Necator americanus (NA) is a member of the phylum Nematoda and primarily found naturally in tropical and temperate regions. Commonly known as the New World hookworm, it is one of only three hookworms that are adapted to living in humans, and the only one used in helminthic therapy. (The other two hookworms are the Old World hookworm, Ancylostoma duodenale, which is abundant throughout the world, and Ancylostoma ceylanicum which is also adapted to cats and found predominantly in Southeast Asia and the Pacific.  )
Dosing with hookworms
See the following page.
Hookworm larvae storage and survival
Ideally, L3 NA larvae should be used as soon as possible after cultivation because, at this stage in their development, the larvae do not feed and therefore rely on their fat stores for nourishment, and they become increasingly weaker as these stores are used up.
Home-grown larvae have been known to survive for as long as 3-5 months if kept in ideal conditions, i.e., out of light, at a temperature of 60-65°F (15-18°C), in a minimal amount of distilled, demineralised or dechlorinated water.   (Researchers working with NA larvae have reported that the pressure from too large a volume of water will shorten their life.) Survival time is also partly determined by the temperature at which the larvae were incubated, with those grown at a lower temperature having greater longevity.
Larvae purchased from the helminth providers may not last as long as home-grown larvae because the providers put them through a cleaning process which reduces their longevity. Variations between the cleaning protocols used by different providers result in differing lengths of shelf life. For example, one hookworm provider (Wormswell [2014-2018]) said that their larvae had a shelf life of 3 to 4 weeks after rinsing, whereas larvae purchased from a different provider (Worm Therapy) only have a stated shelf life of 2 weeks.
Purchased hookworm larvae should therefore be used as soon as possible after receipt. If there is an unavoidable delay in inoculating with the larvae, the provider should be consulted, and, in the meantime, the larvae should be stored in their original packaging at room temperature and out of direct sunlight.
Worm growers have reported a 50% loss of larvae at 50ºF (10ºC) and a 90% loss if the temperature drops to 43ºF (6ºC) for several hours. Hookworm larvae are also susceptible to drying and heat, for example from direct sunlight.  (PDF) Temperatures above 113ºF (45ºC) will kill them, and they may only last for a week or two at 90ºF (32ºC). Personal experiences include the following.
If L3 larvae encounter extreme temperatures during transit, they may be dead on arrival. (See Losses during shipping.) Sitting in an outdoor mailbox in sub-zero temperatures will also be fatal for the larvae, so delivery should be arranged to an address where the package can be received indoors.
NA larvae are applied to the skin on a bandage/dressing and should be used as soon as they are available.
They can survive for as long as 3-5 months if kept at a favourable temperature (70ºF/21ºC), although their longevity is also dependent on the temperature at which they were incubated, with larvae grown at a lower temperature surviving for longer. Since they do not feed at this stage in their development, the larvae are reliant on their fat stores, which obviously diminish over time, rendering them progressively weaker. For more detail, see Hookworm larvae storage and survival, above.
After being pipetted onto a bandage/dressing, the hookworm larvae should be applied to a hairless, convex area of skin to ensure good skin contact, as demonstrated in the following instructional video.
There can be stragglers trapped in water droplets in the vial after the contents have been pipetted onto a bandage/dressing, so at least one rinse should be carried out, and some providers supply a vial of distilled water for this purpose. Even a second rinse can sometimes capture the odd straggler. 
If inoculating with home-grown larvae, use a bandage/dressing with a flat central pad in preference to the quilted type. A Band-Aid dressing with a ⚠️"Quilt-Aid Comfort Pad" has been reported to have prevented successful inoculation in one case. 
If a ready-made bandage/dressing is not available, an effective substitute can be made by using a makeup pad, or by cutting the required size from a large lint pad and securing this to the skin with medical tape. . Alternatively, a piece of toilet paper, or flat paper towel, stuck to a piece of masking/painter's tape will work.  
Some self-treaters have found that placing the larvae on multiple bandages/dressings greatly reduces the itch and rash that often develop following inoculation using a single bandage/dressing.
Some hookworm suppliers have experimented with pre-loading the larvae onto a small damp cotton patch, which the user then placed onto a larger bandage/dressing after receipt, before applying this to their skin. This method was eventually abandoned after if proved to be less successful than the traditional method requiring the self-treater to pipette the larvae from an eppendorf tube onto a bandage/dressing.
After the bandage/dressing has been applied, the larvae will detect the presence of skin   and, responding in particular to warmth,  will swing into action, often producing an itch within just a few minutes. Occasionally, the itch may be delayed for up to several hours,  or even until the next day. 
The bandage/dressing should be left in place for a minimum of four hours and, ideally, for twelve hours. If you need to take a shower before the bandage/dressing is due to come off, cut a piece from a plastic bag and tape this over the area to keep it dry.  Duct tape or electrical insulation tape are ideal for this purpose because they are waterproof.
