Home » Diseases » In cats » Hypereosinophilic Syndrome (Overproduction of Special White Blood Cells of the Immune System in the Bone Marrow)

Hypereosinophilic Syndrome (Overproduction of Special White Blood Cells of the Immune System in the Bone Marrow) in cats

Occurrence: common
Location of illness: General/Whole body

Author: Dr. med. vet. Gisa Löwe | Last modified: 17.02.2026

When to visit the vet?

Non-urgent see a veterinarian within 2–3 days

If the condition worsens / symptoms persist, consult a veterinarian.

Definition

Hypereosinophilic syndromeA syndrome is a group of symptoms that occur together and indicate a specific disease or condition. A syndrome can be caused by various factors and is often complex in its diagnosis and treatment. (HES) in cats is a rare disease characterized by excessive production of eosinophilsEosinophils are a type of white blood cell primarily involved in fighting parasitic infections and allergic reactions. They are characterized by their ability to absorb eosin, a red dye., a type of white blood cell, in the bone marrow. These cells play an important role in fighting parasites and in allergic reactions.

The most important facts at a glance

Hypereosinophilic syndrome (HES) in cats is a disease in which there is excessive production of eosinophilic white blood cells in the bone marrow. These cells are normally important for fighting parasites and allergic reactions, but can cause health problems when overproduced. The exact causes of HES are not yet fully understood, but it is suspected that genetic and environmental factors as well as disorders in the bone marrow play a role. Possible triggers include chronicChronic refers to a condition or disease that is long-lasting or permanent. Chronic diseases often develop slowly and can lead to persistent or recurring health problems over time. infections, parasiticParasitic refers to diseases or conditions caused by parasites. Parasitic infections can affect the digestive tract, skin, and other organ systems. diseases, or autoimmune disorders. Symptoms include general weakness, loss of appetite, fever, vomiting, diarrheaDiarrhea refers to the condition of frequent, watery bowel movements, which can be a sign of infections, digestive disorders, or other health problems., and weight loss. DiagnosisA diagnosis is the identification of a disease or condition by its symptoms and/or results from diagnostic tests. It is the first step in the management and treatment of patients. requires a comprehensive examination by a veterinarian, including blood tests and possibly a bone marrow examination. Treatment focuses on reducing the number of eosinophilsEosinophils are a type of white blood cell primarily involved in fighting parasitic infections and allergic reactions. They are characterized by their ability to absorb eosin, a red dye. and relieving symptoms, often through the use of corticosteroidsCorticosteroids are a class of steroid hormones that occur naturally in the body and can also be produced synthetically. They have anti-inflammatory and immunosuppressive properties and are used in veterinary medicine to treat a variety of conditions. such as prednisone. In severe cases, other immunosuppressants may be necessary. The prognosisThe prognosis is the prediction of the likely course and outcome of a disease based on the general state of health, the nature of the disease, and the response to treatment. It can provide information on how a disease is expected to develop. depends on the severity of the disease and the response to treatment. Regular veterinary check-ups are important to monitor the cat’s health status. PreventionPrevention includes measures taken to prevent diseases or injuries. In veterinary medicine, this includes vaccinations, parasite control, and health monitoring. measures are limited since the exact causes of HES are unclear, but regular veterinary examinations, a balanced diet, and avoiding stressStress refers to the state of an organism reacting to external stimuli or demands. In veterinary medicine, stress can have physical or psychological effects on animals, impacting their health and well-being. can minimize the risk.

Causes

EosinophilsEosinophils are a type of white blood cell primarily involved in fighting parasitic infections and allergic reactions. They are characterized by their ability to absorb eosin, a red dye. are a subset of white blood cells that play a central role in the immune systemThe immune system is the body's defense system against infectious agents such as bacteria, viruses, fungi, and parasites. It comprises a complex network of organs, cells, and molecules that work together to protect the body.. They are primarily responsible for fighting parasiticParasitic refers to diseases or conditions caused by parasites. Parasitic infections can affect the digestive tract, skin, and other organ systems. infections and can triggerA trigger is a stimulus that elicits a specific reaction or state. In veterinary medicine, this can refer to emotional responses, such as fear or aggression, or medical conditions, such as seizures in dogs and cats. allergies and inflammatory reactions in the body. Normally, they are present in small numbers in the blood, but in HES their count increases significantly.

