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Endoparasites - Life Cycles

The life cycles of all parasites involve both immature and mature stages. The animal harbouring sexually mature parasites is called the definitive or principal host. Immature stages of certain parasites must partially develop inside an animal of another species, such as an insect, a snail, or another mammal. These animals are called intermediate hosts. After such development, the immature parasite forms are infective for the principal host. Many tapeworms (cestodes) and all of the flukes (trematodes) require intermediate hosts. These parasites are said to have indirect life cycles. Parasites that reproduce without an intermediate host have direct life cycles. Once infective parasite forms have entered the principal host, they grow to maturity. The time from entry of the infective stage to reproductive maturity is known as the prepatent period.

Most parasites must develop partially before they are capable of infecting their principal host. There are four ways in which host animals can be infected by internal parasites:

  • Direct ingestion of infective larvae. Parasitic infection of the host often occurs after the ingestion of an infective, free-living form or encysted form. This means of infection is typical of many nematodes or roundworms, such as the stomach worms Haemonchus contortus and Ostertagia ostertagi, and also of the liver fluke, Fasciola hepatica.
  • Eating the intermediate host. In other cases, sheep may ingest an intermediate host harbouring the infective stage. This is true of tapeworms such as Moniezia expansa, whose infective larval form lives in oribatid mites.
  • The parasite actively penetrating the principal host. Larvae of the sheep hookworm (Bunostomum trigonocephalum) and the intestinal threadworm (Strongyloides papillosus) may infect their hosts by skin penetration. Following infection, the larvae are transported via the blood to the lungs before proceeding to their sites of predilection where they mature.
  • The parasite may be maternally transmitted. Strongyloides papillosus may cross the placenta, infecting the foetus before birth. The larvae may also pass in the colostrum to the newborn.

A knowledge of life cycles and their timing is important in controlling parasites. In many cases, certain drugs are only effective against specific stages in parasite development. Sometimes control is possible by reducing the numbers of an intermediate host.



Roundworm parasites, with the exception of hookworms, lungworms, and threadworms, have similar direct life cycles. Females lay thousands of eggs, which pass out in the faeces of infected animals. If the environmental conditions of warmth and moisture are favourable, eggs deposited in the faeces will develop to the first larval stage (L1) and hatch in several hours. If deposited on dry ground or if temperatures are low, the eggs develop more slowly or will not survive. Hatched larvae thrive on pastures, feeding mainly on bacteria.

Larval growth is limited by a rigid skin, or cuticle. Larvae increase their size through the process of moulting. When a first-stage larva grows to the limits of its cuticle, it develops a second, larger cuticle underneath the first, then casts off the old one to become a second-stage larva (L2). The L2 grows to its limits and becomes a third-stage larva (L3) which is encased by the shed skin of the second-stage larva. The third-stage larva forms are infective to sheep. During cool nights these larvae stay at the base of grass near the ground. When sunlight warms the pasture, the larvae migrate up wet blades of grass to settle near the top, frequently swimming in drops of dew. In this position they are most likely to be eaten by sheep. Once inside the animal, infective larvae become established in the site of predilection appropriate to their species and become adult worm.

Some roundworms have life cycles that are exceptions to the general cycle just described. For example, the larvae of the sheep hookworm, Bunostomum trigonocephalum, can infect the animal by boring through its skin, as well as by being eaten on grass. Eggs of Dictyocaulus filaria, the sheep lungworm, develop and hatch inside the host. First-stage larvae, not eggs, are passed in faeces.



Life cycle of a typical roundworm

L3 larvae in water droplet on grass leaf   Larvae undergoing exsheathment or shedding cuticle

The intestinal threadworm Strongyloides papillosus has a distinctive life cycle. Strongyloides larvae are capable of both skin penetration and lung migration. Only females of this species are parasitic. Once infective larvae are in the lungs, they migrate up the trachea toward the mouth and are swallowed. They mature in the small intestine, where adult females lay eggs that do not require fertilisation to develop. Eggs passed in faeces hatch to yield first-stage larvae, which develop in the manner, described as typical for roundworms, to become infective third-stage larvae. This life cycle is termed homogonic and includes a parasitic phase inside the host. However, the adult threadworms in the intestine can lay eggs that develop into a different kind of larvae. If warmth and humidity are conducive to survival of free-living forms, these larvae develop and moult into adult worms, which can live outside the host. Males and females of this type mate, producing fertilised eggs which eventually yield infective L3 larvae that are eaten by the host during grazing or actively penetrate host. A life cycle in which adult forms reproduce on pasture is termed heterogonic and required permissive environmental conditions of warmth and humidity.

