(also called "Feline Hemotropic Mycoplasmosis" or infection by Hemobartonella felis, or infection by Mycoplasma haemofelis)

Parasitic organisms survive by attaching themselves to a host and using the host's body to thrive, generally at the host's expense. Parasites find themselves protected from the harsh temperature and moisture changes of the external world when they live within the rich, warm body of their host.  The parasites this article concerns is a bacterium that attaches itself to the actual red blood cell membranes of its host, happily riding around, feeding and reproducing until the host's immune system sees it and begins destroying red blood cells in an attempt to remove it.

Hemobartonella felis (newly renamed "Mycoplasma Haemofelis")

The agent of what has traditionally been called "feline infectious anemia" is an organism called (until recently) "Hemobartonella felis." This creature is technically a bacterium but is a member of a special group of bacteria called "Mycoplasmas." Mycoplasmas are different from other bacteria because they do not have a cell wall surrounding and protecting their microscopic bodies. They cannot be cultured in the lab like normal bacteria because they require living hosts.

Hemobartonella felis was first discovered in Africa in 1942 but it was not recognized as a Mycoplasma until gene sequencing was possible recently. This has led to a renaming of Hemobartonella felis to Mycoplasma haemofelis, though after decades of celebrity we shall see if they new name becomes commonly used. Complicating matters further, gene sequencing has revealed a second species, previously thought to be another variant of Hemobartonella felis. This one, which is smaller, has been named "Candidatus M haemominutum." This smaller mycoplasma seems to only be a problem for cats infected with the feline leukemia virus . The discussion that follows largely concerns Mycoplasma haemofelis.

The term "feline infectious anemia" has also recently been felt to be inaccurate as there are many infectious organisms that might cause an anemia (lack of red blood cells).  For this reason the disease itself has been re-named "Feline Hemotropic Mycoplasmosis" which literally means a blood infection of Mycoplasma organisms in cats.

What happens to the infected cat?

The infected cat's immune system is busy coating infected red blood cells with antibodies. Coated red blood cells are removed from the circulation by the spleen. This kills the organism and the iron is harvested and recycled into new red blood cells. The problem is that so many red blood cells are being destroyed that the cat becomes anemic.

The infected sick cat is pale (sometimes even jaundiced) and weak. Anemic cats often eat dirt or litter in an attempt to consume iron.  A fever may be present. The initial blood tests show not just red cell loss but a very responsive bone marrow (the source of new red blood cells) which means that the cat knows it is losing red cells and is trying to make more as quickly as possible to keep up. Cats with concurrent feline leukemia virus infection tend to have more severe anemias as the virus does not permit the bone marrow to respond.

When a cat is newly infected, it can take up to one month before adequate numbers of parasites are present to actually make the cat sick. Mortality is highest during the month following this initial stage. If the cat recovers, it becomes a carrier though stress can re-activate the infection.

How is diagnosis confirmed?

Confirmation of diagnosis has been problematic since the discovery of the organism.  Because of the lack of cell wall, Mycoplasma haemofelis cannot be cultured which means one cannot simply culture a blood sample and isolate the organism the way one might isolate the organism causing say a urinary tract infection.

Most reference labs scan all feline blood samples under the microscope looking for the characteristic appearance of infected red blood cells (see graphic at the top of the page). Unfortunately, the number of organisms cycles in a matter of hours such that the number of infected cells can change from 90% to 1% in a matter of 3 hours. This makes it very easy to miss infected cells even in a grossly infected cat.

Luckily, PCR technology has made diagnosis easier, though carrier cats can still slip by. Unless, organisms are actually seen on a blood smear, the PCR test is the type of test to request.

What cats are at risk?

The cats at highest risk are those that roam outside in the Spring and Summer (obviously these cats have the highest risk for flea infestation). Cats that are statistically likely to be infected are male cats younger than age 4-6 years, with a history of cat fights, and incomplete vaccination histories (in short, cats with somewhat casual care).   Infection with the feline leukemia virus is also a factor in diagnosis. This may be because this immune-suppressive virus allows proliferation of the organism not possible in normal hosts or perhaps the anemia associated with the virus directly leads to a sicker cat who is thus more likely to see the vet and have testing. Making matters worse, the presence of the mycoplasma seems to enhance the ability of the feline leukemia virus to create bone marrow cancers.

An abnormal immune system is absolutely not a necessity in infection with hemotropic mycoplasmas; normal cats are infected as well. Further, infection with Feline Immunodeficiency Virus does not enhance the severity of hemotropic mycoplasma infection.

Blood sucking parasites such as fleas, ticks, lice, and mosquitoes are the leading candidates for spread of the organism. This makes flea control paramount in protection.

Cats can become infected by blood transfusion, though animal blood banks routinely screen donors so this is an unlikely route.

Infected mother cats appear to be able to infect their unborn or newly born kittens. Oral transmission is definitely possible.

Treatment

If hemotropic mycoplasma infection is suspected, initiating treatment is probably a good idea as treatment is much easier than diagnosis. All mycoplasma infections are susceptible to the use of tetracycline. In cats, the derivative doxycycline tends to be most easily dosed as it comes in an oral suspension. Tablets must be used with caution as they can stick in a cat's esophagus, cause irritation, and scarring. The quinolone class of antibiotics (enrofloxacin etc.) are also effective against hemotropic mycoplasmas. Three weeks of medication is needed to adequately suppress the organism.

Killing the mycoplasma is only part of the therapy, however; it is the host's own immune system which is removing the red blood cells and this must be stopped. Prednisone or similar steroid hormone is typically used to suppress this part of the immune system so that the red blood cells are not removed as quickly. Very sick cats will probably require blood transfusions to get through the brunt of the infection. Happily, prognosis is fair if the diagnosis is made in time as cats generally respond well and quickly to treatment.

Carrier cats are generally not treated.

Can dogs be infected?

There is an organism previously called Hemobartonella canis (now renamed Mycoplasma haemocanis). It is not generally considered to be a problem except in dogs who have lost their spleens and thus cannot effectively remove infected red blood cells.  Gene sequencing suggests that this may actually be Mycoplasma haemofelis able to disguise itself slightly when it lives in a dog's body. Blood from infected dogs, however, will not infect cats.  It is not at this time clear, what the relationship is between these two Mycoplasmas but it appears that cats cannot infect dogs and dogs cannot infect cats. There is a hemotropic mycoplasma disease in dogs but it is separate from that in cats.