Pathology Case of the Month - Raccoon
The raccoon was reported to be lethargic, not moving for several hours, and appeared drunk. It was standing when approached, and it fell over rather than moving away. Collectively at this location, six raccoons were found sick or dead over the course of several months. A distemper outbreak had occurred in the same location the previous year. Distemper was diagnosed in this animal and was the cause of death. In addition there were the following findings.
Gross Findings: On external examination, the oral mucous membranes and the conjunctiva were pale. There was light yellow soft discharge around the vulva. On internal examination, there was scant to adequate fat. There were four fetuses in the uterus. The liver was firm and gray brown. The spleen was moderately enlarged and firm, with scattered small white foci present throughout. Many small, white, nodular foci were present throughout the lungs as well. The pancreas was meaty and firm, with scattered white foci similar to those in the spleen and lung.
Histopathological Findings: In multiple organs including lung, liver, spleen, pancreas, kidney, and in the intestinal wall are numerous, nodular clusters of trematode eggs (Fig. 1A). Eggs are unoperculated and thin-shelled, measuring approximately 50-80 um in diameter, and containing miracidia, consistent with schistosome ova. Clusters of ova were often surrounded by infiltrates of eosinophils, macrophages, and lymphocytes and occasionally fibrosis (Fig. 1A). Adult schistosomes were present in several mesenteric blood vessels (Fig. 1B).
Marked, multifocal, chronic disseminated visceral granulomas with intralesional schistosome ova.
Etiology: Heterobilharzia americana, a trematode parasite (schistosome) that lives within the vascular system of the host.
Distribution: Endemic to the southeastern United States, extending to Texas and as far north as Kansas.
Seasonality: Any time of year.
Host range: Infects a wide range of wild mammalian hosts. Raccoons are the natural definitive host. Domestic species such as dogs and horses may also be infected, and dogs may also act as definitive hosts.
Transmission: The life cycle of H. americana is indirect, involving an aquatic snail (Lymnaea cubensis and Pseudocuccinea columella) as the intermediate host. Adults are blood parasites and live in the mesenteric veins where mating occurs. The eggs penetrate venules to enter the intestinal wall and are shed into the intestinal lumen. Eggs shed into freshwater release ciliated miracidia which infest suitable snails. Cercariae that are released from the snail intermediate host infect mammals through direct skin penetration, completing the cycle.
Clinical signs: Chronic diarrhea, dehydration, weight loss, fever, lethargy, coughing, and anorexia. Clinically significant H. americana infection includes massive inflammation leading to intestinal disorders, dehydration, pancreatic insufficiency, and systemic dissemination.
Disseminated visceral granulomas surrounding schistosome eggs may be seen as white nodules in multiple organs grossly. The liver is firm secondary to periportal fibrosis. Adult trematodes are present in mesenteric veins.
Diagnosis: A history of chronic illness with typical clinical signs and with consistent gross and histologic findings is suggestive of schistosomiasis. Definitive diagnosis is based on identification of the adult trematodes.
Public health concerns: Skin penetration by motile cercariae can occur when humans enter infested waters, resulting in dermatitis (“swimmer’s itch”).
Wildlife population impacts: Unknown
Management: None currently recommended for wildlife. Post signs near water known to contain cercariae to prevent swimmer’s itch.
- Bartsch RC, Ward BC. 1976. Visceral lesions in raccoons naturally infected with H. americana. Vet Pathol 13:241-249. https://doi.org/10.1177/030098587601300401
- Loker E. 1983. A comparative study of the life-histories of mammalian schistosomes. Parasitology 87: 343-369. https://doi.org/10.1017/s0031182000052689
- Verbrugge LM, Rainey JJ, Reimink RL, Blankespoor HD. 2004. Swimmer’s itch: Incidence and risk factors. Am. J. Public Health 94: 738-741. https://doi.org/10.2105/AJPH.94.5.738