The exact mechanism leading to HES is not fully understood. It is believed that the overproduction of eosinophilsEosinophils are a type of white blood cell primarily involved in fighting parasitic infections and allergic reactions. They are characterized by their ability to absorb eosin, a red dye. is due to disorders in the bone marrow, where these cells are formed. Some studies suggest that genetic factors or environmental factors may play a role, but this has not yet been conclusively determined.

Another possible cause is an underlying disease or irritant that prompts the cat’s immune systemThe immune system is the body's defense system against infectious agents such as bacteria, viruses, fungi, and parasites. It comprises a complex network of organs, cells, and molecules that work together to protect the body. to produce excessive numbers of eosinophilsEosinophils are a type of white blood cell primarily involved in fighting parasitic infections and allergic reactions. They are characterized by their ability to absorb eosin, a red dye.. These could include chronicChronic refers to a condition or disease that is long-lasting or permanent. Chronic diseases often develop slowly and can lead to persistent or recurring health problems over time. infections, parasiticParasitic refers to diseases or conditions caused by parasites. Parasitic infections can affect the digestive tract, skin, and other organ systems. diseases, or autoimmune disorders. In some cases, however, the cause remains idiopathicIdiopathic refers to a condition or disease for which the cause is unknown. It is used when no clear external cause or underlying disease can be identified as an explanation for the symptoms., meaning unknown.

Symptoms

Very common symptoms:

Common symptoms:

Rare symptoms:

Symptoms of HES in cats can vary and are often nonspecific, making diagnosisA diagnosis is the identification of a disease or condition by its symptoms and/or results from diagnostic tests. It is the first step in the management and treatment of patients. difficult. The most common signs include weight loss, loss of appetite, and lethargy. However, these symptoms are not specific to HES and can also occur with many other diseases.

Additional symptoms may include vomiting, diarrheaDiarrhea refers to the condition of frequent, watery bowel movements, which can be a sign of infections, digestive disorders, or other health problems., and respiratoryRespiratory refers to breathing or the process of breathing. Respiratory diseases affect the respiratory tract and can impair the ability to breathe effectively. problems, as the excessive number of eosinophilsEosinophils are a type of white blood cell primarily involved in fighting parasitic infections and allergic reactions. They are characterized by their ability to absorb eosin, a red dye. can cause inflammatory conditions in various organs. Skin changes such as itching and rashes may also occur, as eosinophilsEosinophils are a type of white blood cell primarily involved in fighting parasitic infections and allergic reactions. They are characterized by their ability to absorb eosin, a red dye. can infiltrate the skin and triggerA trigger is a stimulus that elicits a specific reaction or state. In veterinary medicine, this can refer to emotional responses, such as fear or aggression, or medical conditions, such as seizures in dogs and cats. reactions there.

In severe cases, enlargement of organs such as the spleen and liver may occur, indicating systemic involvement of the disease. It is important to see a veterinarian if these symptoms appear in order to make an accurate diagnosisA diagnosis is the identification of a disease or condition by its symptoms and/or results from diagnostic tests. It is the first step in the management and treatment of patients..

Diagnosis

Diagnosis of HES in cats requires a comprehensive examination by a veterinarian. First, a thorough medical historyThe medical history is the systematic collection of a patient's medical history through questioning. It includes information about previous illnesses, treatments, allergies, and lifestyle habits. is taken to identify possible triggers or underlying diseases. A physical examination can provide clues to enlarged organs or other abnormalities.

Laboratory tests play a crucial role in diagnosisA diagnosis is the identification of a disease or condition by its symptoms and/or results from diagnostic tests. It is the first step in the management and treatment of patients.. A complete blood count typically shows an elevated number of eosinophilsEosinophils are a type of white blood cell primarily involved in fighting parasitic infections and allergic reactions. They are characterized by their ability to absorb eosin, a red dye., which is an important indicator of HES. Additional blood tests may be performed to rule out other causes of eosinophiliaEosinophilia is a condition in which there is an increased number of eosinophils in the blood or tissues. This may indicate various conditions, including allergies, asthma, parasitic infections, and some forms of cancer., such as parasiticParasitic refers to diseases or conditions caused by parasites. Parasitic infections can affect the digestive tract, skin, and other organ systems. infections.

