Considerations when choosing a therapeutic helminth
Where the different therapeutic helminth species live in humans
The four species are most likely to be found where the red text appears in the following representations of the human digestive tract.
NA (See here for further details.)
HDC (See here for further details.)
TSO (See here for further details.)
TTO (See here for further details.)
Species that have shown benefits in common medical conditions
There is no single "best" worm or worm combination for all conditions, but some worms do have a better track record with certain diseases. 
The following details, based on anecdotal reports, offer a rough guide to which species might be best to try first in some of the most common medical conditions.
Further insights to help with species selection can be found in the following resources.
Even with the benefit of the details presented above, personal experimentation may still be required in order to establish exactly which worm, or worm combination, will be best in the longterm for each self-treater.
The further details presented below should provide additional help in selecting a worm to try first.
The scientific evidence
There have been no head-to-head clinical trials comparing any of the currently available therapeutic helminths, and there may never be any, due to the extremely high cost of mounting such studies and the lack of incentive for anyone to fund them. There is also little scientific evidence for the efficacy of the individual helminths used in therapy, although almost all species of intestinal worm appear to be capable of producing beneficial effects.
In the groundbreaking Correale and Farez study of 2007 in patients with multiple sclerosis, (Abstract | PDF) all the helminth-infected subjects benefited, even though they had 5 different species of helminth between them. Therefore the type of worm used may not make a great deal of difference to the eventual outcome of helminthic therapy.
The only organism to be studied in the treatment of autoimmune disease in well designed trials at therapeutic doses is TSO, and this has been shown to be safe and effective in ulcerative colitis,  (PDF) promising in the management of Crohn's disease  as well as in the treatment of relapsing-remitting multiple sclerosis (RRMS)  and also in dampening allergic airway hyper-reactivity.  TSO has also shown significant results in 70% of subjects with the inflammatory subtype of autism spectrum disorder (I-ASD) who took TSO in its original pH2.7 form. 
Several other trials of TSO were conducted by the now defunct company, Coronado Biosciences, prior to the termination of its research programme early in 2015 following what appeared to be disappointing results. However, the apparent failure of these trials was likely due to poor trial design, including an inadequate duration of only 12 weeks and, in particular, the insistence by researchers on the use of a novel formulation of TSO with a higher pH than the product that had been employed successfully in previous studies and used to good effect by many thousands of self-treaters for more than a decade. ,  For more details about the later, "unsuccessful" TSO trials, see Problems with clinical trials using live helminths.
The first trial of the hookworm, NA, to be carried out over a sufficient period of time to allow the worms to mature and produce consistent therapeutic effects was completed in early 2016 at Nottingham University in subjects with relapsing remitting multiple sclerosis (RRMS). Although the particular statistical endpoint determined for this trial was not reached,  the data reveal that more than half the patients given NA had not developed any new lesions. 
There have been no clinical trials in humans using the other available therapeutic organisms, TTO and HDC, although there was a detailed case study of an individual with ulcerative colitis who experienced almost complete remission of his symptoms after self-infecting with TTO,  and a further case study showing several indicators of mucosal and systemic immune modulation following self infection with TTO.  A study in mice showed that Infection with HDC was superior to daily corticosteroids in the prevention of colitis in this species, and that this did not result in any additional side effects. 
The anecdotal evidence
While data from clinical trials is still limited, the anecdotal evidence gathered by the self-treaters who have pioneered helminthic therapy for more than a decade is already significant and compelling. See Helminthic therapy personal stories and the following paper.
- Overcoming Evolutionary Mismatch by Self-Treatment with Helminths: Current Practices and Experience (PDF)
Apart from this study, there is little recorded anecdotal evidence for TSO and HDC, largely because users of these two organisms, and especially TSO, have been much less active on helminthic therapy social media sites. In contrast, NA has been discussed extensively online since 2007, and, to a lesser extent, TTO, since 2009, originally on the Yahoo Helminthic Therapy Forum and, more recently, in the Facebook Helminthic Therapy Support Group.
