PATHOGENIC ILLNESS INDEX | COMMENT AND FEEDBACK

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3 - Epidemiological Research
3.1 - Early Investigations
3.1 a) - USA and Canada
3.1 b) - United Kingdom
3.1 c) - Early Bathing Water Standards

Forward to Chapter 3.2 - Large Scale Sea Bathing Epidemiological Investigations

Chapter 3 - Epidemiological Research

The quality of recreational water has been a matter of slowly increasing concern for a number of decades, and a series of research projects have been undertaken to find some means of estimating and managing the risk.

Few projects have studied the health of canoeists. Most studies have quite naturally been on the more general topic of bathing, and especially bathing on sea beaches in marine (salt) water. This chapter aims to review a number of these studies to determine what factors are relevant to managing the risk of infection to canoeists.

3.1 - Early Investigations

The first attempts to quantify the possible health effects of bathing in sewage contaminated marine waters took place in the early 1950s, with studies in the UK and USA.

a) USA and Canada

Stevenson (1953) examined three pairs of bathing sites:

• North and South beaches at Chicago on Lake Michigan
• An Ohio River beach and Tacoma Park Pool (a chlorinated recirculating fresh water pool) at Dayton
• Two beaches on the tidal waters of Long Island Sound.

The approach adopted was to select comparative beaches at each site which had either poor or good quality (averaged over the duration of the study which was typically one to two months in mid summer). Water quality at each beach was measured using total coliform organisms only, and observations were made of sanitary conditions, bathing load and weather.

Volunteers for the study were recruited by extensive local advertising. Participants (preferably families) were provided with a calendar on which to record daily swimming and illness experience. Illnesses recorded were eye, ear, nose and throat infections, gastro-intestinal disturbances, and skin irritations.

The data was analysed to detect statistically valid relationships between the reported illness rates, the swimming experience, and the average water quality over the survey period. By recruiting families it was possible to obtain data both from swimmers, and from non-swimmers who had been exposed to similar environmental risk factors away from the beach, and thus control for illnesses not acquired through bathing.

The results of the study are summarised at table 1. It is not possible to extract much useful data from this table, as the "number of persons in study" are totalled for the sites at Chicago and Dayton, and not given separately. It is also not clear how many of these are bathers or non-bathers. Similarly it is not clear whether the "total illnesses recorded" refer to the number of persons reporting illness, or to the total number of symptoms (which could be more than one per person).

It is clear that swimmers showed an increased rate of illness per 1 000 person days of activity over non swimmers. The rates tended to rise with days of swimming experience. South Beach swimmers who swam on more than 24 days of the 60 day study had an illness rate of 13.4 per thousand days, which was a higher rate than for those who swam on fewer days. This rate was exceeded however by swimmers at Dayton pool (with a chlorinated treatment plant) who reported illness rates of 32 per thousand days for those who swam on 10-19 days of the 34 days of that study.

The types of symptoms reported varied with the swimming venue. At Dayton, pool swimmers had an average illness incidence of 13.8 per thousand person days of all illnesses, of which 68% were eye, ear, nose and throat, and 15% gastro-intestinal. In the Ohio River the average 10.1 illnesses per thousand person days consisted of 52% eye, ear, nose and throat, and 24% gastro-intestinal.

One of the most interesting aspects of the study is the lack of correlation between the levels of illness and the levels of coliform bacteria found in the water quality tests. At the two Long Island site the rates of illness are similar although median coliform levels vary by a factor of two. The highest recorded illness rate is at Dayton pool, which is chlorinated and had a coliform count tending to zero.

3.1 b) - United Kingdom

In the UK a different approach was adopted by the Public Health Laboratory Service. A retrospective research programme examined notified cases of poliomyelitis and enteric fever over a five year period between 1953 and 1958. It is not clear why the study was limited to these two diseases, apart from a statement that:

The committee was particularly anxious to assess the risk of contracting enteric fever or poliomyelitis through bathing in sewage polluted sea water. (PHLS. 1959)

This research project, by design, excluded analysis of all of the less serious symptoms and infections of the eyes, ears, nose and skin, and less serious gastro-intestinal infections.