When the bandage is removed, this may reveal the beginnings of a rash. For more about this and how to treat the itch, see the following page.
Best time of day to inoculate with NA
Different self-treaters prefer to inoculate at different times during the day.
Body sites used for hookworm inoculation
The inside of the forearm or bicep is a good choice because it is convenient to access in order to treat the rash with creams or apply hot air from a hair dryer, but inoculation too close to the armpit can cause lymph nodes there to swell. Inoculation high on the inner thigh - and even occasionally on the outer thigh  - can cause painfully raised lymph nodes in the groin, which can be uncomfortable when walking. 
Many hookworm hosts have found that repeatedly using the same site results in a more robust skin reaction, and that varying the site effectively reduces both the rash and the itching.
Sites that have been used include the following, starting with the feet and moving up to the shoulder.
- NB. The underside of the foot may not be a good place for a first inoculation because the skin response is often reduced at this site, increasing the possibility of getting no noticeable response at all, which may lead a new hookworm host to assume that the dose was dead, when it might not have been. This may result in them ordering a replacement, and risking more severe side effects from what would then have become a double dose. The arm is therefore arguably a better choice for a first dose, and the foot can be used once the individual has more experience with the process of inoculation. 
One site that should be avoided is the thumb!
Oral inoculation with NA is not effective
There are several reasons why it is not recommended to swallow NA larvae.
1. While oral inoculation has been shown to be effective with some hookworm species, such as the dog hookworm, Ancylostoma caninum,  and the human hookworm, Ancylostoma duodenale,  oral inoculation with Necator americanus has been found to be ineffective.
2. L3 hookworm larvae are particularly susceptible to acid, so there must be some developmental change that occurs during the few days that they normally take to travel from the skin to the stomach which enables them to survive the harsh, acidic gastric environment. Hotez and colleagues have reported that, following entry into the host via the skin, L3 larvae receive a signal present in mammalian blood and tissue that causes them to resume development and secrete bioactive polypeptides.   Larvae that are swallowed will obviously not experience this trigger.
3. Hookworms have probably taken the same route throughout most of their long co-evolution with mankind, so they are likely to try to follow the same pathway irrespective of where they actually enter the body. Therefore, unless they are encapsulated, larvae that are swallowed may still be looking for skin, and might mistake the lining of the mouth or throat for external skin and enter via these surfaces. Nagahana, et al., have reported that N. americanus L3 larvae will invade the buccal epithelium if they enter through the mouth. (Nagahana M, et al. Experimental studies on the oral infection of Necator americanus. III. Experimental infection of three cases of human beings with Necator americanus larvae through the mucous membrane of the mouth. Japanese Journal of Parasitology. 1963;12:162–167.) Since some people experience significant swelling at the inoculation site, there is a potential for blockage of the throat if larvae were swallowed.
4. Oral inoculation does not provide the visible confirmation of dose viability that is common with successful percutaneous inoculation. So, unless the worms have been cultured by the person inoculating, or they have a microscope with which to check the viability of doses received from other sources, they might unknowingly swallow a dose of dead worms, which could delay the progress of their treatment.
Possible side effects after inoculation with NA
See the following pages.
Checking whether inoculation was successful
The success of inoculation can be confirmed in two different ways.
1. A raised IgE level.
If you had a full blood count (CBC) and/or IgE levels taken just before inoculation, you could retest a couple of weeks after the inoculation. If there has been an increase in eosinophils, this may suggest that colonisation has been successful, although there are other factors that can influence eosinophil levels.
2. The presence of hookworm eggs.
The presence of hookworm eggs provides definitive confirmation that there are mature hookworms in the gut, and egg production begins between 4 and 6 weeks after inoculation. Eggs can then be detected in either one of two ways.
- a) by having a stool sample professionally tested by a laboratory (see Stool testing)
- b) by incubating the individual’s stool and checking for hookworm larvae (see Helminth incubation)
Hookworm side effects
See the following page.
For what to expect in the days, weeks and months after inoculating with hookworms, see the following page.
Source: US Centers for Disease Control 
The developmental stages, migration and diet of Necator americanus
Egg - Hookworms start their life as unembryonated eggs in faeces deposited in soil, and initially gain nutrients from the faeces. When there is adequate warmth, shade and moisture, the eggs embryonate and, within 1-2 days, hatch into first stage L1 larvae.
L1 - In this first, non-infective, juvenile rhabditiform (free-living) stage, the larvae feed on bacteria living in the feces in which they were deposited  and on bacteria and organic debris in the soil until they molt, after approximately 3 days, into second stage larvae, L2s.
L2 - During this second rhabditoform (free-living) stage, the L2 larvae will feed for 6 or 7 days and then molt again into third stage larvae, or L3s.
