Epidemiology

F. hepatica infection was first documented in the Gallo-Roman period (Da Rocha et al, 2006). Today, the estimated number of human infections ranges from 2.4 million to 17 million, and 91.1 million are at risk of infection around the world (Keiser & Utzinger, 2005). In the past, fascioliasis was limited to specific and typical geographic areas, but it is now widespread throughout the world. According to the reported cases, F. hepatica transmission has increased in Europe, the Americas, and Oceania, and in Africa and Asia, where F. gigantica and F. hepatica overlap. The geographical distribution is determined by the intermediate host (Lymnaea spp.) and certain other conditions such as climate, alimentary behaviors, and poverty.

Examples of countries with estimates of the infected population include 830,000 in Egypt, 742,000 in Peru, 360,000 in Bolivia, 37,000 in Yemen, 20,000 in Ecuador, and 10,000 in Iran (Haseeb et al, 2002). Furthermore, other countries that have reported a significant number of cases in years include Argentina (Kleiman et al, 2007), Venezuela (Incani et al, 2003), Chile (Llanos et al, 2006), Ecuador (Trueba et al, 2000), Mexico (Cruz-Lopez et al, 2006), Turkey (Kaya et al, 2006; Turhan et al, 2006), Thailand (Aroonroch et al, 2006), Japan (Inoue K, et al, 2007), Korea (Lee & Kim, 2006), the United States (Fullerton et al, 2006; Graham et al, 2001), Tunisia (Khelifi et al, 2006), and Lebanon (Birjawi et al, 2002), among others. The majority of reported cases in these countries are related to complications from the infection, and the current number of cases is undoubtedly underestimated. On the other hand, globalization and migration of populations from rural areas to large cities have led to a number of cases of fascioliasis in nonendemic areas. In fact, because of clinicians’ lack of familiarity with this parasitic infection in nonendemic areas, the diagnosis can be delayed, which increases the rate of complications (Kang, 2008).

Fascioliasis is distributed globally, with the most affected area in the world being the Andean region of South America. In fact, more than half of the population may carry the infection in some selected regions, with prevalence ranging from 6% to 68% (Marcos et al, 2005b, 2007a; Parkinson et al, 2007). New evidence has shown that proximity of medium- to high-income industrialized cities to rural areas creates a potential source of infection in nonendemic areas because of the importation of contaminated vegetables to the high-consuming markets of big cities. Thus it is not uncommon to see cases of fascioliasis in nonendemic areas. Another factor that contributes significantly to the dissemination of the infection to new areas is the highly adaptable capacity of both the parasite and the lymnaeid snail hosts to challenging meteorologic conditions (e.g., 4200 meters above sea level, Andean Altiplano). Paleoparasitologic studies have shown that the introduction of F. hepatica and its snail host from Europe into the Americas has been relatively recent. Today, fascioliasis as a result of F. hepatica is the vector-borne disease with the widest latitudinal, longitudinal, and altitudinal distribution known. These key factors explain the wide geographical distribution of the disease (World Health Organization [WHO], 2007).

The epidemiologic pattern of transmission of fascioliasis can be classified as follows (Mas-Coma et al, 1999): 1) cases imported to areas where neither human nor animal fascioliasis is transmitted; 2) autochthonous, isolated, nonconstant cases of sporadic infection in areas where animal fascioliasis is present; 3) endemic fascioliasis (hypoendemic ≤1%, mesoendemic 1% to 10%, hyperendemic ≥10%); 4) epidemic fascioliasis (i) in animal endemic areas and (ii) in human endemic areas. Epidemiology is an important determinant of the initial evaluation of a patient with probable fascioliasis.

Life Cycle

The adult F. hepatica flukes are large, flat, brown, and leaf shaped with a broad anterior portion covered with scalelike spines. Flukes measure approximately 25 to 30 mm by 10 to 15 mm, although F. gigantica can measure up to 75 mm. The adult fluke lives in the common and hepatic bile ducts of the human or animal host, and animals susceptible to becoming reservoir hosts for Fasciola species include mainly cattle, sheep, pigs, buffaloes, and donkeys, although it has been also reported in horses, dogs, goats, llamas, alpacas, dromedaries, and camels. The eggs are oval, yellowish brown, and measure approximately 130 to 150 by 60 to 90 μm (Fig. 45.2).

FIGURE 45.2 Adult egg of Fasciola hepatica in microscopic examination of stools by the rapid sedimentation technique.

