bubble_chart Overview

Trichinosis, also known as trichinellosis, is a zoonotic disease caused by the parasite Trichinella spiralis. It is prevalent among mammals, and humans can become infected by consuming raw or undercooked pork containing Trichinella cysts. The main clinical manifestations include gastrointestinal symptoms, fever, muscle pain, edema, and an increase in blood eosinophils.

bubble_chart Epidemiology

(1) Source of Pestilence: Pigs are the primary source of pestilence. Other carnivorous animals such as rats, cats, dogs, sheep, and various wild animals like bears, wild boars, wolves, and foxes can also become infected through mutual killing and consumption or by eating animal carcasses containing Trichinella spiralis cysts. It has been proposed that there are two transmission cycles of this disease: the domestic animal cycle and the wild animal cycle. Humans are a collateral branch of these two transmission cycles, and in the absence of human infection, both cycles can operate independently.

(2) Transmission Routes: Humans become infected by consuming pork, dog meat, mutton, or wild boar meat containing cysts. Outbreaks are closely related to the habit of eating raw meat.

(3) Susceptible Population: Humans are generally susceptible to this disease. After infection, significant immunity can be developed, and the condition of reinfected individuals is much milder than that of first-time infections.

(4) Epidemiological Situation: Trichinellosis is sporadically distributed worldwide, with higher incidence rates in Europe and America. Domestically, it is mainly prevalent in Yunnan, Tibet, Henan, Hubei, Northeast China, Sichuan, and also occurs in Fujian, Guangdong, and Guangxi. Recent surveys across various regions show that the infection rate in pigs is generally 0.1-0.2%, with detection rates reaching 2% or 7% in some areas, and even as high as 50% in pigs sent for slaughter in certain regions. The infection rate and severity in rats are also relatively high.

bubble_chart Pathogen

The female Trichinella worm is 3-4 mm long, while the male is only 1.5 mm long. They typically reside in the duodenum and the upper part of the jejunum. After mating, the female worm burrows into the mucous membrane or reaches the mesenteric lymph nodes, where it releases larvae. These larvae travel through the lymphatic or blood vessels, passing through the liver and lungs, and enter the systemic circulation to spread throughout the body. However, only those that reach the striated muscles can continue to survive. The muscles most commonly affected are the diaphragm, gastrocnemius, buccinator, deltoid, biceps, and psoas muscles, followed by the abdominal, ocular, pectoral, nuchal, and gluteal muscles. Respiratory, tongue, masticatory, and swallowing muscles can also be involved. Five weeks after infection, the larvae form 0.4×0.25 mm olive-shaped cysts between muscle fibers, which mature within three months (becoming infective larvae) and calcify within six months to two years. Due to their small size, these calcified cysts are difficult to detect on X-rays. Larvae within calcified cysts can survive for up to three years (or up to 11 years in pigs). When mature cysts are ingested by an animal, the larvae emerge from the cysts in the upper small intestine, penetrate the intestinal mucosa, and develop into adult worms after four molts. Larvae begin to be excreted within a week of infection. Both adult worms and larvae reside within the same host.

bubble_chart Clinical Manifestations

The severity of Trichinella's pathogenic effects on the human body is related to factors such as the number of larval cysts ingested, their vitality, and the host's immune function status. Mild cases may be asymptomatic, while severe cases can be fatal. The infection process of Trichinella in humans can be divided into the following late stage [third stage]:

(1) Invasion period (small intestine period, about 1 week) scrotal swelling larvae penetrate the intestinal wall to mature, causing extensive duodenal inflammation, membrane congestion edema, bleeding, and even superficial ulcers. About half of the patients experience gastrointestinal symptoms such as nausea, vomiting, diarrhea (loose or watery stools, 3-6 times a day), constipation, abdominal pain (mainly in the upper abdomen or umbilical region, presenting as dull pain or burning sensation), and loss of appetite within a week of infection, accompanied by lack of strength, fear of cold, and fever. A few patients may have respiratory symptoms such as chest pain, chest tightness, and cough.