The PHLS examined the bathing history of all notified cases of the two serious infectious diseases and compared them to a suitably selected control group. Only 4 cases of paratyphoid fever, notified between 1956 and 1958, could be associated with bathing or playing on polluted beaches. Other anecdotal evidence of previous cases was also studied. The committee suggested that:

for patients suffering from poliomyelitis a history of having bathed during the previous three weeks preceding the onset of symptoms is probably irrelevant as a causal factor.

and came to the following conclusions:

It could be argued that bathing waters with median coliform counts of greater than 10, 000 per 100 ml occasionally cause paratyphoid fever, and that a standard of this order can be justified on health grounds. (PHLS, 1959, page 467)

and

1. That bathing in sewage polluted sea water carries only a negligible risk to health, even on beaches that are so fouled as to be aesthetically very unsatisfactory.
2. That the minimal risk attending such bathing is probably associated with chance contact with intact aggregates of faecal material that happen to have come from infected persons.
3. That the isolation of pathogenic organisms from sewage contaminated sea water is more important as evidence of an existing hazard in the populations from which the sewage is derived than as evidence of a further risk to infection in bathers.
4. That since a serious risk of contracting disease through bathing in sewage polluted sea water is probably not incurred unless the water is so fouled as to be aesthetically revolting, public heath requirements would seem to be reasonably met by a general policy of improving grossly insanitary bathing waters and of preventing so far as possible the pollution of bathing beaches with undisintegrated faecal matter during the bathing season.

3.1 c) - Early Bathing Water Standards

Both the PHLS (1959) and the Stevenson (1953) study have profoundly affected policy in their respective countries.

In the USA, the National Technical Advisory Committee of the Department of the Interior developed standards based on the work by Stevenson (1953) and his co-workers. Although the original reports played down the significance of any relationships between the illness rates and bacteria levels due to the lack of statistically valid data, certain findings, although barely significant, were used to develop water quality standards for the USA and Canada.

The methodology was to take a certain three day period at Chicago South Beach when a significantly higher illness rate (12.2 per 1000 person days) was found than for swimmers in other periods. This level was then defined as the level at which a significant excess disease incidence could be observed (see table 2). The total coliform measure for this period of 2300 per 100 ml was then converted to a faecal coliform measure of 414 (as 18% of total coliforms were found to be faecal), and after factoring in a safety margin of 50% and rounding down, the resulting standard was produced:

"Faecal coliforms should be used as the indicator organism for evaluating the microbiological quality of recreational waters. As determined by the multiple tube fermentation or membrane filter procedures, and based on a minimum of five samples taken over not more than a 30 day period, the faecal coliform count of primary contact recreation waters shall not exceed a log mean of 200/100 ml, nor shall more than 10% of total samples during any 30 day period exceed 400/100 ml." (USEPA, 1986)

The weak epidemiological basis for this standard was noted and criticised by many workers. For example:

• It ignored the data from Ohio which did not show significantly differing illness rates on days of highest and lowest MPN.
• The study included all illnesses, some of which might not be related to sewage contamination.
• There was a loose definition of bathing, and a lack of a beach going but non-swimming control group. (Kay 1991).

Although the US standard could be criticised for it's weak statistical basis, it was at least better than the UK experience where the PHLS study dominated UK policy until the mid 1980s, and it's findings were used to justify a less than enthusiastic approach to any attempt to improve recreational water quality. The lack of enthusiasm was perhaps understandable when it is borne in mind that no valid epidemiological study was available which proved the link between illness and pathogen level, or which could indicate what levels of pathogens or indicator organism should be used in setting water quality standards.

After this early work it was clear that further investigations would be needed before scientifically valid water quality standards could be obtained. These studies tended to be of two types. Carefully designed large scale epidemiological studies were developed in an attempt to define the risks associated with bathing at fresh and marine water sites, and to produce appropriate indicator systems. In addition a number of smaller scale projects studied the risk of other watersports, such as canoeing, rowing and snorkelling.