Digestive System Diseases

Fungal, Protozoan, and Helminth Infections of the Digestive System

Fungal Infections

Alimentary canal diseases are rarely caused by fungi in healthy individuals; however, Aspergillus and Candida mycoses are increasingly recognized as complications in immunocompromised patients.

The population of immunocompromised patients is increasing. Chemotherapy treatment, immunosuppression associated with transplantation, and infectious diseases such as human immunodeficiency virus (HIV) leave patients with reduced immunocompetence. Fungal genera such as Candida and Aspergillus opportunistically infect the alimentary canal of the immunocompromised, causing mycosis, a disease resulting from fungal infection. Typical symptoms of alimentary canal mycoses include abdominal pain, diarrhea, fever, hemorrhage with melena, and vomiting. While species of Candida and, especially, Aspergillus are the most common causes of alimentary canal mycosis, several other species have been identified as causative agents, including Basidiobolus ranarum, Fusarium species, members of the order Mucorales, Penicillium marneffei, and many others. Determination of the causative agent determines the treatment course followed. Amplification of the ribosomal RNA gene using PCR and subsequently sequencing the DNA product is the only conclusive diagnostic for most fungal infections. Some species causing mycosis may be cultured, but the process is time-consuming. Antifungal treatment options vary according the causative species, the location of the infection within the digestive system, and the immune status and age of the patient. Common antifungal drugs are amphotericin B, echinocandins, and fluconazole.

Candida species may cause a condition called candidiasis, or thrush, that is characterized by an overgrowth of the yeast in the mouth. The tongue is most commonly infected and becomes covered in cream-colored yeast. Candida species are a normal part of the native microbial community in humans and are regularly found in the intestines, mouth, skin, and vagina. Their overgrowth in the mouth in otherwise healthy individuals is most typical during or immediately following antibiotic therapy that removes much of the native bacterial flora of the mouth. The condition also presents in those with weakened immune systems. Treatment is not always necessary for healthy individuals who experience the condition following antibiotic treatment. Once the antibiotic course is complete, the mouth is recolonized by bacteria, controlling the Candida overgrowth. Otherwise, topical antifungals are most often prescribed.

Protozoan Infections

Several single-celled eukaryotes, protozoa, are pathogenic and result in digestive system disease.
The term protozoa represents the taxonomically diverse and largely unrelated single-celled eukaryotic organisms. Protozoa are widely distributed in the environment and are often associated with aquatic habitats. Several protozoa cause digestive system diseases in humans and other mammals. Protozoan infections are typically associated with poor sanitation and are acquired through the consumption of food or water contaminated with fecal material from other human or mammal hosts.

Protozoan Infections of the Digestive System

Causative Agent Description Symptoms Diagnosis and Treatment
Cryptosporidium parvum and C. hominis Apicomplexa causing cryptosporidiosis Cramps, diarrhea with dehydration, fever, nausea, weight loss Immunoassay, microscopic examination of stool, PCR;
Rehydration, self-limiting, or azithromycin, nitazoxanide, paromomycin
Cyclospora cayetanensis Most prevalent in tropics, colonizes intestinal mucosa Asymptomatic, or cramps, diarrhea with dehydration, fatigue, fever, nausea, vomiting Microscopic examination of stool, especially with UV fluorescence;
Entamoeba histolytica Amoeba causing amoebiasis (amoebic dysentery) Asymptomatic, or mild diarrhea to abdominal distension with colonic infection and fever; may spread, causing abscesses with more serious symptoms Microscopic examination of successive stools;
Metronidazole, tinidazole
Giardia lamblia Diplomonads with two nuclei and four flagella, lack true mitochondria, cause giardiasis Abdominal pain and cramps, diarrhea with dehydration, excessive flatulence, nausea. Chronic infections may include nutrient absorption problems and weight loss. Immunoassay, microscopic examination of stool;
May clear without treatment, or albendazole, metronidazole, tinidazole

Protozoa are a diverse group of organisms causing digestive disorders. Most protozoan infections result from consumption of contaminated food or water and are treated with specific antiprotozoal therapies.

Helminth Infections

Helminths that parasitize humans include nematodes, trematodes, and cestodes.
As adults, helminths are macroscopic worms that are parasitic to humans. Helminths can range in size from 1 millimeter to up to 20 meters long. Helminths are included in microbiology due to the fact that the infectious stages of helminths are microscopic. The many parasitic helminth species include roundworms (nematodes), flukes (trematodes), and tapeworms (cestodes). Helminth infections are spread when helminth eggs are passed from one person to another, typically by ingesting contaminated food. Food sources become contaminated by helminths when soil is fertilized with feces from infected animals or watered with contaminated water. Most commonly, helminth infections are found in tropical or subtropical environments where water is unsanitary.
Enterobius vermicularis, known as the pinworm, is a common intestinal parasite that deposits its eggs on the skin of the anus. Strongyloides stercoralis is a type of helminth that can enter the body through exposed skin, such as bare feet, and cause the disease strongyloidiasis.
Credit: CDC/Dr. George R. Healy (left), CDC/Dr. Mae Melvin (right)
Helminth infections can cause symptoms in organ systems throughout the body, including but not limited to the digestive system. The major group of helminths that infects the alimentary canal are nematodes, leading to approximately 1.5 billion cases worldwide at any given time. Helminth infections can be treated but may lead to chronic infections when treatment is not available. Symptoms of helminth infection can include anemia, or low blood oxygen levels, malnutrition, fatigue, and cognitive decline. Because of the taxing effect combating a helminth infection has on the immune system, helminth infection can also cause susceptibility to secondary infections that the immune system becomes incapable of fighting off. HIV and tuberculosis are two common secondary infections found in patients with helminth infections.

