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As an important zoonotic parasite, Capillaria hepatica poses a threat to human health that cannot be ignored due to its association with high mortality and serious damage to the liver, although there are relatively few human infections. The infection rate of Capillaria hepatica in rodents is very high, which poses a great threat to the health of rodents, and Rattus norvegicus has been found to be the main group carrying Capillaria hepatica. Capillaria hepatica’s unique biological characteristics, including its morphological features and complex life history, determine the specificity of its infection and pathogenicity. In terms of epidemiology, Capillaria hepatica has a worldwide distribution, a wide variety of hosts (mainly rodents), and various transmission routes, all of which increase the difficulty of its prevention and control. Children are more likely to be infected by it, and there is little gender difference among the infected population. Although there are a variety of diagnostic methods for hepatic capillariasis, all of them have certain limitations. In addition, due to its non-specific clinical manifestations, early accurate diagnosis of hepatic capillariasis is still a challenge. This article reviews the biological characteristics and pathogenic mechanism of Capillaria hepatica, the epidemiology of human infection, the epidemiology of animal infection, and the diagnosis and treatment of hepatic capillariasis, so as to provide a useful reference for related research and clinical practice.

Hepatic capillariasis is a rare and neglected zoonosis affecting wild and synanthropic small rodents. It is caused by infection with Calodium hepaticum in liver. Despite the worldwide distribution of the host Rattus norvegicus (brown or street rats) in the urban area, the epidemiological status of this parasitosis remains unknown. In the present study, we examined a total of 27 brown rats from the city centre and a garden (four km from the city centre) of Marseille, France. All rats were autopsied and 52% showed the presence of C. hepaticum eggs in the liver. This result draws general attention to public health risks, since street rats are living near the human population.

Background Capillaria hepatica is a zoonotic parasite in humans and animals and has a worldwide distribution. However, infections in mammals apart from rodents, which are natural hosts of the parasite, have rarely been reported. This report describes the first known case of C. hepatica infection in a horse in Japan. Case presentation A 3-year-old filly without clinical signs was presented at a slaughterhouse in Japan. Gross examination revealed white to tan nodules 0.5 to 1.5 cm in diameter in the parenchyma of the liver. Histologically, the nodules had mature fibrous capsules and consisted of multifocal to coalescing granulomatous inflammations with numerous nematode eggs. The eggs were barrel shaped with an opercular plug on each end and double-layered shells; these findings are consistent with the features of C. hepatica eggs. Conclusions To our knowledge, this is the first case of C. hepatica infection in a horse in Japan. The pathological findings confirmed the presence of this pathogen in this part of the world, and they highlight the importance of this nematode in the differential diagnosis of hepatic granulomatous lesions in horses.

Calodium hepaticum (syn. Capillaria hepatica) and Calodium splenaecum (syn. Capillaria splenaecum) are nematodes that infect the liver and spleen, respectively, of mammals. While the host range, distribution, pathology and zoonotic potential of C. hepaticum are well known, very little is known about C. splenaecum. The observed prevalence of these two parasites, the factors associated with prevalence, and the lesions resulting in the different host species were studied in 408 micromammals captured in two periurban areas of Barcelona (NE Spain) from 2011 to 2013. C. hepaticum was found in 4 % of 322 wood mice (Apodemus sylvaticus) (with local prevalence up to 16 %) and 1 of 2 Norwegian rats (Rattus norvegicus). C. splenaecum was found in 10 % of 38 greater white-toothed shrew (Crocidura russula) (local prevalence up to 30 %). Neither parasite was detected in 29 Algerian mice (Mus spretus) and 17 black rats (Rattus rattus). Prevalence of C. hepaticum was significantly higher in wood mice captured in natural areas (6.4 %) than those from residential areas (0 %), and infected mice were in better body condition. No differences in prevalence were found among age and sex groups, years and seasons. Lesions of hepatic capillariasis in wood mice consisted mainly of mild to moderate multifocal granulomas around degenerating adult parasites and/or eggs, while lesions seen in a rat consisted of multifocal granulomatous hepatitis and bridging fibrosis extending from the necrotic areas caused by the parasites. Splenic lesions found in shrews due to C. splenaecum, representing the first histological description of this parasite, were single nodules that corresponded to finely encapsulated clusters of eggs with adult parasites.

