A variety of micro-organisms that can cause illness have been found in water, these can be introduced into the body by drinking and inhalation, or by exposure to skin or mucous membrane. They include bacteria, protozoa, algae and viruses. This discussion will cover the description, significance, water treatment required for elimination and health significance where applicable.

Aeromonas
Aeromonas species is a bacteria and has many similarities to the coliform group and some similarities to Pseudomonads. They have been isolated from fresh water and other water sources, soil, food (including meat, fish and milk). They are also associated with causing diarrhoea in some people and may cause septicaemia. There are three main species: A. hydrophila, A. sobria and A. caviae, although A. veronii biovar sobria, A. veronii biovar veronii, A. jandaei, A. schubertii and A. trota are also known to cause human infections.

Although Aeromonas species  have been isolated from several drinking waters in Australia, their relationship between the presence and water and causing infection is not clear.

By attaining a free available chlorine residual level of 0.2 to 0.5 ppm, Aeromonas can generally be controlled in water distribution systems.

These bacteria have long been known to cause disease in cold-blooded animals such as fish and amphibians. In humans, three types of infections are described: systemic infections, usually people who are immuno-compromised; would infections and diarrhoea. Many foodborne outbreaks have occurred overseas involving fish, oysters, prawns soup and egg salad. It is believed that an enterotoxin is produced which is responsible for the commonest clinical presentation (watery diarrhoea) of Aeromonas-associated gastroenteritis.

The gastroenteritis associated with Aeromonas is most often reported in children, but can occur in any age group. The incubation period is usually about 1 to 2 days after exposure.  It usually presents as a self-limiting illness of 1 to 7 days, although diarrhoea may persist in up to one-third of patients for periods from 2 weeks to several months. The most common presentation of symptoms is a mild fever associated with the typical watery diarrhoea. In children under 2 years of age, vomiting may also occur. Less commonly, a presentation of blood, mucus and white blood cells may be found in faecal samples. Some studies have suggested that Aeromonas is the second or third most common cause of gastroenteritis in children during summer periods. Formula fed infants appear to have an increased risk of infection with Aeromonas caviae in the first three months of life. In adults, predisposing risk factors include antibiotic treatment and an underlying enteric condition such as gastric and colonic surgery.

The infective dose required to cause gastroenteritis is not known. Only one human trial has been conducted and it yielded inconclusive results. However, levels of 10⁷ to 10⁹ CFU have been reported.

Despite all this research the significance of Aeromonas as an enteropathogenic organism is still in question. In some studies the occurrence of Aeromonas was higher in the faeces of the control group compared to as compared to those patients with diarrhoea, whereas in other studies there was no difference. Nevertheless, Aeromonas species have sometimes been associated with gastroenteritis.

There are no guidelines for acceptable levels of Aeromonas in drinking water. The absence of coliforms, faecal coliforms and E. coli does not indicate the absence of Aeromonas species.

The next BioTalk will deal with Campylobacter.

Source: Australian Drinking Water Guidelines - 6: 1996. National Health and Medical Research Council of Australia & Agriculture and Resource Management Council of Australia and New Zealand.  Foodborne Microorganisms of Public Health Significance. 5th Edition. 1997. AIFST (NSW Branch Food Microbiology Group.