this is Nosema, at 400x magnification.

There are 2 types of Nosema, Nosema apis and the new variant Nosema ceranae, both are very difficult to identify. Nosema-infected bees do not look much different from non-infected individuals, but certain traits are evident even before microscopic analysis and confirmation of disease.
The lifespan of infected bees is often reduced and infected colonies often dwindle in late winter or early spring. In cases of Nosema ceranae, colony death can be even faster than with Nosema apis and may take only a few days – foraging is particularly affected and bees are often too weak to return from foraging flights.
Dysentery is a prime symptom of bees infected with Nosema apis, but it is not a symptom of Nosema ceranae infection.
Nosema apis infection causes young adult bees to change their behaviour. They stop brood-rearing and attending to the queen earlier than normal and instead they start foraging and guarding the hive like older bees. This reduces the number of new foragers and honey yields are reduced. The life span and egg production of infected queens also reduces and this encourages queen supersedure.
Nosema apis infection tends to cause adult bees to become bloated with water and this leads to dysentery. Digestion and production of royal jelly can be severely affected. If infected bees are confined to hives during cold weather, hive parts, combs and frames will become contaminated from faecal material. This is more obvious in spring when yellow-brown faecal spots can be seen on the outside of the hive after the first cleansing flights
Treatments for Nosema are best administered in the autumn, followed by a Spring treatment if still required. There are 3 effective treatments available and all involve treating with a sugar solution.
Fumidil B - is an antibiotic and is known to work well against Nosema spores.
Nozevit – is an all Natural feeding supplement for treating against Nosema Spores.
VitaFeed Gold – is a liquid feed based on natural beet extract and molasses, shown to reduce Nosema Spores.
Nosema Treatments can be found here