The life cycle begins when the parasite eggs in stools are deposited in tepid water (22° C to 26° C); miracidia appear, develop, and hatch in 9 to 14 days. These miracidia then invade many species of freshwater snails, in which they multiply as sporozoites and redia for 4 to 7 weeks. They leave as free-swimming cercaria that subsequently attach to watercress, water lettuce, alfalfa, mint, parsley, or khat. Free-swimming cercaria may remain suspended in water and encyst over a few hours, and infection of the human host begins after consumption of plants or water contaminated with the metacercariae. In the first week, the larvae excyst in the duodenum and migrate through the bowel wall and peritoneal cavity; after 4 weeks, the juvenile larvae penetrate the liver through the Glisson capsule to initiate the acute larval, hepatic, and invasive stages of human infection.

Sometimes the larvae deviate to other locations; these are called extrahepatic forms or ectopic infections. Maturation from juvenile larvae into adult flukes takes approximately 3 to 5 months, during which time the larvae mature and migrate through the liver into the large hepatic and common bile ducts (Fig. 45.3). Mature flukes consume hepatocytes and duct epithelium and reside for years in the hepatic and common bile ducts and occasionally in the gallbladder. Adult fluke worms produce eggs within 4 months after infection (range, 3 to 18 months); these eggs traverse the sphincter of Oddi and intestine and then continue the cycle of infection. Of note, acute and chronic stages can overlap; this is commonly seen in endemic areas, and it is not unusual to find eggs in the stool samples of patients with acute infection.

FIGURE 45.3 Liver biopsy of an infected rat with Fasciola hepatica, adult parasite in a bile duct.

Risk Factors

The main source of infection is the consumption of metacercariae-contaminated raw vegetables—such as watercress, lettuce, alfalfa juice, and mixed green salads—or contaminated water from man-made irrigation channels (Marcos et al, 2004, 2005a). Because of epidemic obesity, alimentary habits have changed; for example, higher consumption of green vegetables has led to an increase in the risk for fascioliasis, because it requires vegetables imported from endemic areas (Marcos et al, 2007b). The infection has been also seen in people who have eaten salads in luxury restaurants or hotels in endemic countries, and watercress has traditionally been the most common known source of infection. Nonetheless, a large variety of plants have been described in recent years, such as Medicago sativa (alfalfa in juice) in Peru; Taraxacum dens leonis (dandelion leaves), Valerianella olitoria (lamb’s lettuce), and Mentha viridis (spearmint) in France; green leafy Nasturtium spp. and Mentha spp. in Iran; Juncus andicola and J. ebracteatus (Juncaceae), Mimulus glabratus (Scrophulariaceae), and Nostoc species (Cianofitas) in the Bolivian Altiplano (Bjorland et al, 1995; Mas-Coma et al, 1999).

More women are affected than men, with higher prevalence rates, more severe infections, and with more reported liver or biliary complications (Marcos et al, 2006). Although, the differences are not well understood, alimentary habits, immunologic factors, and proximity to contaminated sources may play a role; for example, children are affected more than adults (Marcos et al, 2007c) likely because of alimentary habits. Reinfection is commonly seen in highly endemic areas, and the parasite can live in the biliary tract for a long time—between 9 and 13.5 years.

Epidemiologic studies have been carried out in endemic areas to measure the impact of alimentary habits on the acquisition of infection. Drinking beverages made from watercress or alfalfa leaves, called emollients, and living close to irrigation channels were found to be risk factors in a multivariate analysis in the Andean region (Marcos et al, 2004). Eating salads is the common factor among infected families, and it carries a 3.3-fold increased risk of acquiring the infection (Marcos et al, 2005b). In a logistic regression analysis, an age- and gender-matched case-control study comparing 60 infected children found that drinking alfalfa juice carries a 4.5-fold increase in the risk of acquiring fascioliasis, and familiarity with aquatic plants carries a 4.3-fold increased risk (Marcos et al, 2006). Therefore aquatic plants—watercress, alfalfa, and others—and the irrigation channels that carry the metacercariae play a key role in the transmission of fascioliasis in Andean endemic areas. Hypothetically, exportation of plants or other products could lead to transmission in nonendemic areas. Treatment of contaminated plants with high doses of potassium permanganate decreases metacercariae viability and could be used to prevent infection (Ashrafi et al, 2006), although its impact in endemic areas has not yet been tested. A summary of risk factors with studies reporting odds ratios is presented in Table 45.4.

Table 45.4 Imaging Findings in Fascioliasis

3D, three-dimensional

Clinical Manifestations

Fascioliasis has two distinct clinical phases: acute and chronic. Signs and symptoms depend on the worm burden, duration, and phase of infection. In general, the chronic infection is usually diagnosed in epidemiologic studies in endemic areas as a cause of biliary obstruction or in routine stool tests for other reasons. On the other hand, the acute infection has a more florid clinical picture that brings the patient to the emergency room. The clinical manifestations are so variable that mild right upper quadrant pain may call for a step-by-step workup that can lead to the final diagnosis of fascioliasis (Behar et al, 2009).