The life cycle of most helminths consists of three phases: the egg, the larval stage, and the adult stage. Each of these stages of helminth development is associated with different properties of infection. Different species of helminth require very specific living conditions. For example larval liver flukes, Fasciloa species, live in snails, but the adult liver fluke lives in the bile duct of mammals. Helminths that infect humans cannot undergo their life cycle in any other animals. However, the life cycle of helminths is similar regardless of the host they are able to parasitize.

Helminth eggs can exist in the environment or in a host following ingestion. Helminth larvae have the same characteristics. Once in the host, the eggs or larvae continue their life cycle, ultimately reaching their adult phase within the host. Many helminth infections that are contracted through ingestion via the alimentary canal undergo this development in the intestines. The eggs made by adult worms in the intestines are excreted from the host via feces, which can then be picked up by the same or a new individual, continuing the helminth life cycle.

Reinfection with the same parasite is common in areas with poor sanitation or where many individuals live in close quarters. Some helminths that infect via the alimentary canal, however, do not remain there and can instead undergo part of their life cycle elsewhere in the body. The larvae of a helminth known as Strongyloides stercoralis can migrate to the lungs, while the eggs of Enterobius vermicularis develop on the skin of the anus.

Life Cycle of the Hookworm

Ancylostoma duodenale and Necator americanus are nematodes that infect through the skin. They travel in blood to the lungs and then move to the alimentary canal and mature in the intestine. Eggs are released in feces.

Helminth Infections of the Digestive System

Causative Agent Transmission Symptoms Diagnosis and Treatment
Ancylostoma duodenale and Necator americanus (nematode hookworms; larvae feed on blood in the intestines) Larvae penetrate the skin from soil contaminated by domestic animal feces. Larvae migrate in blood to the lungs and, with coughing, are swallowed. Abdominal pain, coughing, diarrhea, rash, reduced appetite Microscopic examination of stool for eggs;
Albendazole, mebendazole, pyrantel pamoate
Ascaris lumbricoides (nematodes; live in tropical climates) Consumption of food contaminated with eggs from soil Asymptomatic, or abdominal pain, bloody feces, diarrhea, nausea Microscopic examination of stool for eggs, visual evidence of worms in feces or vomit;
Self-limiting, albendazole, mebendazole
Enterobius vermicularis (nematodes) Fecal-oral Itching around the anus, particularly during sleep and occasionally leading to insomnia, abdominal pain Visual inspection for worms during sleep, inspecting tape peel of anus for eggs;
Albendazole, mebendazole, pyrantel pamoate
Strongyloides stercoralis (nematodes) Skin penetration, typically the foot. Larvae migrate in blood to the lungs and, with coughing, are swallowed. Asymptomatic, or with immunosuppression and enhanced growth, nonspecific symptoms develop Visual examination of feces, immunoassay;
Albendazole, ivermectin
Taenia asiatica, Taenia saginata, and Taenia solium (cestodes; attach to the intestinal wall and grow continuously while releasing eggs through the host's feces) Consumption of undercooked meat or fecal-oral Asymptomatic, or diarrhea, flatulence, hunger, mild intestinal pain, nausea. When eggs are consumed directly, larvae can migrate into the brain, eyes, or muscle, forming cysts, which may cause headache, pain, and seizures, and may become life-threatening. Microscopic examination of successive bowel movements for eggs or egg-containing body segments (proglottids) Niclosamide, praziquantel
Trichinella spiralis, other Trichinella species (nematodes; Cysts rupture in the stomach, mature into adults, and birth larvae that move into muscle tissue, forming cysts.) Consumption of cysts in undercooked pork Asymptomatic, or abdominal pain, chills, constipation, diarrhea, headache Enzyme immunoassays, muscle biopsy;
Albendazole or mebendazole for larvae in the alimentary canal. Treating cysts is ineffective.
Fasciola hepatica (liver flukes; develop into adults in the bile ducts of infected animals. Consumption of contaminated water Nausea, vomiting, and abdominal pain/tenderness. Fever, rash, and difficulty breathing may occur. Eggs seen in stool, examination of duodenal contents and bile;
Oral administration of triclabendazole.
Fasciolopsis buski (intestinal flukes; trematodes; adult worms attach to lining of the bowel and cause ulcers to form in it) infection is by eating aquatic plants (e.g, water chestnuts) that bear infectious stage Diarrhea, abdominal pain, fever, and signs of malabsorption or intestinal obstruction. Finding eggs or, less commonly, adult worms in the feces;
Administration of praziquantel for one day

Several types of worms categorized as helminths parasitize the digestive system. Helminths often have complex life cycles, leading to infections acquired through mechanisms that are unusual in other groups of pathogens.

Taenia solium is one of three cestodes commonly called tapeworms that infect the intestine and may grow to lengths of several meters.
Credit: CDC/Dr. Mae Melvin