fungal disease Honey bee

Nosemosis

Find a veterinarian All honey bee diseases

Definition Signs Causes Diagnosis Treatment Prevention Complications When to see a vet Prognosis FAQ Sources Affected species

What is it?

Nosemosis is a parasitic disease of adult honeybees caused by a microscopic fungus (microsporidium) that infects the cells of the intestine. Two species are involved: Nosema apis and Nosema ceranae, the latter now being the most widespread in the world, including in Tunisia. The disease progressively weakens colonies without always causing obvious visible signs, which makes it difficult to detect without laboratory analysis.

Commonly observed signs

These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.

Related signs — each one links to its page:

Diarrhea Weakened colony Reduction Weight loss

Signs are often subtle and nonspecific. Diarrhea may be observed with brownish staining on frames and at the hive entrance, particularly with Nosema apis. With Nosema ceranae, infected bees often die away from the hive with no marked visible symptoms, making the disease even more insidious. The colony gradually weakens: fewer bees, less honey produced, and slowed brood development. Affected bees appear lethargic, have a shortened lifespan, and struggle to perform their normal tasks. At an advanced stage, the population may decline sharply, particularly after winter.

When to consult without delay

This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.

Causes and risk factors

Nosemosis results from the multiplication of microsporidium spores (Nosema apis or Nosema ceranae) in the cells lining the midgut of the adult honeybee. These spores damage the digestive wall and disrupt nutrient absorption.

Stress from cold, humidity, or food shortage promotes the onset of disease. An overcrowded or poorly ventilated hive increases the risk of transmission between bees. Prolonged confinement during winter, which prevents bees from taking cleansing flights, worsens diarrhea and spore spread inside the hive. A colony already weakened by other diseases or by varroa mites is more susceptible to nosemosis. Frequent replacement of contaminated frames and equipment promotes persistence of infection in the apiary.

Diagnosis and care

Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.

Definitive diagnosis requires laboratory examination: the intestines of collected honeybees are ground and then observed under a microscope to detect and count spores. A veterinarian or specialist beekeeper technician can recommend this type of analysis when nosemosis is suspected.

Treatment

There is no specific validated treatment to completely eliminate this parasite. Management focuses mainly on supporting the colony: improving nutrition, enhancing winter conditions and ventilation, and replacing contaminated equipment. Some beekeepers use products based on natural substances, but their effectiveness varies and their use should be discussed with a beekeeping professional.

Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.

Prevention

Maintaining strong, well-fed colonies limits disease risk. Good ventilation and protection against excessive humidity in the hive are recommended, especially in winter. Regular renewal of comb and disinfection of beekeeping equipment help reduce spore load. Avoiding colony stress, such as frequent transport or excessive handling, helps limit disease occurrence. Regular monitoring of apiary health by a professional allows early detection.

Possible complications

Uncontrolled nosemosis can lead to lasting weakness of the colony, significant reduction in honey production, and in severe cases, death of the colony, especially if combined with other diseases or parasites such as varroa mites.

When to see a veterinarian

Diarrhea or unusual brown spots on frames or at the hive entrance

Progressive and unexplained weakening of one or more colonies

Abnormal mortality of adult bees, particularly after winter

Notable decrease in honey production with no obvious cause

Prognosis

The outlook depends on how quickly management begins and the initial strength of the colony. Many infected colonies survive with mild weakness, especially if beekeeping conditions improve. However, severe infection combined with significant stress can lead to colony collapse.

Frequently asked questions

Is nosemosis dangerous for humans?

No, nosemosis is a disease specific to bees. It poses no risk to human health, including through honey consumption.

How can I tell if my bees have nosemosis?

Visible signs are not sufficient because the disease is often subtle. Only laboratory analysis, using bees sampled from the hive, can confirm the presence of the parasite.

Does nosema spread easily between hives?

Yes, spores spread through contaminated feces affecting frames, water, or shared equipment. Sharing equipment between apiaries should therefore be avoided or done with caution.

Can a colony with nosema be saved?

Many infected colonies recover, especially if the beekeeper quickly improves their nutrition and wintering conditions. Close monitoring remains advisable.