NA (Necator americanus)
NA, which has the longest lifespan of the currently available therapeutic helminths, produces powerful systemic immunomodulation and it’s use appears to be effective in approximately 70-80% of cases of autoimmune disease, inflammation and allergy.    The extent of its benefits can be seen in the hundreds of reports in the compilation of Helminthic therapy personal stories, most of which are from users of NA.
This organism causes a short-lived, but often itchy, rash at the inoculation site in most subjects, and may produce a very pleasant, if brief, “bounce” in the first few weeks following inoculation, as well as causing other well documented transient side effects during the first few months. (See The hookworm timeline) However, it does not cause any long-term ill effects.
Concern is sometimes expressed about the safety of self-infection with NA, and it has been claimed that this species can act as a vector for pathogens and mis-migrate to organs outside the digestive tract, but neither of these claims is true. Nor is anaemia/anemia an issue for the vast majority of well-nourished NA hosts, and the “tissue damage” caused by hookworms needs to be seen in context. For more detail about all these issues, see The safety of NA.
For information about the use of NA, see the following pages.
TTO (Trichuris trichiura ova)
The colon-dwelling Trichuris trichiura, which has an effect that is less systemic than that of NA and more localised in the colon, is most frequently employed in the treatment of ulcerative colitis and colonic Crohn’s disease. Its effect on these disorders may be related to its ability to reduce the inflammatory Bacteroides species which are thought to be involved in the aetiology of inflammatory bowel disease. 
Some people with colonic diseases seem to do well using TTO alone.
But many others have found it best to combine TTO with NA.
Some people with colonic diseases have had more success using NA alone.
Some people who have tried both TTO and TSO have found the human species to be more effective.
Although TT is used primarily in the treatment of diseases affecting the colon, and its systemic effect is less than that of NA, its immune modulation can nevertheless produce benefits beyond the intestine,  so may assist in the remediation of diseases elsewhere in the body, in a way similar to how its close relative, TS (Trichuris suis, which also lives in the colon) is able to reduce the number of brain lesions in MS patients. 
Since TT produces significant changes in the composition and relative abundance of gut bacterial species,  it is possible that it may also help to ameliorate any condition that is driven by intestinal dysbiosis, such as obesity, diabetes (type I and type II), cancer and neurological disorders.
For information about the use of TTO, see the following page.
TSO (Trichuris suis ova)
TSO has a localised effect in the colon  similar to that of TTO, so is particularly effective in the treatment of diseases affecting this section of the intestine, such as ulcerative colitis. It has also proved to be effective against autoimmune disorders affecting other parts of the body, especially Lupus and Crohn’s disease  but may not be as effective against MS as NA.  TSO can also be effective in autism,  perhaps more so than any of the other worms,  although the extent of its effect is dependent on the sub-type of autism exhibited,  and NA is reportedly helpful in addressing some of the medical co-morbidities that accompany autism. 
TSO has been reported to efficiently dampen allergic airway hyper-reactivity  and has been claimed to be very effective in the treatment of food and contact allergies,  although less effective than NA against seasonal allergies. One user has reported that she found TSO was a huge help with joint and hip pain caused by eating nightshades, especially chilies and peppers. However, while NA had allowed her to resume eating nightshades, it had not helped with aching and stiffness in her joints. These symptoms only resolved after she added TSO. 
TSO does not work as quickly when taken by someone who has previously used NA, so anyone who is unsure which helminth species to use should ideally start with TSO, then add, or switch to, NA, if necessary, at a later date. 
Approximately 80% of people who take at least one full course of TSO (10 doses of 2,500 ova, with one dose being taken every 2 weeks) may achieve disease remission and, according to the producer of TSO, around 60% of these subjects will continue to experience remission for between 1 and 3 years. This was demonstrated by the experience of one TSO user.
During this time, some users of TSO may not need any further doses at all, while others might need to take a periodic maintenance dose, for example 500 TSO every month,  or every 2 months.  This has allowed some subjects to maintain remission for many years, although the level of success is likely to depend on the severity of a subject’s condition at the start of treatment. One individual with ulcerative colitis has reported being in remission for 9 years after taking a single course of TSO during a clinical trial.
