Trichinosis, as known as trichinellosis, is a zoonotic, parasitic disease caused by Trichinella spiralis and is characterized by gastrointestinal symptoms, fever, myalgia, and edema.
Adults and larvae of Trichinella spiralis parasitize the same host, but the hosts must be replaced in order to complete the life cycle.
Adults parasitize the duodenum and anterior jejunum. Females are 3 - 4mm long and are ovoviviparous, while males are only 1.5mm long. The larvae in the eggs are hatched in the uterus. The females directly deliver larvae, and the larvae are widely present in the striated muscle. Humans or mammals are infected after ingestion of meat with cysts containing live Trichinella spiralis larvae. The larvae escape from the cysts in the duodenum and upper jejunum few hours after the Trichinella spiralis cysts enter the body, penetrate the intestinal mucosa of the duodenum and anterior jejunum, and develop into adults after 4 molts within 2 days. After copulation, the males die and are excreted out from the intestinal cavity. Some females reinvade the intestinal mucosa, and some can parasitize the abdominal cavity or mesenteric lymph nodes. The fertilized females begin to deliver larvae 5 - 7 days after infection, and can deliver persistently larvae for 4 - 16 weeks. Each female can deliver about 1500 - 2000 larvae in her life.
Some of the newborn larvae are discharged through the intestinal cavity, and most of them reach the various tissues of the human body through the lymph and blood circulation. The larvae can develop only in the striated muscle, mostly the diaphragm, intercostal muscles, masticatory muscles, tongue muscles, cervical muscles, biceps, deltoids, and gastrocnemius muscles. The larvae grow and develop rapidly after colonization, and are spirally curled in 2 weeks. The larvae stimulate the host tissue to produce fusiform cysts. The outer wall of the cysts is a thin, fibrous, connective tissue derived from the proliferation of fibroblasts, and the inner wall is transparent substances composed of degenerated muscle cells. There are 1 - 2, about 1mm long, curled larvae in each cyst. The newly formed cysts are small in size and can gradually increase to 0.25 - 0.50mm × 0.21 - 0.42mm over time. The long axis of the cysts is parallel to that of the muscle fiber. The cysts can be calcified in 6 months, and the larvae may die. However, some larvae in the calcified cysts are still alive, and can survive for up to 31 years.
Humans are infected by ingestion of undercooked pork, dog meat, or lamb containing larval cysts.
Signs and Symptoms
The incubation period of trichinosis is generally 5 - 15 days, averagely 10 days, as short as few hours, as long as 46 days. The shorter the incubation period is, the more severe the condition is.
According to the infection process in the human body, the disease can be divided into 3 stages.
The decysted larvae penetrate the intestinal wall and develop into adults there, causing extensive duodenal inflammation, mucosal congestion and edema, hemorrhage, and even superficial ulcers. There are nausea, vomiting, diarrhea (loose or watery stools, 3 - 6 times a day), constipation, abdominal pain (mainly in the upper abdomen or umbilical area, with dull pain or burning sensation), anorexia, fatigue, chills, and fever within 1 week after infection in almost half of patients. Few patients may present respiratory symptoms such as chest pain, chest distress, and cough.
In the second week after infection, females deliver a large number of larvae. The larvae invade the blood circulation, migrating to the striated muscle. Vascular inflammation and significant reactions to the foreign proteins can occur where larvae migrate. There is remittent fever, lasting from 2 days to 2 months, averagely 3 - 6 weeks, and saddleback fever is present in few patients. Some patients present with skin rashes such as maculopapular rash, urticaria, and scarlatiniform rash. Trichinella spiralis larvae can invade any striated muscle and cause myositis. There are lymphocytes, large monocytes, neutrophils, eosinophils, and even epithelial cell infiltration around the larvae. Clinically, there are muscle soreness, local edema, tenderness, and significant fatigue. Myalgia usually lasts for 3 - 4 weeks, up to more than 2 months. Myalgia is severe and systemic. Most patients with skin rash present ocular symptoms, such as ocular muscle pain, periorbital edema, facial edema, bulbar conjunctival hyperemia, blurred vision, diplopia, and retinal hemorrhage. In the severe patients, the lungs, myocardium, and central nervous system are also involved, and there are focal or extensive pulmonary hemorrhage, pulmonary edema, bronchopneumonia, pleural effusion, myocardial and endocardial congestion and edema, interstitial inflammation, myocardial necrosis, pericardial effusion，nonsuppurative meningoencephalitis，and increased intracranial pressure. Eosinophils are often increased significantly.
With the formation of intramuscular cysts, acute inflammation subsides and systemic symptoms alleviate, but myalgia can last for a long time. Severe patients may present with cachexia, collapse, or die of toxemia or myocarditis.
Definitive diagnosis is dependent on the detection of larval cysts of Trichinella spiralis, but it is very difficult to find the cysts.
If there are fever, edema, myalgia, and eosinophilia, this disease should be suspected.
Anthelmintics can repel adults in the digestive tract, but are almost ineffective against larvae in the cysts.
The treatment regimen is albendazole 400mg twice a day orally for 8 - 14 days, or mebendazole 200 - 400mg 3 times a day for 3 days followed by 400 - 500 mg orally 3 times a day for 10 days.
Myalgia may require analgesics. For patients with severe allergic symptoms or with myocardial or central nervous system involvement, the treatment regimen is prednisone 20 - 60mg orally once a day for 3 - 4 days, followed by gradual dose reduction until withdrawal in 10 - 14 days.