# Eosinophilia and Parasitic Infections

> NEET-PG Microbiology notes on eosinophilia in parasitic infections, covering helminth causes, tropical pulmonary eosinophilia and Loeffler syndrome.

- Canonical URL: https://prepelephant.com/topics/neet-pg/microbiology/eosinophilia-and-parasitic-infections
- Exam / course: NEET-PG · Subject: Microbiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-15
- Last updated: 2026-10-15
- Reference basis: Paniker's Textbook of Medical Parasitology; Ananthanarayan and Paniker's Textbook of Microbiology
- How to cite: "Eosinophilia and Parasitic Infections", PrepElephant, https://prepelephant.com/topics/neet-pg/microbiology/eosinophilia-and-parasitic-infections

## Direct answer

Eosinophilia is an absolute eosinophil count above 500 cells per microlitre, and helminth infection is its commonest parasitic cause. Tissue-invading larvae and worms stimulate IL-5, which drives eosinophil production, so the count rises during tissue migration and falls once the parasites are cleared. Tropical pulmonary eosinophilia, caused by filarial microfilariae trapped in the lungs, is the classic Indian exam example.

## What you must remember

- **Definition:** An absolute eosinophil count above 500 cells per microlitre, with hypereosinophilia usually taken as 1500 or more.
- **Helminth causes:** Ascaris, hookworm, Strongyloides, Toxocara, Trichinella and schistosomes are the commonest worm causes of eosinophilia.
- **Visceral larva migrans:** Toxocara canis larvae migrate through tissues to cause fever, hepatomegaly, pneumonitis and eosinophilia, and are diagnosed by serology.
- **Tropical pulmonary eosinophilia:** Nocturnal cough and wheeze with very high eosinophilia and raised IgE, while microfilariae are typically absent from the peripheral blood.
- **Loeffler syndrome:** Transient pulmonary infiltrates with eosinophilia, classically from Ascaris larvae migrating through the lungs, which resolve within weeks.
- **Strongyloides hyperinfection:** Corticosteroids suppress the immunity that controls Strongyloides stercoralis and can trigger life-threatening hyperinfection, so screening comes first.
- **Non-parasitic causes:** Allergy, asthma, drug reactions, eosinophilic vasculitis and haematological malignancies must be considered when parasite tests are negative.
- **Treatment principle:** Treating the causative helminth should bring the eosinophil count back to normal, which confirms the link.

## A stepwise workup for unexplained eosinophilia

A patient from an endemic region with a raised eosinophil count and cough is the classic exam vignette. The workup links the organ involved to the likely parasite before any test is sent.

1. **Confirm the count:** Repeat the complete blood count to confirm an absolute eosinophil count above 500 per microlitre before pursuing a cause.
2. **Take an exposure history:** Ask about travel, soil contact, raw meat or fish, pets and drug intake, since each points to a different parasite.
3. **Match the organ pattern:** Pulmonary symptoms suggest Loeffler syndrome or tropical pulmonary eosinophilia, serpiginous urticaria suggests Strongyloides, and hepatomegaly with fever suggests visceral larva migrans.
4. **Send targeted tests:** Stool for ova and larvae, and serology for Strongyloides, Toxocara and filarial antibodies, including IgG4 against Brugia, as the clinical pattern dictates.
5. **Exclude non-parasitic causes:** If parasite tests are negative, consider drug reactions, asthma, allergic disease, eosinophilic vasculitis and haematological disorders.
6. **Treat and monitor:** Treat the helminth and recheck the eosinophil count, since its return to normal confirms the diagnosis.

## Where the exam tries to catch you

Eosinophilia questions usually test whether the candidate looks beyond the blood smear to the organ and the exposure.

- **Trap:** Calling tropical pulmonary eosinophilia unlikely because blood smears show no microfilariae. — **Correct:** Microfilariae are trapped in lung tissue, so the blood is often negative, and filarial IgG4 antibodies or a response to diethylcarbamazine supports the diagnosis.
- **Trap:** Starting prednisolone for eosinophilia without considering Strongyloides. — **Correct:** Steroids can trigger hyperinfection, so Strongyloides serology and stool examination should be done first in endemic settings.
- **Trap:** Assuming every eosinophilia is parasitic. — **Correct:** Allergy, asthma, drug reactions and malignancies also raise the eosinophil count, so the exposure and organ history decide the cause.
- **Trap:** Expecting to find worms in visceral larva migrans. — **Correct:** Toxocara larvae do not mature in humans and are rarely seen in stool, so serology is the usual test.

## Frequently asked questions

### What absolute eosinophil count defines eosinophilia?
Eosinophilia is defined as an absolute eosinophil count above 500 cells per microlitre of blood. Hypereosinophilia, usually taken as 1500 or more per microlitre, prompts a wider search for a primary haematological or clonal cause.

### Which helminth causes tropical pulmonary eosinophilia?
Tropical pulmonary eosinophilia is caused by Wuchereria bancrofti or Brugia malayi microfilariae that are trapped in the lungs. Patients have nocturnal cough and wheeze, and the microfilariae are typically absent from peripheral blood, so the diagnosis rests on serology and response to treatment.

### Why is Strongyloides screening done before steroids?
Corticosteroids suppress the immune response that controls Strongyloides stercoralis and can cause life-threatening hyperinfection. Screening by serology and stool examination before starting steroids in endemic areas prevents this complication.

### Which drug treats tropical pulmonary eosinophilia?
Diethylcarbamazine is the drug used to treat tropical pulmonary eosinophilia, and symptoms and eosinophil counts usually improve after treatment. Corticosteroids are added in some patients with severe lung involvement.

### What is Loeffler syndrome?
Loeffler syndrome is transient pulmonary infiltration with eosinophilia, classically caused by Ascaris larvae migrating through the lungs. Symptoms are usually mild and resolve within weeks as the larval migration ends.