The egg is sometimes ingested by earthworms, which may act as transport hosts.

Eggs may survive for up to nine months in soil and L3 for years within the earthworm or other transport hosts.

It should be remembered in designing a control system that the earthworm transport hosts present a continuous reservoir of infection.

Preparasitic development from egg to L3 is typically strongyloid, though earthworms may intervene as transport hosts.

Adult birds are symptomless carriers, and the reservoir of infection is on the ground, either as free eggs or in earthworm transport hosts.

In some cases a parasite will infect a host, but not undergo any development, these hosts are known as paratenic or transport hosts.

Earthworm transport hosts are important factors in the transmission of Syngamus trachea when poultry and game birds are reared on soil.

Transport hosts such as earthworms are thought to play a role in transmission of A. galli and hence, free range birds tend to have a higher risk of infection.

The most common transport host is the common earthworm, but a variety of other invertebrates including slugs, snails and beetles, may act as transport hosts.

Earthworms may be transport hosts, the eggs simply passing through the gut, or paratenic hosts in which the egg hatches and the L2 travels to the tissues to await ingestion by the fowl.

Infection may occur by one of three ways, firstly by ingestion of the L3 in the egg, secondly by ingestion of the hatched L3 or thirdly by ingestion of a transport host containing the L3.

Given the low concentration of fertile eggs on infected dogs' coats (less than 0.00186% per gram), it is plausible that such eggs were transferred to the dog's coat by contact with fecal deposites in the environment, making dog coats the passive transport hosts.

There is also evidence to suggest that strains of Syngamus trachea from wild bird reservoir hosts may be less effective in domestic birds; if they have an earthworm transport host rather than direct infections via ingestion of L3s, or eggs containing L3s.

The Black Death is traditionally believed to have been caused by the micro-organism Yersinia pestis, carried by the tropical rat flea (Xenopsylla cheopis) which preyed on black rats living in European cities during the epidemic outbreaks of the Middle Ages; these rats were used as transport hosts.