Nosema Disease of Honey Bees

What is nosema?

Nosema disease, or nosemosis, is caused primarily by two species of microsporidians, Nosema apis Zander and Nosema ceranae Fries. Recently, researchers proposed to reclassify the two species into a new genus, Vairimorpha (Tokarev, Huang et al. 2020); however, the change has not been universally accepted (Bartolomé, Higes et al. 2024). As such, we will use Nosema here.

Nosema spp. are single-celled, spore-forming organisms that require a host to complete their lifecycle, called obligate parasites. Nosema apis was first described in honey bees in 1909 (Zander 1909) while N. ceranae was first described in 1994 in the Asian honey bee, Apis cerana (Fries, Feng et al. 1996). It was not until 2007 that N. ceranae was identified in European honey bees, Apis mellifera (Huang, Jiang et al. 2007). While both species collectively cause nosema disease, the two species have specific impacts on the bees and the colony as a whole.

Prevalence of the Species in the US

Nosema ceranae is the predominant species found. There has been debateover whether N. ceranae is displacing N. apis, as it is more commonly found. But N. ceranae has been found in older samples, dating back to 1979 in Brazil (Teixeira, Guimarães-Cestaro et al. 2018) and the1970s in the United States (Traver and Fell 2015). Each species has an infection cycle where its prevalence increases at different times of year. These cycles can vary regionally and latitudinally (Chen and Huang 2010, Bollan, Hothersall et al. 2013, Szalanski, Whitaker et al. 2013, Shutler, Head et al. 2014, Szalanski, Tripodi et al. 2014). Nosema apis tends to be found more frequently in colder regions (Naudi, Šteiselis et al. 2021) because it can withstand only a narrow range of temperatures, whereas N. ceranae thrives in a wide temperature range (Higes, García-Palencia et al. 2010). Depending on where the colonies are located andthe time of year you are testing for Nosema spp., the species and infection levelscan and will vary.

Life Cycle and Transmission

Nosema spp. have two primary stages in their life cycle: a vegetative stage (the obligate parasite) and the spore (the infectious stage). As an obligate parasite, the pathogen requires a host (the bee) to complete its life cycle. When bees are infected, the pathogen hijacks the host cells and uses them to reproduce (make spores). Transmission primarily occurs via the oral-fecal route, although some speculate that the pathogen may also use airborne transmission (Sulborska, Horecka et al. 2019). Honey bees encounter infectious spores from the feces, contaminated water, honey, royal jelly, and/or foraging sites. Healthy bees do not defecate in the hive and use cleansing flights to relieve themselves, while sick bees will defecate on the front of the hive (See image) and within the hive, contaminating the frames, honey, bee bread, and royal jelly (Sulborska, Horecka et al. 2019). Contamination can also occur at foraging sites, thus leading to transmission between colonies, and other species like bumble bees (Goblirsch 2018). In the hive, the workers clean up the infected feces, which are significantly sweeter when infected with Nosema and ingest the spores (Galajda, Valenčáková et al. 2021). Once the spores are in the midgut, they germinate by releasing their sporoplasm (contents) into the midgut cells. Here, the vegetative stages grow and steal resources from the host's cells. Nosema spp. finish their life cycle by producing spores, the infectious stage of the life cycle. Once sufficient spores are produced, the midgut cells open and the spores are released. Spores can infect neighboring cells or can be passed through the digestive tract and out of the bee through the feces to restart the infection cycle.

Symptoms and Impacts

Symptoms

The quintessential symptom that a colony is infected with nosema disease in honey bees is fecal streaking on the outside of the hive and/or on the frames and comb. The fecal streaking is caused by dysentery (aka bee diarrhea). However, nosema disease is not the sole cause of dysentery. Long periods of confinement that prevent cleansing flights, consumption of too much moisture due to honey crystallization, or feeding sugar syrup that is too dilute can also cause dysentery (Alfonsus 1935). Examining the feces under a microscope for the presence of spores will help determine whether the bees are infected with Nosema spp. or if the dysentery is caused by something else.

Careful removal of the alimentary canal of workers can be done in the field to determine nosema infection. Bees infected with Nosema apis show a swollen, white midgut (Fries 1993).

Impact on Colonies

While nosema disease is typically thought of as a stress disease, what exactly triggers the stress and how to mitigate the infection outbreak is unknown. When the infection is always present, known as a persistent infection, individual bees and colonies can be impacted when exposed to other stressors, like pesticides, other pathogens, poor nutrition, and parasites such as varroa (reviewed in Ghramh and Khan 2023). Colonies are unlikely to be nosema-free just as they are unlikely to be varroa-free. The threshold when the infection prevalence (number of bees infected) reaches a tipping point and nosema disease becomes a more serious consideration for beekeepers is also unknown. Reducing stress is what all beekeepers aim for in their management approach (e.g., make sure the bees have food and forage available, keep varroa loads as low as possible, hope for better weather, etc.). Synergistic interactions with pesticides (Alaux, Brunet et al. 2010, Pettis, vanEngelsdorp et al. 2012, Wu, Smart et al. 2012) and other pathogens and parasites can stress bees and make them more susceptible to infection.

