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Legionnaires' Disease In Potting Soil?

Updated on April 9, 2016
M G Del Baglivo profile image

The author holds a degree in Zoology and Physiology from Rutgers University.

Yup! If you’re a gardener like me there’s some important information I recently discovered. Precautions should be taken when working with potting soil and compost. I know I don’t follow all of the safety guidelines you’ll read later in this article. I doubt many of us do because we’ve never been warned of this particular danger.


The Pathogen in the Mix

Legionella longbeachae was first isolated from a pneumonia patient in Long Beach, California in 1980. The source of the infection was never determined.

Following a cluster of Legionnaires’ disease cases in Australia in 1989, researchers suspected the outbreak was associated with gardening, an activity shared by all of the patients. Further investigation identified potting soil and compost as the likely sources of the infection. A 1990 analysis of commercial potting mixes sold in Australia revealed that 79% of samples tested positive for L. longbeachae. A 2001 Japanese study determined that 2 of 24 potting soil mixes contained the same strain of the bacterium. In 2008, analysis of 46 potting mixes sold in Switzerland found 2 harbored this Legionella species. In that same year, 18% of commercial potting soil mixes sold in Britain tested positive for the strain.

The Connection of Legionellosis to Potting Mix

In 2000 the Centers for Disease Control and Prevention reported the first recognized American cases of Legionnaires’ disease caused by L. longbeachae associated with potting soil use by three patients in California, Oregon and Washington. The 45 year old California victim died.

Since then sporadic cases in Europe, Asia and the US have been linked to using commercial potting soil and compost. In fact, the incidence of Legionella from these products has increased worldwide. The Netherlands had five cases between 2000 and 2004. The genotype of the longbeachae found in one patient’s sputum was identical to that of the bacterium in the potting mix the victim had recently used in the garden. Although the results of genotype analysis were indistinguishable for a second patient, that individual reported visiting the same gardening center at the time. The remaining three patients in the cluster died from complications of Legionella pneumonia before testing could be completed.

Medical surveillance in Scotland determined that a cluster of Legionnaires’ disease cases in 2008 to 2009 were associated with potting mix when sputum samples of 2 patients contained the identical genotype of the longbeachae found in the potting mix they had used just prior to the onset of pulmonary symptoms. A third patient had recently worked with the same brand of potting mix. From 2008 to 2013, Scottish authorities reported that a total of 16 people contracted Legionellosis from commercial potting soil and two died from pneumonia.

Legionnaires’ disease

The bacterium Legionella was first recognized as a human pathogen in 1976 when an outbreak of illness among attendees of a Philadelphia convention of the American Legion sickened 182 patients, 29 of whom died. The etiology of the Philadelphia infections was later determined to be the central air conditioning system of the hotel hosting the convention. It is now widely known that the cooling towers of large air conditioning units in office buildings, hotels, hospitals and nursing homes harbor the strain Legionella pneumophila when not properly maintained with chemical treatment. The etiology of the Philadelphia infections was inhalation of aerosolized water (droplets) containing the strain. The illness came to be popularly known as Legionnaires’ disease.The clinical manifestations of the illness range from asymptomatic to acute atypical pneumonia to death from pulmonary and multi-organ failure (acute respiratory distress syndrome or ARDS). The period between exposure and onset of illness for Legionnaires’ disease can vary between 2-14 days.

L. pneumophila


Pontiac disease (fever)

This is a less severe form of Legionellosis discovered after L. pneumophila caused an outbreak of febrile illness in health department workers in Michigan in 1968. All had flu-like symptoms, but no pneumonia. The infamous strain of the bacterium was later isolated in original blood serum specimens obtained from the workers in 1968 and then tested after the 1976 Philadelphia outbreak. Since that initial recognition of Pontiac fever as a type of Legionellosis several other species of the microbe have been discovered as causative agents including L. micdadei and L. anisa. These two strains are found in contaminated whirlpool spas and decorative fountains and spread by infected aerosolized water.

L. longbeachae is a known cause of Pontiac disease as well as life-threatening Legionnaires’ disease. Concentrations of the strain per gram of potting mix have been reported to be comparable to those found in one milliliter of water infected with pneumophila. The onset of Pontiac disease usually occurs between 5 hours to 3 days of exposure. Symptoms of Pontiac disease can include:

  • muscle aches and headache;
  • fever;
  • unexplained fatigue;
  • shortness of breath; and
  • persistent cough.

Worsening of flu-like symptoms of Pontiac disease may suggest advancement of the illness to Legionnaires’ and require medical evaluation especially if the patient has had recent exposure to potting soil. As with all of the currently recognized 50 strains of Legionella, including L. pneumophila, health authorities have never reported person to person transmission of L. longbeachae.

How is Legionellosis Transmitted in Potting Soil

Unlike other Legionella species, longbeachae can cause disease by both inhalation and ingestion of the bacterium. Both pneumophila and longbeachae infect and reproduce in several genus of amoeba, protozoans commonly found in warm water and soil. The amoeba consumes the bacterium. Rupture of the cell membrane of the host releases more bacterium into the medium. After humans inhale or ingest infected water or soil, the bacterium repeats this process by invading lung macrophages to then reproduce, break down the wall of the cell and spill the newly formed microorganisms into healthy alveolar tissue. L. longbeachae illness is especially linked to water dripping from hanging pots due to the high concentration of amoeba. Inhalation or ingestion of these droplets can be a source of the bacterium. Hand to mouth transmission of potting mix produces infection by ingestion.



L. pneumophila (red chains) Multiplying in Amoeba

. | Source

Treatment and Prognosis

Pneumophila and longbeachae Legionellosis may be successfully treated with antibiotics in otherwise healthy patients. Immunocompromised patients are particularly at risk for more serious outcomes. Age is a factor and those over 40 are more susceptible to infection as are smokers and individuals with previously existing pulmonary disease. In the case of both Legionnaires’ and Pontiac illness, some infected individuals may never display symptoms and the condition resolves on its own.

A Growing Global Problem

Initially confined to Australia, New Zealand and Tasmania, the incidence of longbeachae Legionellosis is increasing across Europe and parts of Asia. L. pneumophila is the cause of 95% of Legionnaire’s disease in Europe with L. longbeachae the most common agent in the remainder of patients. In Australia, New Zealand and Japan an equal number of cases of Legionellosis are caused by both strains of Legionella.

All longbeachae cases in Australia and New Zealand can be traced back to the recent use of potting mix. Warnings are now required on bags of commercial planting products in those two nations. The Australian label states:








Sounds like good advice to me.


Centers for Disease Control and Prevention (CDC). “Emerging Infectious Diseases, Legionella longbeachae and Legionellosis.” Accessed November 20, 2015.

CDC. “Legionnaires' Disease Associated With Potting Soil --- California, Oregon, and Washington, May--June 2000.” Accessed November 21, 2015.

National Institutes of Health (US). “Does using potting mix make you sick? Results from a Legionella longbeachae case-control study in South Australia.” Accessed November 21, 2015.

National Institutes of Health (US). “From amoeba to macrophages: exploring the molecular mechanisms of Legionella pneumophila infection in both hosts. “ Accessed November 21, 2015.

Wikipedia. “Legionella longbeachae.” Accessed November 20, 2015.

Wikipedia. “Potting soil.” Accessed November 20, 2015.

WorkSafe New Zealand. “Legionnaire’s disease: What you should know if you work with soil, compost and potting mix.” Accessed November 21, 2015.

© 2015 M G Del Baglivo


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