Diagnosing and Treating Nosema Disease
Eric C. Mussen, Extension Apiculturist, UC Davis – 3/11/11
Causative Agent
Nosema disease in U.S. honey bees is caused by one of two (or both) fungi named
Nosema apis and Nosema ceranae . Nosema species are oblig ate, fungus -like, intra -cellular
parasites that are limited to specific host s species . Nosema apis and N. ceranae cannot be reared
in laboratory culture, as is possible with most bacteria and other fungi. They can multiply in
living honey bee midgut , and perhaps other, cells. There is evidence that, like Nosema bombi in
bumble bees, the N. ceranae may infect other honey bee tissues, but that remains to be
substantiated.
Life Cycle
When a bee ingests Nosema spores, the spores are filtered out of the ho ney sac by the
proventricular valve and released into the midgut. The exact physical and chemical conditions of
the honey bee midgut stimulate germination. The organism penetrates a midgut cell and grows
by absorbing nutrients from that cell. The parasi te increases in size until it is large enough to
divide in half. Each new parasite continues this multiplication process until the nutrients in the
cell begin to become exhausted. That stimulus triggers sporulation. Depending upon the species
of Nosema , approximately 100 spores can begin to develop as early as four days post -infection or
up to nine days later. Some of the early, thin walled spores appear to germinate in side the
infected cells, sending their polar filaments into adjacent cells. In this manner, they can make
their way through the body cavity, infecting other tissues, at least in bumble bees. The nutrient -
depleted host cell s rupture . Environmentally resistant, thick walled spores are released into the
midgut lumen to start the process, a gain , or be excreted to the outside . Heavily infected worker
honey bees can contain an excess of 50 million spores. Damaged intestinal tissue is subject to
secondary infections and "dysentery" (brown diarrhea spots on the combs and exterior of the
hive) is a common sign of infection with Nosema apis , but not seen with N. ceranae . N. apis
infected bees also defecate inside the hive, contaminating combs with millions of infectious
spores.
Effects on Colony
Nosema infections have specific negative effect s on honey bees. Worker bees that ingest
spores when they are less than a week old normally do not digest food well and are not capable
of produc ing brood food secretions . Infected bees tend to skip the brood rearing phase of life
and become foragers at very young ages. Their life spans can be reduced up to 78%. Young
queens that ingest Nosema apis spores normally are superseded within a month. In climates
where winter prohibits supersedures for many months, colonies often go queenless and dwindle
away in early spring. Experience in Minnesota suggests that an average of one million or more
N. apis spores per bee can lead to increased winter losses. When high percentages of workers
are infected and spore counts exceed ten million spores per bee, signif icant numbers of colonies
will die or lose queens during the winter. With Nosema apis , this spike in the level of infection
normally occurr s in early spring, then “ goes away” as the weather improve s and the bees
defecate outside the hive. With Nosema cer anae having become the dominant species , infection
and spore levels can be elevated all year. All levels of infection le d to very slow spring build up
with Nosema apis , eve n when forage and temperatures we re ideal. Frequent ly, reduced honey
yields follow ed this poor population build up. In Spain, year „round infections with Nosema
ceranae did not seem to interfere with spring build up and swarming, but were found to lead to
very high percen tages of lost colonies during summer and through to the next spri ng.
Diagnosis
Nosema disease is difficult to diagnose without using laboratory equipment. Pulling the
last abdominal segments from a bee usually will remove the intestinal tract intact. According to
some authors, a healthy midgut is tan in color, with concentric constrictions. An infected midgut
will become swollen, whitish and lose its visible constrictions. There is so much variation that
this method of diagnosis really cannot be trusted. Besides , other causes of dysentery, such as
inges ting honey dew, fermented syrups, indigestible sugars in cola syrups, m olasses and kitchen
corn syrups can result in similar intestinal changes.
Scientists use a specific methodology to determine levels of infestation. Known numbers
of severed abdomens are homogen ized, using a mortar and pestle. The homogenate is sieved
through two layers of cheesecloth into calibrated centrifuge tubes. The tubes are spun in a
clinical centrifuge at 600 rpm for six minutes to drive the spores to the bottom of the tubes. The
liqu id (supernatant) is poured off (decanted) and the plug (pellet) at the bottom is resuspended in
a specific volume of water ( final calculation is spores in one ml water per bee). The plug is
broken up well (resuspended) by sucking the water in and out many times through a small -tipped
disposable pipette. Then a small droplet of the suspension is placed on a blood cell counting
chamber (hemocytometer). The number of spores counted over certain areas of the chamber grid
can be converted to millions of spore s per bee. If infection levels are below 10,000 spores per
bee, no spores will be seen over the entire grid and the diagnosis is determined to be “not
detected” or “ND.” That does not mean that there is no infection.
