Feline Hemotropic Mycoplasmosis (Hemoplasmosis)

Feline Hemotropic Mycoplasmosis (Hemoplasmosis), formerly known as feline infectious anemia (FIA), is an infectious disease of domestic and wild cats caused by obligate intraerythrocytic bacteria belonging to the genus Mycoplasma [1]. These wall-less, hemotropic bacteria parasitize the surface of red blood cells (RBCs), triggering immune-mediated hemolytic anemia of varying severity [2]. Three principal species infect cats worldwide: Mycoplasma haemofelis (Mhf), 'Candidatus Mycoplasma haemominutum' (CMhm), and 'Candidatus Mycoplasma turicensis'* (CMt), each differing markedly in pathogenicity [2][3]. The disease is globally distributed and has emerged as a significant concern not only in veterinary medicine but also as a potential zoonosis [4].

Causative Agents

Three hemoplasma species are recognized as primary feline pathogens [2][3]:

1. ·Mycoplasma haemofelis (Mhf) — the most pathogenic species, capable of causing severe hemolytic anemia even in immunocompetent cats [1][5].
2. ·'Candidatus Mycoplasma haemominutum' (CMhm)* — the most prevalent species globally; generally causes mild or subclinical disease but can contribute to anemia in immunocompromised hosts [2][6].
3. ·'Candidatus Mycoplasma turicensis' (CMt)* — intermediate pathogenicity; reported to cause anemia especially in the context of co-infection or immune suppression [2][3].

A fourth species, 'Candidatus Mycoplasma haematoparvum'-like organisms, has been reported in some geographic regions including Chile [7].

Pathological Mechanism

Hemoplasmas are obligate epicellular parasites that attach to the outer leaflet of the erythrocyte membrane; they cannot be cultured in vitro, which significantly complicates research and diagnostics [1][5]. Their attachment causes direct physical and biochemical damage to the RBC membrane, but the primary mechanism of anemia is immune-mediated hemolysis: parasitized erythrocytes are recognized as foreign and cleared by the mononuclear phagocyte system, predominantly in the spleen (extravascular hemolysis) [1][2]. Intravascular hemolysis may also occur. The resulting anemia triggers compensatory erythropoiesis and elevated reticulocyte counts, giving the anemia a regenerative character in most cases [1].

Bacteremia is typically cyclic; M. haemofelis parasitemia peaks are followed by periods of low or undetectable organisms, corresponding to clinical relapses and apparent remissions [5]. Immunosuppression (e.g., FIV co-infection, FeLV co-infection, corticosteroid therapy, concurrent illness) markedly worsens outcomes by impairing the immune response to the organism and accelerating RBC destruction [1][2][6].

Epidemiology and Risk Factors

Prevalence varies considerably by geographic region and study population. In Japan, overall hemoplasma prevalence was approximately 21.5% in outdoor-accessing cats, with CMhm being the most common species [6]. Studies from Thailand and Iran similarly demonstrate CMhm predominance among community cats [4][8]. In Chile, combined prevalence reached approximately 14.6% across species [7]. Risk factors consistently identified across studies include: male sex, outdoor or free-roaming access, adult age (>1 year), feral or stray status, FIV or FeLV co-infection, and flea and tick infestation [1][6][7][8].

Transmission

The exact transmission routes are not fully elucidated [1]. The most commonly proposed mechanisms include:

• ·Arthropod vectors — fleas (Ctenocephalides felis) are considered the most likely biological vector; ticks and biting insects may also play a role [1][2]
• ·Cat bites and fighting — mechanical transmission via blood-contaminated saliva is strongly suspected, explaining the higher prevalence in male, outdoor cats [1][7]
• ·Iatrogenic transmission — blood transfusions from infected donor cats represent a documented risk [1][2]
• ·Vertical (transplacental or perinatal) transmission — has been proposed but not definitively confirmed [1]

Clinical Suspicion

Diagnosis should be suspected in any cat presenting with hemolytic anemia, pallor, icterus, or splenomegaly, particularly when risk factors (outdoor lifestyle, fighting wounds, flea infestation, known FIV/FeLV status) are present [1][2].