L3 - In this infective, filariform stage, characterized by a closed mouth, the L3 larva does not feed in its natural environment and will only survive for a few weeks until it exhausts its lipid metabolic reserves, or it finds a host, at which point it will commence feeding on protein in the bloodstream.  . The lifespan of L3 larvae supplied by commercial helminth providers may be further shortened by their exposure to the antibacterial rinse used by these companies to clean them.
Once the L3 larvae have entered the skin, they remain inactive within the skin for about 40 hours before beginning to move into the cutaneous blood vessels and travel via the circulatory system and heart to the lungs, where there's another delay while further growth takes place in the alveoli.  The literature claims that the larvae reach the lungs at around 4-7 days. However, the subjective experience of NA self-treaters suggests an earlier migration through the lungs, with the L3s already ascending the bronchioles to the trachea - during which part of the journey they molt into the L4 stage - before day 4.
Serum and gluthatione provide the signals for L3 larvae to continue their development to the L4 adult stage.
In experimental conditions, exposure to serum has been shown to stimulate about 50% of hookworm L3s to feed and undergo development, and this response can be increased to 90-100% by adding glutathione to the serum.  Starting this process in a petri dish in larvae that are to be used in therapy would make infection impossible, but the serum experiment might indicate that L3 larvae are in fact able to feed once they enter the bloodstream, which might explain successful inoculations with old batches. 
L4 - The L4 larvae transfer from the trachea to the oesophagus by slipping under the epiglottis. They are not necessarily coughed up into the mouth, as is sometimes claimed, unless they have become attached to mucus which is then coughed up. Once in the oesophagus, the larvae continue down to the intestines, which they reach, according to the literature, between days 8 and 14, but possibly much earlier, and perhaps even by days 4 to 7. They then molt into the pre-adult stage around days 17–21 and commence feeding on blood drawn from the intestinal mucosa. Any stragglers should all reach the intestine within 4 weeks.
Contrary to the impression created in many scientific texts, the amount of blood drawn by Necator americanus is minuscule. When discussing the amount of blood lost to hookworms, researchers will often have in mind the Old World hookworm, Ancylostoma duodenale, which draws 9 times more blood than Necator americanus.
L5 - At about 5-9 weeks, the larvae attain their adult (L5) form, fertilisation occurs, and the females begin producing eggs (5,000-10,000 per day per NA female) which are passed in the host's feces.  After 24–48 hours under favorable conditions, the eggs become embryonated and hatch. 
Note. Many of the details above were recorded from observations in a hamster model. 
Once it has reached the intestine, Necator never migrates to other parts of the body, so remains within the gut for the rest of its life, the length of which varies widely depending on the immune response of the individual host. (See Hookworm lifespan.)
For more detail about the developmental stages and migration of NA in humans, see Immune responses following experimental human hookworm infection.
Where hookworms live
According to Wikipedia, mature Necator americanus live at the distal (lower) end of the jejunum and the proximal (upper) end of the ileum, while the other species of human hookworm, Ancylostoma duodenale - which is not currently used in therapy - resides in the duodenum/jejunum. 
Croese, et al, used capsule endoscopy to determine that the distribution of NA was influenced by the parasite’s maturity, and that, early in infestation with NA, the worms were distributed along the length of the jejunum but that, by 20 weeks, the predominant location of surviving worms was the proximal jejunum - the upper end of the jejunum, just below the duodenum.  It could be argued that this latter observation is more likely to be correct than the opinion expressed in Wikipedia and, if it is, mature NA are most likely to be found where the red text appears in the following representation.
Rarely, NA have been observed in the stomach. This exceptional localisation might be caused by one of three factors:
1. the removal of the jejunum and/or duodenum
2. jejuno-duodeno-gastric reflux 
3. an excessively large hookworm colony that has forced some of its members to spread out from their usual localisation site. 
A short video clip of a hookworm in situ.
While it is theoretically possible that NA might be seen during a colonoscopy, this would only happen if the colonoscope were advanced into the lower end of the ileum, and only if some hookworms had taken up residence there, which is unlikely unless the jejunum has been removed or very large numbers of NA - i.e., hundreds - are being hosted, forcing some NA to occupy the ileum. It is very unlikely that any would take up residence in the colon, which they normally only pass through after they die.
Hookworms are not adversely affected by the laxatives used in preparation for a colonoscopy. While it is possible to lose hookworms to very severe diarrhoea/diarrhea, they can withstand the effects of normal, recommended quantities of laxative, including the standard colonoscopy prep.
An upper endoscopy (i.e. via the mouth) will not reach past the second of the four parts of the duodenum, so will not usually reveal, or disturb, any hookworms unless these have been forced to spread beyond their usual location as a result of one of the three factors mentioned above.
Caring for hookworms
See the following page.
