Among many infectious diseases prevalent in the world, a few are emerging and emerging infectious diseases, which include the influenza virus. There are some other infections, which are spread throughout the world causing pandemics like the human immunodeficiency virus (HIV). Human parasitic infectious diseases assume greater significance in public health perspective, as they contribute to majority of the morbidity especially in the developing and economically weak nations usually affecting the pediatric age and the young adults, as well as immunocompromised individuals. Human strongyloidiasis is one such parasitic disease, which is least studied and under reported undermining its clinical significance. This manuscript attempts to reinvent the pathogenic potential of Strongyloides stercoralis infection, laboratory identification of human strongyloidiasis and future perspectives. Keywords: Strongyloides stercoralis, human strongyloidiasis, laboratory diagnosis of human strongyloidiasis
Strongyloides stercoralis is an intestinal parasite belonging to the group of nematodes. It is the only other parasite, which can cause human infection by the penetration of skin by the larval forms along with Ancylostoma duodenale. Humans may acquire S stercoralis infection when walking bare footed in the soil contaminated with infective larval forms of the parasite. The infective form of the parasite, the filariform larva enters the subcutaneous connective tissues of the skin and wanders around under the skin to produce a characteristic serpiginous skin lesion, called as larva currens. The larva may take two routes to reach the intestine. They either enter in to the blood directly through lymphatic’s and move in to the lungs and are coughed out or swallowed back in to the stomach or directly move from the subcutaneous connective tissue in to the intestine. After reaching the intestine, larva then grow in to the adults, wherein the male worms does not help in fertilization but are just passed out of the stool and the female worms attach themselves to the intestinal folds and later produce eggs with the help of parthenogenesis. Being ovoviviparous, the eggs passed out in to the lumen immediately hatch out the rhabditiform larva, which are excreted in the stool [Figure 1].
Rhabditiform larva are noninfective and feeding forms, which might freely live in the environment (soil and water) and later transform in to filariform larva waiting for the entry in to a new human host.[1],[2],[3],[4] Most human infections of S stercoralis remain asymptomatic. Children usually suffer from acute strongyloidiasis.[5],[6],[7] The patients with immunosuppressive conditions are at increased risk of chronic infection with S stercoralis and in such individuals strongyloidiasis may present as hyperinfection syndrome and disseminated strongyloidiasis. This is a condition in which larva of S stercoralis moves in to various organs like the lung, liver, kidneys, and brain resulting in fatal outcome.[8],[9],[10] Laboratory diagnosis of human Strongyloidiasis Laboratory diagnosis of most of the parasitic infections transmitted through fecal-oral route is performed by a simple microscopic method called as stool wet mount. The use of concentration methods like the saturated salt solution, formol-ether sedimentation methods, and zinc sulphate floatation techniques may improve the chances of finding the parasitic forms in the stool. Previous studies have noted that to find the S stercoralis larva one has to collect and observe at least seven stool specimens from the suspected patient.[11] Most clinical microbiologists find it difficult to differentiate between the larva of A. duodenale and the S stercoralis. In-fact, a freshly passed stool of a person infected with A. duodenale usually contains eggs and not larva. But a delay in observing the specimen may result in the hatching out of larva from the egg. Although difficult there are few characters, which help microbiologists to differentiate between larva of A. duodenale and the S stercoralis. Rhabditiform larva of S stercoralis can be identified by the presence of short buccal cavity, clear esophagus, and a prominent genital primordium. The tail end of the larva in case of S stercoralis is slightly curved/dented and sharp when compared with the larva of A. duodenale, which shows a sharp and straight tail [Figure 1]. In view of the low sensitivity of direct stool microscopy, clinical microbiology laboratories are advised to incorporate at least one of the concentration techniques to improve the chances of finding the larva. The Baermann method, Koga agar plate culture technique, Harada–Mori filter paper method, and recently described FLOTAC (Floatation chamber apparatus for qualitative and quantitative copromicroscopic analysis) techniques would assist in the better diagnosis of S. stercoralis infection. Kato–Katz thick stool smears for the demonstration of S stercoralis larva is another method employed to increase the chances of finding larva in stool. Serological evaluations for the detection of antibodies and PCR have been found beneficial for an increase of the sensitivity and specificity in the laboratory diagnosis of human strongyloidiasis.[12],[13],[14]
Among the many tropical parasitic infectious diseases prevalent, human strongyloidiasis assumes greater significance. S stercoralis is a free living parasite that causes mild disease in healthy humans and usually causes mild infections, which are mostly undiagnosed and requires no treatment. The cause of concern is the ability of this parasite to cause chronic infections among school going children, resulting in severe morbidity. Infection with S stercoralis may also prove to be fatal attributed to their ability to cause hyper infection syndrome in immunocompromised patients (HIV seropositive patients) and those taking immunosuppressive drugs (patients suffering from asthma, transplant patients).[15] In view of the various factors highlighted in this manuscript and the literature available, thus far, it is obvious that human strongyloidiasis is a neglected tropical parasitic infection prevalent throughout the world especially in tropics and subtropical regions accounting for severe morbidity among pediatric age population and has a potential to cause significant mortality in immunocompromised individuals. There are only few studies available in literature and most of them are in the form of case reports undermining the significance of human strongyloidiasis.[16],[17] Only fewer studies are reported in literature elaborating the epidemiology of S stercoralis infection worldwide.[18],[19],[20],[21]
Studies in future must be encouraged from developing nations including India to identify and delineate the possible demographic characters, predisposing conditions, laboratory methods, and effective management strategies of human strongyloidiasis. Research studies on the immunological aspects of S stercoralis infection both in immunocompromised, as well as immunosuppressed individuals should be encouraged. Clinical microbiologists should implement advanced laboratory strategies available for improved chances of finding the parasite in the stool specimens that may aid in better patient care. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.
Source of Support: None, Conflict of Interest: None
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