Feline Infectious Anaemia

Summary
Cause:	Mycoplasma haemofelis
Signalment:	Male cats with outdoor access
Associated conditions:	Feline retroviruses
Differential diagnoses:	Other causes of anaemia
Diagnosis:	PCR, blood smear examination
Treatment:	Doxycycline

Also known as: FIA — Haemoplasmosis — Hemoplasmosis; previously known as: Haemobartonellosis

Caused by: Mycoplasma haemofelis (Mhf) (previously known as Haemobartonella felis), Candidatus M. haemominutum (Mhm) — Candidatus M. turicensis (Mtc)

Introduction

Feline Infectious Anaemia is a regenerative anaemia of cats caused by Mycoplasma haemofelis (Mhf). The disease occurs worldwide and should be considered as a differential diagnosis for any cat presenting with anaemia.

Aetiology

Mhf is one of a group of organisms known as haemotropic mycoplasmas which infect many mammalian species. The causative organism was previously called Haemobartonella felis and thus the disease was known as haemobartonellosis. Polymerase chain reaction (PCR) assays allowing DNA analysis resulted in the reclassification of the organism as a mycoplasma in 2001. Two other haemotropic mycoplasmas (Candidatus M. haemominutum (Mhm) and Candidatus M. turicensis (Mtc)) have also been identified in cats but appear to be less pathogenic.

Mhf can vary in shape from cocci to rods and is sometimes present as chains of organisms on the surface of erythrocytes.

Mycoplasma haemofelis on the surface of an erythrocyte

Transmission

The mode of transmission of Mhf is poorly understood. Several possibilities have been proposed but none have been able to be reliably demonstrated. These include:

• Biting and fighting activities. Infection by this route has not been able to be demonstrated experimentally.
• Fleas. The fact that Mhf infection can be prevalent in areas where fleas are uncommon makes this less likely.
• Arthropods. Geographic variation in rates of Mhf infection does exist and could be due to transmission by an arthropod.

As transmission has been shown to occur through blood transfusion due to the ability of the organism to survive in stored blood products for up to 7 days , it is recommended that blood donors be screened.

Signalment

The major group at risk for Mhf infection are young male cats who spend time outdoors. In many areas of the world an association has been shown between Mhf and retroviral infections. It has been demonstrated that cats infected with Mhf and feline leukaemia virus (FeLV) are likely to develop more severe anaemia that cats infected only with Mhf .

Pathogenesis

The pathogenesis of Mhf is not fully understood. It is thought that the presence of the parasite on the surface of the erythrocytes may induce antibody production, resulting in haemolysis which is primarily extravascular. The haemolysis may be due to direct damage of erythrocytes by the organism or as a result of the antibodies produced by the infected animal .

Cats can produce an immune response which results in the disappearance of organisms from the blood. Production of uninfected erythrocytes can then result in an increase in haematocrit.

It is thought that recovered cats can become subclinical carriers, with the potential for the disease to recrudesce if the animal is in a stressed state eg pregnancy, illness.

Mycoplasma haemofelis on the surface of blood cells

Clinical signs

Infected cats most commonly present for lethargy and decreased appetite. Physical examination findings are non-specific and can include signs of anaemia, such as mucous membrane pallor, tachypnoea and tachycardia, pyrexia, and occasionally splenomagaly and jaundice. Pyrexia is frequently intermittent and spikes when parasite numbers are highest . The anaemia can be severe and rapidly fatal in some cases.

Differential diagnosis

There are multiple causes of both regenerative and non-regenerative anaemia in cats which must be considered as differential diagnoses.

For cats showing signs of regeneration, causes of blood loss or haemolysis must be considered:

• primary immune-mediated haemolytic anaemia
• Heinz body haemolytic anaemia (onions, garlic, propofol)
• other infectious causes such as Babesia felis and Cytauxzoon felis
• neoplasia

Major differential diagnoses for cats with non-regenerative anaemia include:

• neoplasia
• chronic inflammatory conditions
• underlying conditions such as diabetes mellitus and renal disease
• Feline Immunodeficiency Virus(FIV) and FeLV infections
• bone marrow diseases

Diagnosis

The most commmon findings from complete blood counts from cats with Mhf infections are a marcocytic, hypochromic regenerative anaemia. Reticulocytes and Howell-Jolly bodies may be identified on cytologic examination.

Mhf infection can be definitively diagnosed by identification of organisms on a blood smear, appearing as cocci or rods and sometimes forming short chains of organisms. However, examination of a single blood smear is less than 50% sensitive as the animal's immune response causes organisms to disappear from the blood stream for several days, often to reappear a few days later. One study found that smears should be examined every four days over a minimum of three four day cycles, making it possible to have eight days between presentation and diagnosis. It also has the potential for misidentification of organisms.

The gold standard for diagnosis of Mhf infection is polymerase chain reaction (PCR), which detects the 16S RNA gene. The test is widely available through veterinary diagnostic laboratories. There are two types of PCR tests; conventional PCR which uses the presence or absence of bands on a gel to assess infection, and real-time PCR assays which uses fluoresence to mark a segment of DNA unique to the organism, making it highly sensitive and species-specific and thus the test of choice . Treatment with antimicrobials may result in false negatives on PCR so collecting before beginning therapy is preferable.

Treatment

The treatment of choice for Mhf infection is doxycycline for at least two weeks. Only cats who are anaemic and have clinical signs and laboratory results consistent with haemoplasmosis should be treated as the drug does not reliably eliminate the organism. For this reason a PCR-positive cat may not become negative despite treatment and therapy cannot be used to ensure blood donor cats are free from infection.

Enrofloxacin is also an effective treatment but is a second choice due to the risk of acute retinal damage in cats.

A blood transfusion may also be required in cases of severe anaemia.

Antimicrobial therapy and blood transfusions should be initiated before PCR results are known, as this may take several days. In cats testing negative for Mhf and failing to respond to therapy, alternative diagnoses should be considered.

The use of glucocorticoids to suppress immune-mediated damage to erythrocytes has been suggested. However, as this treatment may cause reactivation of a latent infection it is recommended that they only be used in cats not responding to doxycycline and/or for whom there is no definitive diagnosis.

Prognosis

Prognosis for infected cats is related to the severity of the anaemia at presentation. If an animal survives the initial anaemia prognosis for clinical recovery is good. However, antibiotics do not clear the infection so many cats become carriers of the organism. Affected animals may also have intercurrent disease, such as retroviral infections, which will also affect the prognosis.

Extra references

Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial