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

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

• 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

• 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.

• In the life cycle of 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

• 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

• 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

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

• 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