Heligmosomoides polygyrus, previously named Nematospiroides dubius, is a naturally occurring intestinal roundworm of rodents. It belongs to the family Trychostrongylidae, and male and female worms are morphologically distinguishable. The parasite has a direct lifecycle, with its larval form being the infective stage. H. polygyrus has the ability to establish chronic infections in rodents and alter host immune responses. This nematode is widely used as a gastrointestinal parasitic model in immunological, pharmacological, and toxicological studies.
Lifecycle and morphology
This parasite has a direct lifecycle with no intermediate hosts. The lifecycle takes about 13–15 days to complete. Infected mice pass faeces containing eggs, and egg sizes vary between 70 and 84 micrometres (µm) in length and 37 and 53 µm in width. Eggs are shed from the host at the 8– to 16-cell stage and hatch in the environment, roughly 24 hours after passing through the host. L1 larvae emerge from the egg and measure from 300–600 µm in length. Three lip-like structures can be seen around a rudimentary mouth. L1 larvae moult to L2 larvae after 2–3 days; they then begin feeding on bacteria in the environment. The L1-stage cuticle loosens from either end of the larvae, but remain loosely associated with the L2 larvae, becoming an outer sheath up until infection. After 3 days, the L2 partially moults into ensheathed L3, the infective nonfeeding stage. Infective larval stages measure 480–563 µm long.
Mice ingest the L3 stage of the parasite and after 18 hours, exsheathed L3 appears in the intestinal lumen. The L1 sheath is shed following ingestion, at which point the larvae shorten slightly and measure 376–540 µm in length. After 24 hours after ingestion, larvae invade the mucosal layer of the intestine. Around 4 days after ingestion, L3 moult into L4 in the submucosa of the intestine. At 6 days after ingestion, they encyst in the muscle layer of the intestine and start maturing in to adult parasites. By day 14, adult male and female worms contact in the lumen of the intestine, mate, and produce eggs that are passed in the faeces, thus continuing the lifecycle. Adult males are tightly coiled and usually measure 8–10 mm in length. The females are also tightly coiled, but larger, measuring 18–21mm in length. Adults are characterized by a dark red pigmentation, whereas the free-living larval forms are mostly translucent.
Epidemiology
In natural infections, H. polygyrus is found almost ubiquitously within populations of wild wood mice (Apodemus sylvaticus). In one study of wood mouse populations in Oxfordshire, England, 70% of all mice sampled carried an infection with H. polygyrus, with an average infection burden of about 12 worms per mouse. Natural infection intensity displays high variability in wood mice, ranging from none to 244 adult worms per mouse. Male and female mice show equal parasitic burdens. Parasite occurrence appears to positively correlate with weight and age of the mouse, showing an increase in prevalence in older, heavier mice. Infection was also seasonally regulated in the wood mouse population, with highest prevalence of infection/worm burden intensity occurring in early spring and reaching their lowest values in late summer/early autumn. This is inversely correlated with typical breeding behaviors of the wood mouse, where the population peaks in late summer or early autumn, and is at its lowest in the early spring. The bulk of research on H. polygyrus has been conducted on the laboratory mouse, Mus musculus, as it is used as a model of human helminth infection to which a spectrum of natural resistance to parasite infection exists.
Pathogenicity
Upon infection with H. polygyrus, innate and adaptive host immune responses are generated to prevent the establishment of the parasite in the gut. A strong wound-healing immune response (Th2-type) associated with intestinal pathology is mounted. Similar to other roundworm infections, Th2 immunity focuses on eliminating the parasite or confining it to minimize host damage.