Several nematode parasites of sheep have indirect life cycles requiring intermediate hosts for particular stages of development. The eyeworm, Thelazia, has larvae which develop to infective stages in certain muscid flies. Red lungworms (Protostrongylus rufescens) and hair lungworms (Muellerius capillaris) use snails as intermediate hosts in which larvae become infective to sheep.



Adult tapeworms in the small intestine of the principal host grow by generating proglottids from the scolex. Proglottids mature and, after fertilisation, become gravid (enlarged and filled with eggs). The enlargement of segments as they mature results in the characteristic widening of the tapeworm body toward its end.

Gravid proglottids break off from the end of the tapeworm and pass in faeces.

Eggs are released as the proglottids decompose, either within the animal or on pasture in the faeces. With some species of tapeworms, eggs will not appear in the faeces even when a heavy infection exists. Rather, intact proglottids are found. Neither the proglottids nor the eggs are infective for the host. To become infective, eggs must first develop to the infective stage in intermediate hosts such as arthropods, crustaceans, or mammals. Because of the need for intermediate hosts the life cycle of tapeworms is indirect.

The embryo that develops within the tapeworm egg is known as a hexacanth. When ingested by the intermediate host, it hatches and develops into an immature stage called a metacestode. Metacestodes vary greatly in structure among tapeworm species, but are generally fluid-filled cyst, the inner lining of which grows one or more immature scolices (heads) depending on the species of tapeworm. Infection of the principal host occurs when an intermediate host or part of its tissue containing a metacestode is ingested. Digestion releases the scolices, which mature in cattle and become adult tapeworms. The egg of the common sheep tapeworm (Moniezia expansa) is ingested by oribatid mites, in which the metacestode develops. The life cycle is completed when sheep eat infected mites.

Certain tapeworms use domestic animals, including sheep, as intermediate hosts. Dogs are the principal host of Echinococcus granulosus. This adult tapeworm is of little concern to the dog, but its metacestode, known as a hydatid cyst, can grow in almost any mammal including man. The considerable size (5 to 10cm or more in diameter) that these achieve may damage animals by putting pressure on neighbouring organs, particularly liver and lungs, impairing their function. Sheep with hydatid cysts are a reservoir of metacestodes that are infective for dogs, which in turn may infect humans. Sheep infected with Echinococcus are, therefore, important because they increase the likelihood of humans contracting hydatid disease.



The indirect life cycles of trematodes are complicated and involve at least one intermediate host. Immature forms change shape greatly during development and may even reproduce without fertilisation (asexually). As a result, many adults may be produced from one egg. This process is called polyembryony.

Adult flukes in sheep lay eggs which pass in the faeces. The miracidium, a free-swimming larval form, hatches from the egg outside the host when optimal moisture and temperature exist. The miracidium penetrates the snail host where it develops into a sporocyst, which may divide and produce "daughter" sporocysts. The sporocyst, in turn, may reproduce asexually through polyembryony to yield a maximum of eight rediae, the succeeding form. In some trematode species, the rediae may divide, doubling the number of adults eventually produced. Cercariae develop from rediae. Cercariae leave the snail intermediate host and are then infective to the principal host. Some cercariae, such as those of Fasciola, swim onto vegetation and secrete a protective cyst. The encysted fluke is called a metacercaria.

After being ingested by sheep, metacercariae hatch to immature flukes. Much of the damage caused by Fasciola occurs during the wandering of the immature flukes within the liver of sheep.

Life cycle of a tapeworm


Life cycle of liver flukes


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