Treating Infected Colonies
Medica ting for Nosema apis is based on the most appropriate times to prevent comb
contamination and development of disease in bees that clean up fecal deposits from combs while
expanding the brood nest. Later in the summer, when bees are defecating outside the hive, N.
apis usually cannot be detected. A few bees are infected all year, but only the diseased late
season bees are of consequence. When they develop high levels of infection, they defecate on
the combs in October, November and December, then die.
Brood rearing never ceases in many parts of California over the winter, but as the days
begin to lengthen in late December, the bees are stimulated to pick up the pace. Availability of
nectars and pollens, along with warming temperatures, accelerate brood rearing. It is at this time
that many bees "cleaning and polishing" cells, in anticipation of egg laying, become in ocula ted.
How severe the disease will get in the colony population depends upon the initial spore load
(amount of contamination) and how mu ch of the time the bees are confined to the hive by non -
flight weather. So, Nosem a apis levels can vary significantly from year to year.
In order to "cover the bases" in Minnesota, if a colony population had one million or
more spores per bee in April, we fed it two gallons of fumagillin -medicated, heavy (two parts
sugar : one part water) syrup the following September. If we had to "feed for weight," that was
done earlier, so that the early syrup could be "ripened" and stored before the medicated syrup
was applied. If the medicated syrup is mixed with other, unripened syrup, it can be diluted to
ineffective concentrations. We anticipated that the medicated syrup would be consumed
throughout the winter. Spore deposition on combs in early winter would b e reduced and the
parasite could not reproduce in medicated bees that became inoculated in the spring. The syrup
would be consumed, totally, long before the bees produced any honey.
Although we have not conducted the experiments, it is likely that two g allons of
medicated syrup may not be required in most of California. Nosema apis levels we re not as high
in California as they we re in Minnesota. Combs should not be so badly contaminated during the
winter months, since intermittent flight is possible. Therefore, first treatments with medicated
syrup should coincide well with the normal practice of providing colonies with "stimulative"
syrup and pollen substitute feeding in late December and January. A gallon, or so, of medicated
syrup should provide pr otection against Nosema apis until the bees are flying well in March and
April. Heavy nectar flows from Manzanita , Eucalyptus , mustard and radish might dilute the
medication significantly, as would later feeding with non -medicated syrup.
Medicating for Nosema ceranae is going to be different. For one thing, fumagillin no
longer is available as Fumidil -B®. A Canadian company , Medivet, sells the product as
Fumigilin -B®. The new fumagillin mixes more readily into solution and it appears to be less
stable in solution, if the label instructions are correct. The label suggests using the antibiotic in
the fall , as always has been the ractice at that time of year for nosema control. Since N. ceranae
tends to persist throughout the year, the label calls for i ncreasing the dosage level , and feeding
repeatedly in small amounts of sugar syrup , in the spring . Beyond that, Spanish researchers have
found that N. ceranae -infected bees tend not to take medicated syrup from feeders, so they had to
pour the syrup over the bees (called “drench”) , down between the frames, to make them consume
the medication.
Expected Results of Treatment
Beekeepers who have fed fumagillin to field colonies years ago had noted significant
differences in colony build up. In fact, many o f them stopped using fumagillin. The colonies
built up too quickly and swarm control became nearly impossible. With so many colonies
succumbing to CCD at this time, I doubt that swarm control will be a major issue any longer .
Decontamination of Equipmen t
The spores of both nosema species are susceptible to irradiation or fumigation with
glacial acetic acid (details in The Hive and the Honey Bee ). Otherwise the spores of apparently
Europe -originated N. apis are very resistant to the elements, except su nshine, and persist for
many years. Nosema ceranae , on the other hand, originated in tropical Asia, and its spores are
susceptible to weather, especially cold weather. The spores can be killed by refrigeration or by
freezing. If possibly spore -contamina ted combs are placed in a freezer and left there until it is
certain that all materials in the combs have reached freezing temperatures (honey holds a lot of
heat for quite a while), N. ceranae spores, all life stages of greater wax moth, all stages of sma ll
hive beetle, and other little critters that tend to get into stored combs will be eliminated.
I am happy to discuss Nosema , its consequences in colonies, and treatments. I can be
reached by telephone at: (530) 752 -0472 or by email at: ecmussen@ucdavi s.edu. Copies of this
"Bee Brief" can be downloaded at:
http://entomology.ucdavis.edu/facutly/mussen/beebriefs/index.cfm.
