Piroplasmida

PARASITES PROTOZOA

Cytauxzoon felis

• Cytauxzoon is classified in the order Piroplasmida and family Theileriidae
  • This family has both an erythrocytic and a tissue (leukocytic) phase

• The Babesiidae, a related family, is characterized by having a primarily erythrocytic phase in the mammalian host
  • Its morphological features are indistinguishable from the erythrocytic form of Cytauxzoon

• Cytauxzoon felis, B. equi, and B. rodhaini have been linked to both the babesias and theilerias by RNA gene sequence analysis
  • It has been suggested that these organisms be reclassified within a separate family

Life Cycle

• Large schizonts of C. felis develop in macrophages
  • In Theileria the exoerythrocytic stage occurs primarily within lymphocytes

• In C. felis, schizonts develop within mononuclear phagocytes, initially as indistinct vesicular structures and later as large, distinct nucleated schizonts that actively undergo division by true schizogony and binary fission

• Later in the course of the disease, schizonts develop buds (merozoites) that separate and eventually fill the entire host cell

• Each schizont may contain numerous merozoites
  • Ultrastructurally, schizonts lack a parasitophorous vacuole, and individual merozoites possess rhoptries

• The host cell ruptures, releasing merozoites into the tissue fluid and blood

• Merozoites are then believed to enter erythrocytes to form the intraerythrocytic stage

• Merozoites appear in macrophages one to three days before they are observed in erythrocytes

Pathogenicity

• Ticks are implicated as the natural vector for Cytauxzoon
  • Most cases of infection have been associated with the presence of these parasites on the hosts
  • Experimentally, Dermacentor variabilis can transmit the organism from bobcats to domestic cats. In a white tiger that developed a natural, fatal infection in Florida, two female Lone Star ticks (Amblyomma americanum) were present on the inguinal skin.

• Clinically, the disease in cats is characterized by fever, depression, dyspnea, anorexia, lymphadenopathy, anaemia, and icterus leading to death in three to six days

• Gross findings include pale or icteric mucous membranes, petechiae and ecchymoses in the lung, heart, lymph nodes and on mucous membranes, splenomegaly, lymphadenomegaly, and hydropericardium

• Microscopically, numerous large schizonts are present within the cytoplasm of endothelial-associated macrophages
  • Infected macrophages become markedly enlarged (up to 75μm) and may occlude the lumens of numerous vessels of many tissues, especially the lungs
  • Minimal inflammatory reaction is present in tissues

Diagnosis

• Merozoites within erythrocytes, best seen on peripheral blood or tissue impressions, are variable in morphology and can occur as round, oval, or signet ring-shaped bodies
  • Are 1-5 micrometers in diameter
  • Small, peripherally placed basophilic nucleus

• Organisms that must be distinguished from the intraerythrocytic phase of C. felis include Babesia and Hemobartonella
  • The blood stage may appear similar to the ring forms of Hemobartonella and to the piriforms of Babesia
  • Unlike Cytauxzoon, babesiosis and hemobartonellosis do not have a tissue stage of infection

• Differential diagnosis for the tissue phase of cytauxzoonosis includes other small (less than 5 μm), intrahistiocytic organisms such as Toxoplasma, Leishmania and Histoplasma