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==Life Cycle==
==Life Cycle==
L1-L3 occurs within the envelope. The L3 then exits the sheath in the small intestine, and then concentrates within the '''Lieberkuhn's gland'''. At this stage they are known as EL3, and have two potential options. Firstly, they may either become '''hypobiotic''', remaining dormant for several months to years. This usually occurs Autumn-Winter in temperate climates. Alternatively EL3 may evolve directly into LL3. They will then evolve into L4, at which stage they will emerge from a nodule. After emergence of the cyst, L4 transforms into L5. They then transform into adults in the lumen.
L1-L3 development occurs on the pasture. The rate and extent of development and therefore infective 3rd stage larval challenge is directly affected by environmental conditions. At temperatures below 6 degrees centigrade minimal egg hatching and larval development occur.
Infective L3 larvae ingested by the grazing horse penetrate the large intestinal mucosa where they are quickly (within a few days) encased in a fibrous capsule to become 'encysted'. At this stage they are known as EL3, and have two potential options. Firstly, they may either become 'hypobiotic', remaining dormant for several months to years. Hypobiosis usually occurs late autumn/ early winter in temperate climates. Alternatively EL3 may evolve directly into LL3 and then L4. After emergence of the cyst, L4 transform into L5 and then adults in the lumen of the caecum and colon. Importantly, since egg laying adults may not yet be present, a horse may be habouring large numbers of encysted small redworm despite a negative or low faecal worm egg count (FWEC).
Highest egg shedding is in the spring months. The larvae, which represent 90% of the population, are at maximal levels in the autumn.
Only two anthelminitics have activity against encysted stages of small redworm, a single dose of moxidectin (MOX) or a 5-day course of fenbendazole (5d-FBZ). At 8 weeks post treatment with moxidectin EL3 stages are eliminated. Due to widespread resistance to fenbedazole a faecal egg count reduction test is recommended prior to using it for treatment of the encysted stages. Killing of encysted mucosal larvae due to 5dy-FBZ has been associated with severe tissue damage, which clinically may correspond to
reactions caused by synchronous mass emergence of fourth stage larvae(larval cyathostominosis). This effect did not occur after treatment with MOX (Steinbach et. al., 2006)
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