What Is Acute Eosinophilic Pneumonia?

  • Asma Hadjadj Master's degree, Pharmaceutical Sciences, Kingston University, UK
  • Ellen Rogers MSc in Advanced Biological Sciences, University of Exeter, UK


Pneumonia is a condition that affects the lungs and is either an acute or a chronic infection. Eosinophilic pneumonia (EP) is a subcategory of pneumonia that is most common in people who were assigned male at birth and 20-40 years of age. EP can be caused by roundworm, smoking, asthma, or allergies. This article focuses on the acute (short-term) form of EP (known as AEP), which is characterised by the accumulation of eosinophils (a type of white blood cell) in the lungs. Whilst AEP is rare and still somewhat poorly misunderstood, it can be fatal - so it is important to understand its symptoms and how to manage it, so you can protect yourself and those around you.

Understanding acute eosinophilic pneumonia

The role of eosinophils in the immune system

Eosinophils account for approximately 0.5-1% of all white blood cells in your body, which protect you against infection. Specifically, eosinophils are involved in our body’s protective mechanisms against parasites - but they also assist general immune responses by moving to areas of inflammation.1

Whilst we know that AEP is caused by the excessive build-up of eosinophils in the lungs, the exact mechanisms underpinning its development are not yet fully understood. For now, research suggests that AEP is triggered by lung damage and inflammation, such as that caused by inhaling irritating chemicals or particles in the air, smoking, or taking steroids2. Damaged lung cells call for help using cytokines - chemical messengers that help to regulate the immune system’s response to infection or allergens. Here, these cytokines recruit white blood cells (including eosinophils) to the area of injury to protect against infection and begin repairs. However, the accumulation of eosinophils can be dangerous in some cases, causing AEP and leading to acute respiratory failure if not treated quickly.

Symptoms of acute eosinophilic pneumonia 

AEP develops rapidly and is usually accompanied by severe symptoms that require immediate treatment. These symptoms include:

  • Chest pain
  • Trouble breathing
  • Fever
  • Dyspnea (feeling like you’re running out of air)
  • Hypoxemia (low blood oxygenation)3

Causes and risk factors of acute eosinophilic pneumonia

AEP can often arise spontaneously without a discernable cause. However, known risk factors include smoking, asthma, allergies, or being exposed to various environmental or airborne triggers.4,5 These triggers include:

  • Inhaling (breathing in) airborne metal ions or dust
  • Inhaling airborne fungi, including Aspergillus species
  • Taking non-steroidal anti-inflammatory drugs (NSAIDs)
  • Parasitic infections
  • Using e-cigarettes

A large proportion of eosinophilic pneumonia cases are due to either asthma, allergic reactions, or a combination of both.6

Diagnosis of acute eosinophilic pneumonia

It is important that AEP is diagnosed quickly. If your doctor suspects you have AEP, they may take the following steps or tests

  • Taking a complete medical history
  • Pulse oximetry: A pulse oximeter is used to measure the oxygen saturation of your blood. To use an oximeter, you should clip it onto your finger and wait for a reading. Normally, your blood oxygen saturation should be around 98%; a lower value is indicative of trouble with your breathing or lungs.
  • Blood testing / IgE testing: IgE is a type of antibody which helps your body recognise and respond to allergens. As such, testing blood IgE levels can help a doctor work out if your pneumonia is due to high numbers of eosinophils.
  • Bronchoalveolar lavage: This test involves introducing liquid into the lungs through a bronchoscope - a tube-like camera that is inserted through the mouth into your lungs. The fluid mixes with the cells and fluids in the lungs and is collected and tested. This test can be used to differentiate between different subtypes of eosinophilic pneumonia, including acute eosinophilic pneumonia (AEP), Löffler syndrome, and chronic eosinophilic pneumonia.7
  • Taking chest X-rays or CT scans: These scans will reveal any abnormalities in your lungs. This includes pleural effusions, which are caused by pleural fluid building up in the space between the lungs and chest cavity.8
  • Lung biopsy (only in severe or atypical cases)

Due to AEP’s similarities to other lung conditions, a differential diagnosis will need to be carried out to distinguish AEP from other diseases, such as bacterial pneumonia and pleurisy. 


