Outbreak of

Cryptosporidium Infection in a swimming pool complex in Merthyr Tydfil South Wales

Summer 2009

Report of the Outbreak Control Team

July 2010


Table of Contents / 1
Summary / 2
Abbreviations / 3
Glossary / 4
Introduction / 7
Methods of Investigation
Environmental / 8
Results of Investigation
Descriptive Epidemiology
Microbiological Findings
Environmental findings & Control Measures
Investigations & Control Measures / 15
Communications / 28
Discussion / 30
Conclusions & Recommendations
Recommendations / 37
References / 39
Appendix 1 List of OCT Membership / 40
Appendix 2 Analytical Epidemiology: The Cohort Study / 41


In the late summer of 2009, an outbreak of Cryptosporidium hominis infection occurred in association with a swimming pool complex in Merthyr Tydfil.

There were 106 cases meeting the case definition, of which 45 were confirmed through laboratory testing. The majority of cases (93) were primary, with only 13 cases of secondary spread.

The outbreak began as a point source. Cryptosporidium oocysts were introduced into the Leisure Pool of the swimming pool complex late in the morning of Saturday 22nd August. The most likely cause of this was a faecal accident involving the smearing of faeces on the toddler slide. Since Cryptosporidium oocysts are resistant to chlorination at usual pool-dosing levels, individuals swimming at the time and shortly afterwards were exposed to contaminated pool water.

The plumbing of the Leisure Pool mushroom water feature into the outlet from the balance tank so that the water in the cascade was taken before it had been through the filters was believed to be a contributory factor to spreading cryptosporidium contamination widely.

In addition gaps and weaknesses in policies and operational procedures and non-adherence to procedures in relation to incidents such as faecal accidents are also likely to have contributed to spreading cryptosporidium contamination widely at the time.

The point source outbreak which resulted was identified on September 1st when the first cases presented. Active case finding immediately commenced, along with investigation of the premises which included systematic review of pool plant, staff training, policies and procedures.

As the investigations progressed, new cases were identified whose exposure history and dates of symptom onset were not consistent with the point source faecal contamination incident described above. As a result, all the pools were immediately closed to allow for further in-depth investigations. These identified unexpected irregularities in the pool design and mechanical functioning.

These irregularities, which included inadequate circulation pump size, aberrant plumbing and intermittent flocculant dosing, may have allowed cryptosporidium to persist in the Leisure Pool water, causing the tail of cases seen after the initial point source outbreak. These irregularities were rectified in all the pools before they were reopened.

This is the largest documented swimming pool associated outbreak of cryptosporidium recorded in England and Wales in recent years, mainly due to the active case finding undertaken. The majority of cases did not seek medical attention, but the proactive follow up of exposed cohorts and widespread publicity allowed cases to be identified and appropriate infection control advice given. This was an important factor in limiting spread and preventing the development of serious illness in potentially more vulnerable household contacts. It may explain the low number of secondary cases seen in this outbreak.


A+E Accident and Emergency Department

CI Confidence Interval

EHO Environmental Health Officer

FSA Food Standards Agency

GP General Practitioner

LA Local Authority

LHB Local Health Board

MTCBC Merthyr Tydfil County Borough Council

NHS National Health Service

NOPs Normal Operating Procedures

NPHS National Public Health Service for Wales

OCT Outbreak Control Team

PWTAG Pool Water Treatment Advisory Group

RCT Rhondda Cynon Taf


Analytical epidemiological study: Any study in which groups are compared to identify and quantify causes of disease to test a theory as to what the cause may be. Case-control and cohort studies are both examples of analytical studies.

Attack rate: The attack rate is the cumulative incidence of infection in a group of people observed over a period of time during an epidemic.

Attack rate = number of exposed persons infected with the disease

total number of exposed persons

Backwash: cleaning water treatment filters by reversing the water flow

Balance tank: a reservoir of water between the pool itself and the rest of the circulation system; maintains a constant pool water level and supply to the pumps

Case: Any person who described symptoms of diarrhoea and/or vomiting and/or abdominal cramps.

