Water as a Vector of Organisms

Key issues

• Water

• Vibrio cholerae

• Microbial growth and survival in water

• Food safety control, and the need for national infrastructure

Challenge

In 1974, cholera spread in Portugal. Not only did potable water supplies become polluted with cholera infected faeces, but so did shellfish and other foods and bottled water. This case study invites you to start thinking about how organisms become disseminated, whether they die in that process, and what affects their growth and survival. You can, of course, extend your thoughts beyond this case and wonder how control systems prevent water in all its various uses in the food industry both becoming contaminated and being a vector for a very wide range of pathogens.

1.1 THE CASE STUDY: CHOLERA IN PORTUGAL, 1974

(Most of the text below comes from Blake et al., (1977a and 1977b, see 1.1.4). The text in the original papers has been slightly shortened and some linking sentences put in to allow textual flow. Paragraphs from the two papers have been mixed to provide a shorter version of the two texts which are used here with the permission of the copyright holders – The Johns Hopkins University School of Hygiene and Public Health.

From Blake et al., 1977a:

On April 24,1974, in Tavira, a town on the southern coast of Portugal, a 33-year old man developed diarrhoea and dehydration so severe that he suffered a cardiac arrest. An alert physician suspected cholera, and the Ricardo Jorge Institute in Lisbon isolated Vibrio cholerae biotype El Tor serotype Inaba from his stool. V. cholerae were subsequently isolated from a Lisbon woman who had contracted a diarrheal illness on May 10 and from a child in Porto on May 14. Within six weeks after the first case, cholera had been reported from eight of Portugal's 18 districts. Eventually 17 of the 18 districts reported cholera, all due to El Tor Inaba organisms. The epidemic peaked in the last week in August, then declined rapidly; on November 29, 1974, Portugal was declared free of cholera. In this seven month period 2467 cases and 48 deaths were reported to the World Health Organisation (WHO). The case-fatality ratio was 1.9%.

Control measures included individual case investigations by public health nurses, distribution of two- or three-day supply of tetracycline to all contacts of cholera patients, a massive health education program, and an extensive effort to promote the chlorination of public water systems. These measures were supplemented by distribution of free bottles of chlorine for disinfection of drinking water from unsafe sources. Cholera vaccine was given only to those who demanded it.

Investigators from the Center for Disease Control [USA] and the Portuguese Directorate General of Health carried out two studies in Tavira and Faro, two cities on the southern coast, and in Lisbon to determine the important modes of transmission of cholera in Portugal.

1.1.1 Background to the Outbreak

Both Tavira and Faro are separated from the open sea by the Ria de Faro, a complex of islands and mud flats that extends approximately 50 km along the southern coast [of Portugal]. Sewage from the coastal towns emptied into the Ria, and water and shellfish in the Ria had high coliform bacteria counts. Most shellfish consumed throughout Portugal are caught in this area. After anecdotal information suggested that shellfish had caused some cases of cholera, the Maritime Biology Institute in Faro systemmatically cultured water and shellfish from the Ria from May to August 1974; V. cholerae was isolated from 24% of 121 seawater samples and 42% of 154 shellfish, including clams (44 of 114 positive), cockles (11 of 22), oysters (7 of 15), and mussels (2 of 3).

Tavira's municipal water supply was chlorinated, but some residents preferred to drink from two springs in the town. One of these, the Fonte do Bispo, emerged from a pipe in a populated area and had allegedly produced clear water until September 1973 when, following blasting of the rock above the spring and a heavy rain, the water was muddy for a few days. Both springs were closed on May 10 and 11, shortly after the cholera outbreak began, because health authorities suspected that some of the cases might have resulted from drinking from these springs. No water from either spring was cultured. Troops travelled back and forth between a military base uphill from the Fonte do Bispo and what were then Portugal's African colonies – Angola, Mozambique and Portuguese Guinea. Untreated sewage from the base and the town entered the Gilao river which bisects the city, and flowed with it into the Ria. V. cholerae was isolated from 9 of 20 water samples taken from the Gilao river May 14–22, 1974.

In the rural area north of Faro, outbreaks of cholera were attributed by the Health Authorities to consumption of water from wells, several of which were culture positive for V. cholerae. The cause of the 59 cholera cases reported in Faro itself in May–September 1974 was unclear, although shellfish were suspect.

In the area around Lisbon, where sewage was sometimes used to irrigate and fertilise vegetable gardens, health authorities initially felt that contaminated raw vegetables, as well as shellfish, might be responsible for most cases ... But

[From Blake et al., 1977b]

... in August, during the peak of the cholera epidemic, Lisbon Health Department noted that some cholera patients reported recent travel to a spa in Lisbon district. At the same time Vibrio cholerae El Tor Inaba was isolated from both the springs which supplied mineral water to the spa and to a commercial water bottling plant. Both the spa and the plant were closed at once ...

The spa had 51718 visitors in 1973, of whom about 20000 came during...