Ascariasis Case File

Ascariasis Case File

Eugene C.Toy, MD, Cynthia Debord, PHD, Audrey Wanger, PHD, Gilbert Castro, PHD, James D. Kettering, PHD, Donald Briscoe, MD

CASE 45

An 8-year-old boy, a child of immigrants from El Salvador who moved to the southwestern United States 3 months ago, presents to the emergency room with abdominal pain and vomiting. He was ill for a day, but his symptoms have worsened in the past few hours, and his parents panicked when they saw a worm in his vomitus. He has no significant medical history and has taken no medications. On examination, he appears very ill and in obvious pain. His temperature is 37.7°C (99.9°F), his pulse is 110 beats per minute, and his blood pressure is normal. His mucous membranes are dry, but his head and neck exam is otherwise normal. He is tachycardic, and his lungs are clear. His abdomen has high-pitched, tinkling bowel sounds on auscultation and is diffusely tender to palpation. There is, however, no rebound tenderness. An abdominal x-ray shows air-fluid levels consistent with a small bowel obstruction. His parents saved and brought in the worm that he vomited. It is 5 inches long and reddish-yellow in color.

◆ What organism is most likely responsible for the patient’s illness?

◆ How did the patient become infected?

ANSWERS TO CASE 45: ASCARIASIS

Summary: An 8-year-old boy with a small bowel obstruction and who vomited a worm.

◆ Most likely organism responsible for the patient’s illness: The nematode, Ascaris lumbricoides.

◆ How did the patient become infected: By ingesting eggs of the parasite that have matured in an external environment to contain a larval form.

CLINICAL CORRELATION

Infections with Ascaris are frequently asymptomatic or may be accompanied by numerous symptoms. The pathology that underlies symptoms is conditioned by the number of worms harbored and the sequential location of larvae in the lungs and adult worms in the intestine.

Larvae released when ingested eggs hatch make a so-called “heart-lung circuit” in which they are carried via the blood to various tissues (Figure 45-1). Migrating larvae elicit eosinophilia and granulomatous lesions. During the larval migration stage, symptoms are associated mainly with larvae present in the liver and lungs. Larvae that reach the lungs may cause pneumonitis. Patients may cough up exudates that are blood-tinged and contain polymorphonuclear leukocytes, mainly eosinophils. Fever may accompany pneumonitis, which usually disappears when the larvae move to the small intestine, as part of their life cycle. Ascaris is highly allergenic, stimulating inordinately high IgE levels. Therefore, pneumonitis may be especially severe in individuals who are sensitized by a primary infection and encounter a challenge or secondary infection. Thus, increased severity of pneumonitis is caused by immune-mediated inflammation.

Gastrointestinal (GI) symptoms depend on the number and location of adult worms in the GI tract. Light infection may cause GI upset, colic, and loss of appetite and impair digestion or absorption of nutrients. In heavy infections, the adult worms may ball up in the small intestine, resulting in physical obstruction, a rare but serious occurrence. From their usual location in the small intestine, adult worms infrequently migrate down and out of the anus or up and through the mouth or nose. Adult worms may cause symptoms by migrating to obstruct the bile ducts or penetrate into the gallbladder or liver. Adult worms may be seen in the stool or vomitus.

Figure 45-1. Life cycle of parasitic nematodes, including Ascaris lumbricoides.

(With permission from Barron S, ed. Medical Microbiology, 4th ed.

Galveston, TX: University of Texas Medical Branch, 1996.)

APPROACH TO SUSPECTED Ascaris INFECTION

Objectives

1. Learn the life cycle of A. lumbricoides and the epidemiology and clinical course of infection.

2. Be able to describe three basic aspects of infection: transmission, diagnosis, and treatment/prevention.

Definitions

Nematode: A helminth characterized by a cylindrical body and separate sexes, in contrast to cestodes and trematodes (tapeworms and flukes) that are flatworms and generally hermaphroditic.

“Giant intestinal roundworm”: Common name for Ascaris lumbricoides.

Egg (ova), larva, and adult: Sequential developmental stages in life cycle of helminths.

