Legionnaires’ Disease Symptoms & Diagnosis

How to Recognize Legionnaires’ Disease and How Doctors Confirm It

Legionnaires’ disease is often described as a severe form of pneumonia, but what exactly should you look out for? And how do doctors know it’s Legionnaires’ and not some other type of pneumonia? In this page, we’ll break down the common symptoms of Legionnaires’ disease, the process of diagnosis, and why timely diagnosis is so important. Whether you suspect you have Legionnaires’ disease or are just looking to educate yourself, this information can help.

Note: This page is for informational purposes and not a substitute for professional medical advice. If you suspect Legionnaires’ disease, seek medical attention promptly.

Early and Common Symptoms of Legionnaires’ Disease

Legionnaires’ disease symptoms typically begin 2 to 10 days after exposure to Legionella bacteria. Early on, it may be hard to distinguish from other respiratory infections because it often starts with symptoms that resemble the flu. Here are key symptoms to watch for:

High Fever – Often 102°F (39°C) or higher. Chills are also common. Patients frequently report fevers that come on suddenly.
Cough – Usually a dry cough at first, which can later produce mucus or phlegm. Sometimes, but not always, people cough up blood-tinged sputum.
Shortness of Breath – As the pneumonia affects the lungs, breathing can become difficult. You might feel tightness in the chest or like you can’t get enough air, especially with exertion.
Muscle Aches and Headache – These systemic symptoms make it feel flu-like. Muscle pain (myalgia) and headaches can be quite pronounced.
Fatigue and Weakness – Legionnaires’ often knocks people off their feet. You may feel extremely tired, to the point of needing to stay in bed.
Chest Pain – Some individuals experience chest discomfort or pain, especially when coughing or taking deep breaths (known as pleuritic chest pain, due to lung inflammation).
Gastrointestinal Symptoms – About 1 in 3 Legionnaires’ disease patients have diarrhea, nausea, vomiting, or stomach discomfort. This is one thing that can tip doctors off that it’s Legionnaires’ and not a typical pneumonia, because GI issues aren’t as common in regular bacterial pneumonia.
Confusion or Mental Changes – In severe cases, especially among older patients, Legionnaires’ can cause confusion, disorientation, or delirium. High fever and spreading infection can affect the mind. Loved ones might notice the patient is “not themselves” or is unusually drowsy, agitated, or unresponsive.

It’s important to note that not everyone will have all these symptoms. For example, not every patient has diarrhea, and not everyone will have chest pain. However, fever, cough, and shortness of breath together are a red flag for pneumonia in general, including Legionnaires’. The presence of extra symptoms like GI problems can further raise suspicion of Legionnaires’, especially if there’s a known outbreak or risk factor.

Also, severity can vary:

Some people develop a relatively mild illness (sometimes diagnosed as Pontiac fever, which is a minor Legionella infection causing flu-like symptoms without pneumonia). They might just feel like they had a bad flu for a few days and recover.
Many Legionnaires’ cases are moderate to severe, requiring hospitalization. The pneumonia can lead to complications like lung abscesses or respiratory failure needing ICU care.
Certain groups are at higher risk of severe disease: older adults (50+), smokers or those with a history of smoking, people with chronic lung disease (like COPD or emphysema), and those with weakened immune systems (due to conditions like diabetes, kidney disease, cancer, or medications like corticosteroids). For instance, an otherwise healthy 30-year-old might get a milder case, whereas a 70-year-old smoker could become critically ill.

How Legionnaires’ Disease is Diagnosed

Diagnosing Legionnaires’ disease involves a combination of clinical judgment (recognizing the symptom pattern and patient history) and specific laboratory tests. Here are the main ways doctors confirm Legionnaires’:

Chest X-ray or CT Scan: This doesn’t confirm Legionnaires’ specifically, but it shows pneumonia in the lungs. Almost all Legionnaires’ patients will have evidence of pneumonia on imaging – often patchy infiltrates in the lungs that can consolidate (solidify) in lobes. If you have symptoms, a chest X-ray is usually the first step to see if it’s pneumonia. A CT scan (or CAT Scan) can provide a more detailed look if needed.
Urine Antigen Test: This is one of the most common quick tests for Legionnaires’ disease. It checks for a part of the Legionella bacteria (antigen) that gets excreted in urine. It’s rapid (results same day) and if positive, it’s a very strong indication of Legionnaires’. The standard urine test mainly detects Legionella pneumophila serogroup 1 (the most common strain causing illness). It might miss other strains, but since that strain causes most outbreaks, the urine test is a very useful tool.
Sputum Culture: If you’re coughing up phlegm, doctors may take a sample (sputum) and try to culture the Legionella bacteria from it in a lab. Legionella is a finicky organism to grow – it requires special media – but when successful, a culture can not only confirm Legionnaires’ disease but also allow further tests (like DNA typing to match it with environmental samples, which is useful in outbreak investigations). Cultures can take several days to grow.
PCR (Polymerase Chain Reaction): Some advanced labs use PCR on sputum or other respiratory samples. PCR detects Legionella DNA. It’s faster than culture and more comprehensive than the urine test (can detect multiple Legionella species), but not all hospitals have PCR testing for Legionella readily available.
Blood tests: There is a blood serology test (looking for antibodies to Legionella in your blood), but it’s of limited use in acute diagnosis, because it usually requires acute and convalescent samples (two blood draws a few weeks apart to see rising antibody titers). That’s retrospective and not so helpful when deciding treatment, but it can confirm a case after the fact.

