Ascariasis

The term Ascariasis refers to disease caused by nematode worms of the genus Ascaris. This disease can occur in a number of different species as described below.

In Pigs

Ascaris suum is the primary cause of ascariasis in pigs.

Diagnosis

The disease is most commonly identified by the presence of milk spots on the liver. This is usually an incidental finding at post mortem or during the slaughter process. The discovery of milk spots on the liver will lead to the comdemning of the infected tissue.

Gross Examination

In the early stage of the disease haemorrhagic tracts will be visible through the liver due to the migratory activity of the larvae. In later stages of the disease diffuse white spots on the capsular surface of the liver will be visible due to the fibrosis of these tracts.

Microscopic Examination

The early disease will show haemorrhage and necrosis of parenchyma along the tracts created by the migrating larvae. There will be fibrosis at these sites in the later disease as well as evidence of inflammatory response. The inflammatory response consists of infiltration of many eosinophils, as well as lymphocytes and macrophages.

Pathogenesis

• 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.
• Even small numbers of migrating larvae can lead to fibroplastic responses in the liver (chronic interstitial hepatitis - the 'milk-spot' lesion).
• Heavy burdens of adult worms retard growth and may even obstruct intestine and bile duct.
• 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.

Immunity and Milk-Spot

• The relationship is complex as several immune mechanisms are involved:
  • initially L4 larvae are killed as they return to the intestine having completed their hepatotracheal migration.
  • later, L2 are stopped before they reach the liver.
• Consequently:
  • milk-spot can occur in immune baconers if larvae are not killed until after they have passed through the liver.
  • milk-spot is less often seen in sows as larvae are killed before they reach the liver in highly immune pigs.

Control of Milk-Spot

• Milk-spot results from ingestion of Ascaris eggs, therefore, milk-spot can only be prevented by decontaminating the environment.
• 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).
• Thus, anthelmintic treatments do not 'cure' this condition immediately - a longer term strategy is needed to control this problem.

Immunity

• Pigs do develop immunity, but it takes several months for this to become strongly protective.
• Even so, a small proportion of adults harbour small adult worm burdens.
• These 'carrier sows' are epidemiologically important as they contaminate the farrowing house with eggs, thereby providing a source of infection for the susceptible piglets.