The histopathology of Fasciola hepatica infections in sheep

An Update on the Pathogenesis of Fascioliasis: What Do We Know?

Fasciola hepatica is a trematode parasite distributed worldwide. It is known to cause disease in mammals, producing significant economic loses to livestock industry and burden to human health. After ingestion, the parasites migrate through the liver and mature in the bile ducts. A better understanding of the parasite's immunopathogenesis would help to develop efficacious therapeutics and vaccines. Currently, much of our knowledge comes from in vitro and in vivo studies in animal models. Relatively little is known about the host-parasite interactions in humans. Here, we provide a narrative review of what is currently know about the pathogenesis and host immune responses to F. hepatica summarizing the evidence available from the multiple hosts that this parasite infects.

Fasciolosis: pathogenesis, host-parasite interactions, and implication in vaccine development

Fasciola hepatica is distributed worldwide, causing substantial economic losses in the animal husbandry industry. Human fasciolosis is an emerging zoonosis in Andean America, Asia, and Africa. The control of the disease, both in humans and animals, is based on using anthelmintic drugs, which has resulted in increased resistance to the most effective anthelmintics, such as triclabendazole, in many countries. This, together with the concerns about drug residues in food and the environment, has increased the interest in preventive measures such as a vaccine to help control the disease in endemic areas. Despite important efforts over the past two decades and the work carried out with numerous vaccine candidates, none of them has demonstrated consistent and reproducible protection in target species. This is at least in part due to the high immunomodulation capacity of the parasite, making ineffective the host response in susceptible species such as ruminants. It is widely accepted that a deeper knowledge of the host-parasite interactions is needed for a more rational design of vaccine candidates. In recent years, the use of emerging technologies has notably increased the amount of data about these interactions. In the present study, current knowledge of host-parasite interactions and their implication in Fasciola hepatica vaccine development is reviewed.

Controversial aspects of the life cycle of Fasciola hepatica

Fasciola hepatica is a well-known helminth parasite, with significant economic and public health importance all over the world. It has been known since more than 630 years ago and a considerable research work has been carried out on the life cycle of this important parasite. In the hepatic phase of the life cycle of F. hepatica, it is assumed that the young flukes, after about 6-7 weeks of migration in the liver parenchyma, enter into the bile ducts of the definitive hosts and become sexually mature. Even though the secretion of cysteine peptidases including cathepsin L and B proteases by F. hepatica may justify this opinion, because of several scientific reasons and based on the experimental studies conducted in different animals (reviewed in this article), the entry of parasites into the bile ducts, after their migration in the liver parenchyma seems to be doubtful. However, considering all the facts relating to the hepatic and biliary phases of the life cycle of F. hepatica, two alternative ideas are suggested: 1) some of the migrating juvenile flukes may enter into the bile ducts immediately after reaching the liver parenchyma while they are still very small, or 2) when newly excysted juvenile flukes are penetrating into the intestinal wall to reach the liver through the abdominal cavity, a number of these flukes may enter into the choleduct and reach the hepatic bile ducts, where they mature. According to the previously performed natural and experimental studies in different animals and human beings, the supporting and opposing evidences for the current opinion as well as the evidences that might justify the two new ideas are reviewed and discussed briefly. In conclusion, our present knowledge about the time and quality of the entry of F. hepaticas into the bile ducts, seems to be insufficient, therefore, there are still some dark corners and unknown aspects in this field that should be clarified.

Transcriptional analysis identifies key genes involved in metabolism, fibrosis/tissue repair and the immune response against Fasciola hepatica in sheep liver

BACKGROUND

Although fascioliasis has been relatively well studied, little is known about the molecular basis of this disease. This is particularly relevant, considering the very different response that sheep have to Fasciola hepatica relative to cattle. The acute phase of this disease is severe in sheep, whereas chronic fascioliasis is more common in cattle.

METHODS

To begin to explore the host-response to Fasciola in sheep and improve the understanding of the host-pathogen interactions during the parasite's migration through liver parenchyma to the bile duct, we used RNA sequencing (RNA-seq) to investigate livers from sheep infected for eight weeks compared with those from uninfected controls.

RESULTS

This study identified 572 and 42 genes that were up- and down-regulated, respectively, in infected livers relative to uninfected controls. Our molecular findings provide significant new insights into the mechanisms linked to metabolism, fibrosis and tissue-repair in sheep, and highlight the relative importance of specific components of immune response pathways, which appear to be driven toward a suppression of inflammation.

CONCLUSIONS

This study is, to our knowledge, the first detailed investigation of the transcriptomic responses in the liver tissue of any host to F. hepatica infection. It defines the involvement of specific genes associated with the host's metabolism, immune response and tissue repair/regeneration, and highlights an apparent overlapping function of many genes involved in these processes.

