Life Cycle of Hookworms

Parasitologists are obsessed with “lifecycles”. What they mean by lifecycle is the sequence of different stages that the parasite undergoes, what it does to earn a living at each stage, and where the different stages occur, particularly inside humans or other animals (hosts) or outside (external environment).

Hookworms are classed as soil transmitted helminths (STH) since they need to spend time developing outside the host in the soil before the larvae reaches the infective stage. During this stage the egg hatches, usually within 24 hours. The first stage larva lives in a mixture of faeces and soil feeding on faecal bacteria. It moults to a second stage larva which also feeds on bacteria. During the final moult the third stage larva retains the skin of the second stage larva as a “sheath”. The third stage is the infective larvae (iL3). It does not feed, but searches for a human host to penetrate. First and second stage larvae have an oesophagus composed of three sections (body, neck, bulb) and are called rhabditiform larvae; the iL3 has a relatively longer, cylindrical oesophagus and is called filariform larvae (see terminology in Fig. 1).

So to summarise, inside the host hookworms suck blood; outside the host they eat bacteria growing in the faeces; and the infective larvae do not feed.

Since a lifecycle is a circle of sorts, one can start anywhere. Here is a typical representation of a hookworm life cycle (Fig. 1)

Figure 1: Life cycle of human hookworm (from Centres for Disease Control – http://www.dpd.cdc.gov/dpdx/html/hookworm.htm)

Summary: Eggs are passed in the stool (1), and under favourable conditions (adequate but not excessive moisture, warmth (25-28°C), shade), larvae hatch in 1 to 2 days. The rhabditiform larvae grow in the faeces in the soil (2), and after 5 to 10 days (and two molts) they become filariform (third-stage) larvae that are infective (3). These infective larvae can survive 3 to 4 weeks in favourable environmental conditions. On contact with the human host, the larvae penetrate the skin (4) and are carried through the blood vessels to the heart and then to the lungs. They penetrate from the pulmonary capillaries into the pulmonary alveoli, migrate up the airways, pass down the oesophagus, through the stomach to the duodenum where the hookworms mature (5). Male locates female, they mate and eggs appear in the faeces (1).

A few numbers:

How long does it take for eggs to be produced? The average prepatent period (time from penetration of iL3 to eggs appearing in faeces) is about 7 weeks for Necator americanus with some individuals becoming positive at 9 weeks (Cline et al 1984, Maxwell et al 1987). However, eggs in some infections may appear at 5 weeks post inoculation (Logan 2009).

How many eggs does a female hookworm lay? In experimental infections of N. americanus in myself the maximum number of eggs produced per day per female hookworm was about 6,700.

How long do the infective larvae survive in the soil? This depends greatly on the temperature and moisture. At higher temperatures the metabolic rate of the iL3 is greater and they exhaust their energy supplies. Augustine (1922) showed that in tropical conditions 90% of the iL3 in the soil had died within 3 weeks of eggs being deposited. However, a small percent did survive to 7 weeks. Augustine's experimental studies confirmed observations made by Cort and Payne (1922) on soil contaminated naturally by human faeces with abundant iL3 detected. Within 3 weeks of preventing additional faecal contamination iL3 had largely disappeared from the soil although a few survivors were found.

References

Augustine DL. Investigations on the control of hookworm disease. X. Experiments on the length of life of infective hookworm larvae in soil. American Journal of Hygiene 1922;2:177-187.

Cline BL, Little MD, Bartholomew RK, Halsey NA. Larvicidal activity of albendazole against Necator americanus in human volunteers. American Journal of Tropical Medicine and Hygiene 1984;33(3):387-394.

Cort WW, Payne GC. Investigations on the control of hookworm disease. VI. A study on the effect of hookworm control measures on soil pollution and infestation in a sugar estate. American Journal of Hygiene 1922;2:107-148.

Logan M. Methods in Improving The Quality of Nector americanus larvae for use in Therapeutic Applications. Honours thesis. James Cook University, Townsville. 2009.

Maxwell C, Hussain R, Nutman TB, Poindexter RW, Little MD, Schad GA, Ottesen EA. The clinical and immunological responses of normal human volunteers to low dose hookworm (Necator americanus) infection. American Journal of Tropical Medicine and Hygiene 1987;37(1):126-134.

Page by Rick Speare 30 August 2012