Doctors will also do routine lab tests – blood counts, liver/kidney function tests – to see how the infection is affecting the body. Legionnaires’ can cause issues like low sodium in blood (hyponatremia) or elevated liver enzymes, which aren’t specific but often present.

Differentiating from Other Pneumonias:

Legionnaires’ disease may be suspected if:

The patient has the pneumonia symptoms plus the unique signs like GI problems or confusion.
The patient doesn’t respond to first-line pneumonia antibiotics (like if they tried a penicillin-type antibiotic which doesn’t work against Legionella).
There’s knowledge of an outbreak or the patient was in a high-risk setting (for instance, stayed at a hotel with a hot tub, or was hospitalized recently – hospital-acquired pneumonia could be Legionnaires’).
Lab hints, such as low sodium, can clue in some doctors toward thinking “this could be Legionella.”

Ultimately, a positive Legionella test (urine antigen, culture or PCR) clinches the diagnosis.

Why Quick Diagnosis Matters

Time is of the essence with Legionnaires’ disease for a couple of reasons:

For the Patient’s Treatment: Prompt appropriate antibiotics save lives. Legionella doesn’t respond to some standard pneumonia drugs (like penicillin or cephalosporins), so knowing it’s Legionella means doctors will use the right antibiotics (usually a fluoroquinolone or macrolide antibiotic). Studies have shown that delays in appropriate treatment can increase the risk of complications or death.
For Public Health: If you have Legionnaires’, finding out quickly can trigger an outbreak investigation, potentially preventing others from getting sick. For instance, your diagnosis might alert the health department to check that hotel you visited, and they might discover and fix a contaminated water system. Early diagnosis of one case can nip an outbreak in the bud.
For Legal Reasons: From a legal perspective, a confirmed diagnosis provides clear grounds for a case (if negligence is found). Also, the sooner an outbreak source is identified, the easier it is to gather evidence before it’s remediated. Occasionally Legionnaires’ disease is missed by doctors and that complicates the case (we then have to prove our client likely had Legionnaires’ even if it wasn’t tested at the time).

Sadly, misdiagnosis or delayed diagnosis happens. Many doctors might first think of COVID-19 or influenza or another type of pneumonia, especially if Legionnaires’ disease isn’t on their radar or if it’s not common in their area. If you suspect Legionnaires’ (for example, you know of an outbreak or multiple people sick after a conference you attended), you can advocate for yourself by asking the doctor, “Could this be Legionnaires’ disease? Should we do a Legionella test?” Good doctors will appreciate the information, since you might have knowledge they don’t (like “I heard the hotel I stayed at had a Legionella problem”).

After Diagnosis: What’s Next?

If you’re diagnosed with Legionnaires’ disease:

Treatment is straightforward in concept: antibiotics, rest, fluids, oxygen if needed. Most people, with proper care, will start to improve within the first week, though full recovery can take multiple weeks or longer.
Hospitalization: Many Legionnaires’ patients need to be in the hospital, at least initially, especially if oxygen is required or if there are complications. Don’t be alarmed – it’s standard for severe pneumonia. Intensive care might be needed if breathing assistance (like a ventilator) is required.
Prognosis: The majority of otherwise healthy individuals recover completely, though it can be a tough road. Unfortunately some people have more lingering and serious issues. The overall fatality rate is around 10%, but that skews higher in hospital-acquired cases and lower in community cases.
Follow-up: You’ll likely get a follow-up chest X-ray after a few weeks to ensure the pneumonia is clearing. Some people have residual lung scarring but many heal well.
Informing: As we discussed in our “What to Do” page, consider next steps like making sure the health department was contacted and contacting a lawyer once you’re on the mend, to explore why you got sick and whether you can collect compensation.

How to Recognize Legionnaires’ Disease and How Doctors Confirm It

Early and Common Symptoms of Legionnaires’ Disease

Differentiating from Other Pneumonias:

Why Quick Diagnosis Matters

After Diagnosis: What’s Next?

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