Hookworms are intestinal, hematophagous parasites belonging to the phylum Nematoda, the nematodes or roundworms. Not all nematodes are considered hookworms; some common genera that include hookworms are Ancylostoma, Necator, and Uncinaria. When these animals infest the body of a larger creature, they cause an infection called helminthiasis. Infection caused by Ancylostoma is termed ancylostomiasis, while infection caused by Necator is termed necatoriasis. Infection caused by Uncinaria is termed uncinariasis.
At a glance, a hookworm can be difficult to tell apart from other types of nematodes. They are generally much smaller than giant roundworms such as Ascaris lumbricoides; most hookworms are measured in millimeters. They are generally tubular, smooth-skinned animals with a typical worm shape. As seen in the above image, the distinguishing feature of a hookworm is its slightly bent head, which is where the name “hookworm” comes from.
Some hookworm species possess pairs of teeth, usually two pairs, which are used to bite into the intestinal wall of the host. Others have a pair of cutting plates instead of teeth. Coloration in hookworms is usually fairly bland because they live in completely dark environments; they may appear off-white, gray, or pinkish.
Hookworms are extremely tiny upon hatching; the newly-hatched larvae, or rhabditiforms, are so small because their mother must be able to carry tens of thousands of mature eggs at a time in addition to the eggs that are still growing. The larvae will go through two rhabditiform stages. After about one week of this, the larvae grow into the filariform stage, which is more mobile and larger in size. They do not feed at this stage of life, instead putting their energy toward finding and entering a host animal. Filariforms can live for about two weeks, attempting to reach the small intestine. To get there, they will swim through the circulatory system until they reach the lungs, prompting the host to cough them up and then swallow them. Once in the intestines, they will mature into adults; this entire process takes five to nine weeks.
Adult worms, depending on the species, can live between six months and fifteen years, though a more common lifespan is one to five years. Larvae, on the other hand, can live in a state of stasis within body tissues for years before emerging into the host.
In most hookworms, the female is considerably larger and thicker than the male. This size difference is often on the order of several millimeters. Males, on the other hand, can be distinguished by their larger reproductive organs.
As adults, hookworms live in the small intestines of animals. Each species has a preference for particular animal types (for example, the common Necator americanus only affects humans). The rhabditiform larvae live in the soil, while the filariform larvae burrow through the skin or are ingested by the host and make a journey to the small intestines.
In order for the eggs and larvae to survive, the ambient environmental temperature must be above 18 degrees Celsius (64 degrees Fahrenheit). The soil must be warm and moist, and exposure to sunlight or dry conditions will kill the larvae.
Hookworms are found around the world, mostly in warm regions that experience larger amounts of rainfall. Due to medical negligence, species which infect humans are common in the tropics, where people often cannot access proper medical care as it is not provided to them. Hookworms can affect most species of vertebrate animals and thrive in places with poor sanitation.
Human infections are the best-measured of all hookworm infection types. In 2015, there were approximately 428,000,000 human hookworm infections. Other animals, particularly livestock, are commonly infected with hookworms around the world, especially in the tropics. Areas such as Central America are ideal habitats for hookworms to grow and infect.
They are known on offshore islands in the Gulf of Fernandez, such as the Muertes Archipelago and Isla Nublar. Hookworms belonging to the genera Ancylostoma and Uncinaria probably thrive here, where abundant mammal and bird populations (both native and introduced species) provide ample hosts. The temperature in the Gulf of Fernandez is warm and humid, which are beneficial to the worms as this ensures a higher larval survival rate.
Birds are among the most common animals on these islands. Brown pelicans, Franklin’s gulls, and other seabirds live on the shores, and tropical forest birds such as collared aracaris inhabit the inland areas. The various bird species probably brought many nematode parasites to the islands; the species Baylisascaris procyonis mostly affect raccoons and dogs, but can survive in birds. The genus Oxyspirura or eyeworm is common in birds, which become infected after eating infected cockroaches. As cockroaches are known on the islands, it stands to reason that Oxyspirura may be found there and infect its birds. Other bird-infecting nematodes include Capillaria contorta and the gapeworm, Syngamus trachea. Not all of these species are considered hookworms, but their life cycle and habits are similar.