In some cases, a bone marrow examination may be necessary to determine the cause of eosinophil overproduction. Imaging procedures such as X-rayX-ray is an imaging technique that uses X-rays to create images of the inside of the body. It is often used to diagnose bone fractures, lung diseases, and other conditions. or ultrasoundUltrasound refers to sound waves that are above the hearing threshold of the human ear. In medicine, ultrasound is used for diagnostic imaging procedures to visualize internal organs, tissues, and blood flow. can be used to detect organ changes and assess the severity of the disease.

Therapy

Treatment of hypereosinophilic syndromeA syndrome is a group of symptoms that occur together and indicate a specific disease or condition. A syndrome can be caused by various factors and is often complex in its diagnosis and treatment. in cats aims to reduce the eosinophil countThe eosinophil count refers to the number of eosinophils in the blood. An increase may indicate allergic reactions, parasitic infections, or certain autoimmune diseases. and relieve symptoms. CorticosteroidsCorticosteroids are a class of steroid hormones that occur naturally in the body and can also be produced synthetically. They have anti-inflammatory and immunosuppressive properties and are used in veterinary medicine to treat a variety of conditions. such as prednisone are often the first choice, as they have anti-inflammatory effects and inhibit the production of eosinophilsEosinophils are a type of white blood cell primarily involved in fighting parasitic infections and allergic reactions. They are characterized by their ability to absorb eosin, a red dye. in the bone marrow.

In severe cases or when corticosteroidsCorticosteroids are a class of steroid hormones that occur naturally in the body and can also be produced synthetically. They have anti-inflammatory and immunosuppressive properties and are used in veterinary medicine to treat a variety of conditions. are not effective, other immunosuppressants may be considered. These medications help suppress the activity of the immune systemThe immune system is the body's defense system against infectious agents such as bacteria, viruses, fungi, and parasites. It comprises a complex network of organs, cells, and molecules that work together to protect the body. and reduce the number of eosinophilsEosinophils are a type of white blood cell primarily involved in fighting parasitic infections and allergic reactions. They are characterized by their ability to absorb eosin, a red dye..

In addition to medication, it is important to identify and treat the underlying cause of the disease, if possible. For parasiticParasitic refers to diseases or conditions caused by parasites. Parasitic infections can affect the digestive tract, skin, and other organ systems. infections, appropriate deworming measures should be taken. Dietary changes or hypoallergenic diets may help if food allergies play a role.

Prognosis and follow-up care

The prognosisThe prognosis is the prediction of the likely course and outcome of a disease based on the general state of health, the nature of the disease, and the response to treatment. It can provide information on how a disease is expected to develop. for cats with HES depends on various factors, including the severity of the disease, the response to treatment, and the identification of an underlying cause. With early diagnosisA diagnosis is the identification of a disease or condition by its symptoms and/or results from diagnostic tests. It is the first step in the management and treatment of patients. and appropriate treatment, the prognosisThe prognosis is the prediction of the likely course and outcome of a disease based on the general state of health, the nature of the disease, and the response to treatment. It can provide information on how a disease is expected to develop. can be favorable, especially if a specific cause can be identified and treated.

In cases where the cause cannot be identified or the cat does not respond well to treatment, the prognosisThe prognosis is the prediction of the likely course and outcome of a disease based on the general state of health, the nature of the disease, and the response to treatment. It can provide information on how a disease is expected to develop. may be less favorable. Long-term medication may be necessary to control symptoms and prevent relapses.

Regular veterinary check-ups are crucial to monitor the cat’s health and adjust treatment as needed.

Prevention

Since the exact causes of hypereosinophilic syndromeA syndrome is a group of symptoms that occur together and indicate a specific disease or condition. A syndrome can be caused by various factors and is often complex in its diagnosis and treatment. in cats are not fully understood, there are no specific prevention measures to reliably prevent the development of this disease. Nevertheless, some general strategies can help minimize the risk and promote your cat’s health.

An important aspect of preventionPrevention includes measures taken to prevent diseases or injuries. In veterinary medicine, this includes vaccinations, parasite control, and health monitoring. is regular veterinary examination. This makes it possible to detect potential problems early and take appropriate measures. Make sure to have your cat vaccinated regularly and maintain prescribed deworming and flea treatments to avoid parasiticParasitic refers to diseases or conditions caused by parasites. Parasitic infections can affect the digestive tract, skin, and other organ systems. infections, which are considered possible triggers for the syndromeA syndrome is a group of symptoms that occur together and indicate a specific disease or condition. A syndrome can be caused by various factors and is often complex in its diagnosis and treatment..