(2) Larval migration period (2-3 weeks) In the second week after infection, female worms produce a large number of larvae that invade the bloodstream and migrate to the striated muscles. Vascular inflammatory reactions can occur along the path of larval migration, causing significant heterologous protein reactions. Clinically, there is remittent high fever, lasting from 2 days to 2 months (average 3-6 weeks), with a few cases showing saddle fever. Some patients have rashes (maculopapular, urticarial, or scarlet fever-like rashes). Trichinella larvae can invade any striated muscle causing myositis: disappearance and degeneration of muscle cell striations, with lymphocytes, large mononuclear cells, neutrophils, and eosinophils, even epithelioid cells infiltrating around the larvae; clinically, there is muscle soreness, local edema, accompanied by tenderness and significant lack of strength. Muscle pain generally lasts 3-4 weeks, and in some cases, it can last more than 2 months. Muscle pain is severe and systemic, and most patients with rashes develop ocular symptoms, including eye muscle pain, eyelid and facial edema, conjunctival congestion, blurred vision, diplopia, and retinal hemorrhage. Grade III infections also involve the lungs, myocardium, and central nervous system, producing focal (or extensive) pulmonary hemorrhage, pulmonary edema, bronchopneumonia, and even pleural effusion; myocardial and endocardial membrane congestion, edema, interstitial inflammation, and even myocardial necrosis, pericardial effusion; non-suppurative meningoencephalitis and increased intracranial pressure. Blood eosinophils are often significantly increased (except in the most severe cases).

(3) Intramuscular cyst formation period (1-2 months after infection) As intramuscular cysts form, acute inflammation subsides, systemic symptoms lessen, but muscle pain can persist for a long time, although there is no definitive evidence of chronicity.

Severe patients may present with cachexia, collapse, or die from toxemia or myocarditis.

bubble_chart Auxiliary Examination

(1) Blood Picture: During the early migratory phase, the white blood cell count and eosinophils increase significantly, reaching 10,000 to 20,000/mm³, but in severe cases, eosinophils may not increase.

(2) Muscle Biopsy: In the fourth week after infection, a small piece of muscle near the tendon of the deltoid or gastrocnemius muscle (or the most prominent area of edema and myalgia) is taken, placed between two glass slides and pressed tightly. Observation under low magnification reveals coiled larvae surrounded by numerous inflammatory cells, forming small granulomas. Muscle biopsy is limited by the area of tissue extraction, making it difficult to detect larvae in the early stages of infection and in grade I infections. For those with mild infections and negative microscopy, the muscle slice can be digested with pepsin and dilute hydrochloric acid, and after centrifugation, the larvae can be examined, which has a higher positive rate than the squash method.

(3) Immunological Tests: Trichinella antigens can be divided into somatic antigens, soluble somatic antigens (including crude soluble antigens from infectious larvae and soluble antigens extracted from α-granules in the rod cells of infectious larvae), surface antigens (soluble antigens extracted or stripped from the surface of the larvae), and excretory-secretory antigens (or metabolic antigens). Various immunological tests have been tried both domestically and internationally, including intradermal tests, complement fixation tests, bentonite flocculation tests, counterimmunoelectrophoresis, circumlarval precipitation tests, indirect fluorescent antibody tests (IFA), indirect hemagglutination tests (IHA), enzyme-linked immunosorbent assays (ELISA), and indirect immunoenzymatic staining tests (IEST). The latter four have strong specificity and high sensitivity and can be used for early diagnosis. ① IFA has diagnostic value for both early and grade I infections. Using whole larvae as antigens, a positive reaction is indicated by fluorescent precipitates around the larval cortex or mouth. Patients may show positive reactions 2 to 7 weeks after infection. ② IHA uses freeze-dried sensitized sheep red blood cells to detect antibodies in the patient's serum. Using dried blood spots on filter paper instead of serum shows no significant difference in results, making it suitable for epidemiological surveys. ③ ELISA has higher sensitivity than IFA. Antigens are often prepared from physiological saline extracts of the larvae. ④ IEST uses frozen sections of infected mouse muscle as antigens to detect antibodies in the patient's serum. Serological tests become positive 2 to 4 weeks after infection, and most are positive by 7 weeks. A change from negative to positive or a fourfold increase in antibody titer is particularly diagnostic. Serological tests have achieved good results in antibody detection, but the persistence of antibodies in the serum after human or animal infection with Trichinella is not conducive to evaluating treatment efficacy. In recent years, monoclonal antibodies against Trichinella larvae have been successfully prepared both domestically and internationally. Using soluble somatic antigens and excretory-secretory antigens combined with monoclonal antibodies, a polyclonal antibody-indirect double antibody sandwich ELISA method is used to detect circulating antigens in the patient's serum. A positive antigen result indicates current infection and has value in evaluating treatment efficacy.

