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Parasitic Gastroenteritis (view source)
Revision as of 17:58, 26 December 2008
, 17:58, 26 December 2008→Epidemiology
In temperate climates, some species can overwinter on the grass, while others cannot. In either case, nematodes may survive inside the host for long periods of time, not as normally developing adult worms (that would be expelled within a few weeks), but as larval worms that have become temporarily arrested in their development and may remain inside the host 'asleep' for many months.
In temperate climates, some species can overwinter on the grass, while others cannot. In either case, nematodes may survive inside the host for long periods of time, not as normally developing adult worms (that would be expelled within a few weeks), but as larval worms that have become temporarily arrested in their development and may remain inside the host 'asleep' for many months.
'''Arrested Larval Development''' (synonyms: inhibited development, hypobiosis, diapause):
*Long lifespan - several months (c.f. adult worms - a few weeks).
*Larvae become arrested at an early stage in their development, e.g. ''Ostertagia'' as EL4 (i.e. early fourth stage), equine cyathostomes as L3.
* Mechanism for ensuring the survival of the parasite when climatic conditions adverse. Note seasonal pattern of arrested development, e.g. ''Ostertagia'' only need to arrest over winter.
*Stimulus for arrested development varies, e.g. falling temperatures - ''Ostertagia''; drought - ''Haemonchus''.
*Termination of arrested development - spontaneous (i.e. a genetic alarm clock). Previously arrested larvae then resume their development and grow to adult worms.
=== Host Immunity ===
Acquired host immunity affects both the establishment of recently ingested infective larvae as well as the course of infection of developing worms. When considering the effects that host immunity has on a worm burden, remember that animals grazing at pasture are continuously exposed to infection and not just challenged by a single large infection.
'''Worm Population Dynamics - Continous Infection''':
It is important to remember that a worm burden inside an animal grazing at pasture does not remain static, but is continually changing, i.e. as older adult worms are lost, more infective larvae are recruited. A state of equilibrium is reached, i.e rate at which L3 establish = rate at which adult worms are lost. In an immune animal, the same equilibrium exists, except that fewer of the L3 ingested become established.
'''Effects of Host Immunity on Worm Burden''':
a) ''Reduction in establishment of larvae'':
e.g. fewer L3 establish in an immune adult cow than a parasite naive calf.