A balanced diet is also crucial for your cat’s health. Ensure that your cat receives high-quality food containing all necessary nutrients. Avoid stressful situations by providing your cat with a safe and enriching environment. This includes sufficient hiding places, toys, and daily interaction.

Additionally, it is advisable to watch for changes in your cat’s behavior or health and to consult a veterinarian quickly if health problems are suspected. Early intervention can help prevent serious complications.

Outlook on current research

Hypereosinophilic syndrome (HES) in cats is a disease characterized by excessive production of eosinophilic white blood cells in the bone marrow. These cells normally play a role in fighting parasites and allergic reactions, but their overproduction can lead to a variety of health problems. The exact cause of HES in cats is not yet fully understood, making it an active area of research.

Current research focuses on deciphering the genetic and immunological mechanisms that lead to this overproduction. Scientists are investigating possible genetic predispositions and searching for specific genetic markers that might make cats susceptible to HES. Additionally, research is examining whether environmental factors or certain infections could play a role in triggering the disease.

Another area of research is the improvement of diagnostic methods. Currently, there is no specific test for HES, and diagnosisA diagnosis is the identification of a disease or condition by its symptoms and/or results from diagnostic tests. It is the first step in the management and treatment of patients. is often based on excluding other diseases. Researchers are working to develop more accurate diagnostic tools that enable earlier and more precise detection of the disease.

On the therapeutic side, intensive research is being conducted on new treatment approaches. While corticosteroidsCorticosteroids are a class of steroid hormones that occur naturally in the body and can also be produced synthetically. They have anti-inflammatory and immunosuppressive properties and are used in veterinary medicine to treat a variety of conditions. are currently the mainstay of treatment, scientists are investigating alternative medications that may have fewer side effects. These include immunomodulatoryImmunomodulatory describes the ability of a substance to modify the reaction of the immune system. Immunomodulatory drugs can either stimulate the immune system to fight infections more effectively or suppress it to prevent overreactions. drugs and targeted therapies that specifically target eosinophilic cells.

An important aspect of research is also understanding the long-term effects of HES on the cat’s health. Long-term studies are underway to better understand the course of the disease and to determine how different treatment strategies affect the quality of life and life expectancy of affected cats.

In summary, research on hypereosinophilic syndromeA syndrome is a group of symptoms that occur together and indicate a specific disease or condition. A syndrome can be caused by various factors and is often complex in its diagnosis and treatment. in cats currently focuses on discovering the causes, developing improved diagnostic methods, testing new treatments, and understanding the long-term effects of the disease. This research is crucial to achieving better outcomes for cats suffering from this complex and often difficult-to-treat disease.

Frequently asked questions (FAQs)