bubble_chart Diagnosis

Diagnostic basis: ① History of consuming raw pork, etc., 1-2 weeks (1-40 days) before the onset of illness; ② Clinical features mainly include fever, muscle pain, edema, rash, etc., with possible gastrointestinal symptoms in the initial stage [first stage], and a significant increase in total white blood cell count and eosinophils; ③ Definitive diagnosis relies on finding larvae in muscle biopsy and/or serological tests.

bubble_chart Treatment Measures

(1) General Treatment: Patients with obvious symptoms should rest in bed and be provided with adequate nutrition and fluids. Analgesics can be given for significant muscle pain. For severe patients with significant heterologous protein reactions or involvement of the myocardium or central nervous system, adrenal corticosteroids may be administered, preferably in conjunction with killing worms medication. The usual dose of prednisone is 20-30mg per day, taken for 3-5 days, and can be extended if necessary; alternatively, hydrocortisone 100mg/day can be administered intravenously, with the same course of treatment.

(2) Pathogenic Treatment: Among benzimidazole drugs, albendazole is the first choice due to its good efficacy and mild side effects. In China, different regimens with doses of 15mg/kg/day and 24-32mg/kg/day, divided into 2-3 oral doses, for a course of 5 days (up to 10 days) have achieved good results. If necessary, 1-2 additional courses can be repeated after an interval of 2 weeks. Generally, 2-3 days after taking the medication, body temperature decreases, muscle pain lessens, and edema disappears. In a few cases, a fever reaction may occur on the 2nd-3rd day after taking the medication. Thiabendazole is effective against both adult worms and larvae (migratory and encysted stages); the dose is 25mg/kg, twice daily, for a course of 5-7 days, and can be repeated after a few days if necessary. This drug may occasionally cause dizziness, nausea, vomiting, abdominal discomfort, dermatitis, decreased blood pressure, slowed heart rate, elevated serum transaminase levels, and other reactions, which can be alleviated by adding prednisone. Mebendazole is 95% effective against larvae of Trichinella spiralis at all stages, and slightly less effective against adult worms; the adult dose is 100mg, taken three times daily, for a course of 5-7 days (for larvae) or more than 10 days (for adult worms).

bubble_chart Prevention

(1) Strengthen health education: Not eating raw or undercooked pork and other mammalian meat or meat products is the simplest and most effective preventive measure.

(2) Control and manage the source of pestilence: Improve pig farming methods, promote enclosed farming, isolate and treat sick pigs; exterminate rats to prevent rat feces from contaminating pig pens; cook feed to prevent pigs from getting infected.

(3) Strengthen meat inspection: Meat that has not been inspected is not allowed to be sold. Stockpiled pork treated with low-temperature freezing, stored at -15℃ for 20 days, or at -20℃ for 24 hours, can kill larvae.

bubble_chart Differentiation

This disease should be differentiated from food poisoning (initial stage [first stage]), diseases with eosinophilia such as nodular polyarteritis, wind-dampness heat, dermatomyositis, leptospirosis, epidemic hemorrhagic fever, etc. Epidemiological data are of significant reference value for differential diagnosis.