Hookworm infections in humans are caused by two important species of nematode parasites (a type of a roundworm), Ancylostoma duodenale and Necator americanus.
A much larger group of hookworms that infect animals can sometimes parasitise humans or penetrate human skin, but those infections are much less common.
In the human organism, the lumen of small intestine represents the habitat of hookworms where they can feed on host mucosa and blood. Female worms can then produce eggs which are passed in the stool to embryonate in the ground.
The rhabditiform first-stage larvae hatch within a couple of days and develop (through two molts) to the third-stage filariform larvae that can penetrate skin and infect the host.
Epidemiology and transmission
Together with trichuriasis and ascariasis, hookworm belongs among the three major soil-transmitted helminth infections.
From a public health perspective, these infections are significant as approximately two billion people inhabiting the world’s tropical and subtropical areas harbour hookworms (according to new data from the World Health Organization).
A negative correlation can be observed between hookworm infections in humans and income level when we look at cross-country comparisons.
China and sub-Saharan Africa are regions containing a largest pool of hookworm infection cases, with approximately 200 million infections found in each country. Likewise, in some villages in Brazil, the prevalence exceeds 80%.
The important prerequisite for hookworm infection is walking barefoot on ground with faeces of people who harbour hookworm (which can also be used as a fertiliser). Hence there is much lesser risk of encountering this condition in countries with adequate waste control and sanitation.
Although hookworm infections are generally considered to be asymptomatic, the insidious course of these parasitic conditions makes them potentially very dangerous.
Soon after infection, a ground-itch (allergic reaction at the site of penetration) may develop, especially in patients infected with Necator americanus.
Oral route of infection with Ancylostoma duodenale can result in Wakana disease characterised by pharyngeal irritation, nausea, vomiting, cough, and hoarse throat. Potential larval pulmonary migration results in (usually mild) Loeffler’s pneumonitis where eosinophils can accumulate in the lungs of the affected patient.
Hookworm is considered a particularly pathogenic nematode since each adult hookworm can cause intestinal blood loss up to 0.2 milliliters per day.
Accordingly, individuals who are chronically infected with a large number of hookworm parasites may experience significant iron deficiency and subsequent anemia.
Diagnostics and management strategies
A diagnosis of hookworm infections relies on finding the eggs in the stool with the help of light microscopy.
In an early stage, the egg is unsegmented, although a larval stage is sometimes seen within the egg if a specimen has been standing at room temperature for a longer period of time.
Several anti-helminthic drugs are at our disposal to remove hookworms from the human intestine. Those that belong to the benzimidazole class – albendazole and mebendazole – are most widely used due to their effectiveness, relative safety, and low price.
Nevertheless, high rates of reinfection hamper adequate control of hookworm infections worldwide.
Albeit certain progress has been made in developing an effective hookworm vaccine, there is still a myriad of uncertainties regarding design and ideal vaccination programmes.
Therefore, new paradigms for hookworm control are needed, alongside proper sewage disposal systems and good hygiene standards.