Imaging Studies

The most common imaging findings in fascioliasis are summarized in Table 45.3.

Diagnosis

The diagnosis of the acute phase of fascioliasis is confirmed by serology or response to therapy. Accurate identification of early disease has been difficult because of the lack of a confirmatory test; the serologic test that requires medium-high technology is not available in most endemic areas. Nonetheless, if the pretest probability or clinical suspicion is unequivocally high, an antiparasitic trial is warranted. Diagnostic criteria are significant clinical improvement and decreasing levels of eosinophils in the 3 to 5 days after a trial of triclabendazole (Marcos, 2008b). The results of the serology and stool sampling, the possible stage of the infection, and the clinical significance are illustrated in Figure 45.4.

FIGURE 45.4 Most common possible scenarios of diagnostic results in fascioliasis. Cases 2 and 3 are the most common found in clinical practice. Cases 1 and 4 are uncommon.

Treatment

Triclabendazole is the treatment of choice for both phases of Fasciola infection (Keiser & Utzinger, 2007a). The cure rate exceeds 90% for acute stages after a single dose of 10 mg/kg (Marcos et al, 2009), and similar results have been obtained in the therapy of chronic infection (Apt et al, 1995; Talaie et al, 2004; El-Tantawy et al, 2007). This drug has a better absorption with a fatty meal, and the most frequent adverse event is biliary colic caused by the passage of dead or dying parasites through the bile ducts. Triclabendazole was introduced in the early 1980s for the treatment of F. hepatica infections in livestock, and it has been placed on the WHO List of Essential Medicines (http://www.who.int/medicines/publications/essentialmedicines/en/) because of its efficacy and cost effectiveness.

Treatment of fascioliasis with triclabendazole was shown to reduce the prevalence significantly, from 5.2% to 1.2% in 7 years (Abdussalam et al, 1995). However, the intensive use of triclabendazole has resulted in the development of resistance in animals (Mottier et al, 2006; Alvarez-Sanchez et al, 2006). On the other hand, in places where triclabendazole for humans is not available, the veterinary form can be effective as well. In Peru, a single dose of 10 mg/kg cured 96%, whereas 10 mg/kg for 2 days cured 100% with no major adverse effects registered (Terashima et al, unpublished data). The most important adverse effect was abdominal pain or biliary colic during the first week of treatment. This pain is likely caused by the passage of dead or dying parasites through the bile ducts (Millan et al, 2000; Richter et al, 1999). Antispasmodics may decrease or completely alleviate the transitory episodes of abdominal pain and should be used in most cases at the beginning of the therapy or even before the first dose.

A single dose of triclabendazole is effective and tolerable, without major side effects, and it may also be used as a diagnostic tool (Marcos et al, 2007c). However, given the unlikelihood of any new drugs against F. hepatica being developed in the foreseeable future, the emergence of resistance represents an important threat (Alvarez-Sanchez et al, 2006), although resistance in humans has not been reported.

Many years ago, parenteral dehydroemetine at doses of 1 mg/kg for 10 days was used. Then, bithionol was applied at doses of 30 to 50 mg/kg every third day for a total of 10 to 15 doses, although it is cardiotoxic and very expensive. Other drugs, such as nitazoxanide, have been evaluated for fascioliasis, but cure rates have been disappointingly low. Adult patients who received a 7-day course of nitazoxanide had a cure rate of 60%, but the rate was only 40% in children (Favennec et al, 2003). Because of this, and because of the increasing triclabendazole resistance in animals, new drugs have been evaluated and tested in animals with fascioliasis. For instance, artesunate, artemether, and OZ78 have fasciocidal properties in animals, and they are promising drugs for the near future, should resistance to triclabendazole become a problem (Keiser et al, 2007a, 2009). Albendazole, metronidazole, and praziquantel are not recommended for fascioliasis treatment.

Future Directions and Vaccines

Because fascioliasis constitutes a major economic impact, development of effective vaccines would be an important advance. Preliminary studies in animals have reported significant advances (McManus & Dalton, 2006; Spithill & Dalton, 1998). Cysteine proteinases released by F. hepatica play a key role in parasite feeding and migration through host tissues and in immune system evasion. A recombinant cysteine proteinase (CPFhW) expressed as inclusion bodies in E. coli was used for enteral vaccination of rats against fascioliasis. In that study, oral vaccination reduced the parasite burden by 78% to 80% after a challenge with metacercariae (Kesik et al, 2007). The glutathione transferase superfamily (GST) from liver fluke plays phase II detoxification and housekeeping roles and has been shown to contain protective vaccine candidates (Chemale et al, 2006). Promising future research will yield meaningful immunologic targets to prevent the infection, especially in the well-recognized endemic areas and particularly in children, but so far the vaccines are targeted only to animals.