Nosemosis Type A: N. apis

Nosema apis primarily impacts the workers, but queens and drones are also sometimes infected (Jay 1966, Bailey 1972, Czekonska, Tofilski et al. 2003, Webster, Pomper et al. 2004). After infection, the hypopharyngeal glands (the brood food glands developed in nurse bees to feed larvae) are reduced, leading to a reduction in royal jelly production (Wang and Moeller 1969). Digestive enzyme production decreases (Fries 1993), which reduces the ability of bees to digest food and absorb nutrients (Fries 1993). Bees eat more when they are infected, but they are still malnourished and cannot adequately make proteins for the fat bodies (Fries 1993). Nosemaapis-infected bees are also often found crawling in front of the hive. Infected bees age faster, reducing their life spans (Wang and Moeller 1970). Nosema apis has not been observed to cause outright colony death, but rather infections impact colony productivity as measured by reduced honey production due to reduced lifespan of foragers and decreased pollen collection (Wang and Moeller 1970, Anderson and Giacon 1992).

Nosema apis infections typically peak in the spring and then a smaller peak in the fall (Fries 1993). N. ceranae has a similar cycle (Gisder, Hedtke et al. 2010, Traver and Fell 2011, Stevanovic, Simeunovic et al. 2013), but in Spain, there is no variation across seasons (Higes, García-Palencia et al. 2010).

Nosemosis Type C: N. ceranae

Similar to N. apis, bees have a higher energetic stress (demand more food), have a decreased lifespan (Mayack and Naug 2009, Woyciechowski and Moroń 2009, Dussaubat, Maisonnasse et al. 2013, Goblirsch, Huang et al. 2013) and are more likely to have an increased progression of tasks, becoming foragers earlier in life (precocious foraging), spending less time as nurse bees (Holt, Aronstein et al. 2013).

Nosema ceranae can suppress the bees’ immune system, whereas N. apis activates the immune response (Antúnez, Martín-Hernández et al. 2009, Paris, El Alaoui et al. 2018). By evading the immune response, N. ceranae is a more challenging pathogen for honey bees and could increase susceptibility to other infections. Nosema-infected colonies also are seen to weaken over time, because the lifespan of workers is shortened and their homing ability is impaired, resulting in a dwindling of the worker population (Higes, Martín-Hernández et al. 2009, Wolf, McMahon et al. 2014).

Diagnosis and Treatment

Diagnosing Nosema Infections

Sending a sample to a USDA ARS or a university laboratory that can do molecular detection will confirm the presence of Nosema spp. and tell you which species. In some cases, the level of infection may also be provided. However, a beekeeper with a microscope can perform a diagnostic test, as previously described (Cantwell 1970, Galajda, Valenčáková et al. 2021), and can be seen online.

Treatment/Control of Active Infections in Bees

To successfully eradicate the disease from a colony, both active infections and spores must be targeted. Treat only the active infection in the honey bees, which will not prevent reinfection from viable spores that remain in the colony and/or on the equipment.

How do you know when treatment is necessary? Many have anecdotally reported a threshold of 1 million nosema spores per bee requires action.

Fumagillin is the only approved treatment for nosema disease in honey bees. Fumagillin is derived from the fungus Aspergillus fumigatus (McCowen, Callender et al. 1951) and is used to treat both N. apis and N. ceranae infections. Fumagillin dicycloheylamine is the active ingredient in Fumidil-B and is sold as a powder from various online sources. As per the manufacturer’s label, the powdered fumagillin is mixed into sugar syrup and can be administered in the spring and/or fall (cannot be used when supers are on). Fumagillin was first used in honey bees in the 1950s to treat for N. apis infections and is effective at decreasing the number of infected bees and increasing survival (Katznelson and Jamieson 1952, Bailey 1953). Note that this treatment only targets active infections. It does not target the spores, which may, therefore, remain on contaminated equipment and cause reinfection. As suggested by Peirson and Pernal (2024), the effective dose of fumagillin requires further research. However, you should always follow the manufacturer's instructions on any product.

A variety of other compounds have been examined for controlling nosema infections, including natural compounds such as thymol, resveratrol, vetiver essential oil, and lysozyme (Maistrello, Lodesani et al. 2008). Nozevit, an herbal compound (Gajger, Petrinec et al. 2009, Gajger, Vugrek et al. 2009) and surfactant molecules (Porrini, Audisio et al. 2010), have been proposed as compounds that can reduce infection. HiveAlive, a liquid feed for honey bees that contains seaweed extracts, has also been found to be effective at reducing infections (Charistos, Parashos et al. 2015, Garrido, Porrini et al. 2024).

Treatment of Equipment

Other, lesser-used treatments currently are fumigating combs with formaldehyde, acetic acid (vinegar), and formalin (Bailey 1955). Heat treatment of equipment at 120°F for 24 h can kill N. apis spores (Cantwell and Lehnert 1968, Cantwell and Shimanuki 1969). Gamma irradiation is also effective at killing Nosema spp. spores (Hornitzky 1986, Liu, Batchelor et al. 1990). However, neither of these methods is readily available to beekeepers. Alternatively, exposing N. ceranae spores to freezing temperatures for one week decreases spore viability (Fenoy, Rueda et al. 2009).

Summary

Nosema disease is a stress disease that can have negative impacts on honey bees, such as decreased productivity, change in population dynamics as bees forage earlier, and therefore a decrease in honey production and survival. However, while nosema infections are commonly detected in colonies, the pathogen does not seem to be a major contributing cause to the increased mortality observed. Acting together with other factors could be a cause for concern, but requires more research.

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