Anatomicopathological Study of Vascular and Biliary Systems Using Cast Samples of Fasciola-infected Bovine Livers

In 117 livers with fascioliasis, this study was focused on the number of Fasciola, the number and intrahepatic localization of affected hepatic ducts and bile ducts, and the degree of fibrosis in the hepatic segments and bile ducts. The degree of pathological changes in bile ducts caused by fascioliasis was classified into five levels. The site of Fasciola habitation was most often the hepatic ducts of the porta hepatis: it was the left hepatic duct in 101 livers and the right hepatic duct in 88 livers. Casts were prepared by infusing synthetic resin into the hepatic arterial, portal, hepatic venous and biliary systems of 15 bovine livers with fascioliasis and then examined. In the left lobe, quadrate lobe, and caudate process where atrophic fibrosis was noted, the bile ducts became rod-shaped by losing branches, and the samples resembled dead branches of liver. Portal branches were thinned or completely terminated with marked fibrosis. Fine and irregular newly formed bile ducts not parallel with portal branches were observed in livers with markedly chronic fascioliasis. Distal portal branches in the right lobe, caudate lobe, and papillary process showed hypertrophic proliferative changes. The arterial system was generally well developed in thickened walls of bile ducts and formed vascular beds, and surrounded the bile ducts as tubes. In livers with severe fibrosis, capillaries were markedly developed and resembled glass cotton.

Experimental hepatobiliary fascioliasis in rabbits: a radiology-pathology correlation

RATIONALE AND OBJECTIVES

The authors sought to correlate the radiologic findings of hepatobiliary fascioliasis with pathologic features.

METHODS

Serial ultrasound, computed tomography (CT), and magnetic resonance findings in seven rabbits with experimentally induced fascioliasis were obtained every other week. Direct cholangiogram was also obtained after the rabbits were killed. Radiology-pathology correlation was done in specimens.

RESULTS

In the parenchymal phase (an acute phase of parenchymal invasion of a larva), CT showed subcapsular clustered areas of low attenuation. Magnetic resonance appearance was similar in shape but better than CT in characterizing the hemorrhagic nature of the lesion. Ultrasound findings were nonspecific in this phase. In the ductal phase (a stationary phase after residing in the bile duct), CT showed dilatation of central ducts with symmetric periportal hypoattenuation (periportal tracking). Magnetic resonance could not depict mild ductal dilatation. Ultrasound was most valuable in demonstrating the moving worm within the dilated duct. Pathologically, the hepatic parenchymal lesions consisted of a cluster of eosinophilic granulomas with hemorrhagic change (migratory tract of the flukes). Ductal changes were observed predominantly in the central bile ducts. Periportal lymphangiectasia was also noted.

CONCLUSIONS

Computed tomography or magnetic resonance can demonstrate the characteristic evolutionary pattern of fascioliasis that reflects the unique life cycle of Fasciola hepatica. The role of ultrasound, although limited in the parenchymal phase, was most useful in the ductal phase in that it demonstrated the moving worms themselves.

An erratic parasitism found in the lungs of sheep during experimental infection with Fasciola gigantica (Japanese strain)

An erratic parasitism was observed in the lungs of sheep experimentally infected with Fasciola gigantica at autopsy 14 weeks after inoculation. Macroscopically, several hyperaemia of 7.5-12.5 mm in diameter were found on the surface of the lungs. Juvenile flukes detected in the lungs were much smaller than those in the liver of the same sheep. A slight inflammatory reaction was observed in the lungs and it is likely that the flukes had migrated in the pleural cavity for some time. Occasionally, a mixed thrombus with many eosinophils was found in the blood vessels adjacent to the bronchia, although no cough was observed clinically. This suggests that diagnosis of juvenile fluke is difficult not only by parasitological but also serological methods.

Acquisition of resistance against Fasciola gigantica by Indonesian thin tail sheep

The plasma levels of GLDH and the dynamics of development of the numbers of surface lesions on livers, and numbers of parasites, within two to three weeks of a challenge infection, indicated that the major component of the high resistance of Indonesian thin tail (ITT) sheep against Fasciola gigantica was acquired, and acted against juvenile parasites. Few parasites reached the livers of ITT (exposed) sheep, but many had been inhibited, probably in the wall of the jejunum. ITT (naive) sheep showed some resistance, compared with control Merino sheep, by three weeks after infection. Manifestation of the resistance of ITT (exposed) sheep was suppressed by administration of the immunosuppressant, dexamethasone. Killing of parasites in ITT sheep appeared to have ceased by 21 to 28 days after infection. The basis of the acquired resistance was deemed to be an exceptional immunological capacity of ITT sheep responding to an antigen, or an immunological suppressant, peculiar to F. gigantica. That molecule, produced by juvenile parasites, warrants further study as a candidate for a vaccine.