Life Cycle

Fish-borne trematodes have a complicated life cycle with two intermediate hosts. Starting from a human host, the adult worms deposit fully developed eggs; the eggs are then passed to the environment through the feces, and they must get into water to hatch and to infect their first intermediate host, a freshwater snail. After being ingested by a suitable snail, the eggs release miracidia, which undergo several developmental stages in the snail: as sporocysts, redia, and cercaria. The snail intermediate hosts for Opisthorchis are Bithynia goniompharus, B. funiculata, and B. siamensis. Parafossarulus manchouricus often serves as a first intermediate host for C. sinensis, and snail hosts also include other Bithynia, Tarebia, Alocinma, and Bulimus species. Once inside the snail’s body, the miracidium hatches from the egg and parasitically grows inside of the snail, where it develops into a sporocyst that houses the asexual reproduction of redia, the next stage. The redia themselves house the asexual reproduction of free-swimming cercaria. This system of asexual reproduction allows for an exponential multiplication of cercaria individuals from one miracidium. Once the redia mature, having grown inside the snail body until this point, they actively bore out of the snail body into the freshwater environment and seek out fish. Cercariae are released from the snail and then penetrate freshwater fish—the second intermediate host (Cyclocheilichthys and Puntius spp., Hampala dispar), encysting as metacercariae in the muscles or under the scales. Once inside the fish muscle, the cercaria creates a protective metacercarial cyst with which to encapsulate their bodies. This protective cyst proves useful for when the fish muscle is consumed by a human or other host, such as a cat, dog, pig, or any other fish-eating mammal. The acid-resistant cyst enables the metacercariae to avoid being digested by the gastric acids and allows them to reach the small intestine unharmed.

Reaching the small intestine, the metacercariae navigate toward the human liver—the final habitat. In contrast with the larger F. hepatica fluke, metacercariae of Opisthorchis and Clonorchis migrate through the ampulla of Vater after excysting in the duodenum; they then travel into the bile ducts, where they mature into adult worms within 4 weeks and deposit yellow, operculated eggs. The parasites may live for up to 45 years in the liver of a human host, producing 1000 to 2500 eggs per day. The adult flukes, measuring 10 to 25 mm by 3 to 5 mm, reside in medium-sized and small intrahepatic bile ducts and, occasionally, in the extrahepatic bile ducts, gallbladder, and pancreatic duct.

Clinical Manifestations

Clonorchiasis and opisthorchiasis infections are associated with a number of hepatobiliary diseases but mainly with disease in the biliary tract, because these flukes do not penetrate the liver parenchyma as they do in fascioliasis. The frequency and types of pathology and clinical disease seem to differ for C. sinensis and O. viverrini. For example, cholelithiasis is one of the more serious complications of clonorchiasis but is a rare complication of opisthorchiasis. In both species the prominent inflammatory process within the biliary tract is the main pathologic characteristic of these infections that can sometimes lead to cholangiocarcinoma, especially with O. viverrini.

Flukes can occasionally gain access to the pancreatic tract, where they can cause obstruction and pancreatitis. The pathologic and clinical consequences of opisthorchiasis are related to the intensity and duration of cumulative infections. Because adult flukes are long lived, they can produce eggs and symptoms long after the human host has emigrated from the area. Most people with these Asian fluke infections have no symptoms, and only 5% to 10% of those heavily infected have nonspecific chronic symptoms such as right upper quadrant abdominal pain, flatulence, and fatigue. Cholangiocarcinoma is a known complication.

Consequences of Chronicity of Infection

Carcinogenesis associated with helminth infections is a complex process that may involve several different mechanisms, but chronic inflammation is a key feature. The human liver fluke O. viverrini infects millions of people throughout Southeast Asia and is a major cause of cholangiocarcinoma, or cancer of the bile ducts. The mechanisms by which chronic infection with O. viverrini results in cholangiocarcinogenesis are multifactorial, but one such mechanism is the secretion of parasite proteins with mitogenic properties into the bile ducts, driving cell proliferation and creating a tumorigenic environment. A possible pathway for the development of cholangiocarcinoma is the presence of Ov-GRN-1, a major growth factor present in O. viverrini excretory-secretory products that induces proliferation of host cells, which supports a role for this fluke protein to establish a tumorigenic environment.

