Eosinophilia is classified as primary or secondary, in addition to the hypereosinophilic syndromes.
Primary eosinophilia is a rise in the peripheral eosinophil count without any underlying condition to account for it. It is subclassified into clonal and idiopathic types.
Clonal primary eosinophilia is the type of eosinophilia that is due to proliferation of a clone of eosinophils in the bone marrow, and is found in leukemias or other myeloproliferative disorders.
Idiopathic primary eosinophilia is a term used to refer to peripheral blood eosinophilia without any detectable cause.
Secondary eosinophilia is the result of other disorders not associated with bone marrow proliferation, such as atopy, asthma, and most commonly helminthic infestations.
Hypereosinophilic syndromes are disorders which are characterized by eosinophilia above 1500/µL persisting for at least 6 months, with no underlying disease condition, but associated with organ dysfunction due to eosinophil recruitment into tissues which suffer resulting damage. These include the syndromes of pulmonary eosinophilic infiltrate with eosinophilia, such as:
- Churg-Strauss syndrome
- Tropical pulmonary eosinophilia
Etiology of Secondary Eosinophilia
The following simple classification may help to understand the manifold conditions which may give rise to secondary eosinophilia.
Allergic conditions, especially of the skin and the respiratory tract, such as;
- atopic dermatitis
- bronchopulmonary aspergillosis
- allergic rhinitis
- non-allergic rhinitis with eosinophilia syndrome (NARES)
- occupational pneumoconiosis
- milk protein allergy
Connective tissue disorders such as;
- Dressler syndrome
- Eosinophilic fasciitis
- Inflammatory bowel syndrome
- Rheumatoid arthritis
- Systemic lupus erythematosus
Immune disorders such as;
- Graft-versus-host disease
- Congenital immunodeficiency syndromes such as Wiskott-Aldrich syndrome or IgA deficiency
Certain tumors, such as Hodgkin’s disease, also result in eosinophilia. Others include
Invasive parasite infestations result in a rise in the number of peripheral blood eosinophils, such as
- Ascariasis (tapeworm infestation)
- Echinococcosis (dog tapeworm infestation)
- Visceral larva migrans
- Schistosomiasis (liver fluke infection)
With regard to parasitosis, a sustained rise in eosinophils is usually seen when the parasites migrate into tissues and come into contact with patrolling immune cells. Once the parasite is walled off within a cyst, or is dormant, eosinophilia tends to fade. Only if the cyst leaks and parasite products again become exposed to immune effector cells (T-lymphocytes) does eosinophilia again appear. Parasites which remain within the lumen of the gut during their whole lifecycle rarely cause persistent eosinophilia.
Again, protozoal infection is not usually a cause of a raised eosinophil count. However, ectoparasites such as the scabies mite are associated with eosinophilia.
Non-parasitic infestations such as:
- Chlamydial lung infection
- Acute coccidiomycosis
- Cat-scratch fever
- Infectious mononucleosis
- Mycobacterial infections
Skin disorders due to hypersensitivity reactions:
- Dermatitis herpetiformis
Asymptomatic drug-induced eosinophilia
Eosinophilia associated with pulmonary infiltrates
- non-steroidal anti-inflammatory drugs (NSAIDs)
Eosinophilia associated with hepatitis
- Selective serotonin receptor inhibitors (SSRIs)
- Semisynthetic penicillins
Eosinophilia associated with interstitial nephritis
- Semisynthetic penicillins
Drug reaction with eosinophilia and systemic symptoms (DRESS)
Other conditions that may be found in eosinophilia include:
- Peritoneal dialysis
- Liver cirrhosis
- Certain cancers such as lung or pancreatic cancer
- Hodgkin’s lymphoma
- Non-Hodgkin’s lymphoma