Roundworm parasites, with the exception of Trichinella spp worms, 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 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 moults again to become a third-stage larva (L2) which is encased by the shed skin of the second-stage larva.

Life cycle of typical roundworm

The third-stage larval forms are infective to swine. Once inside the animal, infective larvae become established in the site of predilection appropriate to their species and become adult worms.

Some roundworms have life cycles that are exceptions to the general cycle just described. The intestinal threadworm Strongyloides ransomi has a distinctive life style. Strongyloides larvae are capable of both skin penetration and oral transmission. Only females of this species are parasitic. Once infective larvae reach 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 gain access by ingestion during grazing or by skin penetration. A life cycle in which adult forms reproduce on pasture is termed heterogonic and requires permissive environmental conditions of warmth and humidity.

The lungworm Metastrongylus apri lays eggs in the bronchi; these are coughed up, swallowed, and passed in the faeces. The eggs hatch after being eaten by earthworms, in which larvae develop to infectivity. Pigs become infected by ingesting these worms.

The pig may serve as both the principal and the intermediate host for Trichinella. Larvae of this worm are laid by the adult female in the intestine. They travel via the blood to muscles all over the body, where they encyst. Further development will occur only if the pig is eaten by another mammal, in which case the larvae excyst and mature, eventually producing more larvae. Trichinella infections are of little concern to the pig; human infections, acquired by eating undercooked infected meat, are of much greater clinical importance. The major Trichinella spp is T. spiralis. There has not been evidence of indigenous trichinellosis in humans or animals in Australia. Confirmed cases in people have invariably been in people who have immigrated from, or visited countries in which trichinellosis is endemic.

Eggs are discharged into the body cavity of the female worm, where they develop into larvae surrounded by a shell of several layers. The embryo, known as an acanthor, has a spiny body and an anterior circle of hooks. The ancanthor inside its thick-walled egg is resistant to the environment and can live for years outside the pig. However, it will develop only if eaten by a dung beetle grub. Pigs become infected by ingesting the grub or adult beetle containing the encysted worm larva.

Certain tapeworms use domestic animals, including pigs, as intermediate hosts. Taenia solium, for example, uses man as a principal host and swine as intermediate hosts. The larval stage in pigs is called a cysticercus. These grow in muscles and are known as pork measles or Cysticercus infected; thus, infected carcasses are condemned at slaughter.

Life cycle of the lungworm