Flagellate Infection in Bees: Hindgut and Rectal Protozoa in Honey Bees

Flagellate infection in honey bees is a gut infection caused by single-celled protozoa with whip-like structures called flagella. In adult honey bees, these organisms are associated with the hindgut and rectum, where they may be found free in the gut contents or attached to the gut lining. Older beekeeping references grouped these organisms under Crithidia or Leptomonas, while newer research shows that Lotmaria passim is now considered the predominant trypanosomatid found in honey bees worldwide, with Crithidia mellificae still present but less common.

This condition can be tricky because it does not always cause dramatic, easy-to-spot signs in a hive. Some infected bees or colonies may appear normal, while others show reduced resilience when the infection overlaps with poor nutrition, heavy mite pressure, viral disease, pesticide stress, or winter stress. That means flagellates are often part of a bigger colony health picture, not always the only problem.

For pet parents and small-scale beekeepers, the key point is that flagellate infections are usually managed at the colony level, not by treating one bee at a time. A confirmed diagnosis helps your vet, extension specialist, or apiary inspector decide whether supportive management, broader disease screening, or changes in husbandry are the most sensible next steps.

These infections are caused by trypanosomatid protozoa, most notably Lotmaria passim and sometimes Crithidia mellificae. Research from the USDA and collaborators clarified that many trypanosomatid infections once assumed to be Crithidia mellificae are actually Lotmaria passim, which appears to be widely distributed in honey bee populations.

The organisms spread through colony-level contact and contamination of shared food or fecal material. Adult bees can pick up infectious organisms while cleaning comb, sharing food, contacting contaminated surfaces, or moving between bees within crowded colony conditions. As with many bee diseases, transmission is easier when bees are stressed, densely packed, or exposed to poor sanitation and nutritional strain.

Flagellates also tend to matter more when they occur alongside other stressors. Heavy Varroa loads, viral infections, poor forage diversity, transport stress, pesticide exposure, and harsh overwintering conditions can all reduce a colony's ability to tolerate gut pathogens. So while the protozoa are the infectious cause, the severity of disease often depends on the colony's overall health and management conditions.

Diagnosis starts with a colony history and a careful review of other likely causes of decline. Your vet or bee health professional will usually ask about recent losses, Varroa monitoring results, feeding practices, queen status, season, and whether neighboring colonies are affected. Because flagellate infection rarely causes unique visible signs, diagnosis should not rely on appearance alone.

Classically, diagnosis is made by dissecting adult bees and examining the hindgut and rectum under a microscope. USDA honey bee disease guidance describes separating the hindgut and rectum from the digestive tract, macerating that tissue on a slide, and examining the suspension under high power for flagellates. This can show whether trypanosomatid-like organisms are present, but microscopy may not reliably distinguish species.

For more specific answers, samples may be sent to a diagnostic laboratory for PCR or other molecular testing. That can help differentiate Lotmaria passim from Crithidia mellificae and may be useful when colony losses are significant, when multiple pathogens are suspected, or when a beekeeper wants a clearer herd-health plan for the apiary. In practice, many cases are managed based on a combination of colony signs, microscopy, and broader disease screening rather than one test result alone.

Prevention focuses on colony resilience more than on a single medication. Strong nutrition, good forage access, regular colony inspections, and prompt control of major stressors help bees tolerate gut pathogens better. Since digestive infections often become more important when colonies are already under pressure, routine Varroa monitoring and seasonal management are central parts of prevention.

Good sanitation and practical biosecurity also matter. Avoid moving equipment from weak or collapsing colonies into healthy ones without a plan. Clean tools between problem hives when possible, reduce robbing pressure, and be thoughtful about combining colonies if disease status is unclear. If you buy bees, queens, or used equipment, source them carefully and ask about health history.

Monitoring is one of the most useful preventive tools. If your apiary has repeated winter losses, chronic weak colonies, or unexplained poor buildup, ask your vet, extension bee program, or apiary inspector whether targeted sampling makes sense. Early detection will not prevent every problem, but it can help you catch overlapping issues before they affect more of the apiary.