Does Potting Mix Cause Legionnaires’ Disease in Gardeners?

The airways were full of a news story that reported, “potting mix caused the death of a Christchurch gardener”.  It got a huge number of comments on Facebook gardening groups and clearly everyone believed the story and was surprised that they could get Legionnaires’ disease, also known as legionellosis from soil and potting mix.

How true was the story? Did this person die from Legionnaires’ disease as a result of exposure to potting mix? What kind of potting mix was it? What is the risk of this happening to another gardener, especially to you? Should you do something different in the garden to protect yourself? All good questions that will be answered in this blog post.

Was the Death in Christchurch Due to Potting Mix?

Lets first examine the story itself. I found numerous reports on this story and all of them used the exact same wording. I guess it is cheaper to cut and paste, than to research and think for yourself.

The story presents zero evidence that the New Zealand gardener got the disease from potting mix. The article never mentioned anything about testing the potting mix for contamination.

Myles McIntyre did die of pneumonia, probably hospital pneumonia. It is reported that he had Legionnaires’ disease prior to the hospital pneumonia but the type of testing used was not reported. You will see why this is important below.

He was a gardener, he did use potting mix, but since there was no testing of the potting mix, there is no confirmed link between it and the disease. It is not even clear which disease he had. It is a sad story, but the data does not support the headlines, which is not surprising since this is a click-bait article that focuses on sensationalism and not facts.

What is Legionnaires’ Disease?

Legionnaires is the result of infection from Legionella bacteria which cause pneumonia-like symptoms. There are some 42 species of this bacteria and 18 have been linked to pneumonia infections in humans.

The EPA reports that, “Legionella are ubiquitous in natural aquatic environments, capable of existing in waters with varied temperatures, pH levels, and nutrient and oxygen contents. They can be found in groundwater as well as fresh and marine surface waters. ”

Most cases of pneumonia are not tested for Legionnaires’ disease so we don’t know the actual number of cases. Some estimates suggest that only 10% are diagnosed correctly.

It was first found in 1976 at the convention of the American Legion in Philadelphia and most reported cases are the result of the species Legionella pneumophila. This infection can be identified by testing the urine and that is the most common test used to confirm Legionnaires’ disease. This bacteria is water borne and is usually contracted by exposure to things like cooling towers, humidifiers, showers and hot tubs.

The ratio of Legionnaires’ Disease due to L. pneumophila, varies by region; US – 75%, Ontario – 60%, Europe – 70%, and New Zealand – 28%.

A different species called Legionella longbeachae, also causes Legionnaires’ disease. It tends to live in soil and not water. We don’t know exactly how this disease is contracted but the best guess suggests it is a result of breathing soil dust, or touching the mouth with dirty hands. It could also be caused by water aerosols (droplets of water) that have contacted soil while watering pots, or watering the garden. What we do know is that it is not passed from human to human.

This infection is not detected by using the above mentioned urine test. It can only be confirmed by culturing the organism and this is a big part of the problem. Hospitals normally test for Legionnaires using the urine test and so infections of L. longbeachae come back negative, resulting in cases being under reported.

History of Legionnaires’ Disease from Potting Mix

This discussion deals only with identified cases of L. longbeachae. 

Following an outbreak of legionellosis in South Australia in 1988 and 1989, a new species L. longbeachae, was identified and found is soil samples.

Outbreaks of this disease were also confirmed in the early 1990’s in New Zealand, with subsequent cases being found in Southeast Asia. At the time there were few, if any, reports from Europe or North America.

In the early 2000’s the disease started showing up in Scotland, the UK and continental Europe. It was still rare in North America. More recently, there has been a steady increase in cases in Europe and North America.

This progression of the disease seemed odd and this, along with other test results, led Dr Tara K. Beattie to suggest that the disease is more prevalent in wood-based potting mix than peat-based potting mix.

New Zealand and Australia have both a large gardening community and have historically used wood-based potting mix. In more recent years the UK has moved away from peat-based mixes to wood-based ones in an effort to save peat lands. Along with this change, came an increase of disease from potting mixes.

In North America, the commercial bags of potting media are still predominantly peat-based and we see fewer cases of the disease.

It should be noted that the logic for moving away from peat-based media for environmental reasons is not supported by the data especially if the peat is replaced by coir.

Does Infected Soil Cause Legionnaires’ Disease?

I have looked at numerous reports that imply an association between potting mix and the disease but most do not show a definitive cause and effect link.

In the medical profession it is common to identify the disease in the patient and if there are a significant number of cases at one time, health inspectors will look for a cause. This is done with surveys and not actual testing. So they will ask patients if they came in contact with items during the incubation period which is about 2 weeks prior to having symptoms.

If a gardener reports that they used potting soil, worked with house plants or worked in the garden, soil is reported as a possible cause. Actual soil samples are rarely tested. As a result of this, there are few reports that provide evidence of a direct link.

