Special Veterinary Pathology

ASCARIASIS

Definition

• Ascarids are extremely common round worms found in the gastrointestinal tract of birds and mammals and are host specific

Pathogenesis

• The infective larvae are released into the gut following ingestion of the embryonated eggs by the host.

First type

• In some larvae penetrate the intestinal wall pass through the liver, lung bronchiole, trachea –Swallowed into the alimentary tract where the adults develops. eg. A.lumbricoides in man and pigs, Parascaris equorum in horses and Toxocara canis in dogs

Second type

• In others larvae migrates not only through the liver and lungs but also through somatic tissues sometimes causing prenatal infection of the foetus.e.g. T.canis and Neoascaris vitulorum in cattle

Third type

• Larvae penetrates the intestinal wall and without migrating to other tissues returns back to the lumen of intestine to become adults eg. Toxascaris leonina and T.cati seen in cats, lion, dogs etc.
• Most of the ascarids does not require intermediate host for development
• prepatent period: Interval between the ingestion of infective form and appearance of eggs in the faeces - Varies for different species of worms

In poultry – A. galli is seen in the intestine and Heterakis gallinae in the caeca, of which the former is more pathogenic

Lesions

• Adult ascarids feed on to intestinal contents and sometimes moves into stomach, hepatic or pancreatic duct
• If in bile duct, it causes “ jaundice "
• Large number of worms may cause obstruction of the intestinal lumen, intussusception and even rupture
  Penetrating worms may cause “peritonitis "
• In liver, as the larvae burrow through the parenchyma is destroyed and the tract formed thus is filled by blood and debris. Neutrophils and eosinophils crowd around such a track. Healing by sacr tissue occurs and the foci appear as " milk spots' on the capsular surface. In heavy infection, the lesions may become confluent and fibrosis may occur to such a degree as to obliterate the whole lobules.
• Adult worms cause inanition and retardation of growth.
• The infective larvae due to their migration causes damage in the various tissues with central caseous necrosis surrounded by epithelioid cells, eosinophils, lymphocytes and neutrophils.
• In the lung of swine, larvae causes haemorrhages, dyspnoea and cough described as “thumps”
• Toxocara larvae migrating in abnormal hosts like man and other animals is desribed as “visceral larval migrans”

Diagnosis

• Eggs are thick shelled with unsegmented yolk