Resistance of Indonesian thin tail sheep against Fasciola gigantica and F. hepatica

High resistance of Indonesian thin tail (ITT) sheep against Fasciola gigantica has been confirmed. Naive ITT sheep had only 17% of the number of mature parasites collected from control St. Croix sheep. In contrast, the level of resistance of ITT sheep against F. hepatica was the same as that of the low resistance Merino breed after both primary and secondary infections. It is suggested that resistance of ITT sheep against F. gigantica was manifested in two phases. The major phase appeared to be specific to the ITT sheep : F. gigantica relationship, acting against immature parasites in both naive and previously exposed hosts and could be innate or acquired. The second phase appeared to be specific to F. gigantica, and was acquired, because it killed young adult parasites only after secondary infection. F. gigantica from ITT sheep were heavier than those from the St. Croix sheep, possible because feeding was impaired by more extensive liver damage in the latter. Approximately 45 F. gigantica killed 25-kg sheep before and during migration from the liver parenchyma into the bile ducts. Death was caused by haemorrhages into the liver parenchyma, bile ducts and peritoneum.

Hepatic and pulmonary pathology of experimental Fascioloides magna infection in guinea pigs

The guinea pig was used to study the pathology of Fascioloides magna, an important pathogen for sheep. Although flukes migrated freely through various tissues in infected guinea pigs, the most serious lesions occurred in the liver and lungs. The sequential development of lesions indicated that flukes first invaded the quadrate lobe of the liver and subsequently migrated to other liver lobes and tissues. Six weeks post-infection, there was a marked drop in the recovery of flukes from the liver along with a dramatic increase in pulmonary involvement. Much of the hepatic and pulmonary pathology in infected animals was secondary to extensive vascular lesions caused by migrating flukes. In the liver, vascular lesions predominantly involved the portal and hepatic veins. Thrombophlebitis and locally extensive necrosis, resembling infarction, were observed. Vascular lesions in the lungs occurred in the pulmonary arteries leading to thrombosis and haemorrhagic infarction. Discovery of a fluke in a pulmonary artery, along with the pattern of hepatic and pulmonary lesions, suggested that flukes probably used the cardiovascular system as a pathway for dissemination. Death in fluke-infested guinea pigs was most often associated with severe pulmonary lesions. The nature and distribution of fluke-induced lesions observed in this study demonstrate that the guinea pig is a suitable animal model for Fascioloides magna infection in sheep.

High resistance to experimental infection with Fasciola gigantica in Javanese thin-tailed sheep

Innate resistance of Javanese thin-tailed sheep to Fasciola gigantica was investigated in animals infected with single doses of 150 or 500 metacercariae and killed 4, 8, 12 or 16 weeks after infection. Infected and non-infected sheep had similar values for packed cell volume, mean corpuscular volume, mean corpuscular haemoglobin concentration, serum glutamate dehydrogenase, serum gamma glutamyl transferase and serum aspartate transferase throughout the trial, except for one animal infected with 500 metacercariae from which the highest recovery of flukes (55) was made. This animal developed pathologically altered values from 12 weeks post infection, coincident with the period of greatest hepatic haemorrhage and destruction of hepatic tissue by migrating flukes and their entry into bile ducts. However, values were altered much less than those reported in other sheep given as few as 200 metacercaria of F. gigantica. Both susceptibility to infection with F. gigantica, as indicated by percentage take of metacercariae and the severity of pathological changes were low in this study in comparison with reports involving other breeds of sheep infected with this parasite. These findings support the conclusion that Javanese thin-tailed sheep have a high innate resistance to F. gigantica.

Studies on naturally-occurring ovine fascioliasis in the Sudan

Haematological, biochemical and pathological changes were investigated in 214 sheep naturally infected with Fasciola gigantica in an endemic area in the Sudan together with 82 uninfected controls. Infected animals showed a clear decrease in erythrocyte counts, haemoglobin concentration and packed cell volume, a normochromic, normocytic anaemia, leucocytosis and eosinophilia. Serum concentrations of the enzymes glutamate dehydrogenase, sorbitol dehydrogenase and glutamate oxaloacetic acid transaminase were also elevated in the infected group, indicating hepatic damage. This was confirmed by histopathological changes, which comprised degenerative and necrotic changes in hepatocytes associated with haemorrhage, fibrosis, increased lobulation of the liver, mononuclear cell infiltration with haemosiderin deposition in fluke tracks and portal areas and the formation of granulomata around fluke eggs and fluke remnants. In the infected group there was slight hyperglobulinaemia and a marked hypoalbuminaemia, with a decrease in A/G ratio. A slight rise in the level of serum bilirubin was also observed.

