fungal disease Bumblebee

Nosema Disease in Bumblebees

Nosema disease in bumblebees is caused by a single-celled microorganism called Nosema bombi. This parasite belongs to a group of microscopic fungi capable of infecting the intestinal cells of bumblebees. It multiplies within the digestive t...

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

What is it?

This disease affects both wild bumblebees and colonies raised for pollination, particularly in greenhouse settings. It is considered one of the possible causes of decline in certain wild bumblebee populations worldwide.

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:

Weakened colony Weight loss Fatigue Reduction Deformed wings

Infected bumblebees often show a general decline in energy and vitality.

The entire colony may appear weakened, with reduced foraging activity and fewer active individuals.

Some infected bumblebees have wings that appear damaged or malformed, making flight more difficult.

Infected queens may struggle to establish a new colony in spring, or produce fewer offspring.

In many cases, infection remains subtle and produces no visible signs to the naked eye, making detection difficult without laboratory analysis.

When to consult without delay

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

Causes and risk factors

The disease is caused by Nosema bombi, an intracellular parasite that develops in the intestinal epithelial cells of the bumblebee.

The parasite produces highly resistant spores in the external environment. These spores contaminate food, nest surfaces or pollen, and are then ingested by other bumblebees.

Colonies weakened by stress, lack of flowering resources, or unfavourable weather conditions appear more susceptible to infection.

Crowding in commercially reared bumblebee colonies in large numbers promotes parasite transmission. Shared foraging sites between wild bumblebees and commercial colonies also contribute to parasite spread.

Transmission

Transmission occurs mainly by the oral route, when the bumblebee ingests spores present on flowers, contaminated pollen or inside the nest.

An infected queen can also transmit the parasite to her offspring when founding the colony. The trade and transport of bumblebee colonies intended for pollination can also promote spread between regions.

Diagnosis and care

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

Diagnosis is based on microscopic observation of characteristic spores in the intestinal contents or in homogenised abdomens of sampled bumblebees.

More advanced laboratory techniques, such as genetic analysis, allow confirmation of the exact parasite species if doubt exists.

Treatment

There is no validated, widely used specific treatment against bumblebee nosemosis at the individual level.

Management relies mainly on hygiene measures and colony management practices rather than targeted medication, unlike what may be available for honeybees.

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

Prevention

It is recommended to use only certified healthy bumblebee colonies for greenhouse pollination.

Regular disinfection of beekeeping equipment and hives used for bumblebees limits the accumulation of spores.

Avoiding the movement of colonies between different geographical regions reduces the risk of introducing new parasite strains.

Preserving natural habitats rich in a variety of flowers helps maintain more resistant wild colonies.

Possible complications

Infection can cause progressive weakening of the colony, a decrease in new queen production, and reduced pollination efficiency.

In severe cases, the entire colony may decline and disappear, especially if already stressed by other factors.

When to see a veterinarian

Commercial bumblebee colony showing unusual decline in activity

Bumblebees observed with visibly damaged wings or difficult flight

Abnormal mortality within a colony reared for pollination

Queen struggling to establish a new colony in spring

Prognosis

The outlook varies depending on the intensity of infection and the overall health of the colony.

Many infected colonies survive without showing obvious signs, while others that are more weakened may gradually decline. Early detection and good hygiene management in commercial operations improve the chances of maintaining a productive colony.

Frequently asked questions

Can nosema from bumblebees contaminate honeybees?

Nosema bombi is specific to bumblebees and is not considered a direct threat to honeybees, even though other Nosema species occur in bees.

Can an infected bumblebee continue to pollinate?

Yes, in many cases the infection remains inconspicuous and the bumblebee continues its activity, but its efficiency may be reduced if the disease progresses.

How to tell if a bumblebee colony you've purchased is healthy? You should check several indicators: **The queen:** She should be visibly present and active. Look for her moving around the nest regularly. An inactive or absent queen is a bad sign. **Worker activity:** The colony should show constant movement and buzzing. Workers should be attending to the brood (larvae and pupae) and building comb. **Brood pattern:** Examine the comb for a good brood pattern—cells should be well-populated with eggs, larvae, and capped pupae. Scattered or sparse brood suggests problems. **Food stores:** There should be visible pollen and nectar stored in cells. This indicates the colony can sustain itself. **Absence of disease:** Look for no signs of mold, fungus, or unusual odors. Dead bees shouldn't be accumulating inside. **Population size:** The colony should match what was promised at purchase. Count the visible workers—a typical healthy colony has dozens to hundreds depending on species and development stage. **Container condition:** The nest box should be clean and properly ventilated. Check that it's not waterlogged or excessively dry. **Behavior:** Bees should seem calm or only mildly defensive. Excessive aggression or lethargy can indicate stress or illness. Request a health certificate from the supplier if available, and inspect the colony within 24 hours of arrival while conditions are still fresh.

It is advisable to source from certified producers who carry out regular health checks on their colonies before sale.

Is this disease dangerous for humans?

No, nosemosis in bumblebees does not affect humans or other domestic animals; it is specific to bumblebees.

References

WOAH — Organisation mondiale de la santé animale (ex-OIE)

EFSA — Autorité européenne de sécurité des aliments

Content reviewed by the TunisieVet editorial team · Updated 25 August 2026

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AT A GLANCE

AgentFungus

SpeciesBumblebee

VaccineNo

Transmissible to humansNo

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