Introduction

“Anemia” is a general term for the reduction below normal in the number of circulating red blood cells (erythrocytes) or in the amount of hemoglobin in the blood. Feline infectious anemia (FIA) is a disease caused by parasitic bacterial microorganisms that attach to the surface of red blood cells, leading to potentially fatal hemolytic anemia. Cats become infected with this organism through arthropod vectors, bite wounds and blood transfusions. Infected queens can transmit the bacteria to their kittens as well.

Causes of Feline Infectious Anemia

The microorganisms responsible for causing feline infectious anemia are Mycoplasma haemofelis and Mycoplasma haemominutum - formerly collectively called Haemobartonella felis. The names were changed to more accurately classify the organisms as mycoplasmas, rather than rickckettsias, based upon relatively recent genetic sequencing.

The bacteria are transmitted from infected to naïve cats by blood-sucking arthropods (fleas and ticks, and possibly mosquitoes). Outdoor cats prone to fighting with other cats can become infected via bite wounds. This is particularly true of the deep bites associated with fights between territorial intact males. Kittens in litters from infected queens are frequently infected as well, although it is still not known whether this vertical transmission is transplacental, transmammary (in the milk) or from exposure to blood during the birth process. Finally, although much less common, cats can acquire the infectious organism through transfusions of contaminated blood.

Outdoor cats that have poor flea and tick control are at an increased risk of developing FIA. Young, intact, male cats are also at a higher risk of becoming infected with this organism. Cats with underlying feline leukemia virus infection or who are otherwise immunocompromised are also at an increased risk of developing feline infectious anemia.

Once in the cat’s blood stream, these mycoplasma organisms parasitize the surface membranes of red blood cells, causing hemolysis and potentially fatal hemolytic anemia through several proposed mechanisms. When the organisms attach to the red blood cell surface, the cat’s immune system mistakenly views those cells as foreign and initiates an immune response to destroy them. This is called immune-mediated hemolytic anemia; “hemolysis” simply refers to the rupture of red blood cells. The liver, spleen, lungs and bone marrow of infected cats probably also participate in destruction of affected red blood cells through a process called erythrophagocytosis, which causes extravascular hemolysis. Finally, certain immune system cells (called macrophages) are able to strip the parasites off of individual red blood cells and return the damaged cells to circulation, where they are fragile, do not function normally and have a shortened life span.