Strongyloides stercoralis.

Strongyloides stercoralis, a nematode not well known to many Canadian physicians, infects 35% of some tropical populations. Larvae can be isolated from the stools in 25% of cases and from duodenal aspirates in 95%. Treatment is with thiabendazole given twice daily in a dose of 25 mg/kg up to a maximum of 1.5 g/d. Frenquently an individual with a previously asymptomatic infection presents with hyperinfection and death rapidly ensues, but usually classical symptoms are present. Such a case is described. Immunosuppression is frequently associated with the hyperinfective state.     

Strongyloides stercoralis infection in the ferret

The ferret (Mustela putorius furo) was evaluated as an animal model for infection with human strains of Strongyloides stercoralis. Results indicate that such infections can be easily and reproducibly accomplished.     

Towards effective control of Strongyloides stercoralis

A widespread intestinal parasite of humans, Strongyloides stercoralis has long been considered very difficult to control in endemic communities. This situation is now changing. In this article, David Conway, John Lindo, Ralph Robinson and Don Bundy review recent advances in diagnosis, chemotherapy and epidemiology of S. stercoralis infection, and highlight new options for control.     

A case of Strongyloides stercoralis infection.

Strongyloidiasis has been recognized as one of the life-threatening parasitic infections in the immunocompromised patients. We report an intestinal infection case of Strongyloides stercoralis in a 61-year-old man. Rhabditiform larvae were detected in the stool examination and developed to filariform larvae having a notched tail through the Harada-Mori filter paper culture. The patient received five courses of albendazole therapy but not cured of strongyloidiasis.     

Strongyloides stercoralis genotypes in humans in Cambodia

Little is known about the genetic variability of the soil-transmitted nematode, Strongyloides stercoralis, in humans. We sequenced portions of the small subunit rDNA (SSU), including the hyper variable regions (HVR) I and IV from S. stercoralis larvae derived from individuals living in a rural setting in Cambodia. We identified three polymorphic positions, including a previously reported one within the HVR I. HVR IV was invariable. Six different SSU alleles existed in our sample. Although different genotypes of S. stercoralis were found in the same individuals, no heterozygous larvae were found. This indicates that there is no or very little interbreeding between the different genotypes. Further studies are needed to examine if this is because sexual reproduction, which is facultative, is rare in our study area's S. stercoralis population or because what is considered to be S. stercoralis today is actually a complex of closely related species or subspecies.     

Amphids in Strongyloides stercoralis and other parasitic nematodes

In this review, Francis Ashton and Gerhard Schad examine the ultrastructure of the amphids of several animal parasitic nematodes. These structures are the main chemosensory organs of these worms and probably play an important role in host-finding behavior and the control of development. Reconstructions made from serial micrographs of the neurons in the amphids of the threadworm Strongyloides stercoralis are shown. These stereo images permit three-dimensional visualization of these complex sense organs. The association between each amphidial neuron and its cell body has not been made previously for a parasitic nematode; however, this has been done for the free-living nematode Caenorhabditis elegans, which served as a model for these studies. Recognition of the cell bodies will provide a point of departure for laser microbeam ablation studies to determine individual neuronal function.     

Transmammary transmission of Strongyloides stercoralis in dogs

Vertical transmission of larvae is a major pathway in the life cycle of several species of Strongyloides, but evidence for it occurring in humans or dogs with Strongyloides stercoralis is absent. In an effort to determine if vertical transmission could occur with S. stercoralis, each of 3 female dogs was infected with filariform larvae at a different stage of the reproductive cycle, i.e., preconception, gestation, or postpartum. Results showed that none of 6 pups born to a female infected before conception or any of 6 pups born to another female infected during gestation harbored any stage of S. stercoralis when necropsied at parturition. Conversely, all 5 pups that nursed from the female infected immediately postpartum became infected with adult S. stercoralis in their small intestines (range, 56-129 adult worms). Significantly, live filariform larvae of S. stercoralis were observed on 2 different occasions from milk samples taken from the lactating female. Because arrested development of larvae is not known in S. stercoralis, there is no reservoir of larvae in the parenteral tissues of females to queue for passage to the pups and, thus, it is not surprising that only timely infections, perhaps very late in gestation and during lactation, can be successful. These data support previous work in dogs with S. stercoralis, which concluded that vertical transmission through prenatal pathways does not occur, but they are the first from the dog to indicate that vertical transmission of this parasite through transmammary routes is possible. Whether transmammary transmission of S. stercoralis occurs in humans remains unknown but given its immense pathological potential, it should not be overlooked.     

