Roundworm infections
Roundworm infections are diseases of the digestive tract and other organ systems caused by nematodes--parasitic worms with long, cylindrical bodies. Roundworm infections are widespread throughout the world, and humans acquire mosttypes of roundworm infection from contaminated food or by touching the mouthwith unwashed hands which have come into contact with the parasite larva. Theseverity of infection varies considerably from person to person. Children are more likely to have heavy infestations and are also more likely to suffer from malabsorption and malnutrition than adults.
The type and symptoms of roundworm infections vary according to the species of nematode which causes them. Anisakiasis infection is caused by anisakid roundworms. Humans are not the primary host for these parasites which infest whales, seals, and dolphins. However, crabs ingest roundworm eggs from the fecesof these animals. In the crabs, the eggs hatch into larvae that can infect fish. The larvae enter the muscles of marine animals further up the food chain, including squid, mackerel, herring, cod, salmon, tuna, and halibut. Humansbecome accidental hosts when they eat raw or undercooked fish containing anisakid larvae. The larvae attach themselves to the tissues lining the stomach and intestines, and eventually die inside the inflamed tissue.
In humans, anisakiasis can produce either a severe syndrome that affects thestomach and intestines, or a mild chronic disease that may last for weeks oryears. In acute anisakiasis, symptoms begin within one to seven hours after the individual eats infected seafood. Patients are often violently sick, withnausea, vomiting, diarrhea, and severe abdominal pain that may resemble appendicitis. In chronic anisakiasis, the patient has milder forms of stomach or intestinal irritation that resemble stomach ulcers or irritable bowel syndrome. In some cases, the acute form of the disease is followed by chronic infestation.
Ascariasis, which is caused by Ascaris lumbricoides, is one of the most widespread parasitic infections in humans, affecting over 1.3 billion people worldwide. Ascarid roundworms cause a larger burden on the human host thanany other parasite; adult worms can grow as long as 12 or 14 inches, and release 200,000 eggs per day. The eggs infect people who eat unwashed vegetablesfrom contaminated soil or touch their mouths with unwashed hands which have become contaminated. Once inside the digestive tract, the eggs release larvaethat penetrate the intestinal wall and migrate (travel) to the lungs throughthe liver and the bloodstream. After about 10 days in the lungs, the larvae migrate further into the patient's upper lung passages and airway, where theyare swallowed. When they return to the intestine, they mature into adults andreproduce. The time period from the beginning of the infection to egg production is 60-75 days.
The first symptoms of infection may occur when the larvae reach the lungs. The patient may develop chest pain, coughing, difficulty breathing, and inflammation of the lungs. In some cases, the patient's sputum (phlegm) is streakedwith blood. This phase of the disease is sometimes called Loeffler's syndrome. It is marked by an accumulation of parasites in the lung tissue and by eosinophilia (an abnormal increase in the number of a specific type of white blood cell). The intestinal phase of ascariasis is marked by stomach pain, cramping, nausea, and intestinal blockage in severe cases.
Toxocariasis is sometimes called visceral larva migrans (VLM) because the larval form of the organism hatches inside the intestines and migrates throughout the body to other organs (viscera). The disease is caused by Toxocara canis and T. cati, which live within the intestines of dogs and cats. Most human patients are children between the ages of two and four years, who become infected after playing in sandboxes or soil contaminated by pet feces, although adults are also susceptible. The eggs can survive in soil for aslong as seven years.
The organism's eggs hatch inside the human intestine and release larvae thatare carried in the bloodstream to all parts of the body, including the eyes,liver, lungs, heart, and brain. The patient usually has a fever, with coughing or wheezing and a swollen liver. Some patients develop skin rashes and inflammation of the lungs. The larvae may survive inside the body for months, producing allergic reactions and small granulomas (tissue swellings or growths produced in response to inflammation). Infection of the eye can produce ocularlarva migrans (OLM), which is the first symptom of toxocariasis in some patients.
Trichuriasis, caused by Trichuris trichiura, is sometimes called whipworm because the organism has a long, slender, whiplike front end. The adult worm is slightly less than an inch long. Trichuriasis is most common in warm,humid climates, including the southeastern United States. The number of people with trichuriasis may be as high as 800 million worldwide.
Whipworm larvae hatch in the small intestine from swallowed eggs and move onto the upper part of the large intestine where they attach themselves to thelining. The adult worms produce eggs that are passed in the feces and maturein the soil. Patients with mild infections may have few or no symptoms. In cases of heavy infestation, the patient may have abdominal cramps and other symptoms resembling amebic dysentery. In children, severe trichuriasis may causeanemia and developmental retardation.
Roundworm infections are diagnosed by several different methods. Since the first symptoms of roundworm infection are common to a number of illnesses, a doctor is most likely to consider the possibility of a parasitic disease on thebasis of the patient's history--especially in children. The definite diagnosis is based on the results of stool (feces) or tissue tests. In trichuriasis,adult worms may also be visible in the lining of the patient's rectum. In ascariasis, adult worms may appear in the patient's feces or vomit and can alsobe detected by x ray and ultrasound tests. In toxocariasis, larvae are sometimes found in tissue samples taken from a granuloma. If a patient with toxocariasis develops OLM, it is important to obtain a granuloma sample in order todistinguish between OLM and retinoblastoma (a type of cancerous eye tumor).
Anisakiasis is one of two roundworm infections that cannot be diagnosed fromstool specimens. Instead, the diagnosis is made by x rays of the patient's stomach and small intestine. The larvae may appear as small threads when doublecontrast x rays are used. In acute cases, the doctor may use an endoscope (an instrument for examining the interior of a body cavity) to look for or remove larvae.
Blood tests cannot be used to differentiate among different types of roundworm infections, but the presence of eosinophilia can help to confirm the diagnosis.
Patients with trichuriasis or ascariasis should be examined for signs of infection by other roundworm species, as well, as many patients are infected by several parasites at the same time.
Treatment also depends upon the type being treated. Trichuriasis, ascariasis,and toxocariasis are treated with anthelminthic medications--drugs that destroy roundworms either by paralyzing them or by blocking them from feeding. Anthelminthic drugs include pyrantel pamoate, piperazine, albendazole, and mebendazole. Mebendazole cannot be given to pregnant women because it may harm the fetus. Treatment with anthelminthic drugs does not prevent reinfection. Patients with an intestinal obstruction caused by ascariasis may be given nasogastric suction, followed by anthelminthic drugs, in order to avoid surgery. Ifsuction fails, the worms must be removed surgically to prevent intestinal rupture or blockage. There is no drug treatment for anisakiasis; however, symptoms usually resolve in one to two weeks when the larvae die. In some cases, the larvae are removed with an endoscope or by surgery.
The prognosis (expected outcome) for recovery from roundworm infections is good for most patients. Ascariasis is the only roundworm infection with a significant mortality rate. A. lumbricoides grows large enough to perforatethe bile or pancreatic ducts; in addition, a mass of worms in the digestivetract can cause rupture or blockage of the intestines. It is estimated that 20,000 children die every year from intestinal ascariasis.
There are no effective vaccines against any of the soil-transmitted roundworms, nor does infection bring about immunity from reinfection. Prevention of infection or reinfection requires adequate hygiene and sanitation measures, including regular and careful handwashing before eating or touching the mouth with the hands.
With respect to specific infections, anisakiasis can be prevented by avoidingraw or improperly prepared fish or squid. Trichuriasis, ascariasis, and toxocariasis can be prevented by keeping children from playing in soil contaminated by human or animal feces, by teaching children to wash their hands beforeeating, and by having pets dewormed regularly by a veterinarian.