What is hypereosinophilic syndrome (HES)?

HES is a condition in which eosinophils in the blood persistently increase and may infiltrate organs, causing a variety of symptoms. Clinical presentation varies; blood and imaging tests are needed to assist evaluation. Please consult your vet for specific diagnosis and treatment.

What tests does the vet usually perform to find the cause?

Common tests include a complete blood count and biochemistry panel, abdominal ultrasound, X-rays, and tissue biopsy or fine-needle aspirate when necessary. Results are reviewed together to identify the likely cause and guide treatment.

How should I monitor and care for my pet at home?

It is recommended to record alertness, appetite, vomiting frequency, and diarrhea each day, and to weigh your pet and check body temperature regularly. If symptoms worsen or acute changes occur, seek veterinary care or contact your vet immediately.

How often is follow-up needed after treatment?

Follow-up frequency depends on the course of the disease. Early on, blood tests and symptom checks are typically needed within a few weeks; intervals are extended as treatment progresses. Follow your vet's instructions for scheduling check-ups.

How much do the tests and treatment usually cost?

Costs vary by clinic and region — ask your vet for an estimate before proceeding.

Disease Guide

Hypereosinophilic Syndrome in Dogs & Cats: Symptoms, Diagnosis & Care [2025]

Hypereosinophilic syndrome (HES) is characterized by persistently elevated eosinophil counts that can infiltrate multiple organs. Ruling out parasitic infections and other underlying causes is essential; treatment typically involves corticosteroids and immunosuppressive therapy.

Take 1 minute to check the symptoms first

Last updated: 2026/01/05

PetHealthMap Editorial Team

Sources: ACVIM, WSAVA, National Taiwan University Veterinary Hospital

When to See a Vet

Seek emergency care immediately

•Persistent lethargy with difficulty rousing or unresponsiveness
•Continuous vomiting or inability to eat for more than two days
•Obvious rapid breathing or significant respiratory distress

Schedule a clinic visit soon (within 24–72 hours)

•Noticeably reduced appetite lasting one to two days
•Repeated vomiting or diarrhea with otherwise normal alertness
•Rapid weight loss or obvious weight loss over a short period

01What symptoms indicate something is wrong?

Hypereosinophilic syndrome presents with varied clinical signs in dogs and cats, most commonly non-specific systemic symptoms. A veterinarian will need to perform tests to identify the underlying cause.

Persistent lethargy or noticeably reduced activity level
Decreased appetite or repeated refusal to eat for more than two days
Repeated vomiting or inability to keep food down
Frequent diarrhea or changes in stool consistency and frequency
Noticeable weight loss or wasting over a short period
Persistent or intermittent fever or abnormal body temperature

02How can you assess the situation at home? Key things to observe and when to seek emergency care

Continue monitoring and recording multiple changes at home — this information helps your vet understand the progression of the condition more quickly.

Record daily changes in alertness, activity level, and sleep patterns
Estimate and log daily food intake, episodes of food refusal, and how many days they persist
Note the number of vomiting episodes, vomit color, and whether blood is present
Record frequency of diarrhea, stool consistency, and presence of mucus or blood
Weigh your pet weekly and track body weight trends
Measure body temperature with timestamps; note any abnormal readings

03How will the vet diagnose this condition? Common diagnostic tests

At the clinic, the vet will conduct a physical examination followed by imaging and histopathology to determine whether the findings are consistent with eosinophil-related disease or another underlying cause.

Complete physical examination

including assessment of the abdomen, skin, and lymph nodes

Blood tests

complete blood count with eosinophil count and biochemistry panel

Abdominal ultrasound

to evaluate organs, intestines, or abnormal masses

Chest and abdominal X-rays

to assess organ positioning or large segmental changes

Tissue biopsy (incisional or fine-needle aspirate) to confirm cellular infiltration and pathology

04How is this condition treated and managed? Medications, dietary adjustments, and care guide

Treatment depends on the underlying cause and severity, and often requires medication alongside long-term monitoring of blood work and imaging to assess treatment response.

Corticosteroids are a common first-line option for initial control; the regimen is adjusted based on disease response
If needed, the vet may consider other immunomodulatory or supportive therapies
Dietary and nutritional support to maintain body weight and overall condition
Regular follow-up visits and repeat blood and imaging tests as directed by your vet
Home care focuses on rest, maintaining a comfortable ambient temperature, and monitoring fluid intake

05What should you bring to the vet? Key steps to help your vet identify the cause quickly

Record the onset date and a timeline of how symptoms have progressed

Bring a log or video from the past week documenting diet, vomiting episodes, and bowel movements

List all current prescription medications, supplements, and past treatment history

Bring previous examination reports, blood test results, and imaging if available

If possible, keep vomit or stool samples chilled and bring them to the clinic

Use a stable carrier and contact the clinic in advance to arrange the appointment

This content is for reference only and cannot replace a professional veterinary diagnosis. If your pet shows signs of illness, please seek veterinary care promptly.