Animalia --> Nematoda --> Secernentea
Animalia --> Nematoda --> Secernentea
Indirect, Two-host cycle
Definitive hosts: Dogs, foxes, wolves, coyotes, cats
Intermediate host: mosquitos
When an infected mosquito feeds on a mammalian host, third-stage filarial larvae in the mosquito's proboscis enter through the wound and remain dormant in muscles for 85-120 days. They molt twice, then enter the bloodstream and migrate to the heart as adults. There, worms reach sexual maturity over 7-9 months and produce microfilariae that circulate in the blood and can be ingested by mosquitos. In mosquitos, microfilariae migrate to the abdomen, molt twice, and become third-stage infective larvae that then migrate to the mosquito's proboscis.
https://www.cdc.gov/parasites/dirofilariasis/biology_d_immitis.html
http://nemaplex.ucdavis.edu/Taxadata/Dimmitis.htm
Transmission rates depend on mosquito and host ecology. Since D. immitis relies on an ectothermic mosquito vector, its infection patterns are seasonally and geographically dependent. Mosquitos thrive in hot and humid environments where they proliferate and feed on hosts. In summer and geographic locations where mosquito populations are high, transmission events rise since mosquitos survive long enough and feed often enough to pick up D. immitis from infected animals and transmit it to susceptible ones.
D. immitis is found in North and South America, Australia, Japan, and Europe.
Indirect, two-host cycle
Intermediate host: cyclops
Definitive host: humans, mammals
Free-floating larvae are consumed by cyclops, small crustaceans/copepods that typically inhabit ponds and still water. Parasites molt twice in infected cyclops to become infective filariform (L3) larvae over two weeks. These are ingested by animals, either through drinking water or by eating fish/amphibians that have eaten infective copepods. The larvae enter connective tissues in the abdomen. Male and female worms copulate there, males die, and females migrate to the skin surface, exiting when there is contact with water and releasing larvae in the process.
https://www.cdc.gov/parasites/guineaworm/biology.html
https://www.who.int/water_sanitation_health/resources/vector324to336.pdf
D. medinensis infects humans and other mammals. Its definitive hosts' long life spans support a 10-14 month developmental timeframe within the host before the adult parasites emerge from the skin. Additionally, frequent contact with water presents opportunities for emergence into a safe environment in which the parasite has a high chance of survival and sexual reproduction.
Since D. medinensis poses a threat to humans, efforts to eradicate it have been successful in Asia and are ongoing in the six African countries where infections persist but are very rare.
https://www.cartercenter.org/resources/pdfs/health/guinea_worm/gw_map_jan09.pdf
Direct, one host: Horned passalus beetle, Odontotaenius disjunctus
Nematode larvae develop in the gut of the horned passalus beetle. Adults of this species are difficult to come by, and the life cycle is not fully studied.
There may be thousands of nematodes in a single beetle, yet they do not kill the beetles. Actually, despite host behavioral manipulation and the suppression of the immune response by the parasite (one study showed a lack of increase in hemocyte density among infected beetles compared to uninfected ones), infection results in larger beetle size and enhances ecosystem services provided by the beetles (Davis et al. 2016). Beetles compensate for infection by increasing consumption of decaying wood, and this aids nutrient cycling.
This parasite follows its host's distribution in forests in the eastern US.
https://www.nytimes.com/2019/05/02/science/parasites-beetles-wood.html
researchgate.net/publication/309956681_Parasite_Manipulation_of_Its_Host's_Physiological_Reaction_to_Acute_Stress_Experimental_Results_from_a_Natural_Beetle-Nematode_System
Andrew K. Davis, David Vasquez, Jake LeFeuvre, Stuart Sims, Meghan Craft, and Anna Vizurraga
Physiological and Biochemical Zoology 2017 90:2, 273-280
Two-host, Vector-Transmitted
Vector: Fly (Two species from the genus Chrysops)
Definitive hosts: humans
Adult worms reside in human subcutaneous tissues and produce microfilariae that circulate through bodily fluids and in peripheral blood by day, returning to the lungs at night in a diurnal cycle. When flies take a blood meal, they pick up microfilariae that develop in the gut. Microfilariae and larvae reside in the gut of flies, whereas infective juveniles migrate to the mouth parts where they are transmitted to more susceptible humans through bites at the skin.
Human infection with these filarial worms results in the disease filariasis, which is often asymptomatic. More than 10 million people are estimated to be infected with Loa loa, although it can not transmit directly from person-to-person. L. loa contributes to health concerns due to the complications presented in treating other infections, such as onchocerciasis, while L. loa microfilariae circulate the bloodstream, since ivermectin causes adverse effects in these individuals that make the disease difficult to control. Long human life spans support worm life spans of up to 20 years, so, alongside a potential for re-infection, infection with L. loa can be lifelong, especially for people who live near densely forested and populated areas.
L. loa is found in forested areas in West and Central Africa.
https://www.cdc.gov/dpdx/loiasis/index.html
https://www.cell.com/trends/parasitology/fulltext/S1471-4922(17)30305-7