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| | ''A. suum'' is seen globally, however in temperate regions infection normally occurs in the summer months due to the warmer temperatures. Young piglets up to about 6 months old are most susceptible although they begin to develop immunity at about 4 months. Sows carrying adult worms will contaminate the farrowing house rapidly, as female worms can produce 200,000 eggs per day, causing a high incidence of infection in the litter. The highly resistant eggs can survive in the environment for up to 4 years with suitable conditions, this makes control very difficult especially in free range systems. | | ''A. suum'' is seen globally, however in temperate regions infection normally occurs in the summer months due to the warmer temperatures. Young piglets up to about 6 months old are most susceptible although they begin to develop immunity at about 4 months. Sows carrying adult worms will contaminate the farrowing house rapidly, as female worms can produce 200,000 eggs per day, causing a high incidence of infection in the litter. The highly resistant eggs can survive in the environment for up to 4 years with suitable conditions, this makes control very difficult especially in free range systems. |
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| − | ==== Pathogenesis ====
| + | ===Immunity=== |
| − | *Large numbers of migrating ascarid larvae can damage the lungs causing respiratory distress and exacerbating other pulmonary infections, as well as providing a portal of entry for pyogenic bacteria.
| + | Young piglets are most susceptible to infection, in older animals immunity does develop but can take a few months to become strongly protective. Even 'immune' animals can carry small numbers of adult worms and therefore contaminate the environemnt. |
| − | *Even small numbers of migrating larvae can lead to fibroplastic responses in the liver (chronic interstitial hepatitis - the 'milk-spot' lesion).
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| − | *Heavy burdens of adult worms retard growth and may even obstruct intestine and bile duct.
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| − | *The 'milk-spot' lestion is the biggest economic loss in the UK attributable to ''Ascaris'' - approximately 7% of pig livers are trimmed or condemned at the bacon factory.
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| − | ===== Immunity and Milk-Spot =====
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| − | *The relationship is complex as several immune mechanisms are involved:
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| − | **initially L4 larvae are killed as they return to the intestine having completed their hepatotracheal migration.
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| − | **later, L2 are stopped before they reach the liver.
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| − | *Consequently:
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| − | **milk-spot can occur in immune baconers if larvae are not killed until after they have passed through the liver.
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| − | **milk-spot is less often seen in sows as larvae are killed before they reach the liver in highly immune pigs.
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| − | ===== Control of Milk-Spot =====
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| − | *Milk-spot results from ingestion of ''Ascaris'' eggs, therefore, milk-spot can only be prevented by decontaminating the environment.
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| − | *''Ascaris'' eggs can survive in pig pens for weeks, months or years (depending on the level of hygiene), therefore pigs will still be exposed to eggs after anthelmintic treatment (unless moved to a clean environment).
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| − | *Thus, anthelmintic treatments do not 'cure' this condition immediately - a longer term strategy is needed to control this problem.
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| − | ==== Immunity ====
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| − | *Pigs do develop immunity, but it takes several months for this to become strongly protective.
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| − | *Even so, a small proportion of adults harbour small adult worm burdens.
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| − | *These 'carrier sows' are epidemiologically important as they contaminate the farrowing house with eggs, thereby providing a source of infection for the susceptible piglets.
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| | ===In Cows=== | | ===In Cows=== |