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 belonging to the genera Ancylostoma and Uncinaria could potentially have lived on Isla Nublar, where abundant mammal and bird populations (both native and introduced species) would have provided ample hosts. The temperature on Isla Nublar is warm and humid, which are beneficial to the worms as this ensures a higher larval survival rate. Necator americanus may have lived on the island once humans arrived, hundreds or thousands of years ago. Domestic pigs are known to be intermediate hosts for this species; larvae will live in pigs and eventually transmit to humans when the humans eat the pigs. In the early to mid-2010s, domestic pigs were kept on the island for use in Jurassic World, though it does not appear that they were being raised for human consumption. Worms that affected these pigs would probably have been unsuccessful, failing to reach their definitive hosts.
Other domestic mammals such as goats and cattle, invasive species such as brown rats, and wild mammals such as mantled howler monkeys and collared peccaries could also have hosted hookworms. Birds, too, can be affected by nematode parasites; 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 Isla Nublar, 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.
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 constrictor, and could have housed many hookworms each.
Between 1988 and 2018, dinosaurs and pterosaurs inhabited Isla Nublar, introduced and/or cloned on-site by International Genetic Technologies following their loaning the island from Costa Rica. Dinosaurs are a group of advanced warm-blooded reptiles that include the birds, and thus, it is likely that they could be infected by hookworms that are found in naturally-extant birds and other reptiles. Pterosaur species are also warm-blooded reptiles, though not as closely related to birds. Most of these dinosaur species were considerably larger than indigenous reptiles and birds; this larger size would have ensured that many more hookworms could have inhabited the intestinal tracts of the dinosaurs. Between 1993 and 2002, the island was essentially abandoned by InGen, leaving natural forces to control the island’s environment. Hookworm populations would have increased, though the island’s small size prevented excess growth of the dinosaur population. While the carnivores would have been affected the most by adult hookworms, larvae and some adults would have affected some of the herbivorous dinosaurs as well.
In 2002, InGen returned to the island to finish the Jurassic Park project, now under the moniker Jurassic World, as a subsidiary of Masrani Global Corporation. Many animals were shipped back and forth between Isla Nublar and Isla Sorna during construction, allowing disease to spread; between 2004 and 2005, most or all of Isla Sorna’s dinosaurs were shipped to Isla Nublar permanently, many of which were likely infected with hookworms. Disease in prehistoric life was now better understood, and further research would give rise to the field of paleoveterinary science. Hookworm populations likely dropped during this time period due to the use of antihelminthic medicine and environmental cleaning by Jurassic World’s paleoveterinarians and maintenance staff. Following the 2015 incident which resulted in Jurassic World permanently closing, most of the animals were permitted to roam the island as they wished, but paleoveterinarians and environmental managers were unable to reach Isla Nublar to provide care. Thus, hookworm infection rates likely spiked between 2015 and 2018 as the de-extinct animals intermingled and lived in non-sterile environments.
On June 23, 2018, the island’s volcano Mount Sibo violently erupted, burning much of the island and polluting its air and water sources. Even prior to this, increasing volcanic activity leading to the eruption as well as the island’s overpopulated environment were leading to high dinosaur death rates and likely reduced numbers of hookworms. Following the eruption, hookworms would have suffered from ecosystem loss; much of the soil in the northern island was covered by lava or cleared by a pyroclastic cloud, while the island’s fauna was drastically reduced. Migratory birds such as the brown pelican and survivor species such as rats would have been the only likely hosts following the eruption.
Much like Isla Nublar, Isla Sorna is a humid and warm tropical island where hookworms would be able to thrive. It has large native populations of birds, reptiles, and amphibians, though its more isolated nature would mean a smaller population of mammals than Isla Nublar. Host animals here could have included milk snakes, at least seventeen other snake species, three-toed sloths, opossums, monkeys, strawberry poison dart frogs, iguanas, and many other birds and reptiles.
Also like Isla Nublar, InGen cloned dinosaurs and pterosaurs on this island. Cloning took place between 1986 and 1993, after which the island was abandoned due to financial difficulties following the failure of Jurassic Park. A much larger dinosaur population survived here until the early 2000s, after which illegal cloning activities hastened the collapse of the de-extinct ecosystem. Hookworm populations would have dramatically risen between 1993 and 2001, after which the de-extinct animal populations began to fall. By 2004, the populations were falling rapidly and likely taking the hookworm populations with them. In late 2004 and continuing until Jurassic World’s opening date in 2005, the dinosaur population continued to fall; it was supplemented only barely by dinosaurs being shipped to the island from Isla Nublar during Jurassic World’s construction. By the time Jurassic World opened on June 1, 2005, most or all of Isla Sorna’s de-extinct life had been relocated to Isla Nublar, where they would have received treatment for hookworm infections. Thus, the hookworm population of Isla Sorna may have returned by now to its pre-InGen levels, assuming that the island has not been interfered with since 2005.
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.
In particular, the game Jurassic World: Evolution portrays hookworms as thriving on every island in the Gulf of Fernandez.
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 opposite 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.
Relationship to Humans
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.
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.