Fascioloides magna

Wednesday, May 26, 2021

(NY) Two pregnant captive deer on a small farm in northern NY were found dead over the course of two weeks. They had shown signs of weight loss and hair loss prior to death. Field necropsies revealed their livers were atrophic and contained black gelatinous material. Fecal floatation revealed many Fascioloides magna eggs. Anaerobic culture was positive for Clostridium novyi and Clostridial FA panel was positive for C. novyi and Clostridium sordelli. Histopathology of liver showed operculated, brown, metazoan eggs within connective tissue. Associated with these eggs was black amorphous extracellular material, likely fluke exhaust. A final diagnosis of Fascioloides magna infection was made.

Fascioloides magna, also known as the giant liver fluke, is a digenean trematode that infects wild ruminants (such as the white-tailed deer, elk, caribou, etc.) as the definitive host and Lymnaeid snails as the intermediate hosts. Definitive hosts are infected through ingestion of encysted metacercaria on emergent and submerged vegetation around bodies of water such as ponds and streams. Upon activation in the intestine the larval flukes migrate to the liver where the host will encapsulate them in the hepatic parenchyma. Within the liver cysts, flukes mature to adults and begin laying eggs. A patent communication between the liver cyst and bile duct allows eggs to be voided in the feces of definitive hosts. The eggs hatch to release free swimming miracidia which seek out aquatic snails to continue this life cycle, eventually producing thousands of cercariae. Released cercaria will encyst on aquatic vegetation as metacercaria until ingested by a susceptible definitive host to perpetuate the life cycle. In dead end hosts, such as cattle, F. magna will become encapsulated within the liver, but the cyst is not in communication with the bile duct and no eggs are shed in the feces. Domestic small ruminants are aberrant hosts of F. magna, in which larval flukes fail to mature but migrate extensively through the liver causing inflammation and severe hepatic damage which is potentially fatal to the host. The lack of eggs in the feces of dead end and aberrant hosts of F. magna makes ante-mortem diagnosis difficult. Fascioloides magna is known to track clostridial species into the liver, resulting in infectious hepatic necrosis or "Black’s disease", particularly associated with C. novyi2. In the U.S, there is no approved treatment for F. magna, however high doses of albendazole have been shown as efficacious for mature flukes, but not against immature forms under 12 weeks of age3,4. Other methods of F. magna control include eliminating the aquatic snail intermediate host, preventing wild ruminants from entering pasture, and removing access of livestock to bodies of water.

For more information about Fascioloides magna, please visit the Cornell Wildlife Health Lab: https://cwhl.vet.cornell.edu/disease/fascioloides-magna#collapse11

  1. Navarro, M., & Uzal, F. A. (2016). Infectious Necrotic Hepatitis. In Clostridial Diseases of Animals (pp. 275–279). Wiley Blackwell.
  2. Pybus, M. J. (2001). Liver Flukes. In Parasitic diseases of Wild Mammals (2nd ed., pp. 121–149). Iowa State University Press.
  3. McKellar, Q. A., & Scott, E. W. (1990). The benzimidazole anthelmintic agents - a review. Journal of Veterinary Pharmacology and Therapeutics, 13(3), 23–247.
  4. Rojo-Vazquez, FA; Meana, A; Martinez-Valladares, M. (2012). Update on trematode infections in sheep. Veterinary Parasitology, 189, 15-38.