Because the number of Cryptosporidium-positive samples in the Netherlands, the UK and Germany increased during the same period, common exposures or influencing factors might be expected. However, no single source has been found that could explain the increase of cryptosporidiosis seen in these countries. Foreign travel has been an important risk in the UK and bottled mineral water raised as a hypothesis in the Netherlands.
However, plausible factors might include multiple sources, extreme weather conditions, person-to-person transmission and other, still unidentified risk factors. Alternatively, the increase in different countries may have developed independently which could explain the difference in age distribution compared to previous years in the Netherlands but not in the UK and Germany.
Yesterday, the HPA announced that – at least in the UK – they’ve identified the likely culprit of last summer’s outbreak, albeit through the use of circumstantial evidence.
Investigation into an outbreak of Cryptosporidium infection in spring 2012
19 March 2013
The Health Protection Agency (HPA) can confirm that findings of an investigation into an outbreak of Cryptosporidium infection that affected around 300 people in England and Scotland in May 2012 showed strong evidence of an association with eating pre-cut bagged salad products which are likely to have been labelled as ‘ready-to-eat’. The outbreak was short lived and the numbers of cases returned to expected seasonal levels within a month of the first cases being reported. Most of those affected had a mild to moderate form of illness and there were no deaths associated with the outbreak.
The HPA conducted an extensive investigation which involved interviewing people who became unwell about their food history and shopping habits and comparing these with a similar number of people who were not unwell.
In the analysis of the exposure to different salad vegetables (irrespective of retailer) a significant statistical association was found between infection and the consumption of pre-cut spinach. When specific retailers were included in the analysis, the strongest association with infection was found to be with consumption of ready to eat pre-cut mixed salad leaves from a major supermarket chain. In this analysis, exposure to pre-cut spinach only reached conventional levels of significance for one retailer - a second major supermarket chain. A link to spinach from a number of other retailers was also suggested but these were not statistically significant. Together these findings suggest that one or more types of salad vegetables could have been contaminated.
The Food Standards Agency (FSA), who were part of the outbreak control team led by the HPA, gathered information on the production and distribution of salad vegetables to try to identify the likely source of the outbreak. Investigation of the food chain including practice and procedures throughout each stage of growing, processing, packing and distribution of salad vegetables has not identified a source of contamination. Bagged salad on sale in supermarkets is often sourced from the same suppliers for most leaf types, often with common production lines packing product for several retailers at the same time. This was the situation in this case.
Dr Stephen Morton, regional director of the HPA’s Yorkshire and the Humber region and head of the multi-agency Outbreak Control Team, said: “This outbreak was fortunately short lived but it was important to see if we could find the source. Our findings suggest that eating mixed leaf bagged salad was the most likely cause of illness.
“It is however often difficult to identify the source of short lived outbreaks of this type as by the time that the outbreak can be investigated, the affected food and much of the microbiological evidence may no longer be available.
The deduction that mixed leaf bagged salad was behind the UK outbreak is not terribly surprising, as Crypto is often a foodborne illness.
According to an EID Journal study published in 2011, there are estimated to be nearly 750,000 Crypto infections in the United States alone each year (see Foodborne Illness Acquired in the United States—Major Pathogens).
For most healthy individuals, a Crypto infection is an unpleasant, but not life threatening illness. The most common symptoms (which generally last 1 to 2 weeks) are:
- Watery Diarrhea
- Stomach cramps or pain
- Dehydration
- Nausea
- Vomiting
- Fever
- Weight loss
Although rarely fatal in healthy individuals, `Crypto’ can be deadly for the very young, the very old, and those with compromised immune systems.
Given its prevalence in the environment, and the number of human infections each year, prevention is key. The CDC maintains an extensive webpage devoted to Prevention & Control of Cryptosporidiosis.