Hematological Findings

Complete blood count (CBC) is central to diagnosis and monitoring [1][2][5]:

• ·Hematocrit (HCT) / Packed Cell Volume (PCV): Markedly decreased; HCT may fall below 15% in severe M. haemofelis infections. The anemia is typically macrocytic, hypochromic, and regenerative (elevated reticulocytes, polychromasia, anisocytosis, Howell-Jolly bodies) [1][5]
• ·RBC count: Significantly reduced
• ·Reticulocyte count: Elevated in regenerative cases, reflecting compensatory erythropoiesis [1]
• ·White blood cell count (WBC): Variable; leukocytosis or leukopenia may be seen; neutrophilia is common in acute infection [1]
• ·Platelet count (PLT): May be decreased (thrombocytopenia) in some cases, potentially due to immune-mediated destruction or concurrent tick-borne co-infections [2]
• ·Blood smear: Historically, organisms could be visualized as small coccoid or rod-shaped bodies on the erythrocyte surface using Romanowsky stains (e.g., Diff-Quik, Wright-Giemsa); however, sensitivity is low and organisms can be confused with staining artifacts, making PCR the gold standard [1][2]

Serum Biochemistry

• ·Total bilirubin (TBIL): Elevated due to hemolysis [1][2]
• ·ALT (Alanine Aminotransferase): May be mildly to moderately elevated if hepatic involvement or hypoxic hepatopathy occurs [2]
• ·BUN / CREA (Blood Urea Nitrogen / Creatinine): Generally within reference ranges unless concurrent renal disease exists or severe hemodynamic compromise has occurred [1]
• ·Total protein, Albumin (ALB), Globulin (GLOB): Hypoproteinemia may develop with chronicity; globulins may be elevated due to inflammatory response [2]

Molecular Diagnostics (PCR)

Polymerase chain reaction (PCR) targeting the 16S rRNA gene is the diagnostic gold standard, offering high sensitivity and specificity and the ability to differentiate between hemoplasma species [1][2][4]. Conventional PCR, real-time quantitative PCR (qPCR), and species-specific assays are available. qPCR is preferred as it provides quantification of bacteremia, which correlates with disease severity and treatment response [2][5]. Because bacteremia is cyclic, false negatives are possible; repeat testing may be warranted in strongly suspected cases [1][2].

Retroviral Testing

All cats diagnosed with hemoplasmosis should be tested for FIV and FeLV co-infection, as these significantly alter prognosis and management [1][2].

Coombs' Test

A direct Coombs' (direct antiglobulin) test may be positive, confirming the immune-mediated component of hemolysis and helping guide treatment decisions regarding immunosuppressive therapy [1][2].

Antibiotic Therapy

Antimicrobial treatment is the cornerstone of managing hemoplasmosis [1][2]:

• ·Doxycycline (tetracycline class): The most widely recommended first-line agent. Typical dosing is 10 mg/kg orally once daily (or divided twice daily) for a minimum of 4–6 weeks [2]. Doxycycline effectively reduces clinical signs and bacteremia but does not reliably achieve complete PCR-negative clearance, and many cats become chronic carriers after treatment [1][2]. Esophageal stricture is a known risk in cats; the tablet should always be followed with a water bolus or a small amount of food, or a liquid formulation should be used [2].
• ·Fluoroquinolones (e.g., pradofloxacin, marbofloxacin, enrofloxacin): Increasingly supported as effective alternatives, with some evidence of superior bacteriological clearance compared to doxycycline, especially for M. haemofelis [2]. Pradofloxacin has shown particular promise. Enrofloxacin should be used cautiously in cats due to the risk of retinal toxicity at higher doses [2].
• ·Combination therapy (doxycycline + fluoroquinolone) may be considered in severe or refractory cases, though evidence-based protocols remain limited [2].

Immunosuppressive Therapy

Given the significant immune-mediated component of hemolysis, glucocorticoids (prednisolone: 1–2 mg/kg/day) are often used adjunctively in cats with severe anemia or a positive Coombs' test [1][2]. The rationale is to reduce RBC destruction while antibiotics address the underlying infection. Glucocorticoids should be tapered carefully once clinical improvement is documented.