A Critical Study

A detailed study of L. longbeachae in 7 patients was done in Scotland in 2013. They not only tested patients to confirm this species of Legionnaires’ disease, but they also tested related soil samples.

They found:

• 6/7 cases had recently bought growing media and had used it in the two weeks prior to onset. No cases had used farm produced compost or manure.
• All 7 cases were confirmed to have contracted L. longbeachae.
• For 6/7 cases, bagged shop-bought growing media had been handled directly
  in the two week incubation period.
• 6/7 cases had soil that tested positive for L. longbeachae.
• All 7 cases had spent time in the garden prior to reporting symptoms.

This team then went on to do some DNA analysis to confirm that the strain infecting patients matched the strains found in the soil samples. DNA confirmed that in 5 cases, both patients and matching soil samples had identical strains.

Based on this work it is clear that humans can contract the L. longbeachae disease from bagged potting mix.

Legionella longbeachae in Soil Samples

Soil samples have been tested for Legionella longbeachae, in a number of countries.

• Australia, 26 (79%) of the 33 potting soil samples tested positive.
• Nineteen (100%) soil samples in Europe and the United Kingdom were negative for L. longbeachae.
• In Japan, 8 of the 17 samples (47%) contained L. longbeachae. The samples were newly purchased bagged potting mix, all of which contained wood products.
• A British study found that 4 of 22 brands of potting soil contained L. longbeachae.

It should be noted that L. longbeachae has also been found in old potted plants as well as soil from the ground.

Note: Some of the work reports the pathogen in ‘compost’, but these are British studies and compost in British English is another name for potting mix (American English).

How High is the Risk?

The next important question deals with risk, because it is the risk factor that determines if gardeners should be concerned about this problem.

Some facts to consider.

• Most cases of pneumonia are not tested for Legionnaires’ disease, so we don’t know how many people get the disease.
• If testing is done for Legionnaires’ disease it is almost always a urine test which does not confirm L. longbeachae.
• The disease seems to affect certain individuals and many others seem to be immune. The factors that contribute to this are mostly unknown.
• Smokers, heavy drinkers and people with immune diseases seem to be more susceptible, but the data about this is limited so far, mostly because very few cases have been studied.
• Wood based potting mix may increase the risk.
• Sealed plastic bags contain high levels of moisture and nutrients, the perfect place for bacteria to grow, especially if the bag is stored warm.
• When the pathogen is found in potting mix, it is found from various manufacturers. This is not a case of a few contaminated sources, it exists in most commercial products.

The numbers for this are vague, but here is some data. There are about 18 reported cases of Legionnaires’ disease, per million, in the US each year (2015). Of these 5% might be from L. longbeachae, which is 1 case per million people. In Scotland the incidence in 2013 was 1.6 cases per million with an average of 1 case per million for the years 2008-2012.

Because many cases are not identified correctly the actual number could be 10 to 15 times higher.

About 10% of reported cases are fatal.

It is important to keep this problem in perspective. Many bacteria and fungi living in soil can infect humans; potting soil is no different. Lots of people talk about “sterile” potting soil but there is no such thing, even if the bag says it is sterile. The reality is that we are constantly exposed to potential pathogens and fortunately the risk of disease is extremely low.

The risk of getting this disease from potting mix is very low and unless you have one of the risk factors mentioned below, you should not be concerned.

What Can Gardeners do to Minimize Their Risk?

There is limited good data on risk factors due to the very small number of cases. Reported risk factors for L. longbeachae disease include:

• Purchasing and using bagged potting mix
• Long time smoker
• Immune disorders
• Gardening
• Being male (males are twice as likely to get the disease)

Assuming you don’t want to stop gardening, what can you do to reduce your risk?

• Wash after handling soil
• Don’t eat while gardening
• Don’t touch your face with dirty hands
• Let potting mix dry out before using it (kills bacteria)
• Just before you use it, wet potting mix gently and don’t splash
• Keep stored bags cool and out of the sun
• Buy peat-based potting mix

Lots of sites suggest using gloves and face masks. In one study that looked at risk factors these did not reduce your chance of getting the disease. Gloves probably don’t help since the pathogen is not transmitted through skin. Since the pathogen is probably inhaled to cause infection, the face mask should reduce incidence of the disease.

Unfortunately, this puts an end to nude gardening day. Everyone will now be wearing a face mask 🙂

Warning Labels on Bags of Potting Mix

Some are calling for warning labels to be put on products that could potentially cause the disease. These requests usually come from people who have suffered, either directly or indirectly from the disease.

Australia and New Zealand have had labeling regulations in place since 2003 and 2005 respectively. Labeling has not resulted in a decline of the disease and most customers completely ignore the labels.

More education is valuable, but labeling will not make much of a difference. They won’t prevent people from buying the product and they won’t change peoples gardening habits.