Chief Scientific Consultant, BioMedical Research and R&D of Health Care, Biofirm Pty Ltd, Australia
Corresponding author details:
Chief Scientific Consultant
BioMedical Research and R&D of Health Care Biofirm Pty Ltd
Copyright: © 2019 G-Halli Rajasekariah. This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 international License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
The developmental cycle of adaptation of human hookworm, Necator americanus into
laboratory rodent host hamster, is being critically reviewed. This laboratory model is found
to be reliable one and recommended for various biological experimentations. This model
is undoubtedly representing all aspects of human hookworm biology, and for elucidating
activity of chemotherapy and planning control experimentation. Adapted - hookworms are
now available and can be sourced either from University of Nottingham, England or from
Prof SH Xiao Shanghai, China. Further this model may prove useful for assessment of newer
anthelminthics and to study the protective immune responses against the infection provided
careful planning and windows of assessment be carried out under controlled conditions.
Human hookworm; Necator americanus; Adaptation; Laboratory hamsters; Generations
More than1.5 billion people are infected with greatly-neglected soil- transmitted intestinal helminthic infections, caused by different species of parasitic worms for eg the round worms, (Ascaris lumbricoides), the hookworms (Necator americanus and Ancylostoma duodenale), the thread worms, (Strongyloides stercoralis), the whip worms, (Trichuris trichiura), and the pinworms (Enterobius vermicularis); these infections are widely distributed in tropical and subtropical countries. About480 million suffer from hookworm infection alone which affect the poor and most deprived communities. They are transmitted by eggs present in human faeces, which contaminate the soil in areas where sanitation is poor, hookworm eggs hatch in the soil releasing larvae that mature into a form that can actively penetrate the skin. People become infected with hookworm primarily by walking barefoot on the contaminated soil . Hookworms are major human pathogen and infected children are nutritionally deprived and physically impaired and they are affected mentally. These worms produce iron-deficiency anaemia and protein malnutrition in developing countries. Hookworms are commonly seen in most of the countries and usually more abundant in rural as opposed to urban communities . Because they feed heavily on blood, infected persons suffer from iron deficiency anaemia.
Hookworms research has been greatly impeded by a lack of suitable animals for experimental work . A laboratory model for hookworms is vital. Hookworms can be infected into large animals such as dogs pre-injected with cortisone and are expensive to maintain . Alternatively, a rodent model is most -wanted for laboratory maintenance and controlled experimentations. Necator americanus-hamster model is well established one and used for several indications over many years. As early as 1967, Sen and his associates published an article in “Nature” on adaptation of human hook worm, Necator americanus in hamsters. Human derived NaL3 were used to infect 2- to3-day old (neonatal) baby hamsters. The NaL3 migrated inside the body, reached lungs and swallowed down the gullet and established at the gut level and later on differentiated into male and female worms. The male and female worms mate and produced eggs and thus completing the entire life cycle in hamsters. This full development of N. Americanus in the experimental host initiated with human derived infective larvae (NaL3 ) is a great achievement indeed. Sen & Seth  concluded that the hamsters are suitable experimental hosts for human hookworm infection. Initially Sen  adapted human parasite, Necator americanus on injecting cortisone to experimental animals (up to 4 generations) and intensively made susceptible to Necator pathogen. The source of infective larvae (NaL3 ) was originally derived from infected humans (as early as 1965). This human- derived NaL3 formed the basis of adaptation in laboratory animals. As early as 1966 Necator was established in hamster. Hamster-adapted Generation No 1 started on May 31, 1966 and then on adapted parasite has been passaged in hamsters successfully. Sen  reported certain imbalances in earlier generations. Cortisone injections in to the hamsters were made altogether a different picture for worms to establish in the gut initially for 5 generations; subsequent generation were successful, in absence of cortisone.
Cortisone acetate is known to suppress the cell-mediated immune response as well as humoral response in animals  against a particular pathogen; effector macrophages migrate to the site of foci where larvae are migrating and kill them. Without cortisone, in hamsters, many larvae migrating inside the body succumb to immune attack before reaching the gut. Those avoiding this mechanism ultimately reach the gut and establish as adult worms. Hamsters, aged ~45days- old, carry about 60% of worms (out of 100 NaL3 used for infecting neonatal ones) and about 40% worms could not establish thereby eliminated by the immune attack possibly at the gut level. The early adults have the ability to resist the rejection force and possibly certain factors able to help them in establishment. Here at the gut level some resistance will come into force against the establishment of worms . Worm expulsion was considerably reduced and animals continue to harbor a greater population of parasites by about 12th generations . Recently, it has been demonstrated that Muc5ac, a mucin in the intestinal tract may be responsible for rejection of enteric nematodes .
