Strongyloides stercoralis (strongyloidiasis) - Microscopic exam; Stools culture (Harada-Mori method or method Baermann); IgG antibodies; Molecular diagnosis (PCR)
Strongyloides stercoralis is a nematode genus Strongyloides, family Strongyloididae. Strongyloides stercoralis is endemic in rural geographical regions of tropical, subtropical temperate weather or where developmentally appropriate conditions exist. This nematode affects people in causing a disease called strongyloidiasis.
The important ways for the diagnosis of this parasitosis are rhabditiform larvae that appear in the intestinal mucosa from eggs deposited in the intestinal mucosa by parthenogenetic females in the absence of males. Rhabditiform larvae have a prominent esophageal bulb and evolve filariform are infective forms, about 600 microns in length and elongated fusiform esophagus. This nematode in its adult stage is filiform body, esophagus and rectum pointed posterior end, and can be up to 2.5 mm long and 50 microns wide.
The life cycle of Strongyloides stercoralis is unique and unusual, because the females of this species can give rise to progeny without the existence of males that fertilize, by a process known as parthenogenesis. As a result, they can have two types of cycles: free life cycle and parasitical cycle. Free life cycle, as the name suggests occurs in the external environment, without mammalian host, while the parasitic cycle occurs within a mammalian host. Invasion of Strongyloides stercoralis in individuals occurs by permeation through the skin of filariform present in the soil or in water, with the exception of endogenous or exogenous autoinfectivos cycles (see below), which have evolved from rhabditiform larvae appeared from eggs laid by females and males free life.
- Free Life cycle: In the free life cycle, rhabditiform larvae (L1) hatched from eggs deposited in the intestinal mucosa by females, they are excreted in the feces. These larvae L1, develop a second stage (L2) and a third stage larvae (L3 larva or filariforme). Alternatively, free - living larvae rhabditiform can lead to a generation of adults, male and female, free life that are fertilized and deposited eggs from which larvae born L1, which evolve infective larvae filariform. Filariform larvae can penetrate the skin of the host mammal to initiate the parasitic cycle.
- Parasitic cycle: filariforme larva (L3) present in the contaminated soil penetrate the skin of the host into the lungs through the bloodstream and then reaching the light of lung alveoli are swallowed and may reach the small intestine. To this end, the lungs, the larvae penetrate the pulmonary capillaries and go to the alveolar spaces. Then migrate into the trachea and throat to be swallowed, reaching the small intestine where the female will penetrate the mucosa and deposit eggs. Subsequently, the eggs hatch leading to first instar larvae (L1 or rhabditiform) coming into the lumen of the intestine and are excreted out through the feces. Alternatively, rhabditiform larvae can develop into filariform in the host organism, thus establishing a cycle of endogenous or exogenous, well autoinfection because filariform perform the infective cycle through the intestinal mucosa, reaching circulation, pulmonary, swallowing and intestine (endogenous autoinfectivo cycle), or through the anus because go outside and through the perianal skin to reach the venous circulation to the right heart, lung alveoli, swallowing and intestine (exogenous autoinfectivo cycle). In cases of self - infection, the disease can persist for long periods of time without accompanying symptoms.
The course of infection by Strongyloides stercoralis depends on the immune response of the individual concerned and the parasite load, since the immune response can control the infection but not eradicated. When infection occurs in immunocompetent individuals, the infection is usually asymptomatic, as occurs with clinical manifestations, they usually include general symptoms unspecific (fever, malaise, chronic wasting), dermatologic (generalized urticaria, pruritus, skin granulomas), respiratory (pneumonitis cough, bronchitis, expectoration) and gastrointestinal (abdominal pain, vomiting, diarrhea, nausea), usually accompanied by eosinophilia greater or lesser degree. The respiratory profile is produced by larval infiltration of vascular and alveolar spaces, which in the most severe cases results in intra - alveolar pulmonary edema, bronchopneumonia and microhemorrhage may cause pneumonitis and respiratory failure, clinically characterized by ARDS (Syndrome acute respiratory distress syndrome) and in extreme cases multiorgan failure. In immunocompromised individuals, as occurs in HIV patients, those undergoing immunosuppressive therapy for organ transplantation by antineoplastic therapy in deep malnourished, alcoholics, of anvanzada age, treated with corticosteroids, coinfected with virus HTLV-I, or any circumstance lead inhibition of Th2 immune response that prevents adequate eosinophilic response, no infection control and hyperinfestation box occurs. This process can lead to disseminated strongyloidiasis that cause organ dysfunction which can be fatal, even in 87% of cases.
Recommended tests for diagnosis:
The diagnosis is based on microscopic identification with or without stool culture, antibody detection (ELISA), or molecular diagnostic methods (PCR).
To improve diagnosis is recommended several samples because the elimination of larvae is scarce and it is not constant. Serological diagnosis can have nonspecific results because there are crossed with other helminths, and also the antibody response may be decreased in HIV - infected or subjected to any type of immunosuppressive therapy (see before) individuals reactions. Molecular methods may give false negative results for the presence of inhibitors of the PCR reaction in feces or by an efficient lysis of the larvae had not been made.
The diagnosis of infection Strongyloides asymptomatic stercoralis is important when an individual will be subjected to immunosuppressive therapy (transplantation, neoplasia, administration corticosterioides, ...), because in case of being infected, a state hyperinfection could occur, to through a parasitic cycle by endogenous or exogenous autoinfection. In these cases it make a diagnosis with the most sensitive methods available, to avoid the risk of infections that compromise life, especially in patients who show no other justification eosinophilia (50-80% of asymptomatic infections).
Tests in IVAMI:
- Microscopic examination without stool culture.
- Microscopic examination after preculture (Method Harada-Mori, or method Baermann).
- Detection of IgG antibodies by enzyme immunoassay (ELISA).
- Molecular diagnosis (PCR), to detect DNA of Strongyloides stercoralis in stool or mucosal biopsies of small intestine.
- Microscopic examination with or without culture: newly issued feces.
- Detection antibody: serum (0.5 to 1 mL).
- Molecular diagnosis: newly issued feces or mucosal biopsies of small intestine.
Preservation and shipment of sample:
- Refrigerated (preferred) for less than 2 days.
- Frozen: over 2 days (for molecular diagnostic tests only).
- Microscopic examination without culture: 24 hours.
- Microscopic examination with preculture (method Harada-Mori, or method and Baermann): 5 to 7 days
- Molecular diagnosis (PCR): 24 to 48 hours.
Cost of the test:
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