Larva: Preadult or juvenile stage of helminths; nematodes have four larval stages in their life cycle prior to reaching the adult stage. Each stage is preceded by a molt or shedding of the cuticle or “skin.”

DISCUSSION

Characteristics of Ascaris

Ascaris is one of several nematodes that infect the GI tract and, like all of these, develops through egg, larva, and adult stages. Ascaris lumbricoides is very similar in size and life cycle to Ascaris suum, which is the species found in pigs. Parasitologists debate the point that there is only one species of Ascaris that infects both pigs and humans. If a human ingests eggs of A. suum, the larvae will migrate to the lungs and die, but in the process can cause a serious form of “ascaris pneumonia.” Adult worms of A. suum do not develop in the human intestine. Other ascarids that infect humans are Toxocara canis and Toxocara cati, which are parasites of dogs and cats, respectively. Although humans can become infected by ingesting eggs of these species, neither worm develops to maturity. After hatching in the intestine, Toxocara larvae migrate chronically in visceral tissue, giving rise to the condition termed “visceral larvae migrans.”

Infection Transmission

Typical of nematodes, A. lumbricoides has separate sexes and a life cycle that involves egg, larva, and adult stages. Growth involves four larval stages. Transition from one stage to another and to the adult stage is preceded by a molt. A person becomes infected by ingesting eggs that are usually acquired through hand-to-mouth transmission from the soil or via contaminated food or

water. Eggs hatch in the duodenum, releasing larvae that penetrate the small intestinal wall to enter the bloodstream. Larvae are carried hematogenously to various organs. On reaching the lungs, they penetrate into the alveoli. Larvae reside in the lung for approximately 3 weeks, growing and advancing to a subsequent larval stage. They then migrate up the bronchi and trachea, where they are then swallowed and returned to the small intestine. In the intestine, larvae develop to the adult stage and reach reproductive maturity in approximately 2 months. Adults can survive for up to 2 years and grow to 15–35 cm in length. A single female worm lays approximately 200,000 eggs daily that are passed in the feces. Eggs can survive for years in the soil, tolerating a wide range of temperatures and other environmental variables. Under optimal conditions, eggs reach an infective, larvated stage in 2–3 weeks. Development is arrested at this stage until the egg is ingested.

Ascariasis is a chronic disease of the small intestine and can be transmitted as long as adult worms are in the intestine and feces containing Ascaris eggs are allowed to contaminate the environment. Ascariasis is more prevalent in tropical climates but is found in temperate regions of the world. It is most common where sanitation is poor and where human feces are used as fertilizer in agricultural practices. Infections occur in all ages but are more common in children. In endemic areas, most of the population has some worm burden. With the exception of pinworm infection, ascariasis is the most common helminthic infection of humans on a global scale.

Diagnosis

A presumptive diagnosis may be based on clinical symptoms. However, symptoms are not pathognomonic. The pneumonitis phase of infection cannot be diagnosed as Ascaris-induced because it is a generalized symptom that precedes the intestinal phase of infection by several weeks. Intestinal symptoms are normally absent or mild and, in most cases, go undetected.

A definitive diagnosis is based on identifying egg or adult stages. As in this case, the first clue of infection may be adult worms that are vomited or passed in a stool. Intestinal obstruction, especially in children, often prompts medical attention. Because of its uniquely large size, an adult A. lumbricoides is unmistakable. The adult females are reddish-yellow in color and can measure up to 7–13 inches long (males are generally shorter) and one-fourth of an inch in diameter. Although a definitive diagnosis can be made by identifying adult worms, ascariasis is more commonly diagnosed by identifying eggs in the stool. Both fertile and infertile eggs are passed in the stool. Because female worms pass thousands of eggs daily, diagnosis can be made by direct examination of a stool sample without the need for specialized laboratory techniques to concentrate eggs. The eggs are ovoid, measure 45 by 75 μm and have a thick transparent inner shell covered by an albuminous coat that is wrinkled and usually stained light brown by bile pigments. Unfertilized eggs are commonly seen in a stool. These are more elongated, measuring 40 by 90 μm. The inside of the unfertilized egg is amorphous instead of containing a well defined single cell of the fertilized egg.