Confirming hookworm infection
There are three practical options for determining the presence of hookworms in the gut.
1. Stool test
From about six weeks post inoculation with hookworms, eggs will be detectable in the host's stool. Checking a stool sample for these eggs is the best way to establish whether hookworms are present. Unfortunately, while most pathology labs will have a faecal (stool) test called something like "Ova, Cysts, and Parasites" or "Ova and Parasite", they typically do not have sufficient experience to accurately identify hookworm eggs, a problem that is exacerbated by the relatively low numbers of worms used in helminthic therapy. Testing is therefore best carried out by a helminth provider who offers a stool testing service, a laboratory associated with a school of tropical medicine, a veterinarian (who will have experience with helminth eggs), or at home using a microscope and fecalyzer. For more details about these options, see the following page.
Some hookworm hosts find it easier to check for the presence of worms by incubating a stool sample rather than attempting to count the eggs via a fecal float.
3. Blood test for eosinophils
Another way to determine whether or not you are hosting helminths is to have a blood draw to see if your eosinophil level is elevated. Although this is not a foolproof test for the presence of helminths, it is fairly reliable. It is also quick, and is a method that a medical insurer might pay for.
Once inside a host, NA are reported to survive for 3-10 years   but to be capable of living for up to 15 years,  and possibly even 18 years.  However, the experience of hookworm self-treaters suggests that they typically only survive for approximately 1-3 years, and sometimes for as little as 2-3 months, depending on the strength of the individual host’s immune response. For more detail about this, see the following page section.
Hookworms, like other nematodes, have digestive, nervous and reproductive systems but no circulatory or respiratory system. They pick up oxygen and give off carbon dioxide via the surface of their bodies using diffusion, which occurs whether they are in water or air. They are also able to extract oxygen from their host's blood. 
Is there genetic degradation in laboratory-reared hookworms?
Concern is sometimes expressed about the possibility that the hookworm larvae being sold by the commercial helminth providers may have become weakened as a result of genetic degradation in the domesticated hookworm stock. However, this is extremely unlikely.
Genetic degradation is not an automatic sequel to the restriction of a colony of microorganisms. Some insect species are able to withstand the effects on their gene pool of high levels of inbreeding, and still produce healthy offspring, a feature also seen in cockroaches and bed bugs.  And helminths have a particularly limited capacity for genetic alteration due to their size, and their prolonged life cycles and generation times when compared with smaller, simpler microbes, or ‘microparasites’.  In fact a study comparing mitochondrial cytochrome oxidase 1 DNA sequences from NA that had been maintained for 100 generations in laboratory-reared golden hamsters with those from natural human infections, concluded as follows.
There is also no evidence of any diminution of therapeutic effect in the NA obtained from the helminth providers, confirming that the possibility of genetic degradation is nothing that today’s NA self-treaters need be concerned about. Also see: Comparing NA larvae from different sources.
Any attempt to obtain new stock from the wild would introduce a significant level of risk for anyone who was to host worms obtained in this way. Walking barefoot in open-air latrines is not a good idea, as is explained here by someone who did this. There is a risk of inadvertently acquiring a different type of helminth, such as the less desirable species of hookworm, Ancylostoma duodenale, which causes nine times more blood loss than NA, can be passed in a mother’s milk and can even cross the placenta to infect a foetus. Even more risky is the roundworm, Strongyloides stercoralis, which is autoinfective and potentially hyperinfective, with a risk of fatality.  
Since the eggs of both these species are virtually identical to those of NA, anyone attempting to obtain “wild” NA would need a considerable level of expertise in species identification in order to be certain that they were not harbouring something much less desirable. They would also need to undertake multiple terminations of their hookworm colony, followed by re-inoculation with individually selected larvae that had been definitively determined to be NA by means of genetic testing.
Can hookworms cause a positive result on fecal occult blood tests?
Some hosts of NA have expressed concern that the minuscule amount of blood shed from hookworm feeding sites might cause a positive result on a faecal occult blood test. However, this is thought to be unlikely, especially given the poor sensitivity and specificity of stool guaiac tests.
Is hosting NA contraindicated for someone with a gastric bypass?
The hosting of hookworms does not present any risk to someone with a gastric bypass, and the surgery does not interfere with the hookworms’ ability to colonise and produce therapeutic effects.
Although gastric bypass surgeries create a detour around, or reduce the volume of, the stomach, food is still able to pass through the reconstructed section of the intestine, and, at the point at which hookworm larvae travel through this section, they are still microscopic, so pass through easily. They then continue along the intestine and typically settle in the first part of the jejunum, which is some distance from the stomach. See Where hookworms live.
See the following page.
- Personal stories from users of NA and other helminth species. Use the search function on your device (Control+F on a PC, or Command+F on a Mac) to search the page for "hookworm", "HW" and "NA".