There are, however, some users of TSO who do not continue to experience benefit after completing a course of 10 doses.
These individuals may need to maintain the treatment long-term.
For information about the use of TSO, see the following page.
HDC (Hymenolepis diminuta cysticercoids)
While anecdotal evidence about this latest addition to the list of therapeutic organisms is still limited in comparison with that available for NA and TT, posts to the helminthic therapy groups indicate that the HDC experience follows a similar course to that seen with other helminths, beginning with the possibility of initial transient side effects and leading to eventual success in many cases.
As with the other therapeutic helminths, HDC can cause initial side effects, some of which have been found to be quite challenging by a few users.
And it has been reported that approximately 1% of paediatric patients may experience severe gastrointestinal pains with the use of HDC, although these symptoms are resolved by the administration of an anthelmintic drug. 
A number of successes have been reported by HDC users, e.g.,
as well as some failures, e.g.,
There are reports from subjects who have previously used NA that the effects of HDC can be more gentle and subtle than those produced by hookworms.
but others have obtained more relief after adding, or switching to, NA.
One helminthic therapist has reported that, in her experience, HDC is the most calming on the gut of all the helminths, citing someone with chronic diarrhoea who had excellent relief from HDC. 
HDC users frequently report a welcome elevation in mood, especially in the first few days after inoculation.
However, HDC can occasionally have an adverse effect on mood.
As larger numbers have begun using HDCs, more reports of adverse side effects have emerged. These have revealed transient problems very similar to those seen with hookworms, but have also included reports of a need for larger doses once the “honeymoon period” of the first 6 months is over, and continuing into the long term.
The fact that a fresh dose of HDC needs to be taken every two weeks, coupled with their very short shelf life, means that maintaining a regular dosing schedule can be an issue with this species.
More examples of users’ experience with HDC can be found in The HDC experience.
In June 2016, a researcher at Duke University commented that William Parker’s team had been unable to determine much difference between the effect of HDC and TSO. 
For information about the use of HDC, see the following pages.
Comparing human helminths with non-human helminths
NA and TT have both become very well adapted to living in humans during millions of years of cohabitation and coevolution with mankind. This longstanding relationship allows them to grow to maturity in the human gut and enter into an ongoing, dynamic, two-way chemical dialogue with their host’s immune system. The resulting immune modulation prevents the worms from being overwhelmed, and provides their host with benefits that are maintained for as long as the worms survive.
The survival of human helminths depends to a large extent on the vigour of the host’s immune response. NA is reported to survive for 3-10 years   but to be capable of living for up to 15 years,  and possibly even 18 years.  However, in a few hosts, it may die in as little as three months. (For more about this, see Hookworm lifespan.) TT can survive for more than 2 years, and occasionally in excess of 5 years.  There has even been one report of TT showing up during colonoscopy 7 years after inoculation, but they are more likely to die in under a year. 
TSO and HDC are both adapted to non-human species, the natural host of the former being the pig, and the latter normally being found in rats. As the human gut is an alien environment for both these organisms, they only survive in humans for a short time, typically just a couple of weeks, during which time the molecules they excrete/secrete distract the host’s immune system.
The fact that TSO and HDC rarely mature in humans has led some commentators to speculate that these nonhuman helminths may not be as effective, therapeutically, as the human-adapted worms, NA and TT, and it may be that helminth efficacy is enhanced in the case of mature, egg-producing worms. 
The view that human-adapted helminths such as NA and TT might be more effective, therapeutically, than nonhuman worms like TSO and HDC, was discussed in detail in a support group thread.  This idea has also received support from William Parker, who has referred to human helminths as our true “old friends”, and TSO, HDC and other helminths that prefer non-human hosts as, at best, our "old semi-acquaintances".
People who have switched from using TSO or HDC to either NA or TT have often done so because they are much cheaper. However, many who have made this transition have subsequently commented that they have found the therapeutic benefits of NA and TTO to be greater than those of TSO or HDC.
However, there are others whose experience has led them to prefer a non-human helminth.
