Sarcocystis species are one of the most commonly found tissue cyst-forming coccidians of cattle in Australia. The thin-walled sarcocysts (‘sarx’ means flesh) are located in the muscles of cattle, and in some cases nerve cells. The cysts are microscopic and cannot be detected by eye. Up to 100% of some cattle populations may be infected.
Dogs and cats are the definitive hosts of this parasite which takes advantage of predator / prey relationships.
Figure 1. Cyst of Sarcocystis spp. in a histologic muscle preparation from a cow. Image courtesy of the National Centre for Veterinary Parasitology, ncvetp.org
Sarcocysts are located within the muscles.
Most infections in Australian cattle herds are of low pathogenicity and cattle appear clinically normal.
Acute infections are rare. They are characterized by protracted fever, anaemia, loss of appetite and loss of weight, excessive salivation and loss of hair most noticeably at the tail switch. Cows infected in the last trimester of pregnancy may abort.
Dogs usually do not show any symptoms of infection.
Sarcocystis inhabits the small intestine of dogs or cats (the predators) and produces oocysts (type of egg) that release infective sporocysts into the faeces.
Cattle become infected by ingesting feeds contaminated with dog or cat faeces carrying these infective sporocysts. Once in the small intestine, the sporocysts release further developmental stages that penetrate the intestinal wall and undergo multiplication phases in the walls of blood vessels and capillaries before invading heart and skeletal muscles and very occasionally nerve cells to form sarcocysts (muscle cysts). Immature sarcocysts are present in muscles about one month after infection and mature muscle cysts are present in about three months.
When dogs or cats are fed carcass meat containing sarcocysts, or are scavenging, the cysts burst open in the small intestine releasing infectious stages that progress to oocyst and sporocyst development in the intestinal wall. The pre-patent period after feeding infective cysts in meat to dogs is 10-12 days and sporocyst shedding can continue for about 20-35 days.
Most cattle are asymptomatic, and the parasite is so small that many are missed during meat inspection. Numerous genetic tests are available for herd testing.
Because most adult cattle harbour cysts in their muscles, dogs or cats should be prevented from eating raw meat, offal, or dead animals, and prevented from contaminating food and water supplies of cattle with faeces. Amprolium fed prophylactically has been shown to reduce illness in cattle experimentally inoculated with sarcocysts.