The wild mammals of the islands include monkeys (such as the mantled howler), opossums, three-toed sloths, collared peccaries, and the endemic Nublar tufted deer. These species serve as hosts for other hookworm species, with endemics such as the tufted deer likely hosting hookworm species found only on their island. Reptiles and amphibians may also be infected by hookworms and other roundworms, including members of the genus Rhabdias, Kalicephalus, Strongyloides or threadworm, Capillaria, Physaloptera, and Dracunculus. Some of the reptile species found on Isla Nublar are rather large, such as the common boa, and could have housed many hookworms each. At least seventeen venomous snake species, as well as nonvenomous species such as the milk snake and lizards such as iguanas, are native to these islands. Amphibians such as strawberry poison-dart frogs and chorus frogs live there as well. The biodiversity of islands in the Gulf of Fernandez means that they are home to a wide range of parasite species despite their small geographic size.
Necator americanus may have lived on the islands once humans arrived, thousands of years ago. Their domestic animals, namely goats on Isla Nublar in particular, may have become host to native hookworms or brought their own parasites. The population levels of native hookworm species depended on the populations of their various hosts; when ecological crises occurred on the islands, hookworms would become scarce. Between 1986 and 1993, both Isla Nublar and Isla Sorna experienced ecological disruption due to the construction of facilities on the islands; Isla Nublar’s natural habitats were further reduced between 2002 and 2015, and in 2018, that island’s primary volcano erupted, causing most of its forest habitat to be destroyed. Host extinctions and habitat loss shrunk the number of hookworms on Isla Nublar; though its ecosystem is now beginning to recover as of 2022, only hookworms that infect seabirds and hardy survivors such as rats are probably living there today.
The spread of hookworms to non-native areas is mostly caused by the transportation of infected animals and contaminated soil. Human activity is a primary cause of hookworms being introduced to areas where they do not normally live. Using medications to kill off hookworms can limit their spread. However, hookworms in general are found throughout the tropics and subtropics, as well as many temperate or seasonal regions, and so are native to places all over the world.
In the Gulf of Fernandez, the impact of human activity on hookworm spread can be studied in detail. Originally, native hookworms were only those that infected indigenous animal life as well as local humans and their livestock (on Isla Nublar, this was mainly goats). For thousands of years, this remained the case. In the 1980s, the islands caught the attention of International Genetic Technologies, a company which leased the Muertes Archipelago and Isla Nublar from the Costa Rican government for the development of de-extinction facilities. The first de-extinct animals, dinosaurs and pterosaurs from the Mesozoic era, were cloned on Isla Sorna in 1986 and shipped to Isla Nublar in 1988. To feed the carnivorous specimens, livestock including goats and cattle were shipped to the islands. Cattle are known to be infected by the hookworm Bunostomum phlebotomum, which could have been introduced to the islands at that point. Invasive species such as brown rats were also introduced, bringing more unchecked parasites.
Isla Nublar was abandoned in 1993, and Isla Sorna was fully evacuated not long after. The animals, which were assumed to be short-lived without human help, survived in the wild, now unregulated as they transmitted parasites. The huge size of some of these animals meant they could host plenty more hookworms than native island creatures could, and hookworms that affected birds and reptiles probably readily adapted to the dinosaurs and pterosaurs. InGen returned to Isla Nublar to continue building their animal park, now called Jurassic World, in 2002; the dinosaurs were recaptured and shipped back and forth between Isla Nublar and Isla Sorna for a few years. Eventually, Isla Sorna’s de-extinct animal population began to collapse, with little information about its population levels available after 2004. Most of the animals were shipped to Isla Nublar at this point, and hookworms on Isla Sorna are probably reduced from their numbers in the 1990s. On Isla Nublar, construction of Jurassic World and the captive containment of the de-extinct animals reduced the number of hookworms as well. By that point in time, paleoveterinary science had advanced enough to properly care for prehistoric creatures, so the park’s staff were able to use antihelminthic medicine and environmental cleaning to reduce infection rates. Hookworms were probably less common on Isla Nublar, and with natural habitats of native species cleared to make room for the park, indigenous hookworms were also depopulated.
