Black rats (Rattus rattus) (unknown age and gender).In November 2011, a trapping campaign was organized in a French zoological park to assess the prevalence of capillariasis in wild rodents. The campaign was initiated after the diagnosis of capillariasis in a primate from the zoo. Eighty rats were trapped and their livers were sampled and fixed in 10% buffered formalin.
Most livers were grossly unremarkable. In a few cases, livers had a slightly irregular surface and, in some areas, contained tightly packed pinpoint yellow foci and small irregular tracts.
There is some variability between slides regarding the severity and the type of lesions. Livers show various degrees of bridging fibrosis characterized by fine fibrous septae connecting portal tracts to portal tracts or to centrilobular spaces (the so-called septal fibrosis). As a consequence, in the most affected areas, the architecture is reminiscent of the porcine liver. This pattern is highlighted with Massons trichome stain. In addition, there are various degrees of lymphoplasmacytic and, to a lesser extent, eosinophilic infiltration in septae and around portal tracts and centrilobular veins. Sections usually have one or two aggregates of parasitic eggs surrounded by macrophages and multinucleated giant cells, and a peripheral rim of fibrosis with lymphocytes, plasma cells and eosinophils (granulomas). In older granulomas, eggs are mainly surrounded by fibrosis. Eggs are typically barrel-shaped, about 80 μm in length, bi-operculate and have a thick shell, the inner layer of which is refractile and the outer layer striated (Fig. 3). The content is eosinophilic and granular (morphology consistent with unembryonated eggs of Capillaria hepatica). Some granulomas also contain areas of necrosis and/or dystrophic mineralization. Some slides show sections of adult worms: they measure 100-150 μm in diameter, have a pseudocoelom and a digestive tract (nematodes), a polymyarian-coelomyarian musculature and two bacillary bands (hypodermal bands with nuclei) (aphasmid nematodes) (Fig. 4). This morphology is consistent with adults of Capillaria hepatica. Some worms are surrounded by numerous eosinophils and macrophages.
Liver: Hepatitis, granulomatous and eosinophilic, chronic, multifocal, moderate, with intralesional eggs and adult nematodes consistent with Capillaria hepatica (adults are not present on all slides).
Liver: Porto-portal and porto-central bridging fibrosis, multifocal, moderate to severe (septal fibrosis).
Capillaria hepatica, rat
Capillaria hepatica (also known as Calodium hepaticum), the cause of hepatic capillariasis, is an aphasmid nematode that mainly infects rodents and lagomorphs, and occasionally other vertebrates such as dogs or primates (including humans).(8,11) Rodents of the genus Rattus are considered the main reservoir. As such, they tend to be highly infected and are often the only source of parasites in urban environments.(9,10) Infection of other mammals and humans results from incidental ingestion of water or food contaminated by embryonated eggs.(10)
Capillaria hepatica is the only known nematode with a direct life cycle requiring death of the host to be completed. Following their ingestion, embryonated eggs hatch in the intestine and release first stage larvae that cross the cecal barrier and reach the liver through mesenteric and portal veins. The migration of larvae within the hepatic parenchyma produces areas of hepatic necrosis.(10,11) After three weeks, mature females start to lay eggs around portal tracts until they die (up to seventy days later). After their death, adults progressively disintegrate. Eggs elicit a mixed inflammatory response composed of macrophages, multinucleated giant cells, lymphocytes, plasma cells and eosinophils that leads to the formation of granulomas.(10,11)
The peculiarity of hepatic capillariasis is that eggs are kept within the hepatic parenchyma instead of being released through the biliary tract as for other hepatic parasites. Eggs can only be released in the environment after death and decomposition of the host or after its ingestion by a predator. This predator represents a paratenic host that, although not necessary for completing the cycle, greatly promotes the maturation and the dissemination of the eggs.(10,11)