Case Definition: A list of criteria that must be fulfilled in order to identify a person as a case of a particular disease. It is used in outbreaks of illness to identify who should be included on a list of cases. The criteria can include the symptoms of the illness, laboratory test results, the time and place of illness.

Chi Squared: A statistical test that is used to detect whether two groups differ from one another in a way that is greater than chance alone.

Cohort Study: A study looking at a defined group of people e.g. children attending a particular school and comparing how often illness developed in those with a certain exposure such as eating school meals with how often illness developed in those without the exposure (i.e. did not eat school meals).

Communicable Disease: Any disease that can be passed from one person to another.

Confidence Interval: A way of expressing, statistically, the certainty about the precision of the findings from a study. The 95% confidence interval represents the range of measurements, calculated from a study, within which we are 95% certain that the true effect lies.

Consultant in Communicable Disease Control (CCDC): A fully trained doctor in a branch of medicine that is responsible for the prevention and control of communicable disease in the community.

Contingency table (2x2): A table (usually 2 rows and 2 columns) that is used in epidemiology to show the relationship between disease and exposure, for example a food source. The table is used to divide people into the categories of diseased and exposed, diseased and not exposed, not diseased and not exposed and not diseased and exposed.

Control: Any person that does not have the illness that can be compared to a case in an analytical epidemiological study.

Cryptosporidium: A protozoan parasite that can cause gastro-intestinal illness

Descriptive Epidemiology: Describing the characteristics of cases i.e. time, place or person characteristics such as date of onset of illness, place of residence, age or sex.

Environmental Health Officer (EHO): An individual fully trained in environmental health issues such as housing, sanitation, food, clean air, noise and water supplies. Responsibilities include inspecting restaurants, swimming pools (private providers) and other premises and following up cases of food and waterborne disease.

Epidemiology: The study of the patterns, causes, and control of disease in groups of people.

Epidemiological link: Cases linked by close social or household contact.

Fisher’s exact test: A statistical test used to determine the association between the exposures and outcomes of interest in an analytical study.

Flocculation: Aggregation of fine particles to form larger particles (floc) which can be removed by filtration as part of the swimming pool water treatment process

Microbiological Sampling: Taking a sample e.g. stool/faeces and testing it to see if an infectious agent is present.

Microbiologist: A doctor, mainly laboratory based, who specialises in the diagnosis, treatment and control of infectious agents such as parasites, bacteria, viruses and fungi.

Oocyst: The environmentally robust transmissible form of Cryptosporidium shed in faeces from an infected person or animal

Outbreak: An increase in the number of people with an illness or disease that is above what you would normally expect in the population at that particular time, or two or more linked cases with the same illness.

Outbreak Control Team (OCT): A team of people from different, usually public bodies, brought together, according to official guidance primarily to control the spread of disease during an outbreak. This is done through assessing the range and extent of the outbreak; identifying the source of the problem if possible, implementing prevention and control measures and communicating with relevant parties and the public.

P-value: P-values are calculated in statistical tests to estimate how likely it is that the associations observed between an exposure (e.g. swimming) and an outcome (e.g. becoming a case of Cryptosporidium) could have been due to chance alone. A p-value of 0.05 means, therefore, that there is a 1 in 20 probability that the difference levels of illness between an exposed and an unexposed group occurred by chance alone (and hence a 19 in 20 chance that there is some other “real” explanation). The lower the p-value the more likely the difference between the two groups is real.

Primary case: The first individual within a group or family to get the disease. There may be several primary cases in a group if they are exposed to the same source around the same time.

Regional Epidemiologist: A doctor specialising in communicable disease epidemiology in a population, working at the all Wales level.

Relative risk: Is used in epidemiological studies to quantify the risk of disease in a group of people exposed to a group of people unexposed. The risk of getting a disease in a group with the risk factor is divided by the risk of getting a disease without the risk factor.

