Sarcocystis

Sarcocytis Life Cycle Diagram - Dennis Jacobs & Mark Fox RVC

Sarcocytis - Joaquim Castellà Veterinary Parasitology Universitat Autònoma de Barcelona

Sarcocystis in sheep oesophagus - Adam Cuerden

Equine protozoal myeloencephalitis - Wikimedia Commons

Sarcocystis cruzi - Courtesy of the Laboratory of Parasitology, University of Pennsylvania School of Veterinary Medicine

• Most infections are asymptomatic

• Heavy infections are causes of chronic wasting in large animals, hide condemnation and downgrading of carcasses

• Sarcocystis should be differentiated from other tissue-cyst forming coccidia

• There are many species of Sarcocystis which differ in size from microscopic to several centimetres in length
  • S.neurona is an important equine pathogen in the USA

• Infective cyst in the intermediate host is called a sarcocyst

Life Cycle

• The individual life cycle of some species is incompletely understood

• Indirect life cycle

• Life cycle alternates between the final and the obligatory intermediate host

• Only one final and one intermediate host

• Sporulated oocyst has 2 sporocysts containing 4 sporozoites
  • Naked sporocyst usually seen in faeces as the oocyst wall is very delicate
  • Oocyst measures 15μm in length

• No schizogony in final host

• Gametogeny occurs deep in subepithelial tissue

• Faecal oocyst count is low

• Oocysts are sporulated when passed
  • Difficult to find on faecal examination as the sporocysts are few in number and small

• Ingestion of sporocyst by intermediate host
  • 2 phases of rapid asexual reproduction in vascular endothelial cells
  • Slow multiplication of bradyzoites in muscle tissue
  • Sarcocyst forms with bradyzoites inside, surrounded by a cyst wall and divided into compartments

Epidemiology

• Final hosts are carnivores and omnivores

• Intermediate hosts are herbivores and omnivores

• Humans are the final host for some species and the intermediate hosts for others
  • Final host for species infecting cattle and pigs

• Dogs are final hosts for species infecting cattle, sheep, goats, pigs and horses

• Cats are final hosts for species infecting cattle, sheep and pigs

Pathogenesis

• Widespread infection but mostly asymptomatic

• Cause meat inspection losses

• Sarcocystis in myositis

• Experimental infections cause severe, acute pyrexic disease when the organism multiplies in the vascular endothelium

• Can cause chronic wasting disease in cattle and horses
  • Causes abortion and post-natal disease in sheep

• Equine protozoal myeloencephalitis
  • Necrotising encephalomyelitis affecting the grey and white matter of the CNS
  • Caused by S.neurona
  • Opossum thought to be the definitive host
  • Horses thought to be accidental hosts
  • Natural intermediate hosts currently unknown
  • Western Blotting shows 50% of horses in the USA are seropositive
  • Risk factors poorly understood
  • Causes spinal cord dysfunction
    • Ataxia and paralysis