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Endoparasites - Life Cycles

The life cycles of most parasites involve both immature and mature stages. Some of the life cycles of horse parasites are particularly variable and very complex; consequently they are often misunderstood. The animal harbouring sexually mature parasites is called the definitive or principal host. Immature stages of some parasites must partially mature 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 to 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 of the infective stage to reproductive maturity is known as the prepatent period.

All 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:

  • Directly eating infective larvae. Parasitic infection of the host often occurs after the ingestion of an infective free-living form or in the egg. This means of infection is typical of many nematodes or roundworms, such as the large roundworm Parascaris and the strongyles.
  • Eating the intermediate host. In other cases, the principal host may ingest the intermediate host harbouring the infective stage. This process is true of tapeworms such as Anoplocephala magna (Not in New Zealand), whose infective larval form lives in oribatid mites.
  • The parasite actively penetrates the principal host. Larvae of the intestinal thread- worm, Strongyloides westeri, may infect horses by skin penetration. Onchocerca (Not in New Zealand) and Setaria larvae (Not in New Zealand) are injected into the blood as the intermediate host, a fly, feeds.
  • The parasite may be maternally transmitted. Strongyloides westeri may cross the placenta, infecting the foetus before birth. These nematodes 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 several exceptions, 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) (see illustration) 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 moults again to become a third-stage larva (L3). The third-stage larva forms are infective to horses. 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 horses. Once inside the animal, infective larvae either become established in the site of predilection appropriate to their species and become adult worms, or migrate through several organs and once in their predilection site become adults.

Strongle egg in horse faeces   Third Stage Nematodes
Strongyle egg in horse faeces   Infective third-stage larvae of nematodes suspended in drop of dew

Some roundworms have life cycles that are exceptions to the general cycle just described. Horses become infected with pinworms (Oxyuris) or with large roundworms (Parascaris) by eating the developed eggs which contain second-stage larvae. The intestinal threadworm, Strongyloides westeri, has a distinctive life cycle. Strongyloides larvae are capable of both skin penetration and lung migration. Only adult 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 penetrate directly through the skin. 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 horses have indirect life cycles requiring intermediate hosts for particular stages of development. With the exception of Draschia, the following parasite examples are not found in New Zealand but they illustrate indirect life cycle examples in which the larvae of the eyeworm (Thelazia), the ligament worm (Onchocerca - not in New Zealand), and the abdominal worm (Setaria - not in New Zealand) are ingested by specific insect intermediate hosts as the insects feed. After developing inside the insects, the larvae migrate to the mouth part of the insects and are deposited on a new horse the next time the insect feeds. The eggs of the gullet worm (Gongylonema - not in New Zealand)) and the equine stomach worm (Habronema - not in New Zealand) pass in the faeces (see illustration). Habronema requires a fly maggot intermediate host, while Gongylonema (not in New Zealand) eggs must develop in a dung beetle.



Adult tapeworms in the small intestine of the principal host grow by generating proglottids from the scolex. Proglottids mature and, after fertilization, 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 the faeces (see illustration). 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.

Horses are the principal host for a common tapeworm, namely, Anoplocephala species Eggs and proglottids are passed in the faeces. An infective egg contains a hexacanth larvae with six hooks, which is released if ingested by an oribatid mite, common on pastures. A tadpole-like cysticercoid larval form develops inside the mite. Mites are ingested by horses along with vegetation. As the mite is digested, the cysticercoid is released and matures to an adult tapeworm in the intestine.

Roundworm Life Cycle
Life cycle of typical gastrointestinal roundworm

Life cycle of Habronema / Draschia

Life cycle of tapeworm Anoplocephala magna (Not in New Zealand).
 A perfiolata is present.

The horse may serve as intermediate host for two tapeworms that have dogs as principal hosts. After a horse eats the eggs of either Multiceps or Echinococcus granulosus the larvae hatch and the embryos migrate to the blood stream, which carries them to various organs. Multiceps develops into a coenurus, which is a fluid-filled vesicle with several invaginated tapeworm heads developed along its inner wall. This structure is also known as "bladder worm." It may cause damage if it develops inside the host´s brain. Echinococcus embryos develop to hydatid cysts within horses or other mammals. Hydatid cysts are large fluid-filled bladders. These structures may put pressure on neighbouring organs, causing atrophy or wasting.




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 fertilization (asexually). As a result, many adults may be produced from a single egg. This process is called polyembryony.

Adult flukes in horses 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. Cercariae develop from rediae. Cercariae leave the snail intermediate host and are then infective to the principal host. Equine fluke cercariae swim onto vegetation and secrete protective cysts. These structures are called metacercariae. After being ingested by horses, metacercariae hatch to immature flukes.


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