Relative risk (RR) = probability of disease in exposed group

probability of disease in non-exposed group

Schematic: A diagram that represents the elements of a system using abstract, graphic symbols. A schematic usually omits all details that are not relevant to the information the schematic is intended to convey.

Secondary case: A secondary case is a person that has caught the disease from a primary case.

Turnover: when the volume of water equivalent to the entire pool water passes through the treatment plant and back to the pool


On September 1st 2009, the Environmental Health Officers from Merthyr Tydfil County Borough Council received a telephone call from a member of the public stating that many of those who had attended a birthday party at a local swimming pool (Premises A) had subsequently become unwell with abdominal pain and diarrhoea. The Environmental Health Officers investigated this report and confirmed that many of those attending this birthday party had indeed been ill. The CCDC was informed of the potential incident. Immediate investigations were initiated, these cases were interviewed, and stool specimens were taken where appropriate.

An incident meeting was convened for the 3rd September 2009. At this meeting, it was reported that three stool specimens were positive for cryptosporidium. An outbreak was declared, and an outbreak control team was therefore formed to investigate and control the outbreak.

This report is a record of the activities of the Outbreak Control Team.

Methods of Investigation

Case Definition

This was agreed at the OCT meeting of the 3rd September 2009 as follows:

“Persons attending Premises A from the 15th of August 2009 onwards, or their household contacts, who subsequently developed symptoms of vomiting and/or diarrhoea and/or abdominal cramps”

During the course of the OCT meetings, it was necessary to refine the case definition in order to provide additional clarity. Cases were classified as primary or secondary, and confirmed or probable as follows:

Primary Case

“An individual who attended Premises A, and subsequently developed symptoms of vomiting, and/or diarrhoea and/or abdominal cramps”

Secondary Case

“An individual who developed symptoms of vomiting and/or diarrhoea and/or abdominal cramps after a member of their household had visited Premises A, and where the most likely cause was thought to be person-to-person spread from a primary case.”

Confirmed Case

“An individual with a laboratory confirmed diagnosis of cryptosporidium infection.”

Probable Case

“An individual who had symptoms consistent with cryptosporidium infection (diarrhoea and/or vomiting and/or abdominal cramps), but for whom a laboratory confirmed diagnosis was not obtained.”

An individual may be classified as a probable case for more than one reason, for instance:

An individual reporting a history of symptoms may have recovered before being interviewed by EHOs, and a stool sample may not have been requested or provided.

Standard laboratory tests used to identify cryptosporidium from stool samples are not 100% sensitive. Occasionally, the test may fail to detect cryptosporidium in a sample especially if the parasite is present in low numbers. Therefore occasionally a negative test result may be obtained for someone who does indeed have cryptosporidium infection.

Case Ascertainment

Cases were initially sought by Environmental Health Officers identifying and contacting individuals who had attended swimming pool parties at Premises A over the weekend of the 22nd and 23rd of August 2009. Cases were also sought among other groups who had attended pool parties, swimming classes and other pool based activities from the 15th of August onwards. In addition, the NPHS contacted GPs, GP Out Of Hours services and hospitals and asked them to let public health know of suspected cases of cryptosporidium infection associated with Premises A. In practice, once the outbreak was widely reported in the media, cases contacted the Pool directly to report illness. All these reports were followed up by the EHOs.


Descriptive Epidemiology

All cases were contacted by EHOs and interviewed using standard questionnaires. The questionnaires collected basic demographic details such as age, gender, address, occupation etc, and enquired about exposures over the preceding incubation period of up to a fortnight before illness. As well as asking about exposure to swimming pools, this included information on other potential exposures such as drinking water, contact with animals, food consumed and overseas travel etc. Where swimming pool use on Premises A was identified as a potential exposure, the EHOs sought further detail of the nature of this exposure including which of the three pools (main, learner, leisure) had been used. Where food had been consumed on Premises A, details were obtained.

Symptomatic cases were provided with specimen pots for the submission of faecal samples.