In general, these flukes cause inflammation around the biliary tree that leads to severe hyperplasia of epithelial cells, metaplasia of mucin-producing cells in the mucosa, and progressive periductal fibrosis. There are clear associations between O. viverrini infection and cholangiocarcinoma in the context of the intensity of infection, parasite-specific antibody response, and abnormalities of the biliary tract.

The higher the intensity of anti–O. viverrini antibody titers, the higher the risk for cholangiocarcinoma (Honjo et al, 2005). The International Agency for Research on Cancer recognizes this parasite as a category I carcinogen. The lesions that predispose to malignant changes in O. viverrini are evident as a dilation of subcapsular medium and large bile ducts with a prominent fibrotic wall, periductal inflammatory cell infiltration, goblet cell metaplasia, epithelial and adenomatous hyperplasia, and periductal fibrosis. The pathogenesis of O. viverrini–mediated hepatobiliary changes may be due to mechanical irritation or to its metabolic products (Sriamporn et al, 2004). Several N-nitroso compounds and their precursors occur at low levels in fermented food, such as preserved mud fish paste (pla ra), a condiment ubiquitous in the cuisine of northeastern Thailand and Laos (Sripa et al, 2007). The study of O. viverrini genes should expedite molecular studies of cholangiocarcinogenesis and accelerate research focused on developing new interventions, drugs, and vaccines that might help in controlling O. viverrini and related flukes (Laha et al, 2007). Similarly, recent studies show a strong association between C. sinensis and the development of cholangiocarcinoma (Choi et al, 2006). For instance, a recent epidemiologic study in Korea correlates the prevalence of C. sinensis and the incidence rate of cholangiocarcinoma: C. sinensis prevalence was 2.1% in Chuncheon, 7.8% in Chungju, and 31.3% in Haman; cholangiocarcinoma incidence rate was 0.3, 1.8, and 5.5 per 100,000 population, respectively (Lim et al, 2006). Hepatocellular carcinoma also has been associated with clonorchiasis, along with hepatitis B virus and alcohol consumption as cofactors (Tan et al, 2008). It seems plausible that cholangiocarcinogenesis associated with clonorchiasis is the cumulative end result of a multifactorial carcinogenic mechanism, although the mechanisms involved are not completely understood.

Improving diagnosis with new serologic tests may be helpful, but such tests cannot distinguish between recent or past infection. Currently, the Ov-CP-1–based ELISA shows a sensitivity of 95% and specificity of 96% in serum coinfected with hookworm, minute intestinal fluke, S. stercoralis, Taenia spp., Giardia lamblia, and E. coli infection (Watthanakulpanich et al, 1997). The sensitivity and specificity are similar to other studies using an ELISA based on recombinant trematode cysteine protease such as C. sinensis (sensitivity 81.3% to 96%, specificity 92.6% to 96.2%) (Nagano et al, 2004; Zhao et al, 2004).

Human clonorchiasis and opisthorchiasis are primarily diagnosed by the detection of eggs in feces. The Kato-Katz method is accepted as the best for fecal examination, although sometimes the eggs may not be detected because of biliary obstruction or intermittent egg excretion similar to that encountered with fascioliasis. In light infections, with less than 10 adult worms in the biliary tract, a polymerase chain reaction (PCR) detecting the DNA of the adult parasite in stools may be helpful (Duenngai et al, 2008). Intrahepatic duct dilation is the most common finding on US imaging (76% of patients), and increasing periductal echogenicity and gallbladder sludge are seen only in patients with heavy infection (Choi et al, 2005). Recently, Ruangsittichai and colleagues (2006) reported high sensitivity and specificity using a recombinant eggshell protein with potential for the serodiagnosis of human opisthorchiasis. However, detection of O. viverrini DNA is expensive and requires skillful personnel.

Treatment

Praziquantel, a derivative of pyrazino isoquinoline, is the drug of choice for O. viverrini, O. felineus, and C. sinensis treatment. For O. viverrini, a single dose (40 to 50 mg/kg) of praziquantel treatment is indicated, with a cure rate between 91% and 97%. For clonorchiasis, the recommended dose of praziquantel is 25 mg/kg three times at 5-hour intervals in 1 day (total dose 75 mg/kg), with a cure rate of 83% to 85% (Rim, 2005).

Future Directions

Opisthorchis and Clonorchis are medically important foodborne trematodes endemic in Asian countries, and infection is associated with cholangiocarcinoma and other hepatobiliary diseases. The epidemiology is found in some rural areas because of the tradition of eating raw or uncooked fish products.