Слайд 2: Classification
Kingdom: Animalia Phylum: Nematoda Class: Chromadorea Order: Rhabditida Family: Strongylidae Genus: Strongyloides Species: S. stercoralis
Слайд 3: General Characteristics
The Strongyloides stercoralis nematode can parasitize humans. The adult parasitic stage lives in tunnels in the mucosa of the small intestine. The genus Strongyloides contains 53 species, and S. stercoralis is the type species. S. stercoralis has been reported in other mammals, including cats and dogs. However, it seems that the species in dogs is typically not S. stercoralis, but the related species S. canis. Non-human primates are more commonly infected with S. fuelleborni and S. cebus, although S. stercoralis has been reported in captive primates.
Слайд 4: Morphology
MALES: grow to only about 0.9 mm (0.04 in) In length Males can be distinguished from females by two structures: the spicules and gubernaculum. FEMALES: It can grow from 2.0 to 2.5 mm (0.08 to 0.10 in )
Both sexes also possess a tiny buccal capsule and cylindrical esophagus without a posterior bulb. In the free-living stage, the esophagi of both sexes are rhabditiform.
Слайд 6: Life Cycle
The strongyloid's life cycle is heterogonic —it is more complex than that of most nematodes, with its alternation between free-living and parasitic cycles, and its potential for autoinfection and multiplication within the host. The parasitic cycle is homogonic, while the free-living cycle is heterogonic. The heterogonic life cycle is advantageous to the parasite because it allows reproduction for one or more generations in the absence of a host.
In the free-living cycle, the rhabditiform larvae passed in the stool can either molt twice and become infective filariform larvae (direct development) or molt four times and become free-living adult males and females that mate and produce eggs from which rhabditiform larvae hatch. In the direct development, first-stage larvae transform into infective larvae via three molts.
First stage larva of S. stercoralis The indirect route results first in the development of free-living adults that mate; the female lays eggs, which hatch and then develop into IL. The direct route gives IL faster (three days) versus the indirect route (seven to 10 days). However, the indirect route results in an increase in the number of IL produced. Speed of development of IL is traded for increased numbers. The free-living males and females of S. stercoralis die after one generation; they do not persist in the soil. The latter, in turn, can either develop into a new generation of free-living adults or develop into infective filariform larvae. The filariform larvae penetrate the human host skin to initiate the parasitic cycle. The infectious larvae penetrate the skin when it contacts soil. While S. stercoralis is attracted to chemicals such as carbon dioxide or sodium chloride, these chemicals are not specific. Larvae have been thought to locate their hosts via chemicals in the skin, the predominant one being urocanic acid, a histidine metabolite on the uppermost layer of skin that is removed by sweat or the daily skin-shedding cycle.
Слайд 10: Distribution
S. stercoralis infection is associated with fecal contamination of soil or water. Hence, it is a very rare infection in developed economies. In developing countries, it is less prevalent in urban areas than in rural areas (where sanitation standards are poor). S. stercoralis can be found in areas with tropical and subtropical climates. Strongyloidiasis was first described in the 19th century in French soldiers returning home from expeditions in Indochina. Today, the countries of the old Indochina (Vietnam, Cambodia, and Laos) still have endemic strongyloidiasis, with the typical prevalences being 10% or less. Regions of Japan used to have endemic strongyloidiasis, but control programs have eliminated the disease
Слайд 11: Means of spread
It is mostly transmitted by zoonotic i.e animals. Dogs can act as a host for this parasite both in the wild and in the laboratory but transmission from dog to human has been difficult to prove. Molecular genetic analyses have shown that there are two populations of this parasite in dogs, one of which is exclusive to dogs and a second that is common to dogs and humans. These two genotypes may be separate species. The identity of the genes suggests that dog to human transmission may occur.
Слайд 12: Symptoms
Many people infected are asymptomatic at first. Symptoms include dermatitis: swelling, itching, larva currens, and mild hemorrhage at the site where the skin has been penetrated. Spontaneous scratch-like lesions may be seen on the face or elsewhere. If the parasite reaches the lungs, the chest may feel as if it is burning, and wheezing and coughing may result, along with pneumonia-like symptoms ( Löffler's syndrome). The intestines could eventually be invaded, leading to burning pain, tissue damage, sepsis, and ulcers. Stools may have yellow mucus with a recognizable smell. Chronic diarrhea can be a symptom.In severe cases, edema may result in obstruction of the intestinal tract, as well as loss of peristaltic contractions
In addition, diseases such as human T- lymphotropic virus 1, which enhance the Th1 arm of the immune system and lessen the Th2 arm, increase the disease state. Another consequence of autoinfection is the autoinfective larvae can carry gut bacteria back into the body. About 50% of people with hyperinfection present with bacterial disease due to enteric bacteria. Also, a unique effect of autoinfective larvae is larva currens due to the rapid migration of the larvae through the skin. Larva currens appears as a red line that moves rapidly (more than 5 cm or 2 in per day), and then quickly disappears. It is pathognomonic for autoinfective larvae and can be used as a diagnostic criterion for strongyloidiasis due to S. stercoralis.
Strongyloidiasis in immunocompetent individuals is usually an indolent disease. However, in immunocompromised individuals, it can cause a hyperinfective syndrome (also called disseminated strongyloidiasis ) due to the reproductive capacity of the parasite inside the host. This hyperinfective syndrome can have a mortality rate close to 90% if disseminated
Слайд 16: Treatment
Ideally, prevention, by improved sanitation (proper disposal of feces), practicing good hygiene (washing of hands), etc., is used before any drug regimen is administered. Ivermectin is the drug of first choice for treatment because of higher tolerance in patients. Thiabendazole was used previously, but, owing to its high prevalence of side effects (dizziness, vomiting, nausea, malaise) and lower efficacy, it has been superseded by ivermectin and as second-line albendazole. However, these drugs have little effect on the majority of these autoinfective larvae during their migration through the body
Слайд 17: Diagnosis
Locating juvenile larvae, either rhabditiform or filariform, in recent stool samples will confirm the presence of this parasite. Other techniques used include direct fecal smears, culturing fecal samples on agar plates, serodiagnosis through ELISA, and duodenal fumigation. Still, diagnosis can be difficult because of the day-to-day variation in juvenile parasite load.
Слайд 18: Prevention and Control
The best way to prevent Strongyloides infection is to wear shoes when you are walking on soil, and to avoid contact with fecal matter or sewage. Proper sewage disposal and fecal management are keys to prevention. Furthermore, if you believe that you may be infected, the best way to prevent severe disease is to be tested and, if found to be positive for disease, treated.
Taking steroids or other immunosuppressive therapies About to start taking steroids or other immunosuppressive therapies A veteran who served in the South Pacific or Southeast Asia Infected with Human T-cell Lymphotropic Virus-1 (HTLV-1) Diagnosed with cancer Going to donate or receive organ transplants