Giardiasis in children

　　1. Etiology

　　This worm was first discovered in Leeuwenhoek's (1681) own feces. Lambl (1859) described its morphology in detail and named it Lamblia intestinalis. Stiles (1915) proposed to rename it giardialamblia to commemorate the two discoverers of this worm, Giardia and Lambl. This worm belongs to the phylum Sarcodina, class Zoomastigophora, family Hexamitidae, order Diplomonadida, genus Giardia. In addition to Giardia lamblia, which is parasitic in humans, there are many species of Giardia flagellates that are parasitic in mammals, birds, amphibians, such as Giardia bovis, Giardia eugi, and Giardia muris, etc.

　　1. Morphology

　　This worm has two forms: trophozoite and cyst. The trophozoite looks like a half of a longitudinal-cut pear, hence the name.

　　The anterior end is rounded, the posterior end is pointed, the back is convex in a semicircular shape, and the ventral surface is flat. The worm is 9-21μm long, 5-15μm wide, and 2-4μm thick. The anterior half of the abdomen is sunken to form a sucker, with the worm attaching to the intestinal mucosal surface through the sucker. The worm is bilaterally symmetrical and has four pairs of flagella. Two pairs of lateral flagella are located on both sides of the worm, one pair of ventral flagella is located on the ventral surface of the worm, and one pair of tail flagella extends backward from the worm. The flagella movement can make the worm perform rapid flip or lateral movement. After staining, the trophozoite cytoplasm appears granular, with two cell nuclei on both sides of the anterior median line of the worm, containing a large nucleolus. There are four basal bodies, of which two are clearly visible and are connected to the axoneme and the anterior lateral flagella, respectively. The axoneme extends backward to connect a pair of tail flagella. There is a semilunar median body in the middle of the axoneme. The cyst is elliptical, 8-12μm long, and 7-10μm wide. There is an uneven gap between the cyst wall and the worm. The immature cyst has two nuclei, and the mature cyst has four nuclei. The structure of the worm inside the cyst is the same as that of the trophozoite, except that it lacks free flagella. Scanning electron microscopy shows that the trophozoite back is convex and the surface is similar to an orange peel. The ventral sucker is an asymmetric spiral structure composed of a single layer of microtubules. The periphery of the worm has prominent pseudopodia-like marginal wings. Transmission electron microscopy reveals that the flagella originate from the basal body, with the basal body first emitting an axoneme composed of naked cytoplasm, which extends outside to form flagella. The cross-sectional structure of the flagella is composed of nine pairs of peripheral microtubules and two central microtubules wrapped in a sheath. The entire worm is supported by microtubules. The median body is located at the posterior end of the two nuclei, which is a pair of tubular structures without a membrane, known as the old name of the secondary basal body. The worm back has a layer of flat vesicles. The cytoplasm of the worm is filled with free ribosomes and polyribosomes, but lacks mitochondria, rough endoplasmic reticulum, Golgi apparatus, and lysosomes, etc. Scanning electron microscopy shows that the surface of the cyst wall is similar to an orange peel, uneven, with fine textures. The cyst wall is composed of more than ten layers of membrane structures.

　　2. Life cycle

　　The life cycle of this parasite is simple. The trophozoite resides in the small intestine, especially in the duodenum, and can also be found in the gallbladder, liver, pancreas, and other organs. The trophozoite attaches to the intestinal mucosal epithelial cells with a suction cup to obtain nutrients by osmosis and reproduces by longitudinal binary fission. Some trophozoites detach from the intestinal wall, enter the distal end of the small intestine with the contents, and form cysts, which are excreted out of the body with feces. After humans ingest the cysts orally, they enter the duodenum through the stomach, and the four-nucleated trophozoites inside the cysts emerge and divide into two trophozoites. In the acute stage, the diarrhea contains trophozoites, while in the chronic stage, the stool mainly contains cysts. A large number of cysts can be found in human feces, and the amount of cysts excreted in a day and night can reach hundreds of millions even billions. Giardia lamblia can survive in artificial culture media, and two pure culture strains have been obtained in China (Beijing strain and Sichuan strain).

　　2. Pathogenesis

　　1. Pathogenicity of Giardia lamblia

　　It is generally believed that the onset of the disease is related to various factors such as the virulence of the parasite strain, the immune status of the host, and the symbiotic internal environment. The trophozoite adheres to the surface of the intestinal mucosa with a suction cup, causing mechanical stimulation and injury that lead to mucosal inflammation. When the parasites reproduce in large numbers, they can cover a large area of the intestinal mucosa, affecting the absorption of fats and fat-soluble vitamins, and compete with the host for nutrients in the lumen. Changes in the intestinal flora can lead to intestinal dysfunction to varying degrees.

　　2. Immune response

　　In recent years, it is believed that immune factors in the human body are the main pathogenic mechanism. The surface antigen components of the trophozoite can induce the body to produce a protective immune response, activate immune cells, and inhibit or kill the parasites. Among them, the 82kDa/88kDa antigen exists on the surface of trophozoite cells and flagella. The 56kDa/57kDa antigen also exists on the surface of the parasite cells, and this antigen can stimulate the body to produce protective IgA and IgG antibodies during the natural infection process. In individuals with normal immune function, infection usually results in a specific humoral immune response, producing specific IgM, IgG, and IgA antibodies through direct cytocidal action, complement-mediated lysis, and opsonization, leading to the killing, lysis, or phagocytosis of the parasites. In addition, secretory IgA antibodies can be detected in the intestinal mucosa, and a certain degree of immunity can be obtained, which can affect the activity of the parasites through agglutination; or act on the surface components of the parasites involved in adsorption, blocking the parasites from adhering to the intestinal mucosa. AIDS patients and individuals with immune deficiency due to the inability to produce effective immune responses are more susceptible to this parasite and have a higher incidence rate.

