Nosema occurs in adult bees and is caused by a one celled organism, Nosema apis. Spores of nosema are ingested with the food and germinate in the midgut of the bee. Each sends out a long thread, known as the polar filament, which penetrates the cells lining the gut. The living 'germ' of the spore passes through this filament and into a gut cell. Here the organism multiplies and soon fills the infected cells with spores.
In diseased bees, the cells, which are released into the lumen of the gut, are frequently packed with nosema spores. These spores, on release, may either infect other cells of the gut lining or may pass out of the bee with its waste products. The infected cells in the gut lining upset the metabolism of the bee by interfering with the digestion and absorption processes. The protein reserves of the infected bee are severely reduced and little brood food can be produced. This is probably why about 15% fewer eggs develop into mature larvae at the height of infection.
Infested workers start foraging earlier in their lives than usual and their lives are shorter than average.
Feces of diseased bees in the hive spread Nosema spores to bees cleaning up. Defecation inside the hive is aggravated if the bees are confined by cold or rainy weather or during normal migration practices of the beekeeper. The disease spreads between colonies if infected bees drift into healthy colonies or if robber bees become infected.
SYMPTOMS
No definite diagnosis is possible without microscopic examination. The only outward signs of Nosema are a weakening of a colony or failure to build up normally when conditions are favorable. However, in severe cases the diseased bees will soil the hive, inside and at the entrance. Bees may be seen crawling out of the hive with abdomens slightly swollen. Heavily infected bees may give the impression of being clumsy and lethargic.
Although definite diagnosis of Nosema is only possible with microscope examination, there is a method which beekeepers can put to use with a little practice. The last abdominal segment (with the sting) of an adult bee is grasped with a fine pair of forceps and the gut pulled out. In healthy bees the midgut is brownish-yellow or mustard coloured, and its constrictions or rings are clearly seen. In bees that are heavily infected with nosema, the midgut is white and somewhat swollen, obscuring the constrictions.
NOSEMA IN SOUTH AFRICA
In the South Western Cape Nosema manifests itself in spring during poor weather, but also at other times during dearth periods in fine weather, when the bees consumed infected pollen stores. Nosema have been found in all provinces. In the summer rainfall region the infections were higher during the summer when more brood were reared. No colonies were severely infected and no colony deaths could be attributed to nosema.