All four helminth species have shown themselves to be capable of producing surprisingly rapid results in some cases, although benefits rarely become consistent with the human helminths, NA and TTO, until at least 12 weeks after the first inoculation.
Some self-treaters have reported that the non-human helminths, TSO and HDC, have produced benefits more rapidly than NA and TTO.
While evidence from clinical trials is still lacking, feedback reported by one researcher and a commercial supplier of HDC suggests that a significant proportion of people using this species seem to respond in days or weeks rather than months, and this provider advises clients that, if they have not experienced any benefit in 12-16 weeks, it is unlikely they will see a positive response.
However, HDC and TSO have not always delivered benefits so quickly.
And there are reports of NA delivering rapid benefits in some cases.
HDC. After extraction from beetles, HDC will not remain viable for more than 48 hours if kept in saline, but will survive for 2 to 3 weeks if stored in the special nutrient solution developed for this species by Biome Restoration. Unless such a solution is used, HDC deteriorate rapidly after extraction. Fresh HDC should therefore be used within 24 hours of being harvested - not 24 hours after receipt. For more detail, see Storage and survival of HDC.
NA. NA larvae can survive for as long as 3-5 months if kept at a temperature of 60-65°F (15-18°C),   but, ideally, should be used as soon as they are available. L3 larvae do not feed, so are reliant on their fat stores, which obviously diminish over time, rendering them progressively weaker. For more detail, see Storage and survival of hookworm larvae.
TSO / TTO. Whipworm ova (both TSO and TTO) can remain viable for a couple of years if stored in a refrigerator, so keeping a supply of these in reserve can be a useful insurance for users of NA against the loss of their colony, or an interruption in its benefits following the use of antibiotics. Since TSO usually produces results more rapidly than TTO, this is the ideal species to keep for such an emergency.
The non-human helminths (TSO and HDC) need to be re-dosed every 2 or 3 weeks in most cases. A disadvantage of the need for such regular re-dosing is that self-treaters can forget to take doses, and a few HDC users have reported experiencing a return of symptoms as a result of deviating from the usual fortnightly dosing regimen. These include the mother of a child with Crohn’s disease who reported that her son began to bleed on two occasions as a result of delays in taking his doses of HDC. Others have reported suffering a sudden return of symptoms due to delays in taking HDC.
It is also possible to forget to re-dose with NA and TTO but, since human worms don’t usually all die at the same time, the return of symptoms in their case is normally much more gradual, so users of NA and TTO can forget about dosing until they begin to see a gradual return of the symptoms of their disease, which typically only occurs every few years.
Hosting multiple worm species
It is clear, both from scientific studies and the personal experience of helminthic therapy self-treaters, that a single species of worm is all that is required in most cases to modulate host immunity and halt existing immune-related disorders, as well as protecting the host from the development of many other conditions associated with a helminth deficiency. And it has been shown that the beneficial effect of helminths is not specific to any single helminth species.  However, different species of helminth can produce somewhat different effects.
It is possible that hosting one species of helminth might reduce a host's susceptibility to infection by other parasites,  although no difficulty has been reported by self-treaters in becoming infected by a second, or third, species of therapeutic helminth.
Combining two species
There are several reasons why self-treaters might consider adding a second species.
To speed up the initial response to helminthic therapy
Starting with a non-human helminth species - either TSO or HDC - could help to ensure a more rapid response in preparation for eventual long-term therapy with either NA or TT.
However, introducing two species together when first beginning helminthic therapy can result in greatly increased side effects.
To increase benefits
Exposure to two worm species at the same time - provided that one of these has already been established - may promote even stronger immunological regulation   and this has been confirmed by self-treaters.
Combining two species can provide a greater range of benefits.
When combining two species, it may not be necessary to dose both of them at the same level. For example, if adding TSO to an existing colony of NA, additional benefits may be gained by using only very small doses of TSO. Even a dose of 250 TSO, taken once every 2 weeks, can deliver significant additional benefits to hosts of other species.
Small doses of TSO can be created easily by dividing the contents of a bottle containing a larger dose, as is explained at the following link.