　　3. Pathology

　　The lesions often involve the duodenum and the upper segment of the jejunum, and in severe cases, the gallbladder, bile duct, distal small intestine, appendix, colon, pancreatic duct, and hepatic duct can all be invaded. The small intestinal mucosa is congested, edematous, with inflammatory cell infiltration and superficial ulcers. The intestinal microvilli are edematous, degenerative, and form vacuoles. In severe infections, the microvilli thicken and atrophy, and there is a large number of neutrophils and eosinophils infiltrating the submucosal and lamina propria.

　　Pediatric giardiasis can cause water and electrolyte disorders, anemia, malnutrition, growth retardation, and other symptoms. If not treated for a long time, it will seriously affect the physical health, growth, and development of children, so it is necessary to treat it in a timely manner.

　　The onset situation is related to many factors such as the virulence of the strain, the immune status of the body, and the symbiotic internal environment. Most patients show malabsorption syndrome with diarrhea as the main symptom, feces are watery, may contain mucus, usually without pus and blood, large in volume, foul-smelling, often accompanied by nausea, vomiting, abdominal distension, abdominal pain, and low fever, fatigue, and decreased appetite. Children may develop anemia and malnutrition due to diarrhea, resulting in weight loss, growth retardation, and other symptoms. Invasive biliary tract manifestations include symptoms of cholecystitis and cholangitis.

　　Strengthening water source hygiene management, paying attention to dietary hygiene, thoroughly treating patients and asymptomatic cyst carriers, eliminating cockroaches, flies, and other vectors, doing fecal harmless treatment, maintaining normal immune function, and other important measures are all important for preventing the occurrence or spread of the disease. Strict personal hygiene can prevent interpersonal transmission, treating asymptomatic cyst excretors can reduce the spread of infection, and reducing the cost of asymptomatic infected children in blood treatment day care centers. Boiling water or heating to 70℃ for 10 minutes can achieve disinfection purposes. Giardia cysts are resistant to routine chlorination concentration and must be disinfected with iodine-containing disinfectants for more than 8 hours. Certain filter devices can also remove Giardia cysts from polluted water.

　　First, pathogen examination

　　Cysts can be found in fresh diarrhea feces, while trophozoites are mostly found in paste-like and formed stools; trophozoites can be found directly on fecal physiological saline smears. After staining with iodine solution, cysts become easily identifiable. Concentration methods such as zinc sulfate floatation can improve the detection rate of cysts. Fecal examination should be conducted three times with three inspections, which can raise the positive rate to 97%. Enterohepatic drainage, small intestinal mucus, or biopsy tissue can all detect the parasites.

　　Second, immunological tests

　　It can be divided into two categories: detecting antibodies in serum and detecting fecal antigens.

　　1. Antibody detection

　　Since the pure culture of Giardia lamblia has been successful, due to the possibility of high-purity antigen preparation, it has greatly improved the sensitivity and specificity of immunodiagnosis. China has established two strains of Giardia lamblia culture, providing conditions for the development of immunodiagnosis in China. Enzyme-linked immunosorbent assay (ELISA) and indirect fluorescent antibody test (IFA) are used to test patient serum antibodies, with the former reaching 75% to 81% positive and the latter reaching 66.6% to 90% positive.

　　2. Antigen detection

　　Enzyme-linked immunosorbent assay (double sandwich method), dot enzyme-linked immunosorbent assay (Dot-ELISA), immunoelectrophoresis (CIE), and other methods can be used to detect antigens in fecal dilution, with the positive rate of double sandwich ELISA reaching 92%, Dot-ELISA reaching 91.7%, and CIE reaching 94%. The detection of fecal antigens can not only be used for diagnosis but also to evaluate the efficacy.

　　Third, molecular biological diagnosis

　　In recent years, polymerase chain reaction (PCR) has been used to detect the ribosomal RNA (rRNA) gene products of the Giardia lamblia, which can detect amplified copies equivalent to the amount of genomic DNA of a trophozoite, and radioactive-labeled chromosome DNA probes can also be used to detect trophozoites and cysts. Molecular biological methods have high specificity and sensitivity, thus having broad application prospects.

　　Diet should mainly include light and easily digestible foods, pay attention to eating more foods rich in protein and vitamins appropriately, eat in small and frequent meals, avoid eating cold, spicy, and刺激性 foods, avoid overeating, avoid eating foods that are difficult to digest such as fatty, sweet, and greasy foods, to prevent exacerbating diarrhea. Avoid catching a cold, avoid excessive activity, avoid staying up late, and pay attention to keeping the body and hands clean.

　　1. Treatment

　　Metronidazole is the first-line drug for the treatment of this disease, taken continuously for 5 to 7 days. Albendazole is also commonly used for clinical or on-site prevention and treatment. Other treatments include tinidazole, nimorazole (nitromorphimimazole), and astragalus root extract tablets, etc.

　　2. Prognosis

　　If the lesion is extensive and prolonged, it can cause malnutrition and anemia in children, which is easy to lead to secondary bacterial infection and death.