A peculiar finding in rats infected with Capillaria hepatica is the development of septal fibrosis, a type of bridging fibrosis in which portal tracts are connected to portal tracts (or rarely to centrilobular spaces) by thin strands of connective tissue containing collagen, fibroblasts and lymphocytes. This pattern gives the hepatic parenchyma a porcine liver-like architecture. In humans, septal fibrosis is an important, frequent and non-specific finding in chronic liver diseases. For this reason, rats infected with Capillaria hepatica represent a good model for hepatic fibrosis. It has been shown that in this model, septal fibrosis is not preceded by hepatic necrosis or overt chronic inflammation. The first step of septal fibrosis appears to be star-shaped expansions of portal spaces containing fibroblasts and blood vessels that sprout from periportal spaces. The presence of proliferating blood vessels is a characteristic feature.(2,3,5,7)
In the present case, other rodent species (ex: Greater white-toothed shrew (Crocidura russula)) have been trapped and revealed to be infected by Capillaria hepatica but only rats developed septal fibrosis. For veterinary pathologists, this example illustrates how different can be the patterns of tissue reaction among species, even with the same initiating cause. Another well-known example for that is tuberculosis.(1)
Table 1. Hepatic parasites in Domestic Species
|PARASITE||DEFINITIVE (DH) OR|
INTERMEDIATE (IH) HOSTS
|WANDERING LARVAE (NUMEROUS SPECIES)|
|IH: sheep, goat, cattle, squirrel, swine||Cysts in peritoneal cavity / Fatal hepatic hemorrhages in lambs severely infected. Associated with Black disease|
|IH: rabbit, squirrel, small rodents||Cysts in liver capsule|
|IH: rodents, rabbit||Cysts in liver / Associated with the development of fibrosarcoma|
|Larvae of Ascaris suum||DH: swine||Causes milk-spotted liver. Adults in intestine|
|Larvae of Stephanurus dentatus||DH: swine||Adults in urinary system|
|Larval strongyles (Strongylus edentatus,S. equi, S. vulgaris)||DH: horse||Causes villous perihepatitis|
|Stilesia hepatica||DH: ruminants (Africa)||Only cestodes that inhabit the bile ducts|
|Thysanosoma actinioides||DH: ruminants (America)|
|Echinococcus granulosus||IH: humans, cattle, swine, sheep, deer, horse, small rodents, moose etc.||Liver|
|Echinococcus multilocularis||IH: humans, cattle, swine, sheep, deer, horse, small rodents, moose etc.||Liver|
|Mesocestoides corti / M. lineatus||IH: dog, cat, other mammals, reptiles, rodents||Liver, peritoneal and pleural cavities, lungs, other organs|
|Capillaria hepatica||DH: small rodents, rabbit, humans etc.||Liver|
|Fasciola hepatica||DH: ruminants, dog, cat, horse||Bile ducts / Associated with Black disease and bacillary hemoglobinuria.|
|Fasciola gigantica||DH: cattle, sheep||Bile ducts|
|Fascioloides magna||DH: cattle, sheep, horse, pig||Liver|
|Eurytrema pancreaticum||DH: sheep, goat, cattle||Pancreatic and bile ducts and duodenum|
|Concinnum procyonis||DH: raccoon, fox, cat||Pancreatic and bile ducts|
|Dicrocoelium dendriticum||DH: sheep, goat, dog, pig, deer||Bile ducts / Associated with Black disease.|
|Dicrocoelium hospes||DH: cattle (Countries south to the Sahara)||Bile ducts|
|Platynosomum fastosum||DH: cat (North America and Amazonia)||Liver, bile ducts|
|Athesmia foxi||DH: New World monkeys||Bile ducts|
|Metorchis albidus||DH: dog, cat, fox||Bile ducts|
|Metorchis conjunctus||DH: cat, dog, fox, mink (North America)||Bile ducts|
|Metorchis bilis||DH: red fox||Bile ducts|
|Opistorchis felineus||DH: cat, dog, fox (Europe, Russia)||Bile ducts|
|Opistorchis sinensis||DH: dog, cat, pig, humans, fox||Bile duct and duodenum. Associated with cholangiocarcinoma in humans.|
|Opistorchis tenuicollis||DH: dog, cat, fox, pig||Pancreatic and bile ducts|
|Pseudamphistomum truncatum||DH: dog, cat, fox||Bile ducts|
|Gigantocotyle explanatum||DH: cattle, buffalo||Bile ducts|
|Schistosoma japonicum, S. mansoni, S. bovis, S. spindle etc.||DH: various species (humans, monkey, cat, dog, ruminants etc.)||Adults reside in veins (portal v., mesenteric v. etc.)|
Liver: Hepatitis, granulomatous and eosinophilic, chronic, multifocal, moderate with bridging fibrosis and adult nematodes and eggs.
