Overview

Pneumonia is an infection that causes inflammation in the lungs. It can be caused by bacteria, viruses, or fungi. Globally, there are an estimated 450 million cases of pneumonia each year, with most affecting people over the age of 65.^1,2

Typical pneumonia is usually caused by bacteria such as Streptococcus pneumoniae, but a less common form — atypical pneumonia — is caused by different microorganisms. Unlike “classic” pneumonia, which can cause sudden and severe illness, atypical pneumonia often develops gradually and may begin with only mild symptoms.

For people who have received an organ transplant, atypical pneumonia poses extra challenges. This is because transplant patients take medicines called immunosuppressants to prevent their bodies from rejecting the new organ. While these medicines are essential, they also weaken the immune system, making it harder to fight infections.³

What is atypical pneumonia?

Atypical pneumonia is sometimes called “walking pneumonia” because its symptoms may be mild enough that people can continue with daily activities at first. However, in transplant patients, even mild infections can escalate quickly and cause serious complications.

The main difference between typical and atypical pneumonia lies in the type of microorganisms that cause the infection. Atypical pneumonia can be triggered by certain viruses, bacteria, or fungi that do not usually cause the classic form of pneumonia.²

Common causes of atypical pneumonia in transplant patients

Viruses

• Cytomegalovirus (CMV): One of the most common viral infections in transplant recipients, CMV can be transmitted through the donor organ or arise from dormant infection in the recipient. It is a well-recognised cause of viral atypical pneumonia in this group³
• Influenza and RSV: Lung transplant patients are especially vulnerable to influenza (the flu) and respiratory syncytial virus (RSV). Because their immune systems are suppressed, these viruses can spread to the lungs and lead to pneumonia⁴

Atypical bacteria

• Mycoplasma pneumoniae: This bacterium causes slowly developing symptoms, such as a dry cough, tiredness, and low fever. In people with healthy immune systems, it is usually mild, but in transplant patients it can lead to more severe illness²
• Legionella pneumophila: Found in contaminated water sources (such as air conditioning systems, hot tubs, and humidifiers), Legionella can cause Legionnaires’ disease, a more severe type of atypical pneumonia that requires urgent treatment⁵

Fungi

• Pneumocystis jirovecii: This fungus is a major cause of pneumonia in people with weakened immune systems, including transplant patients, those living with HIV/AIDS, and people receiving chemotherapy. It causes dry cough, shortness of breath, and dangerously low oxygen levels, and can be life-threatening if untreated⁶

Symptoms to look out for

Atypical pneumonia symptoms can overlap regardless of whether the cause is viral, bacterial, or fungal. They often begin subtly and get worse over time:

• Dry, persistent cough
• Fever (usually mild at first)
• Shortness of breath
• Fatigue or weakness
• Confusion (especially in older adults)

Because symptoms can be hard to spot, transplant patients should contact their doctor promptly if they feel unusually tired, develop a cough, or notice any breathing difficulties. Early detection is key to avoiding complications.

How is atypical pneumonia diagnosed?

Doctors use a combination of clinical history, physical examination, and investigations to diagnose atypical pneumonia. Common tests include:

• Chest X-ray or CT scan: Shows inflammation or fluid build-up in the lungs
• Blood tests: Helps assess immune response and detect infection
• Sputum or nasal swabs: Samples from the airways can identify viruses, bacteria, or fungi
• Bronchoscopy: In complex cases, a camera is inserted into the airways to collect samples directly from the lungs²

Treatment options

The treatment plan depends on the cause of infection, the patient’s type of transplant, and overall health.

Antibiotics and antivirals

• Bacterial infections (e.g., Mycoplasma pneumoniae or Legionella) are treated with antibiotics
• Viral infections (e.g., influenza, RSV, or CMV) may require antiviral medicines
• Medicines may be taken orally or, in severe cases, through a drip in hospital

Adjusting immunosuppressants

In some cases, doctors may reduce the dose of immunosuppressants temporarily to help the immune system fight infection more effectively. This decision must be carefully balanced, as reducing these medicines increases the risk of the transplanted organ being rejected.⁷

Oxygen therapy

Patients with low oxygen levels may require additional oxygen, delivered through a mask or tubes. In severe cases, hospitalisation and use of a ventilator may be necessary.

Hospital care

Because transplant patients are high risk, hospital admission is often required to monitor the infection, adjust medicines, and protect the transplanted organ.

Prevention strategies

Prevention is especially important for transplant patients because of their weakened immune systems. Key strategies include:

• Vaccination: Recommended vaccines include the pneumococcal, flu, RSV, and COVID-19 vaccines¹
• Avoiding exposure: Limiting contact with people who are unwell reduces risk of infection
• Hygiene measures: Regular handwashing and mask-wearing during outbreaks can help
• Routine check-ups: Regular monitoring by a healthcare team allows early detection of infection and quicker intervention

FAQs

Why is atypical pneumonia more dangerous for transplant patients?

Because they take medicines to weaken their immune system and prevent organ rejection, transplant patients are less able to fight infections, making atypical pneumonia harder to control.

Can atypical pneumonia be prevented?

Yes. Vaccination against flu, RSV, COVID-19, and pneumococcal infections can lower the risk. Good hygiene and avoiding contact with sick individuals are also important preventive measures.

What should I do if I have a cough after an organ transplant?

Even a mild cough should be reported to your healthcare team. In transplant patients, early signs of infection can progress quickly if not treated.

Will reducing my immunosuppressants help me recover faster?

Sometimes doctors may temporarily lower the dose to help the immune system fight infection, but this must only be done under close medical supervision to avoid the risk of organ rejection.

Is atypical pneumonia treatable?

Yes. With the right diagnosis and treatment — including antibiotics, antivirals, antifungals, and supportive care — most patients can recover, although close monitoring is essential.

Summary

Atypical pneumonia is caused by uncommon microorganisms, including certain viruses, bacteria, and fungi. While it may start with mild symptoms such as a dry cough or fatigue, in people with organ transplants it can progress quickly and cause serious illness due to suppressed immunity.

Diagnosis relies on imaging, blood tests, and sometimes bronchoscopy. Treatment varies depending on the cause and may involve antibiotics, antivirals, or antifungal medicines. In some cases, oxygen support and hospitalisation are needed.

Preventive steps such as vaccination, good hygiene, and regular medical check-ups are essential for reducing the risk. With close monitoring and prompt medical care, transplant patients with atypical pneumonia can recover well.