What is Feline Infectious Anemia?
“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.
How Feline Infectious Anemia Affects Cats
Infected animals develop a progressive and usually cyclic decrease in the number of healthy circulating red blood cells, which leads to weakness, inappetance, lethargy, depression, splenomegaly, weight loss, pallor, often a fluctuating fever and sometimes sudden death. Once a cat is infected, it typically takes about 10 days for clinical signs to develop, if they develop at all. Cycles of parasitemia (detectable levels of parasites in the blood) then occur, lasting 1 to 2 days with an average of 6 days between episodes. The level of parasites in the blood can vary markedly within a matter of hours. Adult cats, especially males, are reportedly more frequently affected, as are free-roaming unvaccinated animals and FeLV-positive cats.
In acute cases, affected animals suddenly become weak, lethargic, depressed, anorexic and pale. They may or may not have a fluctuating fever and bouts of vomiting. Their mucus membranes tend to turn blue-ish, and their heart and respiratory rates become elevated. In extreme cases, FIA can cause sudden death. Cats with chronic clinical disease have similar symptoms, but their signs linger and eventually are accompanied by weight loss, poor body condition, an enlarged spleen and chronic fatigue.
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.
Preventing Feline Infectious Anemia
Cats kept exclusively indoors are much less likely to develop FIA, because they are not exposed to cat fights and uncontrolled feral cat populations. Routine flea and tick control is especially important if cats are allowed to go outdoors or if indoor cats are in contact with other pets with outdoor access (dogs, cats, others). Neutering and spaying can reduce roaming tendencies and decrease the chances of territorial cat fights.
Treating Feline Infectious Anemia
The therapeutic goals for cats with infectious anemia are to resolve the clinical signs and restore red blood cell volume. In acute cases, affected cats may need hospitalization so that intravenous fluid replacement, blood transfusion and nutritional support are readily available. Doxycycline/tetracyclines are the current drugs of choice to clear the infective bacteria from the bloodstream. They should be given for a minimum of 21 days. Doxycycline tends to be better tolerated in cats, although it can contribute to esophageal inflammation and strictures in cats. Fluoroquinolone antibiotics are reportedly effective in cats as well, but they can cause blindness in cats if given at high doses. Antibiotic therapy will not actually “cure” feline infectious anemia. However, it can dramatically reduce the number of blood-borne bacteria to the point where the cat is no longer clinically affected.
When the infectious microorganisms trigger the cat’s immune system to destroy “marked” red blood cells, immunosuppressive doses of glucocorticoids are often given for a few days. Prednisone, prednisalone and dexamethasone are among the drugs a veterinarian may consider in these cases. Of course, concurrent diseases that might be exacerbated by steroid administration should be ruled out before steroids are given. When feline infectious anemia becomes life-threatening, whole-blood transfusion may be necessary.
Outlook & Prognosis
Most cats with feline infectious anemia have a good to excellent prognosis, as long as they receive aggressive supportive care and appropriate antibiotic treatment. Cats that have underlying disease, such as feline leukemia virus or feline immunodeficiency virus infection, have a more guarded prognosis and are more prone to recurrent bouts of anemia. Once infected, whether clinically or subclinically, all cats become “carriers” for life and may become symptomatic during periods of excitement, stress or immunosuppression from whatever cause. Cats with chronic infection but without clinical signs do not require treatment, as no treatment has been identified to completely clear the carrier state.
Finding the causative organism microscopically on a blood smear is diagnostic of FIA. However, these organisms are difficult to detect given the cyclic nature of the disease, and serial blood samples may be recommended. A polymerase chain reaction (PCR) test has been developed to identify the particular causative organisms, but these may not yet be commercially or widely available.
The more pathogenic strain of this organism can cause fatality in up to 30% of untreated cats, making prompt treatment essential. Cats with concurrent viral infections are usually more severely affected, and co-infection unfortunately is rather common in cats with FIA.
Antibiotics, steroid therapy and perhaps intravenous fluids and whole-blood transfusions may be required. Therapy never completely clears the parasite, although relapse is uncommon.