NEMATODES The phylum Nematoda is a class of the phylum Nemathelminthes and includes the true roundworms as well as those called threadworms. Six separate orders of Nematoda contain species that infect humans and other mammals, and it should be noted that a number of species also are capable of infecting plants and other forms of life, such as mollusks and insects. WHIPWORM INFECTIONS Whipworms are a type of roundworm of which there are perhaps as many as 60 different species. Humans are infected chiefly by a species of whipworm originally called Trichuris trichuris, but currently identified as T. trichiura, where they inhabit the large intestine of humans and animals. The name of the organism refers to the characteristic shape of the adult organism. Life Cycle TRICHURIS TRICHIURA The human whipworm (Trichuris trichiura or Trichocephalus trichiuris) and previously known as Trichuris trichiura, is one of the most prevalent parasitic roundworms found worldwide. The existence of the organism has been documented for many years and in the older literature it was described as causing a condition called trichuriasis when it infects the large intestine of the human. Morphology The Trichuris trichiura parasite is commonly referred to as the “whipworm” because of its resemblance in the adult worm to a whip. The name refers to the shape of the worm with the appearance of an old-fashioned buggy whip with a long and slender thread-like anterior portion and with thicker, wider “handles” at the posterior end that is long and slender, described as a threadlike caudal portion. The male is characterized by a slightly coiled tail, whereas the female has a rounded and somewhat blunt posterior that is not coiled. The adult worm ranges from 30 to 50 mm with a range of 35 to 50 mm for the female. The male is slightly smaller than the female, ranging from 30 to 45 mm in length. The eggs have a mucogelatinous plug at each terminal end of the elongated egg that is bilestained and barrel-shaped. A smooth but thick shell covers the egg, which is from 45 to 55 μm in length and 20 to 23 μm in width. A simple and direct life cycle for T. trichiuria involves the passage of feces containing eggs that mature in about 2 weeks and become infective. Embryonated eggs with first-stage larvae are ingested from contaminated and packed clay soil that clings to root vegetables and leaves of other vegetables in many areas of the world where environmental conditions suitable for the propagation of whipworms are present. Development of the larvae occurs in the duodenum and cecum, where the worm attaches to the mucosa of the intestine by its anterior mouthparts. In about 3 months, the adult is able to lay eggs. A female may produce 2,000 to 20,000 unembryonated eggs per day, which may be deposited in the human feces and into the soil. These unembryonated eggs from the stool of the infected individual incubate in the soil and reach an infective capability when they achieve an embryonated stage after 2 to 3 weeks. The embryonated eggs contain the first stage of larval development and are contracted primarily from hard clay soil, where heavy rain fall may leach other organic nutrients from the soil. It is not uncommon to find a T. trichiura infection accompanied by a second parasite such as A. lumbricoides, and perhaps along with other common intestinal parasites. Whipworm organisms are spread to humans through a fecal-oral transmission when contact with soil containing whipworm eggs occurs. Whipworm infections are more common in children through playing outside and introducing focally contaminated dirt containing whipworm eggs into the mouth. Although whipworms are distributed on a worldwide basis, they survive best in tropical or semitropical climates including the southeastern United States where the weather is warm and humid and plentiful rainfall is present. In addition, recent studies have shown a possible genetic predisposition to infection with the whipworm. Disease Transmission Symptoms Whipworm infections in humans can cause a range of symptoms, by presenting no symptoms at all to only mild symptoms that may progress to somewhat severe symptoms. As in the case with hookworm infections, severely heavy numbers of whipworms in humans can cause stomach pain with loss of appetite and iron deficiency, bloody diarrhea, weight loss, rectal prolapse (detached rectum), and fecal incontinence. Ingestion of embryonated eggs from both contaminated water and food is the most common route leading to a whipworm infection. But direct infection from the soil contaminated with feces and close contact during activities such as gardening and farming also provides for a major portion of the cases of trichiuris. In addition to placing dirty fingers into the mouth, another common way of becoming infected is the ingestion of T. trichiura eggs due to poor preparation of foods (e.g., eating unwashed vegetables from soil that is contaminated. As with a number of parasitic infections, whipworms are frequently transmitted to a host either through eating vegetables grown in contaminated soil or by direct contact by an individual with the soil where infective eggs are introduced to the mouth. It is not uncommon to find a T. trichiuris infection accompa nied by a second parasite such as A. lumbricoides Treatment and Prevention Laboratory Diagnosis Stool specimens and identification of the eggs, which are characteristic with their elongated shape and the polar plugs found in the terminal ends, are evidence of a T. trichiura infection. Identification and differentiation of eggs is the most common method for identifying an infection by the whipworm. Routine