Plentiful and essential, water is considered the universal solvent for biological life. Indeed, life is thought to have originated in the aqueous environment of the planet, and living organisms rely on blood, extra- and intracellular solutions and digestive juices for biological processes. Furthermore, research has demonstrated positive health outcomes, including lower blood pressure, heart rate and stress, from simply looking at fish swimming in aquatic environments. Yet, water can also be a place where dangerous and deadly microbes lurk, waiting for a host to arrive.

In the Dec. 2021 issue of Emerging Infectious Diseases, the Centers for Disease Control and Prevention (CDC) reported that a 56-year-old woman, hospitalized on Sept. 20, 2019, had likely acquired melioidosis via novel transmission of Burkholderia pseudomallei from a freshwater home aquarium. The Maryland woman is the first known person to have acquired the severe tropical infection by this transmission route.

Life in An Aquarium—Burkholderia and Mycobacterium spp.

CDC epidemiologist Patrick Dawson, Ph.D., told Medscape Medical News that, although outbreak investigators always ask about pet ownership, the environment (e.g., soil, water, etc.) and travel, they do not traditionally ask about fish, and in this case, the patient did not volunteer the exposure. However, a visit to the patient's home revealed multiple aquariums. Seeing the water, and knowing that most freshwater tropical fish in the U.S. are imported from Southeast Asia, led investigators to specifically culture for B. pseudomallei, a bacterium that is ubiquitous throughout the tropics and can be difficult for the microbiology lab to identify via automated identification systems (such as Vitek 2 and/or MALDI). Identification of this pathogen is challenged by the specificity of species-level identification and regionally dependent database data. However, when B. pseudomallei is isolated, it is a tier 1 select agent, which, if part of a potential multistate outbreak, not only creates a national alert, but must also be handled in a very careful and specific manner.

One of the reasons that aquatic infections are not immediately considered in differential diagnoses is that, like B. pseudomallei, many aquatic zoonoses are not endemic in the U.S. Melioidosis, the disease caused by B. pseudomallei and formerly known as Whitmore’s disease. It was first described in 1912 and consists of a wide range of symptoms, including localized, pulmonary, bloodstream and disseminated infection. Cases were historically identified primarily in northern Australia and areas of Southeast Asia, such as Thailand, and almost all cases of infection reported in the U.S. have been related to international travel to endemic regions. However, 11.5 million U.S. households have pet fish. Most exotic aquarium animals are caught in the wild and transported to a major hub for transport to the U.S. where a quarantine may not be in place, and outbreaks have recently been reported in the U.S.

Mycobacterium marinum is another bacterium that is commonly found in salt water and freshwater and is the causative agent of disease in many species of fish, and occasionally in humans. The infection is commonly known as fish tank granuloma, and it produces nodular or ulcerating skin lesions on the extremities of healthy hosts. Fishhook injury is a common route of infection. Diagnosis is usually delayed, and invasion into deeper structures such as synovia, bursae and bone occur in approximately 30% of reported case-patients. M. marinum, or fish handler's disease, is not a nationally notifiable condition in the U.S.

Aquatic Zoonoses in Recreational Bodies of Water

Many aquatic infections occur during outdoor play or exercise. Water sports, like knee-boarding, water skiing, swimming and diving can all be associated with exposure to zoonotic and other waterborne microbes. Simple things like getting a cut or abrasion while outdoors working or playing, and then entering a body of water (stream, lake, river or ocean) can be a potential portal of entry for many microbes.

Aeromonas and Vibrio species represent examples of bacteria that are often naturally endemic to warm marine and estuarine waters and threaten life and limb when infection occurs in humans. Aeromonas hydrophila can cause necrotizing fasciitis  and cellulitis, which may lead to life-threatening infections. While Vibrio vulnificus can cause sepsis and characteristic hemorrhagic bullae—large, discolored blisters filled with body fluid—during the summer. V. vulnificus infections require intensive care or limb amputations, and about 1 in 5 people with this infection die, sometimes within a day or 2 of becoming ill. Furthermore, eating raw seafood, contaminated with V. vulnificus has a 50%-60% death rate.

Fishing and/or cleaning and preparing fish may provide additional exposure risk to water-loving bacteria. For example, Streptococcus iniae has caused cellulitis, arthritis, endocarditis and meningitis following superficial or puncture injuries, notably from cleaning tilapia. Other infections from contact with fish include primarily topical skin infections from Erysipelothrix rhusiopathiae and gastroenteritis from Plesiomonas shigelloides, Campylobacter spp. and Salmonella spp.

Perhaps the microbe that can strike the most fear in everyone, the deadly parasite known as the brain-eating ameba, Nagleria fowleri. This free-living, microscopic ameba is a single-celled, living organism. N. fowleri causes the rare and devastating infection of the brain called primary amebic meningoencephalitis (PAM). This microbe is usually found in warm, freshwater (e.g., lakes, rivers and hot springs) and soil. It usually infects people when contaminated water enters the body through the nose. Upon entrance to the nose, the ameba travels to the brain where it causes PAM, a condition that is usually fatal. In fact, once the ameba crosses the blood brain barrier, mortality approaches 100%.

Microbes in Drinking Water

In the U.S. and most developed countries, clean, safe drinking water is readily accessible. However, according to the World Health Organization, approximately 2.2 billion people worldwide still lack regular access to safe drinking water, and untreated or improperly treated drinking water can be a hospitable environment for many dangerous pathogens. Cryptosporidium parvum is a water-borne parasite that can cause massive food-borne (drinking water) outbreaks, as exemplified during the deadliest waterborne outbreak in U.S. history in Milwaukee, WI in 1993-94. Other parasitic protozoa that are transmitted through water and cause human infections include Toxoplasma gondii, Entamoeba histolytica, Cyclospora cayetanensis, Isospora belli, Blastocystis hominis, Balantidium coli, Acanthamoeba spp., Sarcocystis spp. and Naegleria spp. However, cryptosporidiosis and giardiasis remain the most common water-related parasitic infections and are more often identified during outbreaks caused by contaminated drinking water.

How to Prevent Waterborne Infections

Keeping some basic safety measures in mind when outdoors (or indoors) interacting with water can help limit the exposure and spread of water-borne infections. For example, when swimming or playing in recreational facilities, keep water out of the mouth and ears. Likewise, individuals who have been sick with diarrhea in the past 2 weeks should stay out of the water. Disinfection with chlorine or bromine and properly regulated pH is the first defense against the germs that cause recreational water illnesses in pools, hot tubs/spas and water playgrounds. If one suspects something is not right with the water they are about to enter, they should stop and report it to someone who can inspect the facility.

People should not participate in outdoor water sports in lakes, rivers and streams without proper eye and nasal protection to protect against the rare, but deadly, parasite N. fowleri. The importance of careful hand hygiene when caring for aquariums can’t be stressed enough. Aquarium filters, filter floss, biofilm, charcoal and gravel might have exceptionally high concentrations of bacteria. Gloves are critical when cleaning aquariums, and immunocompromised individuals should avoid this task altogether. Finally, one should never drink water from an unknown source or if there is uncertainty about whether the water has been properly treated.