Supportive Care

• ·Blood transfusion: Indicated in cats with life-threatening anemia (HCT typically <12–15%) or severe clinical decompensation. Whole blood or packed RBCs may be administered; all donor cats should be screened for hemoplasmas by PCR [1][2]
• ·Intravenous fluid therapy: To correct dehydration, support cardiovascular function, and maintain tissue perfusion [2]
• ·Nutritional support: Assisted feeding (syringe feeding or nasogastric tube) may be necessary in anorexic cats [2]
• ·Hematinics: Iron supplementation is generally not indicated unless concurrent iron deficiency is documented, but B-vitamin supplementation may support erythropoiesis [2]
• ·Anti-nausea medications: Maropitant or ondansetron may be used if gastrointestinal side effects from antibiotics occur [2]

Management of Co-infections

If FIV or FeLV co-infection is confirmed, management of the underlying retroviral disease and enhanced supportive care are necessary, as these cats carry a substantially worse prognosis [1][2].

Overall Prognosis

Prognosis depends heavily on the infecting hemoplasma species, the severity of anemia at presentation, immune status, and presence of co-infections [1][2]:

• ·Mycoplasma haemofelis: Carries the most guarded prognosis. Without treatment, severe anemia can be fatal. With prompt and appropriate antimicrobial and supportive therapy, the majority of immunocompetent cats recover clinically, though complete microbiological cure (PCR-negative status) is inconsistent [1][2][5].
• ·CMhm and CMt infections: Generally associated with a favorable prognosis in immunocompetent cats; many infections remain subclinical or cause only mild, self-limiting anemia [2][3].

Carrier State and Recurrence

A critical prognostic consideration is the chronic carrier state: even after clinical resolution and antibiotic therapy, many cats remain PCR-positive and serve as persistent reservoirs [1][2]. Relapse can occur under conditions of stress or immunosuppression. In the low-dose experimental M. haemofelis model, all infected cats developed detectable bacteremia, and clinical anemia was documented despite varying inocula [5], underscoring the pathogenic potential of even low-level infection.

Impact of Co-infections

Cats co-infected with FIV or FeLV have significantly worse outcomes; these immunocompromised individuals are more likely to experience severe anemia, treatment failure, and death [1][2][6].

Mortality

Formal population-level mortality statistics are not consistently reported in the reviewed literature. However, M. haemofelis infection is acknowledged as potentially fatal without treatment, particularly in immunocompromised cats [1][2]. In clinical case series and experimental models, treated immunocompetent cats generally survive, but mortality risk increases substantially in the presence of severe anemia (HCT <10%), late presentation, FeLV co-infection, or systemic complications [1][2][5]. Two clinical cases from Korea involving CMhm and CMt demonstrated clinical recovery with appropriate treatment [3], indicating a favorable outlook when diagnosis and therapy are timely.

Vector Control

Since arthropod vectors — especially fleas — are considered primary transmission routes, rigorous and consistent flea and tick prevention is the most important preventive measure [1][2]. Monthly topical or oral ectoparasiticides (e.g., selamectin, fipronil, imidacloprid, fluralaner) are recommended for all at-risk cats, particularly those with outdoor access [2][7].

Lifestyle Management

• ·Restricting outdoor access significantly reduces exposure risk, as outdoor and free-roaming cats consistently demonstrate higher prevalence than indoor-only cats [6][7][8]
• ·Preventing cat fights through neutering and indoor confinement reduces the risk of mechanical transmission via bite wounds [1][7]
• ·Avoiding contact with stray or feral cats minimizes exposure to high-prevalence reservoir populations [8]

Blood Transfusion Safety

All blood donor cats should be screened by PCR for all three hemoplasma species prior to donation to prevent iatrogenic transmission [1][2]. Blood banking protocols should include regular donor monitoring.

Routine Screening

Cats at high risk (outdoor, male, FIV/FeLV-positive) may benefit from periodic PCR screening to detect subclinical infections before clinical deterioration occurs [2][4].

Vaccination

No vaccine is currently available for feline hemoplasmosis [1][2]. Prevention relies entirely on vector control and management strategies.

Retroviral Disease Management

Because FIV and FeLV co-infection dramatically worsens outcomes, routine testing and management of these underlying conditions is an integral part of reducing hemoplasmosis severity and spread within cat populations [1][2].