concentration methods may be necessary in order to recover both ova and adult parasites. Adult worms are rarely seen in fecal samples but are useful in identifying an infection of T. trichiura; if seen, females are larger than males, as previously described, although this overlapping range provides for little diagnostic value. The females have a rounded posterior end compared to the male counterparts which possess a coiled posterior end. Their characteristic eggs have a smooth shell and are barrelshaped, brown, and have bipolar plugs. During the early stage of infection there may be only limited signs of infection in fecal samples. This is due to the cycles of periodic skin shedding and growth, which must occur for a period of approximately 3 months before adults mature and begin egg production. It should be remembered that the mature Trichuris trichiura has a narrow anterior esophageal end and shorter and thicker posterior anus, a characteristic necessary to differentiate adult worms from other species of parasites. These pinkish- white worms extend through the intestinal mucosa and attach to the host through their slender anterior end where they feed on tissue secretions. Mechanical damage may occur in the intestinal mucosa when toxic or inflammatory damage to the intestines of the host occurs, resulting in blood loss and anemia in victims with heavy infections. Whipworm infections may also be accompanied by concurrent infections with Giardia, Entamoeba histolytica, Ascaris lumbricoides, and hookworms. This fact requires that the parasitologist be especially careful in ruling out other infections upon identification of one particular species. A drug commonly used is that of mebendazole but prevention is the best course as for most other parasites. Good personal hygiene and the avoiding of contaminated water and food and providing for safe disposal of human wastes will eliminate many of the cases. It is necessary to teach children to both avoid infection and reinfection. This is a practice that will stand them in good stead for many years, and will help to avoid other parasitic and bacterial infections such as pinworm as well as many infections. During treatment and following treatment, all family members should adopt the practice of thoroughly washing hands and cleaning fingernails thoroughly and regularly with warm water and soap. Food preparation should be started only after careful washing of the hands. Keep finger nails short and clean, and avoid scratching the anus and genitals and nail biting will prevent reinfection. Daily bathing and showering are important, as well as washing bed linens and nightwear with hot water and laundry detergent. TRICHINOSIS, AN INFECTION OF TRICHINELLA SPIRALIS Trichinosis is also known as trichinellosis or as a trichina infection and is caused by the intestinal nematode worm Trichinella spiralis. This organism requires two hosts in its life cycle. The female worms produce larvae that encyst in muscle upon which the new host becomes infected when the encysted muscle is eaten. Because human infections are usually acquired by eating pork containing the encysted larvae, this might have initially given rise to the Mosaic and Islamic traditions of avoiding pork. T. spiralis is most important as a parasite of humans as it does not have specific hosts that it infects, and the disease is distributed throughout the world. The association between trichina infections and pigs has been long recognized but the encysted larvae in the muscle were not seen until 1821. The discovery of the worm in humans was made in 1835 by James Paget, who was then a medical student at St. Bartholomew’s Hospital in London. Sir James Paget was later knighted for his accomplishments in this and other areas of medicine. Another important medical finding attributed to this observant physician is the bone disease osteitis deformans, or softening of the bones, which is a condition called Paget’s disease. The definitive report of trichinellosis was written by Richard Owen who attempted to minimize Paget’s role, although he failed to associate that the wormlike organism embedded in human muscle was the larval stage of a nematode. Religious proscriptions relating to eating pork by Muslims and Jews perhaps stem from basic knowledge of cestodal infections and well as Trichinella spiralis. Morphology The adult worms were discovered by Rudolf Virchow in 1859 and Friedrich Zenker in 1860. Zenker finally recognized the clinical significance of the infection and concluded that humans became infected by eating raw pork. A number of good accounts by many investigators from more than a century ago relating to the history of trichinosis exist, as trichinosis is one of the oldest and most documented parasitic infections found in humans. The condition where the muscle tissues are infested by the larval form of T. spiralis is not identifiable in general by examination of stool specimens, as is the case for a number of other species of parasites. The most specific identification for the infection is examination of biopsies of the muscles in which the larvae are encysted. It appears that the only way the organism is transmitted to humans is through ingestion of raw or undercooked meat, primarily pork, but several other types of animal meat are also be capable of causing the contraction of the disease. Symptoms The first stage of the infection is the intestinal phase, where the ingested larvae invade the intestinal mucosal tissues. The first symptoms develop within a day and the dose of worms ingested relates directly to the severity of these initial symptoms. The victim may experience symptoms