Honey Bee Amoebiasis (Malpighian Tubule Disease): Signs, Spread, and Colony Consequences

Honey bee amoebiasis, also called Malpighian tubule disease or amoeba disease, is a parasitic disease of adult honey bees caused by Malpighamoeba mellificae. The organism targets the Malpighian tubules, which help bees regulate waste and fluid balance. When these tubules fill with amoebae and cysts, they can become swollen, damaged, and less able to function normally.

This disease is considered worldwide in distribution wherever Apis mellifera is kept, but it has likely been underrecognized because it is easy to miss without microscopy or dissection. Worker bees are the caste most often affected. Drones are not commonly reported with infection, and queens appear to be affected much less often.

For beekeepers, the challenge is that amoebiasis rarely announces itself with one unique sign. Instead, colonies may look weak, messy, or slow to build, especially in winter or early spring. Because the outward signs overlap with Nosema and other stress-related colony problems, a visual hive check alone usually cannot confirm what is going on.

The good news is that finding amoebiasis does not always mean immediate colony collapse. Some colonies improve as the season progresses. Still, when infection is heavy or combined with other stressors, it can contribute to reduced worker performance, dwindling populations, and poorer colony momentum.

Honey bee amoebiasis is caused by infection with the protozoan Malpighamoeba mellificae. The parasite forms cysts that can survive outside the bee long enough to be picked up by other bees. After ingestion, the organism infects the Malpighian tubules and can damage the lining of these structures.

Current evidence suggests the main route of spread is fecal-oral transmission. In practical terms, that means bees are exposed when they contact or consume material contaminated with infected feces. Contaminated comb, water sources, and trophallaxis, the mouth-to-mouth exchange of food between bees, are all considered plausible ways the organism moves through a colony.

Amoebiasis often becomes more noticeable when colonies are under stress. Winter confinement, poor weather, crowding, moisture problems, weak nutrition, and concurrent disease may all make signs easier to see. Researchers and field observers have noted that amoebiasis and Nosema may occur together, which can make the colony impact worse and the diagnosis less straightforward.

Importantly, the presence of the organism does not always predict the same outcome in every colony. Some colonies carry low-level infection with limited visible effects, while others show clear dysentery, worker loss, and reduced performance. That is why colony context matters as much as the lab finding.

Diagnosis starts with suspicion, not certainty. If a colony has dysentery, unusual adult mortality, or spring dwindling, your vet or bee health advisor may recommend sampling rather than guessing. Because the signs overlap with Nosema and other adult bee problems, diagnosis usually depends on microscopy or laboratory testing.

One field-friendly approach is microscopic examination of bee feces. University of Florida guidance notes that M. mellificae cysts can be seen under a compound light microscope at about 400x magnification. These cysts are described as round, colorless, and translucent. Some programs also use a hemocytometer approach similar to Nosema screening to estimate organism load.

A more direct method is dissection of adult bees and examination of the Malpighian tubules themselves. Older USDA diagnostic guidance describes removing the digestive tract, isolating the tubules, and checking them microscopically for amoeba cysts. Infected tubules may appear abnormal or darkened. Histopathology and PCR can provide stronger confirmation when available, especially in research or diagnostic lab settings.

For beekeepers, the most useful next step is often practical: collect a fresh sample from affected colonies and ask your vet, state apiary inspector, extension service, or bee diagnostic lab what testing they currently offer. Since availability varies by state and lab, it helps to confirm sample size, shipping method, and turnaround time before you send anything.

Prevention centers on reducing fecal contamination and lowering colony stress. Because spread is thought to be fecal-oral, good apiary hygiene matters. Avoid moving dirty comb, feeders, or equipment from weak colonies into healthy ones without a clear reason and a sanitation plan. If a colony has heavy dysentery or repeated adult bee losses, treat it as a signal to slow down equipment sharing until you know more.

Strong general colony management also helps. Keep colonies dry, well ventilated, and appropriately fed for the season. Winter confinement and early spring stress appear to make amoebiasis more noticeable, so moisture control, adequate stores, and timely spring evaluation are practical prevention steps. Colonies already struggling with mites, poor queens, or nutritional gaps may be less able to tolerate additional parasite burden.

Because amoebiasis can resemble Nosema, prevention should include a broader adult-bee health strategy rather than focusing on one organism alone. Routine monitoring for Varroa, attention to forage and supplemental feeding when needed, and prompt investigation of dysentery or dwindling can all reduce the chance that a manageable problem turns into a colony-level setback.

Finally, use your local support network. Your vet, extension bee program, and state apiary inspector can help you decide when a colony needs testing, when comb replacement is worthwhile, and whether a pattern across multiple hives suggests a larger management issue. Early sampling is often more useful than waiting for a weak colony to declare itself too late.