Frequency of Strongyloides stercoralis infection in alcoholics

Several studies have shown that chronic alcoholics have increased susceptibility to infections due to higher exposure to infectious agents as well as breakdown in their immune defenses. As Strongyloides stercoralis infection is usually more relevant in immunocompromised patients, the aim of this study was to evaluate the frequency of S. stercoralis infection in alcoholics. Thus, coproparasitological examination was carried out in 145 subjects, from which 45 were chronic alcoholics (mean age of 45.7 +/- 11.0 years), 10 were nonalcoholic cirrhotic patients (mean age of 50.2 +/- 13.1 years), and 90 were asymptomatic nonalcoholic subjects (mean age of 46.7 +/- 10.1 years), which served as controls. From the alcoholics, 9 had hepatic cirrhosis, 9 had chronic pancreatitis and 27 had neither cirrhosis nor pancreatitis. For the diagnosis of strongyloidiasis, the Baermann-Moraes and Lutz methods were used in three fecal samples from each subject. Samples were collected at alternated days, and three slides of each sample were analyzed for each method, thus totalizing 2,610 slides examined. The frequency of strongloidiasis in the total alcoholic group (33.3%) and in the subgroups of alcoholics, i.e., patients with hepatic cirrhosis (44.4%), with chronic pancreatitis (33.3%), and those with no cirrhosis or pancreatitis (29.6%) was statistically higher than that found in the control group (5.5%). None of the individuals with nonalcoholic hepatic cirrhosis had S. stercoralis infection. Our results showed that the chronic alcoholism itself is an important factor that predisposes to strongyloidiasis.     

Community control of Strongyloides stercoralis by thiabendazole.

Mass treatment of a community in Costa Rica with thiabendazole in 1965 reduced the prevalence of S. sterocoralis from 19.2% to 0.3%, and the rates remained low 2 years thereafter. Re-examination of the treated population 7 years after initial treatment showed S. stercoralis prevalence still at a 0.5% level.     

The small subunit ribosomal RNA sequence of Strongyloides stercoralis

The published small subunit rRNA (ssrRNA) gene sequences for Strongyloides ratti and Strongyloides stercoralis are remarkably divergent, particularly in the 5' 400 bases of the approximately 1700 base pair (bp) sequences. This level of divergence between species nominally in the same genus was unprecedented. We have redetermined the ssrRNA sequence of S. stercoralis and find that the published sequence is a chimaera of parasite and fungal segments. The true sequence for S. stercoralis ssrRNA is very similar to that of S. ratti.     

Cryopreservation of first-stage and infective third-stage larvae of Strongyloides stercoralis

Infective third-stage larvae of Strongyloides stercoralis were frozen over liquid nitrogen and remained infective to dogs when thawed. Successful cryopreservation depended on a 30-60-min incubation in a cryoprotectant (10% DMSO and 10% dextran) before freezing and thawing the frozen larvae into RPMI. First-stage larvae could also be frozen by this method. Thawed first-stage larvae remained viable and continued their development to third-stage larvae, which were shown to be infective to dogs.     

Strongyloides stercoralis: immunobiological considerations on an unusual worm

In the highly competitive world of parasites, where survival is indissolubly tied to the presence of suitable hosts in the same ecological niche, Strongyloides stercoralis occupies a unique position. This versatile nematode has developed a successful relationship with two ecosystems, the terrestrial environment and the mammalian body, by becoming facultatively independent of either (Box 1). It requires external maturation for transmission, but it also has the ability to multiply within a single host. It prefers man, but may infect apes, monkeys, dogs and cats. As a human parasite, S. stercoralis establishes self-perpetuating intestinal colonies and negotiates their survival with the host's defense mechanisms. In this article, Robert Genta reviews the immunobiological aspects of this unusual relationship.     

Species-specific differences in heterogonic development of serially transferred free-living generations of Strongyloides planiceps and Strongyloides stercoralis

The direction of free-living development (homogonic vs. heterogonic) in Strongyloides stercoralis and Strongyloides planiceps was examined by successive transplantation of the uterine eggs of free-living females into a test tube culture system containing fresh feces. The eggs from the first-generation free-living females of S. stercoralis did not develop into second-generation free-living adult worms, but all developed into filariform larvae. However, the majority of S. planiceps eggs from the first-generation free-living females developed into second-generation free-living adults. By successive transfer of uterine eggs of each generation, 9 generations developed, and in every cycle more adult worms developed than filariform larvae. However, the number of free-living generations was not infinite; in experiments repeated twice, the number of worms developing from the eggs of eighth or ninth generations was too small to continue further culture. These findings indicate that the pattern of free-living development is different between the 2 species.     

How effective is the agar plate method for Strongyloides stercoralis?

The sensitivity of the agar plate method for the diagnosis of Strongyloides stercoralis was studied experimentally. Results demonstrated that this method was sensitive enough to detect S. stercoralis even when only a few worms were present.     

Dissociation of the protective immune response in the mouse to Strongyloides ratti

The generation of protective immunity by various stages in the life-cycle of Strongyloides ratti and the phases against which resistance is directed has been examined in murine strongyloidiasis. Mice were exposed to natural, complete infections, were treated with thiabendazole (which largely resembles the natural infection), were treated with cambendazole (which restricts infection to the larval stage), or infected directly by oral transfer of adult worms. Mice that were infected with infective larvae alone did not become resistant to infective larvae or the complete infection but were resistant to adult worms implanted directly into the gut. Mice exposed to adult worms alone were resistant to natural infections and adults worms implanted directly but were not resistant to infective larvae. On the other hand, mice that had received prior natural infections showed evidence of resistance to infective larvae, adult worms, and natural, complete infections. It is concluded that there is immunological cross-reactivity between infective larvae and adult worms but that under certain circumstances the infective larvae are able to evade the host's protective immune response.