Pediatric visceral larva migrans

　　一、发病原因

　　本病的病因尚不清楚，可能是一种自身免疫性疾病，也可能与寄生虫(钩虫、蛔虫等)感染有关。引起本病征的变应原种类甚多，常见的可能有下述数种。

　　1、寄生虫感染

　　以蛔蚴感染为最多见。人食蛔虫卵后在小肠内孵出幼虫，然后经小血管沿门静脉进入肝脏，循血流至肺，引起肺部浸润病变。其他尚有钩虫、鞭虫、血吸虫、肺吸虫、华支睾吸虫、丝虫等感染。

　　最近研究发现犬弓首线虫(Toxocaracanis)感染，是本病征的病因之一。犬弓首线虫是狗常见的寄生虫，其幼虫也可感染人体而出现内脏幼虫移行综合征。随着养狗增多，儿童犬弓首线虫感染的机会也增多。以海联免疫吸附试验(ELISA)测患儿血清中犬弓线虫抗体检测有助病原学诊断。有人对20例内脏幼虫移行综合征患儿作血清抗体测定阳性10例占50％。

　　2、药物或食物过敏

　　药物中较多见的有磺胺类药、阿司匹林、青霉素及抗毒血清等；食物中如牛奶、鸡蛋、虾、蟹、鱼类等。停药或停食过敏食物后症状可消退。

　　3、变态反应性疾病

　　原有过敏体质的小儿，如支气管哮喘、荨麻疹等患儿，发生呼吸道感染时，可引起肺部的病变。

　　4、吸入过敏的物质

　　如凤仙花、枯草花粉或藜状菌(aspergillus)的孢子等。

　　二、发病机制

　　当致敏者再次吸入某种有机粉尘或动物蛋白微粒时，在肺的终末段引起炎症反应。Ⅲ型超敏反应是本病的主要发病机制。特异IgG抗体存在于50％以上的暴露者和90％以上有临床表现的患者血清中。症状开始于抗原攻击后4～8h，在肺的间质中形成并沉积了抗原-抗体复合物，接着复合物又激活补体引起急性炎症和组织损伤。后来又证实细胞介导的Ⅳ型超敏反应在本病的发病机制中也起重要作用。Ⅰ型超敏反应在典型的过敏性肺炎的发病中作用不明显。一般外周血嗜酸粒细胞和血清IgE水平正常，常见吸入变应原皮试速发反应阳性率不高，特应性疾病的发生率也低，这两点说明特应性不大可能是一个重要罹患因素宿主的易感性主要与个体的遗传体质和先前存在的肺疾患有关。

　　反复发作者造成肺部不可逆性组织损伤，可发展为肺纤维化至慢性呼吸衰竭和心衰。重者可有发绀，肺部出现湿啰音，白细胞常增多，不可逆的肺组织损伤。呈隐袭进展直到肺纤维化，典型表现为逐渐加重的干咳、呼吸困难、厌食和疲乏等，这慢性呼吸衰竭和充血性右心衰竭。

　　1. Acute allergic pneumonia

　　It often occurs after a short and strong antigenic exposure, and its manifestations are similar to those of acute bacterial and viral pneumonia. Sensitive individuals may suddenly start to cough, fever, chills, malaise, and significant dyspnea 4 to 8 hours after exposure to the antigen. Physical examination shows an acute appearance, rapid breathing, and in severe cases, cyanosis. Wet rales may appear in the lungs, and white blood cell counts are often increased. Recovery usually occurs a few days to several weeks after the antigenic exposure stops, and if the patient is hospitalized, they will improve rapidly as well. This improvement is often mistakenly attributed to the effect of antibiotics, but the most likely cause is the natural result of avoiding the antigen. Due to the unclear diagnosis, the patient returns to the original place after discharge, and the symptoms may recur. Such repeated exposure eventually leads to irreversible lung tissue damage in the patient. Chest X-ray examination during the acute phase shows small nodular infiltration in the interstitium and alveoli, which is often patchy or disseminated. The diameter of the nodules varies from one to several millimeters. Adenoidal shadows may also be seen, and the mediastinal lymph nodes generally do not enlarge. The degree of abnormality in the chest X-ray examination reflects the severity of the disease. Occasionally, the lungs may be completely normal during the early examination after exposure, especially in the early stage of exposure.