Conference participants also noted slide variation as mentioned above, with absence of adult nematodes in some sections and variation in the degree and type of inflammation around nematodes. Eosinophilic inflammation is present in some sections and others are dominated by granulomatous inflammation. The stichosome, which is a row of basophilic esophageal gland cells surrounding the esophagus and is one of the most distinctive characteristics of aphasmid nematodes,(4) is seen only in a few sections and the eggs are described as being unembryonated. Other ancillary changes described include the pitted and undulating surface of the liver as well as the hepatocellular degeneration and necrosis directly surrounding adult nematodes in some sections.
Aphasmid nematodes derive their name from the lack of a tiny set of sensory papillae (phasmids) on their caudal end, which are not identifiable histologically, but differentiate them from the phasmid nematodes (i.e. strongyles, rhabditoids, ascarids, oxyurids, rhabditoids and spirurids). Histologically visible characteristics that help distinguish them from the phasmids include absence of lateral cords, the presence of hypodermal/bacillary bands, and as mentioned above, the presence of a stichosome, which would only be seen in sections with esophagus. The eggs of some species are bipolar plugged / bioperculate as seen in this case, but may be embryonated or unembryonated depending on the specific parasite species.(4)
As mentioned above by the contributor, portal bridging fibrosis is a prominent feature of this entity. However, unlike many other types of portal fibrosis, hepatic stellate cells do not seem to participate in the pathogenesis of this lesion. Experimentally, septa are visible around the 23rd and 27th day after rat inoculation with embryonated eggs, and prominent angiogenesis precedes deposition of collagen, which is a key component in the development of hepatic fibrosis. The earliest changes are seen in portal areas, which may be located some distance away from the location of the actual nematodes,(7) and may emanate from multiple portal areas simultaneously.(3) In addition to angiogenesis, studies have found that proliferating cells responsible for the formation of prominent septa include fibroblast like cells(7) as well as pericytes and myofibroblasts.(3) Other cells, such as eosinophils, may also be present, but evidence for involvement of hepatic stellate cells is absent.(7) Interestingly, angiogenesis also seems to play a role in regression of fibrosis as it occurs in rats infected with C. hepatica.(3)
1. Caswell JL: Tuberculosis. In: Maxie, MG ed. Jubb, Kennedy, and Palmers Pathology of Domestic Animals. Vol 2. 5th ed. Philadelphia, PA: Saunders Elsevier; 2007:606-610.
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3. Gaban L, Ramos CDL, Barbosa J+â°nior AA, Souza MM de, Andrade Z de A. Dynamics of Capillaria-hepatica-induced hepatic septal fibrosis in rats. Rev Soc Bras Med Trop. 2010; 43: 643646.
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6. Jones TC, Hunt RD, King NW. Diseases caused by parasitic helminths and arthropods. In: Veterinary Pathology. Wiley-Blackwell; 2007:601667.
7. Maria De Souza M, Tolentino M, Assis BCA, Cristina De Oliveira Gonzalez A, Maria Correia Silva T, Andrade ZA. Pathogenesis of septal fibrosis of the liver. (An experimental study with a new model). Pathol Res Pr. 2006; 202: 883889.
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9. Redrobe SP, Patterson-Kane JC. Calodium hepaticum (syn. Capillaria hepatica) in captive rodents in a zoological garden. J Comp Pathol. 2005; 133: 7376.