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Endoparasites - Life Cycles

The life cycle 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 some 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. All tapeworms (cestodes) and 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 on 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.
  • Ingestion of an intermediate host. In other cases, the principal host may ingest an intermediate host harbouring the infective stage. This is true of tapeworms such as Moniezia benedeni, whose infective larval form lives in oribatid mites.
  • The parasite actively penetrating the principal host. Larvae of the cattle hookworm Bunostomum phlebotomum, 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. Toxocara (Neoascaris) vitulorum and Strongyloides papillosus may cross the placenta, infecting the fetus before birth. These nematodes may also pass in the colostrum to the newborn.

Nematodes

Roundworm parasites, with the exception of hookworms, Toxocara, lungworms, and threadworms, have similar 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 survive 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 cattle. 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 location they are most likely to be eaten by cattle. Once inside the animal, infective larvae become established in the site of predilection appropriate to their species and become adult worms.

 
Infective third stage larvae of Nematodes suspended in a drop of dew   Larvae undergoing exsheathment or shedding cuticle


Life cycle of a typical roundworm

Some roundworms have life cycles that are exceptions to the general cycle just described. For example, the larvae of the cattle hookworm, Bunostomum phlebotomum, can infect the animal by boring through the skin. Eggs of Dictyocaulus viviparus, the cattle lungworm, develop and hatch inside the host. First-stage larvae, not eggs, are passed in faeces. Infective lungworm larvae are less mobile than larvae of other roundworms. Consequently, they do not migrate from the faecal pat to herbage except during heavy rainfall; other mechanical factors such as farm implements and hooves are also important in spreading the larvae. Without such transfer, cattle will not be infected since they do not eat grass soiled with manure. Another way in which larval lungworms can be transferred to clean grass is for them to accumulate on the spore structure of the fungus Pilobolus, which grows on cattle faeces. When the sporangium explodes, larvae can be propelled as far as ten feet, and be deposited on pasture where cattle graze. When ingested, these larvae penetrate the wall of the animal´s intestine and are carried by the lymph and circulatory systems to the lungs, where adult worms develop. The intestinal threadworm Strongyloides papillosus has a distinctive life cycle. Strongyloides larvae are capable of both skin penetration and oral infection. 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 containing first-stage larvae are passed in faeces and hatch to liberate the 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. A life cycle in which adult forms reproduce on pasture is termed heterogonic and requires permissive environmental conditions of warmth and humidity.

Several nematode parasites of cattle 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. Each species of Onchocerca uses a particular fly genus as its intermediate host. Fortunately neither of these species is found in NZ.


Cestodes

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.


Life cycle of a typical roundworm

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 (cysticercoid). Metacestodes very greatly in structure among tapeworm species, but are generally fluid filled cysts, the inner lining of which grows one or more immature scoleces (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 scoleces, which mature in cattle and become adult tapeworms. The egg of the common tapeworm (Moniezia benedini) is ingested by oribatid mites in which the metacestode develops. The life cycle is completed when cattle eat infected mites.

Certain tapeworms utilise domestic animals, including cattle, as intermediate hosts. Taenia saginata, for example, utilises man as principal hosts and cattle as intermediate hosts. The metacestode in cattle is called a cysticercus. These grow in muscles and are known as beef measles or Cysticercus bovis. Consumption of "measly beef" is a means by which man may be infected; this, infected carcasses are condemned at slaughter. Dogs are the principal host of Echinococcus granulosus. This 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 10 cm. or more in diameter) that these achieve may damage animals by putting pressure on neighbouring organs, particularly liver and lungs, impairing their function.


Trematodes

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

Adult flukes in cattle 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 cattle, metacercariae hatch to immature flukes. Damage can be caused by Fasciola during the wandering of the immature flukes within the liver of cattle, and when it enters the liver bile duct to become an adult.


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