Page 177 - The Veterinary Laboratory and Field Manual 3rd Edition

146 Susan C. Cork and Mani Lejeune

Figure 3.24 Life cycle of the dog roundworm (Toxocara canis). This worm has a fairly complex life cycle
that is specialized to ensure infection of young puppies. (A) Ingestion of larvated (L2) roundworm eggs
from the environment. (B) The eggs hatch in the dog’s stomach and the larvae invade the intestinal wall to
spread in the blood stream to the lungs. (C) Larvae pass from the circulation into the alveoli of the lungs
or, depending on the immune status and/or age of the animal, travel back to the heart in the circulation
(D) and are carried away to the tissues. Larvae may then encyst in the tissues as an arrested infective
stage. This commonly occurs in adult animals and in a bitch these arrested stages may reactivate during
pregnancy to infect the developing puppies. (E) Larvae that do enter the lungs migrate to the trachea and
travel to the small intestine to become adults. (F) Adult worms mate and produce eggs which pass out in
the faeces. (G) Eggs embryonated in the environment to form the infective stage. (H) Larvated eggs may
be ingested by an accidental or ‘paratenic’ host in which the larval parasites survive but do not develop
(for example, rodents, humans, sheep). (I) Puppies may ingest infective eggs from the environment or by
ingestion of larvae in the bitch’s milk (J), or before birth if dormant larvae reactivate in the pregnant bitch
and cross the placenta. Illustration: Louis Wood.

cases or bloating and abdominal discomfort in which causes ‘ascariasis’. It is especially common
moderate infections. in warm areas. Ascaris suum is the ‘roundworm’
Toxocara vitulorum is the roundworm found in of piglets and is common in extensively reared
cattle and buffaloes (syn. Neoascaris vitulorum), young stock. Parascaris equorum is the ‘roundworm’

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