BIOMEDICAL RESEARCH AND REVIEWS

ISSN 2631-3944

The Establishment of Human Hookworm, Necator americanus in Hamsters - A Retrospective Loo

G-Halli Rajasekariah

Chief Scientific Consultant,  BioMedical Research and R&D of Health Care, Biofirm Pty Ltd, Australia

CitationCitation COPIED

Rajasekariah GH. The Establishment of Human Hookworm, Necator americanus in Hamsters - A retrospective L. Biomed Res Rev. 2019 Oct;2(2):112

Abstract

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.

Keywords

Human hookworm; Necator americanus; Adaptation; Laboratory hamsters; Generations

Introduction

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 [1]. 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 [2]. 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 [3]. 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 [4]. 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 [5] concluded that the hamsters are suitable experimental hosts for human hookworm infection. Initially Sen [6] 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 [6] 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 [7] 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 [8]. Worm expulsion was considerably reduced and animals continue to harbor a greater population of parasites by about 12th generations [6]. Recently, it has been demonstrated that Muc5ac, a mucin in the intestinal tract may be responsible for rejection of enteric nematodes [9]. 

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 [6] has achieved the migration of Necator and resulted the adult worms to establish in the gut. With this, Sen [6] 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 [6] 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 [10]. 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 [5] 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].