To mitigate unwanted effects caused by a different species
The addition of a second species can sometimes modulate unwanted effects caused by another species. For example, one individual who had experienced increased anxiety after initially inoculating with NA was able to resolve this by adding TSO.
To reduce the attrition of another species
One particular benefit provided by hosting a second species of therapeutic helminth is that this may help to reduce the increased attrition sometimes experienced in cases where subjects have a disease affecting the preferred location of the worm species they are already using. For example, subjects with Crohn’s disease are susceptible to inflammation in the small intestine, which is where NA resides, with the result that this species can have more difficulty attaching in order to feed. Consequently, they tend not to survive in Crohn’s patients for as long as they might do in hosts with other diseases.
It may therefore be advantageous for Crohn’s sufferers to host a second species that lives elsewhere, such as TT or TSO, both of which live in the colon. Similarly, subjects using TT to treat colitis may have difficulty maintaining a whipworm colony due to colonic inflammation hampering attachment by these worms, in which case they might benefit from also hosting NA which contribute systemic inflammation control.
To offset the effects of a sudden loss of human worms
TSO or HDC could be useful to hosts of NA if their colony were to sustain accidental losses, for example following the use of some types of antibiotic or after inadvertently ingesting a food or other substance with anthelmintic effects.
TSO is ideal for this purpose since, unlike HDC, this organism remains viable for years if kept in a refrigerator. A supply of TSO can therefore be kept in reserve by users of NA to provide assurance that they will never be without the benefits of worms.
Keeping a different species of worm in reserve may not be necessary for users of TT, if they are dosing on a regular basis, because they will usually have a supply of TTO in their fridge that they can reach for if required.
To modulate the severity of an NA inoculation rash
One NA host has found that taking a small dose of 250 TSO two hours before inoculating with NA reduces his skin response to the hookworms, perhaps by distracting his immune system from the activity at the inoculation site. For how to divide the contents of a bottle of TSO to create small doses, see the details at the following link.
Two more NA hosts have found that, while regularly taking 250 TSO every 2 weeks, their NA inoculation rashes have resolved much more quickly. They both commented, in particular, on a marked shortening of the oozing stage.
And a fourth self-treater has had success with even a very small dose of TSO.
The ideal species combination varies between individuals
Experience has shown that different individuals can require different combinations of species, so experimentation is required.
Combining two species may not benefit everyone
Switching between two species
If switching from one species to another, there may be a hiatus in benefits until the new species is able to fully take control of immune modulation.
Because of this, it is advisable to maintain the existing dosing regimen for the established species until the new one ceases to cause any side effects.
It would also be advisable to initially limit the dose size of the new species to further minimise the risk of side effects.
Combining three or more species
Adding a third therapeutic helminth has worked well for one self-treater.
There has been very little research into the results of hosting more than two species of helminth at one time, but, according to one study, the outcome of interactions between two parasite species can be significantly modified by the addition of a third, and the consequences of this may not always be desirable. In this study, investigators were looking at the effects of co-infection by four gastrointestinal helminth species on the development of cerebral malaria among Plasmodium falciparum-infected patients, and they found that the addition of a third species led to a reduction in one of the benefits provided by an existing species.
The study found that, while infection by TT and the non-therapeutic helminth, Ascaris lumbricoides, were both associated with direct reductions in cerebral malaria risk, this benefit of TT infection was halved in the presence of hookworms, revealing a strong indirect effect. 
Newcomers to helminthic therapy will often ask in the support group for personal recommendations about the best place to buy helminths, but purchasing worms for therapy needs to be approached on an individual basis.
None of the helminth providers sell all the available species, ship to all global destinations, or accept all payment options. And, while most sell individual doses, with discounts for purchasing multiple doses at one time, a few offer longer term contracts. Therefore, the details for each provider need to be carefully scrutinised before making a decision about which source to use.
- ... weighing potential cost/benefit ratios of various helminths along with other factors, such as feasibility of production, we argue that the four helminths currently in use for CIAD treatments in humans were selected more by happenstance than by design, and that other candidates not yet tested may prove superior.