Treatment and Prevention

Adult worms are the target of several drugs available to treat ascariasis. The drug of choice is mebendazole, a benzimidazole derivative that has a high therapeutic index. The effectiveness of treatment can be assessed by the disappearance of eggs in stool samples and alleviation of symptoms. In cases of intestinal obstruction, the first action should be directed at ridding the patient of worms through the use of chemotherapeutic agents, such as mebendazole. If the worms can be dislodged the patient may void them in the stool. If the bolus of worms cannot be dislodged, surgery may be an option. If surgery is chosen, it is imperative that the worms be killed or paralyzed with a drug such as piperazine prior to physically removing them from the intestine. The reason for this procedure is that a live, active worm releases an aerosol of eggs through its uterine pore. Eggs are invisible to the naked eye. However, if eggs fall on exposed viscera they elicit granulomatous lesions and adhesions that can lead to severe complications. Preventive measures relate to instructing the patient on how the infection was acquired and on the proper disposal of feces to avoid soil contamination and reinfection.

Synopsis

The synopsis of ascariasis is presented in Table 45-1.

Table 45-1

COMPREHENSION QUESTIONS

[45.1] A definitive diagnosis of ascariasis can be made by observing which of the following?

A. An eosinophilia in a differential white blood cell count

B. Motile larvae in a stool sample

C. Larvae in x-ray of lungs

D. An adult worm passed during a bowel movement

[45.2] Ascariasis is most effectively treated with which of the following drugs?

A. Mebendazole

B. Metronidazole

C. Niclosamide

D. Praziquantel

[45.3] A person presents to his physician complaining of chronic GI symptoms. A diagnosis of A. lumbricoides is made. Human nematodes infect individuals via different routes. This patient is most likely infected by which of the following?

A. Larvae penetrating unprotected skin

B. Ingesting larvated eggs

C. Eating uncooked pork

D. Internal autoinfection

Answers

[45.1] D. Identification of adult worms passed by an individual can ensure a definitive diagnosis. Although an eosinophilia accompanies ascariasis, this condition can be caused by other infections and by various allergic conditions. Motile larvae in a stool are indicative of infection with Strongyloides stercoralis. Nematode larvae cannot be seen in an x-ray.

[45.2] A. Mebendazole is a broad spectrum antihelminthic that is the drug of choice for treating Ascaris and several other intestinal nematodes. Metronidazole is used primarily to treat infections with protozoa, such as Giardia lamblia and Trichomonas vaginalis. Niclosamide is the drug of choice in treating most adult cestode or tapeworm infections. Praziquantel is used to treat infections with blood flukes or schistosomes and can also be used to treat adult tapeworms.

[45.3] B. Ascaris is acquired by ingesting infective eggs, as are the whipworm, Trichuris trichiura, and the pinworm, Enterobius vermicularis. Filariform larvae penetrating unprotected skin is the route by which a person becomes infected by other intestinal nematodes, hookworms and Strongyloides; eating uncooked pork could lead to trichinosis; and internal autoinfection is caused only by Strongyloides.

MICROBIOLOGY PEARLS ❖ The fertilized eggs are unsegmented when laid by the female worm. ❖ Mebendazole, one of several available benzimidazole compounds used to treat nematodes, is the drug of choice in treating Ascaris infection.

REFERENCES

Centers for Disease Control. DPDx. Laboratory identification of parasites of public concern. Ascariasis. 2007. http://www.dpd.cdc.gov/dpdx/HTML/Ascariasis.htm. 

Centers for Disease Control. DPDx. Laboratory identification of parasites of public concern. Image Library. Ascariasis. 2007. http://www.dpd.cdc.gov/dpdx/ HTML/ImageLibrary/Ascariasis_il.htm. 

Medical Letter on Drugs and Therapeutics. Drugs for parasitic infections. New Rochelle, NY. 2004. http://www. medletter.com/freedocs/parasitic.pdf. This reference has been updated and “superseded by the special report Drugs for Parasitic Infections, which can be purchased (on-line) for $25.” This reference has been provided because it is a comprehensive and clinically useful reference that is regularly updated and could be of value to those who are involved in treating parasitic infections.

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