Trichinosis (Trichinellosis)

How trichinosis is caught

Animals become infected by eating infected meat. The larvae form cysts in their muscles. When a person eats raw or undercooked meat containing these cysts:

1. stomach acid releases the larvae, which mature into adult worms in the small intestine within days
2. adult worms produce new larvae that enter the bloodstream
3. larvae travel to skeletal muscles and form cysts, where they can survive for years

Trichinosis is one of several parasitic diseases spread through food, and it does not spread from person to person.

In the US, cases are now rare, thanks to commercial pork regulations, and most come from bear meat and other wild game. Some Trichinella species in Arctic animals are freeze-resistant, so freezing does not make wild game safe.

Symptoms

Intestinal stage (first week)

• nausea and vomiting
• diarrhea
• abdominal pain
• fatigue and mild fever

Many people have mild or no intestinal symptoms, and the illness can be mistaken for food poisoning.

Muscle stage (2 to 8 weeks)

• muscle pain and tenderness, especially in the jaw, neck, arms, back and legs; pain with chewing, swallowing and breathing
• swelling around the eyes and face (periorbital edema, see swollen eyelids)
• fever and chills
• headache
• itching and rash
• conjunctivitis and small bleeds under the nails (splinter hemorrhages)
• weakness

Severe infection

Heavy infections can cause:

• inflammation of the heart muscle (myocarditis), with arrhythmias and heart failure
• brain inflammation (encephalitis), with confusion and seizures
• lung involvement and breathing difficulty

These complications are the main causes of death, which is rare.

Diagnosis

• history: eating raw or undercooked game or pork; others who ate the same meat being ill
• blood count: high eosinophils (eosinophilia), a hallmark
• muscle enzymes (CK) often raised, reflecting muscle damage (myositis)
• antibody tests: may not turn positive until 2 to 3 weeks after infection, so repeat testing may be needed
• muscle biopsy: occasionally, showing larvae
• testing leftover meat in outbreaks
• ECG and other tests for heart or brain involvement

Treatment

• Antiparasitic medicines: albendazole or mebendazole kill adult worms in the gut and are most effective early in infection. They are less effective once larvae have encysted in muscle.
• Corticosteroids: prednisone reduces inflammation and allergic-type symptoms in moderate to severe disease; given together with antiparasitic medicine.
• Pain relief: acetaminophen (paracetamol) or NSAIDs at the label dose; rest.
• Hospital care for heart, lung or brain complications.

Mild infections may resolve without treatment, but anyone who may have eaten infected meat should see a doctor, as early treatment can prevent muscle invasion.

Recovery

Most people recover fully. Muscle pain and fatigue can last for months as cysts calcify. Rarely, mild muscle aches persist for years.

Prevention

• cook meat thoroughly: pork and wild game to an internal temperature of at least 160 °F (71 °C) for ground meat and 145 °F (63 °C) with a 3-minute rest for whole cuts of pork; cook wild game to 160 °F (71 °C)
• use a meat thermometer; color is not a reliable guide
• do not rely on freezing, smoking, salting, drying or microwaving to kill larvae in wild game
• clean meat grinders thoroughly after use
• keep pigs from eating raw meat or rodents

Frequently asked questions

What are the symptoms of trichinosis?

Early symptoms are nausea, diarrhea and abdominal pain. Weeks later, people develop fever, muscle pain and tenderness, swollen eyelids, headache, itching and weakness. Severe infections can affect the heart and brain.

Can you still get trichinosis from pork?

It is now rare from commercial pork in the US, but it can still occur from pork raised on small farms, backyard or wild pigs, or from pork in countries with less regulation. Wild game, especially bear, is the most common source in North America.

How is trichinosis treated?

Treatment uses antiparasitic medicines such as albendazole or mebendazole, ideally early, plus corticosteroids for moderate to severe symptoms. Rest and pain relievers help muscle pain.

Does freezing kill trichinosis?

Freezing can kill the larvae in pork if done properly, but some Trichinella species in wild game such as bear and walrus resist freezing. Thorough cooking to a safe internal temperature is the only reliable prevention.