Still, the constant influx of livestock for feeding the carnivorous animals provided introduced hookworms with a way to get on the island. Cattle, goats, and pigs were shipped to Isla Nublar on a regular basis, with live animals as well as prepared meat being used as food. Pigs in particular are known to be intermediate hosts for certain hookworms, including similar species to cattle. Larve live in pigs and are transmitted to animals that eat those pigs. Their definitive hosts are humans, though, not birds or reptiles, so they would not have survived in dinosaurs. Still, some other types of hookworms could have infected the prehistoric animals due to contaminated meat or livestock slipping through the security measures.
Jurassic World closed in 2015, and the animals were allowed to roam the island without human assistance. Now without medical care, the small numbers of hookworm infections were able to increase. Between 2015 and 2018, the dinosaurs began to succumb, not just from disease and predator overpopulation but from natural disasters; the island suffered from volcanic activity in 2017. A major eruption began in June 2018, destroying most of the island’s forest habitat and killing off the majority of de-extinct life there. After these events, Isla Nublar’s hookworm population is probably highly reduced, with many of the introduced species dying out.
Behavior and Ecology
Hookworms are essentially blind and do not rely on diurnal cycles to act. For much of their lives, they live in complete darkness within their hosts, mating often and feeding almost continuously.
Diet and Feeding Behavior
Hookworms and many other roundworms are obligate hematophagous endoparasites. This contrasts with ectoparasites such as ticks, which attach to the outside of the host; an endoparasite attaches on the inside. During the rhabditiform larval stages, the worm feeds on microorganisms in the soil; during the filariform stage, it does not feed and instead seeks out a host. Once it reaches the adult stage, the worm uses its mouth to attach to the intestinal lining of its host and drink blood. Some hookworms can consume quite large quantities of blood from the host, which can lead to significant medical conditions such as anemia.
Aside from mating, most hookworms do not engage in social interaction. They spend nearly all of their time feeding once they reach adulthood and are not very intelligent; large numbers of worms in a single host are due to either the host stumbling across a large group of related filariforms living in once place, or due to adults breeding within the intestines.
Mating in most hookworms is not very complicated and does not involve a significant courtship process. They mate within the intestines of the host. An adult female may produce up to thirty thousand eggs every day, though the number is smaller in most species. Most of these eggs will be passed out in the host’s feces into the external environment. Larvae that hatch within the host may live in stasis within the host’s tissues for years, emerging as the older worms die.
Hookworms, like most worms, do not communicate with one another aside from using sensory signals to locate the compatible sex for mating.
The defining trait of hookworms is their parasitic lifestyle. They cannot survive without a host, as they are highly specialized to feed on blood. However, their activity can have detrimental effects on the host if their population grows too large.
Symptoms of hookworm infection, which is considered a type of helminthiasis, become more severe if the population of worms is great in size. Common symptoms include skin rashes, abdominal distress, and diarrhea. These eventually lead to weight loss and chronic tiredness. Blood loss in the host, especially anemic hosts, can be life-threatening in extreme cases.
Hookworms spread via environmental contamination. The most common route is through feces; the eggs are passed out of the body this way, and hatch in the soil. Larvae may infect smaller animals in the environment as intermediate hosts or directly infect the definitive host. If an intermediate host is infected, the worms may pass to the definitive host if the intermediate is eaten; because of this, carnivorous animals are more likely to become infected by hookworms.
Hookworm infection is most common in poor communities where health and sanitation services are inaccessible, especially in countries affected by colonialism. Therefore, hookworms are often seen as coming hand-in-hand with poverty; communities where animal handling is commonplace are especially susceptible. Like most endoparasites, depictions of hookworms in culture focuses mainly on the symptoms of disease rather than the worms themselves.
In the United States, parasitic worms such as these are most common in the South. Warm conditions mean that impoverished communities with poor healthcare are more likely to become infected; for generations, widespread hookworm infections in the South have caused chronic fatigue to be common. This possibly influenced the stereotype of the “lazy Southerner.” This stereotype has become less common over time, but this is not due to people understanding the effects of helminthiasis; rather, it is due to the political changes occurring in the United States.
Technically, a large number of hookworms already exist in captivity, though their “handlers” are largely unaware. The species of hookworm that most commonly infect humans are Necator americanus and Ancylostoma duodenale. Hundreds of millions of humans are still infected with these worms every year, mostly in tropical regions where medical care is unavailable and sanitation is poorly funded. In more northern climates and other colder parts of the world, fewer humans are infected as the environment is not suited to hookworms; also, northern regions tend to have better access to the money necessary to treat hookworm infections.