Once adult worms developed in hamsters, the eggs produced from these adapted worms were used for culturing infective L3 for further propagation. Eggs were uniformly cultured and the resultant L3 was used for re-infect new neonatal baby hamsters for succeeding generations. This is how adaptation occurred and adapted worms were propagated for successive generation. Thus, the L3 infection of adapted worms initiated in baby hamsters (neonatal) has produced adapted worms for succeeding generations into hamsters. This is how kinetics of adaptation occurred in permissive hosts. At least, Sen  has achieved the migration of Necator and resulted the adult worms to establish in the gut. With this, Sen  has shown that the hamsters as permissive host for human hookworms, Necator. It is a great achievement indeed!. A suitable model was required for gut helminths and its lacuna was filled successfully. A model parasite system in laboratory hamsters has achieved. Subsequently, over the years, this model has been extensively used for screening synthetic compounds and to study the activity of newer anthelmintics and identify the potential newer chemotherapeutic compounds.
It may be desirable to narrate certain specific details entailing the establishment of hookworm model in hamsters. Prior to Necator - hamster model, there was no suitable model for soil- transmitted nematode parasites. This could be used as a retrospective aspect. Once the model is set some of the details were not documented. How the adaptation came about and how the hamsters were considered as the suitable host for experimental purpose. The trend appears to be laboratory animals such as rats and mice which are not permissive host for soil transmitted nematodes and rather hamsters are relatively permissive and considered to be appropriate to use them for experimental purpose. Cortisone injections given to hamsters initially have provided the required condition for Necator worms to establish in the gut  and made any non-specific immune response nullified. In mice, they did not observe any such changes in the behaviour of the migration of larvae under the influence of cortisone . In hamster, under the influence of cortisone, many worms could able to establish at the gut level [6,11]. The established adult female worms under the cortisone influence have laid eggs. These eggs became the source of adapted-NaL3 which in-turn set the standard for infecting neonatal baby hamsters for future generation. The kinetics of adaptation occurred at all level such as the egg production, larval development and infection of L3 to baby hamsters, larval migration inside the body and ultimately the establishment of adult worms at the gut level. These parameters have influenced the kinetics of Necator in hamsters.
Using this hookworm model well over 5000 synthetic compounds have been screened for antihelminthic activity.
Development in Hamsters: The life cycle stages are thoroughly
worked out in humans and there was indication of how they progress
inside the body and how they will establish in the small intestine of
humans. The development of human parasite, Necator americanus
is not readily amenable to the hamster’s system. It took some time
to adapt itself to the bodily condition of hamsters. The sheathed
infective larvae (NaL3
) were deposited on to the tender skin of
2-to 3 –day- old neonatal hamsters. On close examination, it was
observed that deposited NaL3
were able to completely penetrate the
skin and all larvae reached interior successfully for onward progress
of development. After reaching the blood circulation, a period has
reached that these developing larvae accumulate in the lungs. During
the phase of migration, many succumbed the resistance mechanisms
and only a few reaches the lungs. This period in the laboratory
animals may takes place after 6 days of infection and sometimes it
may take even longer. These excysted (sheath-free) L4
larvae in the
lungs are grown into size, larger in appearance and can be seen under
naked eyes. The L4
stages can be recovered from the lungs under
fairly clean conditions and readily be used maintenance of in vitro
condition. These stages are best suited for production of antigen.
They do release the antigen into the suspended medium. These stages
are excellent for recovery of antigens. These larvae are being coughed
up and ultimately reach the gut to establish in the small intestines.
As per Sen & Seth  adult worms measured (male 2.35 to7.75 mm;
female 2.31 to11 mm). These are at 40 days after infection and need
to grow little bit longer. (cf, Necator from humans: Male 5-11 mm
Female 9 to 13 mm) (Figure 1 - 3) [Table 1].