Infectivity and pathogenicity of Fasciola gigantica in West African dwarf sheep and goats

The infectivity and pathogenicity of Fasciola gigantica for sheep and goats were investigated in animals infected with single doses of between 200 and 2,000 metacercariae. Acute fascioliasis occurred in sheep infected with 1,000 to 2,000 cysts in which 10 to 17% of the cysts became established. The infected animals died 80 to 90 days after infection. In goats given the same infective doses death from acute fascioliasis occurred earlier (70 to 83 days after infection) and 12 to 38% of cysts became established. Subacute fascioliasis could occur in both hosts with an infective dose of 200 cysts, the animals surviving for more than 100 days. It therefore appears that F. gigantica is more infective for goats and that the disease was more severe in this host than in sheep.

Evolution of fascioliasis after eating wild watercress

Fascioliasis is rarely reported in humans although it is endemic in sheep and cattle. We describe the illness of a 60-year-old widow who ate wild watercress which is the usual source of infestation. Laparotomy for suspected liver abscesses revealed necrotic tracts on the surface of the liver left by the invasion of numerous flukes. Diagnosis was made during the latent phase by the detection of serum antibody to fasciola hepatica antigen obtained from a sheep. Symptoms returned during the cholestatic phase. Mature flukes were then present in the large bile ducts and ova appeared in the stools. Symptoms resolved twelve weeks after presentation.

Resistance to Fasciola hepatica in sheep harbouring primary Schistosoma bovis infections

A substantial level of resistance to a heterologous challenge with Fasciola hepatica was demonstrated in sheep harbouring primary non-patent (two- to three-week-old) and newly patent (seven- to eight-week-old) Schistosoma bovis infections, the liver-fluke burdens being reduced by 70% and 93%, respectively, (p less than 0.01) in these groups compared with that of the challenge control group. The resistance was also reflected in less pronounced F. hepatica-induced liver damage and presumably also in a reduction of the egg-production capacity per established liver-fluke. In contrast, a corresponding heterologous resistance to challenge with F. hepatica could not be demonstrated in sheep harbouring primary 16- to 17-week-old S. bovis infections. Possible, presumably immunological, mechanisms involved in the observed cross-resistance are discussed, as is the possible practical relevance of this cross-resistance phenomenon in attempts to vaccinate sheep against F. hepatica.

Fasciola hepatica human infection. Histopathological study of sixteen cases

Sixteen human cases of Fasciola hepatica infection are described. The liver was involved in 13 cases, the gall bladder in 9 cases and the stomach in 2 cases. Lesions containing parasitic remnants or fluke eggs were rarely seen. Surface scarring of the liver, scar tracks and granulomas within organs were the most characteristic changes seen and were the most useful for the histopathological diagnosis of the disease. The associated liver, bile and gastric lesions are briefly discussed.

The pathogenesis of primary and secondary infection with Fasciola hepatica in mice

Primary and secondary infections of F. hepatica in mice were compared to determine how prior exposure to the parasite affected host response. Mice with primary parenchymal Fasciola infections initially had hemorrhagic tunnels filled with inflammatory cells and connective tissue. These lesions were progressive and became most severe 30 days after exposure as the parasites entered the bile ducts. At this time there was much hyperplasia and thickening of all layers of the duct system near the parasites and occasionally severe periportal fibrosis. By 2 months after exposure regeneration of the damaged liver cells was complete although hyperplasia persisted in bile ducts containing flukes. In mice with secondary infections (mice exposed 40 to 50 days after first infection), the inflammatory response was faster and shorter. Most lesions were resolved by 30 days after the second exposure. There was little difference in histology of primary and secondary infections during the chronic phase of the disease.

The pathology of natural Fasciola hepatica infection in cattle

The pathology of bovinc livers during the parenchymal and chronic phases of natural Fasciola hepatica infections is described. Immature infections occurred from September to the middle of January. Significant changes of the hepatic parenchyma caused by immature flukes occurred in association with large migrational tracks. The changes included frequent thrombosis and haemorrhages, extensive disintegration and coagulative necrosis of hepatic cells, and abundant infiltrates of inflammatory cells. In chronic infections the proliferation and destruction of the mucosa of main bile ducts were pronounced. Normal mast cells were lacking in the portal areas surrounding fresh migrational tracks. In chronically infected livers mast cells occurred in great numbers.