I just tested another round of bees from my sick hive, I took 4 random bees out of 100 that I sampled a few days ago (stored in freezer). 100% infected with Nosema, 3 with obvious signs of Amoeba and 2 with tubules stones. One showed high levels of amoeba in the ileum and another the cysts had already reached the rectum.
From my reading Amoeba disease infection lasts about 21 days from start to finish at which point infectious cysts make it to the feces and the bee likely dies soon after. The disease is likely transmitted via feces at the end of the infectious cycle. The increased mortality during winter (piles of dead bees out front) is likely what keeps the disease at bay. In early spring, the population drops from say 10,000 bees to zero in 30-45 days. The rate of infection seems to increase as most of the tested bees have both Nosema and Amoeba.
From Hassenein, Some Studies on Amœba Disease:
Cysts did not appear in the Malpighian tubules of healthy bees introduced into the two infected colonies until the 19th and 21st days; in bees that had been fed with viable cysts of M. mellificce immediately before introduction, they were found on the 18th and 20th days. By the 21st day viable cysts were found in 100% of the marked bees examined which had been fed with cysts, but in only 24% (March 20th experiment) and 13% (April 12th experiment) of the bees fed with untreated syrup. Details are given elsewhere (Hassenein, 1950).
In this study, the infection is Amoeba only and it has an transmission rate of non-inoculated bees of 13-24%.
So what late winter behaviour contributes to the accelerated transmission rate?
My best guess is that housekeeping (preparing nest for spring brood) is likely the main driver on the rate increase. My early April inspections typically have 10-16 frames of bees. Other than my 2018 poop fest (honeydew/Nosema/Amoeba), the frames have some feces but nothing excessive so that may not be the trigger.
I found this reference by Bailey on infectious diseases . https://www.era.rothamsted.ac.uk/eradoc/OCR/ResReport1959-204-215
Malpighamoeba mellifcae Prell was studied at Rothamsted first by Hassanein (1952b), who confirmed that an annual epidemic, similar in character to that of Nosema apis, occurs in adult bees of endemically infected colonies. This annual epidemic appears to have the same explanation as that of Nosema apis; transference of infected colonies to noncontaminated combs in early summer eliminated infection, and transference of combs from infected to uninfected colonies in autumn introduced infection which became epidemic the following spring (Bailey, 1957d). Cysts of M. mellifcae on combs were killed as readily as spores of N. apis by vapour of acetic acid, but fumagilin did not affect the development of M. mellifcae in infected bees (Bailey, 1955d).
My original goal was to try and understand the infection cycle to see if there was a window of opportunity to save them. Prevention is key (culling infected frames) however could the colony be saved by moving them into clean equipment (may not be worth the effort or you may just end up infecting more equipment). I actually took the queen from this infected colony and transferred her into a queenless colony. To my surprise she seems to be doing well, laying out a great pattern and ruling the frames. This reenforces the findings that Nosema infected bees make poor nurse bees.
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0110237
Effects of Infection on Honey Bee Population Dynamics: A Model
Matt I. Betti, Lindi M. Wahl, Mair Zamir
Published: October 16, 2014
https://doi.org/10.1371/journal.pone.0110237
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