What is Hypereosinophilic SyndromeA syndrome is a group of symptoms that occur together and indicate a specific disease or condition. A syndrome can be caused by various factors and is often complex in its diagnosis and treatment. in cats? Hypereosinophilic syndrome in cats is a disease in which there is an overproduction of eosinophilic white blood cells. This overproduction can lead to inflammatory conditions and damage to various organs.
What symptoms does a cat with HES show? The most common symptoms include rashes, itching, respiratory distress, weight loss, and loss of appetite. In some cases, signs of organ failure may also occur, depending on which organs are affected.
How is HES diagnosed in cats? DiagnosisA diagnosis is the identification of a disease or condition by its symptoms and/or results from diagnostic tests. It is the first step in the management and treatment of patients. of HES is often made through a combination of blood tests showing a high number of eosinophilsEosinophils are a type of white blood cell primarily involved in fighting parasitic infections and allergic reactions. They are characterized by their ability to absorb eosin, a red dye. and ruling out other causes of the symptoms. Imaging procedures and tissue samples may also contribute to diagnosis.
What are the causes of HES in cats? The exact causes are not yet fully understood, but it is suspected that genetic factors, infections, or allergic reactions could play a role. Research is still investigating these aspects.
How is HES treated in cats? Treatment often includes the administration of corticosteroidsCorticosteroids are a class of steroid hormones that occur naturally in the body and can also be produced synthetically. They have anti-inflammatory and immunosuppressive properties and are used in veterinary medicine to treat a variety of conditions. to reduce the number of eosinophilsEosinophils are a type of white blood cell primarily involved in fighting parasitic infections and allergic reactions. They are characterized by their ability to absorb eosin, a red dye.. For some cats, other medications or treatments may be necessary, depending on the specific symptoms and severity of the disease.
Can HES be cured in cats? HES is currently not curable, but symptoms can often be controlled with proper treatment. The disease typically requires lifelong monitoring and adjustment of therapyTherapy refers to the treatment of diseases or disorders with the aim of alleviating symptoms, promoting healing, or improving quality of life. Therapies can be medicinal, surgical, or through other medical interventions..
Is HES contagiousContagious means infectious. Diseases that can easily be transmitted from one animal to another or from animals to humans are referred to as contagious. in cats? No, HES is not contagiousContagious means infectious. Diseases that can easily be transmitted from one animal to another or from animals to humans are referred to as contagious.. It is a disease related to the individual response of the cat’s immune systemThe immune system is the body's defense system against infectious agents such as bacteria, viruses, fungi, and parasites. It comprises a complex network of organs, cells, and molecules that work together to protect the body. and is not transmitted from animal to animal.
Which cats are most at risk of developing HES? There is no specific cat breed known to have a higher risk for HES. The disease can affect cats of any age and breed, although certain genetic predispositions cannot be ruled out.
Can a cat’s diet influence HES? While specific dietary changes are not considered a primaryPrimary refers to the first or original cause of a disease or condition, as opposed to secondary causes or symptoms that develop from it. treatment for HES, a balanced diet can support the cat’s overall health. In some cases, special diets may be recommended to minimize allergic reactions that could worsen the disease.
What can I do to help my cat with HES? The best support for a cat with HES is to follow the veterinarian’s treatment plans closely and conduct regular check-ups. Monitor your cat carefully and report any changes in condition to the veterinarian immediately so that therapyTherapy refers to the treatment of diseases or disorders with the aim of alleviating symptoms, promoting healing, or improving quality of life. Therapies can be medicinal, surgical, or through other medical interventions. can be adjusted if necessary.

Frequently asked questions (FAQs)

1. What is Hypereosinophilic Syndrome in cats?

Hypereosinophilic syndrome in cats is a disease in which there is an overproduction of eosinophilic white blood cells. This overproduction can lead to inflammatory conditions and damage to various organs.

2. What symptoms does a cat with HES show?

The most common symptoms include rashes, itching, respiratory distress, weight loss, and loss of appetite. In some cases, signs of organ failure may also occur, depending on which organs are affected.

3. How is HES diagnosed in cats?

Diagnosis of HES is often made through a combination of blood tests showing a high number of eosinophils and ruling out other causes of the symptoms. Imaging procedures and tissue samples may also contribute to diagnosis.

4. What are the causes of HES in cats?

The exact causes are not yet fully understood, but it is suspected that genetic factors, infections, or allergic reactions could play a role. Research is still investigating these aspects.

5. How is HES treated in cats?

Treatment often includes the administration of corticosteroids to reduce the number of eosinophils. For some cats, other medications or treatments may be necessary, depending on the specific symptoms and severity of the disease.

6. Can HES be cured in cats?

HES is currently not curable, but symptoms can often be controlled with proper treatment. The disease typically requires lifelong monitoring and adjustment of therapy.

7. Is HES contagious in cats?

No, HES is not contagious. It is a disease related to the individual response of the cat's immune system and is not transmitted from animal to animal.

8. Which cats are most at risk of developing HES?

There is no specific cat breed known to have a higher risk for HES. The disease can affect cats of any age and breed, although certain genetic predispositions cannot be ruled out.

9. Can a cat's diet influence HES?

While specific dietary changes are not considered a primary treatment for HES, a balanced diet can support the cat's overall health. In some cases, special diets may be recommended to minimize allergic reactions that could worsen the disease.

10. What can I do to help my cat with HES?

The best support for a cat with HES is to follow the veterinarian's treatment plans closely and conduct regular check-ups. Monitor your cat carefully and report any changes in condition to the veterinarian immediately so that therapy can be adjusted if necessary.