Nosema is a highly specialised parasitic Microsporidian fungal pathogen. In England and Wales we have two species, Nosema apis and an Asian species, Nosema ceranae. Both invade the digestive cells lining the mid-gut of the bee and then multiply rapidly, so that within a few days these cells are fully packed with spores. This is the resting stage of the parasite. When one of these host cells ruptures the spores are released into the gut where they accumulate in masses, to be later excreted by the bee. The spores can germinate and once more become active by being ingested by another bee, starting another round of the infection and thus multiplication.
Symptoms of Nosema
You should not assume your colony has Nosema if you see dysentery, as although this is often seen in association with the disease as ‘spotting’ at the hive entrance and/or across the frames, it is not caused by the pathogen but as a consequence of the bees being being confined in the hive during poor weather, especially in Spring during the normal build up period. The bees may be forced to defecate in the hive, and if Nosema is present in the colony this will lead to more contamination. There are no outward symptoms of the disease. You may see a steady decline in the number of bees and secondary diseases will often appear such as chalk brood. Eventually this leads to insufficient bees remaining to carry out basic tasks, and the colony will collapse. As Nosema is readily spread through the use of contaminated combs (spores can remain viable for up to a year), you should adhere to best practice and not transfer possibly contaminated combs between colonies. Good husbandry and apiary management, maintaining vigorous and healthy stocks is always the best advice!
Diagnosis and Treatment
Diagnosis of infections by microscopic examination is the most effective and simplest method. Both Nosema species are identifiable in adult bee samples using a standard adult disease screen. The spores appear as white/green, rice shaped bodies. Instead of using medicines for treatment beekeepers are generally advised to apply good husbandry practices to create and maintain well fed, strong and disease tolerant colonies. Replacing queens regularly generates prolific queens; ideally these should be sourced from more tolerant stocks of bees which are better able to cope with Nosema infection.