Keeping these worms on purpose is more difficult and quite uncommon. The larvae are easier to keep than the parasitic adults, though some minor amounts of success can be had getting adults to attach to fresh meat rather than live hosts. This is only a temporary solution, since they need active blood flow to feed and survive. Adult hookworms are more commonly harvested from the bodies of animals that were infected, and kept for study until they expire.
Hookworms are among the species most commonly studied by parasitologists. Their life cycle and feeding habits are now well understood and they make useful teaching and research models. Most study in a professional capacity focuses on how to reduce worm populations and guard against infection, which has resulted in a number of effective preventative measures people can use to keep themselves healthy and protected against hookworms.
Medicine is invariably intertwined with politics, especially where pharmaceutical business is concerned, and disease has a strong impact on economic success as well. Hookworms, as described above, have run rampant in the rural American South and caused widespread damage to the region’s economy for generations by making people too exhausted to work effectively. For some time, Southerners were stereotyped as lazy, since most people did not yet understand the impact that hookworm parasites were having on the South.
As with all diseases, helminthiasis is caught in a political tug-of-war in many countries. On the one hand, treating it and eliminating hookworm parasites would save millions of dollars every year by relieving stress and fatigue from the working population. On the other hand, the pharmaceutical industry makes a massive profit off of medicines used to treat the disease. However, the people who need medicine most are unable to afford it. Some countries have already made the decision to favor public health over corporate profit. Time will tell whether the rest follow suit.
Hookworms are a source of economic and medical hardship and provide few to no beneficial services to humans. They can infect people as well as domestic animals, causing losses to agricultural productivity, the pet trade, and workplace efficiency where they are common. The only benefit they can be said to bring is to the pharmaceutical industry, which profits from creating medical treatments for the worms. However, the communities most grievously afflicted by hookworms usually cannot afford treatment.
Along with infecting livestock and pets, hookworms could infect de-extinct life. In many cases, these animals brought back from extinction would not have any natural defenses against hookworms from the present day, and so would be more vulnerable to them. Jurassic World’s paleoveterinarians would regularly check each animal in the park, with even a single instance of hookworm infection as cause for concern; a mass treatment of every animal in the habitat would be the standard response, followed by a sterilization of the affected environment. Preventative maintenance was the best method to use against this parasite. Hookworms were among the common infections the Dinosaur Protection Group was prepared to treat, following the animals’ planned relocation to a new suitable region.
You are most likely to become infected with hookworms in an area with poor public sanitation, often from lack of monetary resources or infrastructure. If you live in such an area, or are visiting one, ensure that you carry clean drinking water with you wherever you go; hookworm larvae are transmitted through feces. When fecal matter is emptied into a water source, that water can become contaminated with parasites. Personal water filtration devices are available in some parts of the world: if you cannot reliably obtain bottled water, it may be pertinent to carry such a device.
Food is another way you may become infected with hookworms. In the middle stages of their life cycle, they live in hosts such as pigs. When these intermediate hosts are eaten by the definitive host, the definitive host becomes infected, and it is here that the worms mature and breed. Only eat food that has been thoroughly cooked, since high temperature will kill any parasites in the meat.
Some hookworms take a more direct route to infect their primary host. The filariform larvae of some species, such as the common Necator americanus, penetrate the skin of the host and migrate to the intestines. They most often enter through the skin of the feet, since they live in soil. Barefoot walkers are more at risk of infection because of this. Appropriate footwear is advisable for many reasons, and hookworms are one of them.
There are several ways to reduce the risk of hookworm infection in an environment. Sanitation is the best option: your community can work together to support sewer infrastructure, separating wastewater from potable water. Always defecate indoors, and do not use human feces as fertilizer. Sanitation of lawns and kennels can be accomplished using sodium borate, which will reduce rates of infection in animals as well. If infection (in humans or other animals) is detected, it can be treated with antihelminthic medicines such as albendazole and mebendazole, with the exact medicine required varying based on the species infecting hosts. A professional should examine the hosts to determine what species is involved, and can then recommend a medication to kill the worms.