　　2. Chronic allergic pneumonia

　　Due to prolonged exposure to a not very concentrated antigenic environment or repeated acute attacks, 50% of the cases present insidious progression to pulmonary fibrosis. The typical manifestations are gradually worsening dry cough, dyspnea, anorexia, and fatigue, and it is often not noticed until the lungs are widely fibrotic and accompanied by pulmonary dysfunction, at which time there may already be signs of chronic respiratory failure and congestive right heart failure, making it difficult to distinguish from primary pulmonary fibrosis. Chest X-ray examination shows extensive interstitial fibrosis with thickening of the bronchovascular imaging (streaks).

　　1. The primary issue in the prevention and treatment of allergic patients is to make every effort to find the trigger of allergy and avoid it. This has preventive significance for patients in the non-attack period, and therapeutic significance for patients in the attack period. The methods to avoid allergen triggers can be summarized in four words: 'avoid', 'abstain', 'substitute', and 'move'.

　　Prevention of visceral larvae migrans syndrome should also emphasize hygiene and the development of good hygiene habits, such as washing hands before and after meals and defecation, preventing parasitic infection, and regular deworming during the early childhood period. Specific desensitization should be adopted for prevention and treatment when necessary.

　　1. Peripheral blood examination

　　Eosinophils in peripheral blood are larger than normal, contain large granules, increase in number, accounting for 20% to 70% of the total white blood cell count, and the direct count of eosinophils is usually around 3.0×10^9/L.

　　2. Immunological examination

　　Parasite antibody tests in the blood may be positive; IgE levels may be as high as 2300 ng/ml, and those with liver enlargement often show hypergammaglobulinemia.

　　3. Examination of parasitic eggs

　　The lavage fluid of the alveoli and the examination of sputum for 24 hours for parasitic eggs may show positive findings.

　　4. Skin test examination

　　Skin test with parasitic skin test fluid can be positive.

　　5. Chest X-ray

　　Show cloud-like patchy shadows, which can vary in size, shadows can disappear in a short period of time and reappear soon, the location can migrate and is not fixed, and sometimes it can show atelectasis.

　　6. Pulmonary function tests

　　This disease is mainly a restrictive pulmonary dysfunction, with a significant decrease in lung volume during the acute phase, but there is no change in lung volume measurement, FVC decreases, FEV1 slightly decreases, interstitial inflammation reduces lung compliance, and significant air-blood flow abnormalities lead to a decrease in lung diffusion capacity and arterial oxygen partial pressure. Like clinical and X-ray changes, pulmonary dysfunction during the acute phase is reversible. In the chronic stage with extensive pulmonary fibrosis, restrictive and obstructive pulmonary dysfunction becomes irreversible.

　　Patients with pediatric visceral larva migrans should not eat randomly, follow the doctor's advice on diet, and it is best to eat some light foods. It should be food that invigorates the spleen and benefits the lung, especially porridge-like foods.

　　I. Treatment

　　For patients with obvious symptoms, adrenal cortical hormones can be used first to relieve symptoms, reduce blood eosinophil counts, and prednisone (prednisone) is administered at a dose of 1-2mg/kg per day for 3-5 days consecutively. After obtaining temporary efficacy, the cause can be further investigated to facilitate etiological treatment.

　　If caused by ascaris, piperazine (driving ascaris) can be taken orally at a dose of 160mg/(kg·time) per day, once a night for 2 consecutive days; or levamisole 3mg/(kg·d), taken for 2 consecutive days. After worm expulsion, symptoms and eosinophil counts can gradually decrease and recover.

　　When suspected of filarial infection, diethylcarbamazine (Hetrazan) can be tried, with a dose of 12-15mg/(kg/d), taken orally in three divided doses, for 4-5 days consecutively, which can improve asthma and pulmonary signs.

　　When suspected of hookworm infection, mebendazole (driving worm, dihydroxynaphthalene acid mebendazole) can be used, with a dose of 5-10mg/kg, once a day, for 2-3 days consecutively.

　　II. Prognosis

　　This condition is a self-limiting disease with a good prognosis. However, if there are recurrent migratory attacks and pulmonary complications, it will affect the prognosis.