A female ring-tailed lemur (Lemur catta) and her two cubs held in a zoo in Chile exhibited signs of severe hepatic insufficiency. In spite of supportive treatment, the three animals died a few days after the onset of signs. Postmortem examination revealed ascites and fibrotic lesions in the liver of all the individuals. Histologically, the liver of two of them showed a severe parasitic ova infection and lipidosis, the morphologic characteristics of the parasitic ovas were consistent with Capillaria hepatica (syn. Calodium hepatica) eggs. To the authors' knowledge, this is the first clinical case report of hepatic capillariasis in prosimians, and its implications are discussed.

Septal fibrosis of the liver regularly develops in rats infected with Capillaria hepatica. To find out whether such fibrosis also occurs in mice, 20 animals were submitted to infection with either 100 or 300 embryonated eggs and histologically examined after several periods of time, from 30 to 110 days afterwards. Results showed that mice developed acute, severe, diffuse and focal hepatic lesions that were soon modulated to focal areas of fibrosis containing eggs and worm remnants, despite the fact that a few worms remained alive, at least up to 110 days after inoculation. Areas of perisinusoidal fibrosis appeared in the proximity and around focal parasitic lesions, but clear-cut septal fibrosis was not observed. Why septal fibrosis forms in rats, but not in mice during C. hepatica infection, only further studies can clarify. Mice seem to show better host/parasite relationship than rats in regard to C. hepatica infection. A fibrose septal do fígado se desenvolve regularmente em ratos infectados pela Capillaria hepatica. Com o fito de se verificar se tal tipo de fibrose também se desenvolve em camundongos, 20 animais foram infectados com 100 ou 300 ovos embrionados e examinados histologicamente em vários períodos de tempo, entre 30 até 110 dias após inoculação. Os resultados demonstraram que os camundongos desenvolvem inicialmente uma intensa hepatite aguda, difusa e focal, mas logo estas lesões são moduladas, desaparecendo a hepatite reacional difusa e persistindo as lesões em torno de ovos e restos de vermes. Alguns poucos vermes sobreviveram por todo o período experimental. Embora a fibrose perisinusoidal tenha aparecido em torno das lesões fibróticas focais, a fibrose septal não foi observada. Não se sabe por que a fibrose septal se forma no rato infectado por C. hepatica, mas não no camundongo, mas o assunto merece investigação, pois importantes aspectos da fibrogênese hepática poderiam estar implicados. Os camundongos parecem melhor adaptados à C. hepatica que os ratos; nos primeiros, a infecção é nitidamente autolimitada.

Hepatic capillariasis is a global neglected zoonotic disease with small rodents and lagomorphs as host animals although cases have occurred in over 140 mammals, including in humans to underscore its health implications to both animals and humans. A total of 20 free-living Cricetomys gambianus were sampled from the wild within the Federal Capital Territory, Nigeria, for the presence of and effects of Capillaria hepatica between August 2023 – May 2024. The lungs, liver, spleen, and kidneys of the sampled rats were promptly harvested, observed and recorded, and fixed in 10% formalin for histopathological evaluations after humane animal sacrifice according to standard procedures. There were no obvious gross lesions while cellular changes occurred only in the liver that were characterized by the presence of clusters of parasitic eggs with hepatic necrosis and inflammatory responses. The presence of the ellipsoidal-shaped and double layered straited shelled-eggs with operculum and the hepatic cellular changes led to a definitive diagnosis of Capillaria hepatica with a 10% prevalence rate. The findings highlighted the health risk to the animal and the human consumers of infected. Cricetomys gambianus whose meat is considered a delicacy within the study area and other parts of the world. Outside the probable role of host animal’s adaptation strategy being responsible for the non-typical granulomatous inflammation and hepatic septal fibrotic cellular changes, there is a need for more studies to further elucidate the pathogenesis of the disease condition in this particular animal species in the patho-epidemiology of hepatic capillariasis within the study area.