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Among the worms, numerous species are parasites of all kinds of animals (including humans) or plants and are called helminths. They occur worldwide, with a species-specific geographic distribution that depends on ecological and climatic conditions.

Most parasitic worms of veterinary importance affect internal organs of their hosts (lungs, heart, kidneys, liver, gut, stomach etc.) and this is why they are often grouped into the internal parasites, also called endoparasites (as opposed to external parasites or ectoparasites such as ticks, mites, liceflies, fleas, etc.).

Infestations with parasitic worms are also called helminthiasis.

Parasitic worms are highly adapted to their way of life inside their host's organism. Most helminth species have no circulatory system (heart, blood, blood vessels, etc.), tapeworms have no digestive system, etc.: they simply don't need them.

From top to down: adult roundworm (Asacaris suum), adult giant fluke (Fasciola gigantica), tapeworm (Taenia taeniaeformis)All parasitic worms are obligate parasites, i.e. they cannot complete their life cycle without spending some time on their host.

Some helminth parasites are very host specific, i.e. they are able to complete their life cycle only on that particular host species or closely related ones (e.g. Toxocara vitullorum only on cattle; or Chabertia ovina, which infests only sheep and goats). Other species can develop on many different host species (e.g. liver flukes, Fasciola hepatica).

The incidence of parasitic worms and the harm they cause to livestock and pets depends a lot on climatic and ecological conditions, as well as on the type of livestock (e.g. extensive versus intensive farming). Pets in rural regions are exposed to other parasites than those in urban environments. As a general rule helminth parasites tend to be more of a problem in humid regions.

Whereas some parasitic worms are live threatening for livestock and pets, other species are quite benign and well tolerated by their hosts without apparent damage. Among the harmful species, several ones can strongly damage essential organs of the host, which become sick. Other parasitic worms are mainly competitors for food: what the host feeds is virtually "stolen" by the worms dwelling in the host's digestive system. This causes significant productivity losses in livestock: weight loss, insufficient growth, reduced milk or egg production, impaired fertility, etc.

Well-nourished, healthy animals often tolerate considerable worm burdens without major harm, i.e. without developing a disease, somehow in a natural balance. But if they lose condition (e.g. due to stress, droughts, malnutrition, etc.) the worm infestation turns into an excessive burden and the diseases breaks out. Young and old animals, or otherwise sick or weak ones are always more susceptible towards helminth parasites and more likely to become sick. 

Life-cycles of parasitic worms

Limnaea, a snail that often acts as intermediate host  of several parasitic worms. Picture from

Many helminth species have complex life cycles, alternating stages on the host and stages off the host. The duration of each development stage depends strongly on ecologic and climatic conditions. Understanding the life cycle is very important for establish and managing preventative measures.

Adult worms reach sexual maturity and reproduce inside their host and produce thousands (some species millions) of eggs that are expelled mostly through the feces. Young L-1 larvae hatch out of these eggs and follow a more or less complex development that goes at least through two more stages, L-II larvae and L-III larvae, with a molt between the stages. In many worm species L-I and L-II larvae are non-infective for the final host i.e. if they get into a potential host, they will not complete development. Shortly after molting L-III larvae become infective: if they get into a suitable host they will complete development to adult worms. In some species there are various other development stages. The period of time between penetration into the final host and detection of eggs (usually in the feces) is called prepatent period.

Once inside a host, immature stages often accomplish more or less complex migrations through the host's tissues and/or organs before reaching their final location. Migrating stages are often more harmful for the host than the mature stages at their final location. Most parasitic worms have a so-called predilection site, an organ of the host (e.g. liver, stomach, kidneys, etc.) where they usually reach sexual maturity and reproduce.

Direct and indirect life cycles

There are two major types of life cycles, depending basically on the number of host:

  • Direct life cycles, with a single host that is called definitive or final host, where the worms reach maturity and reproduce. Roundworms often have direct life cycles.
  • Indirect life cycles: with at least one obligate intermediate host in addition to the final host. Typical intermediate hosts are often small invertebrates such as snails, insects, crustaceans, etc. The worms complete certain development stages inside these intermediate hosts, with or without harming them. In some species the immature stages can multiply asexually inside the intermediate hosts. The final hosts get infected mostly by ingestion of infected intermediate hosts. Flukes (trematodes) and tapeworms (cestodes) often have indirect life cycles. Some species have even two obligate intermediate hosts (e.g. Dicrocoelium dendriticum).

As a general rule, most parasitic worms can complete development only in a reduced number final hosts, e.g. only in ruminants (cattle, sheep, goats, etc.), or only in carnivores (dogs, cats, foxes, etc.), or only in certain bird species, etc. If infective larvae get into an unsuited host, they will not complete development and will mostly die without causing any harm. But some worm species can be very harmful for such non-final hosts, also called facultative, transport, or paratenic hosts, in spite of not being capable or completing their development. Typical facultative hosts are small invertebrates such as frogs, lizards, rodents, squirrels, etc. The final hosts get infected by ingestion of infected paratenic hosts or carrion. Some tapeworm species (e.g. Echinococcus spp, Taenia spp.) use livestock, dogs, cat and even humans as facultative hosts and can be quite harmful for them (e.g. cysticercosis).

Routes of entry into the host

Infective stages of parasitic worms have several routes of entry into their hosts, whereby each species usually follows one of these routes:

  • Entry through a natural opening, mostly through the mouth, quite seldom through the anus, the nose, or other openings. Ingestion through the mouth is mostly passive, i.e. infective stages are ingested with contaminated food.
  • Entry through the skin, either actively penetrating it by its own efforts, or passively through the bite of a vector (insects, ticks, etc.) that transmits infective stages. A special case is transplacental transmission, i.e. are those worms that can be from mothers to their young inside the uterus.

Transmission from mother to their young can also occur through the mother's milk.

Types de of parasitic worms of veterinary importance

There are three major types of helminths of parasitic importance, each one with similar anatomic characteristics and comparable life cycles:

  • Roundworms (= nematodes). They belong to the group of Nemathelminthes. They look just like worms, more or less long and thick. Close to 30'000 species are known, more than 16'000 parasitic ones.
  • Flukes (= trematodes). They belong to the group of Platyhelminthes (= flat worms). There are about 20'000 species, most of them parasitic of mollusks and vertebrates. They often have an oval form.
  • Tapeworms (= cestodes). They belong also to the group of Platyhelminthes (= flat worms). About 1'000 species are known, all parasitic of vertebrates, including livestock, pets and humans.

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