Treatment of AEP is mainly via corticosteroids, which are given to reduce lung inflammation and reverse the symptoms of AEP. It is essential that treatment begins immediately for improved outcomes.The longer the lungs go without sufficient oxygen, the greater the risk of the lung tissue becoming thick and scarred (fibrosis) or dying (necrosis). Fibrosis and necrosis of the lung tissue are irreversible and permanently reduce your lung capacity. This might leave you struggling to get enough oxygen into your lungs during exercise or other physical activity. The dosage and duration of corticosteroid therapy will vary depending on the individual. Supplementary oxygen is given to patients to increase oxygen saturation and to reduce the effects of hypoxia (lack of oxygen) on the brain and other organs.10


There are positive outcomes with timely and appropriate treatment. However, it is important to diagnose AEP early to prevent respiratory failure. There are potential complications of AEP, such as the need for mechanical ventilation in severe cases. Chronic eosinophilic pneumonia (CEP) is associated with longer-lasting symptoms and, if treatment is not given promptly, respiratory failure.5

Management and follow-up

After the episode of AEP has resolved, it is important that your doctor works with you to minimise your chance of further attacks. Follow-up appointments may involve further blood tests to monitor the levels of eosinophils in your blood. It is also important to identify any potential triggers that may cause another flare-up of AEP, such as pollen, dust, or airborne allergens like cat and dog dander.11 If your symptoms do not resolve after treatment, you may be diagnosed with chronic eosinophilic pneumonia, which requires more extensive follow-up care and long-term management.


Eosinophilic pneumonia (EP) is associated with high levels of white blood cells known as eosinophils in the lungs. The accumulation of eosinophils in the lungs can be dangerous and make it difficult for you to breathe. In severe cases, AEP can cause acute respiratory arrest, so it is vital that affected individuals seek medical attention immediately. EP can be acute or chronic in nature, and a large percentage of patients are asthmatic and non-smokers. Eosinophilic pneumonia can be triggered by non-infectious, bacterial, viral, or parasitic stimuli and usually requires treatment to help alleviate the symptoms of dyspnea and chest pain.12 


What are the initial symptoms of AEP?

  • Breathlessness 
  • Chest pain 
  • Fever 

What are the main medications associated with treating AEP?

  • Corticosteroids, such as prednisone


  1. Gleich GJ, Adolphson CR. The Eosinophilic Leukocyte: Structure and Function. In: Dixon FJ, Austen KF, Hood LE, Uhr JW, editors. Academic Press; 1986. p. 177–253. (Advances in Immunology; vol. 39). Available from: https://www.sciencedirect.com/science/article/pii/S006527760860351X
  2. Tsarev S.V. Eosinophilic pneumonia in allergological practice. Russ. J. Allergy. 2021;18:32-40.
  3. Hayakawa H, Sato A, Toyoshima M, Imokawa S, Taniguchi M. A clinical study of idiopathic eosinophilic pneumonia. Chest. 1994;105:1462–1466.
  4.  Chaaban T. Acute Eosinophilic Pneumonia Associated with Non-Cigarette Smoking Products: A Systematic Review. ARM. 2020;88:142–6.
  5. Umeki, S. ‘Reevaluation of Eosinophilic Pneumonia and Its Diagnostic Criteria’. Arch Intern. Med. 1992;152:1913–19.
  6. Seifert M, Gerth J, Gajda M, Pester F, Pfeifer R, Wolf G. [Eosinophilia--a challenging differential diagnosis]. Med Klin. 2008;103:591–597. 
  7. Pahal P, Penmetsa GK, Modi P, Sharma S, 2023. Eosinophilic Pneumonia, In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023. Cited 2023 Aug 10 via: http://www.ncbi.nlm.nih.gov/books/NBK537169/.
  8.  Rosenberg, C. E., & Khoury, P. Approach to Eosinophilia Presenting With Pulmonary Symptoms. Chest. 2021;159:507–516.
  9. Rhee CK, Min KH, Yim NY, Lee JE, Lee NR, Chung MP, et al. Clinical characteristics and corticosteroid treatment of acute eosinophilic pneumonia. Eur Respir J. 2013;41:402–9.
  10. Delgado BJ, Bajaj T. Physiology, Lung Capacity. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023. Cited Aug 10 2023 via: http://www.ncbi.nlm.nih.gov/books/NBK541029/
  11. NORD. Acute Eosinophilic Pneumonia - Symptoms, Causes, Treatment [Internet]. Accessed Nov 19 2023 via: 
  12. Crowe M, Robinson D, Sagar M, Chen L, Ghamande S. Chronic eosinophilic pneumonia: clinical perspectives. Ther Clin Risk Manag. 2019;15:397–403. 
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Rachel Siobhan Smith

MSc Biomedical Science, University of Greenwich

Rachel is a Post Graduate student reading Biomedical Science at the University of Greenwich. They have years of experience working in Biomedical laboratories, but also a keen interest in medical communications. Rachel previously worked in a Covid Testing laboratory to help enable the UK, to increase testing capacity during the Covid-19 pandemic.

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