similar to influenza or a similar viral illness. Some mistakenly believe they are suffering from acute food poisoning as nausea and vomiting, diarrhea, general malaise, acute edema of upper eyelids, abdominal cramps and pain, and fever soon emerge as major complaints. More serious symptoms may ensue for heavy infections but often occur after the initial crisis when only vague muscular pains arise that may persist for weeks. The prognosis for the patient is positive and the severity and length of illness depends on the number of worms ingested. There may also be a generalized appearance of poor health during the initial phase of the illness. Life Cycle The development of the T. spiralis organism appears to be quite simple but the life cycle of T. spiralis includes several different stages. Once larvae are ingested through the eating of infected meat, the adult organism lives in the intestinal lining of such meat-eating animals. Following the mating of a pair of T. spiralis organisms, the male worm dies while the female proceeds to produce the offspring. These roundworms or nematodes have a stage of development called the embryonic stage, which occurs in many species of parasites and other biological organisms. In trichinae, however, this stage occurs within the uterus of the female. The offspring are then released in the larva’s second stage of life into the host’s intestinal lining. Up to 1500 larvae may be produced from each female worm and these travel through the circulatory system to the heart, and from there to striated skeletal muscle. Those larvae that reach striated muscle will grow to a length of about 1 mm before coiling themselves into a cyst (encysting) for protection. Encysted worms may live up to 10 years in this stage. Humans are considered a dead-end host, as few animals have the opportunity to feed on humans. In order to reproduce, larvae are released from the encapsulated cysts of the muscle tissue that was eaten and mature to the adult stage in the intestines. Females then produce larvae that are able to penetrate the intestinal mucosa and then enter into the blood circulatory system. These organisms have an affinity for striated skeletal muscle and form multiple cysts in the fibers of skeletal muscle. They grow and mature, reaching adulthood in approximately a month. As the encysted larvae grow they coil into the cavity in the muscle and may remain alive for a number of years before the cyst calcifies and the larvae within die (Figure 9-1). The encysted larvae may cause a great deal of pain but during the intestinal phase of development few if any symptoms are experienced, except for vague abdominal discomfort and perhaps slight diarrhea. After producing larvae, the adult dies, so reinfection is required for a continuous cycle of reproduction. Disease Transmission The contracting of trichinosis requires ingesting raw or uncooked meat containing Trichinella larvae, leading to human infection. The most common type of meat containing the Trichinella parasites that infect humans is pork and, in the past, feeding scraps of meat and other foods to pigs provided a ready source of the larvae that infected the animals. The larvae are released from the muscles of the food source where they go on to mature and reproduce in order to encyst in the host’s muscle tissues. Laboratory Diagnosis Trichinella organisms are rarely found in a patient’s stool, blood, or cerebrospinal fluids. Clinical chemistry tests such as creatinine kinase reveal muscle damage through the encysting larvae, providing indirect evidence that a Trichinella infection has likely occurred. The assumption that an individual is suffering from trichinosis is bolstered when accompanied by a patient’s history of eating certain meats, particularly pork. Serological testing has been somewhat unreliable as a substantial percentage of falsely negative results occur with the use of current testing methods. The most definitive diagnosis entails a muscle biopsy, which is prepared for a microscopic examination, and shows the presence of encysted larvae. Laboratory tests, such as a complete blood count to evaluate the number of white blood cells and a differentiation of the various types may reveal an elevated eosinophil count. Treatment and Prevention For heavy and symptomatic infections, antiparasite medication is recommended. Anti-parasite (antihelminthic) medication is the first line of treatment against trichinosis. If the Trichinella parasite is discovered early, in the intestinal phase, albendazole (Albenza) or mebendazole is usually effective in eliminating the intestinal worms and larvae before considerable damage to skeletal muscles occurs. The adult trichina also lives in the intestinal lining of many meat-eating animals as wild swine, bears, walruses, horses, rodents, and a number of other animal species including humans. The best defense against contracting trichinosis is accomplished through proper food preparation. Humans should avoid undercooked meat by ensuring that the meat is cooked to an internal temperature of 170°F (77°C) before eating it. A food thermometer is preferable to ensure that meat is thoroughly cooked. Other methods exist for ensuring meat is safe by storing the meat for at least 3 weeks or by the process of irradiation. Irradiation will kill parasites in wild-animal meat, and deep-freezing for 3 weeks kills Trichinella in some meats; however, Trichinella organisms in bear meat do not ordinarily die by freezing. Neither irradiation nor freezing is necessary if you ensure that the meat is thoroughly cooked. Smoking or pickling of meat will not always kill Trichinella in infected meat.
