The Feline Centre, Langford Veterinary Services, University of Bristol, Langford, Bristol, UK
What Are Feline Haemoplasmas?
The feline haemotropic mycoplasmas (haemoplasmas) are small bacteria that parasitise feline red blood cells (RBCs). They were previously classified as Rickettsial organisms (when they were named Haemobartonella spp.), but they have now been reclassified and renamed as Mycoplasma organisms.
Three main feline haemoplasma species exist:
Mycoplasma haemofelis: present in 0.4–46.6% cats
'Candidatus Mycoplasma haemominutum': 8.1–46.7% cats
'Candidatus Mycoplasma turicensis': 0.4–26% cats
M. haemofelis is the most pathogenic feline haemoplasma species. Acute infection often results in severe haemolytic anaemia (especially in young cats), although at other times only mild anaemia is seen. Chronic infection is not usually associated with anaemia. Cats do not need to be immunocompromised or splenectomised to succumb to M. haemofelis clinical disease. Epidemiological studies have only variably demonstrated associations between anaemia and M. haemofelis infection, likely because these studies usually include chronically M. haemofelis-infected asymptomatic cats. Persistent autoagglutination or positive Coombs' testing, indicating the presence of RBC-bound antibodies, are found in anaemic cats with acute M. haemofelis infection,1 although these antibodies usually appear only after the start of the development of anaemia, suggesting that they are as a result of haemoplasma-induced haemolysis rather than initiating it.
Although 'Ca. M. haemominutum' and 'Ca. M. turicensis' infections can each cause a drop in RBC parameters, anaemia is not usually induced except in cats with concurrent problems or immunosuppression. However, cases of so-called primary 'Ca. M. haemominutum' anaemia have been reported, so infection with this species cannot be ruled out as a cause of anaemia in an individual case. 'Ca. M. haemominutum' has also been associated with the development of myeloproliferative disease in cats with FeLV infection in one experimental study.2 Determining the pathogenicity of 'Ca. M. turicensis' in naturally infected cats has been difficult in epidemiological studies, as cats are often coinfected with other haemoplasma species, confounding disease associations.
As well as different species, different strains of each of feline haemoplasma species may exist, and these may vary in pathogenicity too. Retrovirus infection is believed to be a risk factor for haemoplasma infection and may exacerbate severity of disease too.
Long-term carrier status appears to be especially common following 'Ca. M. haemominutum' infection, although suspected clearance of this infection has been reported with and without antibiotic treatment. M. haemofelis-infected cats may spontaneously clear infection from peripheral blood after infection without antibiotic treatment, and such clearance has also been reported with 'Ca. M. turicensis' infection. It is clear that variation exists in the long-term host-organism interaction, making long-term predictions of outcome impossible. In any haemoplasma-infected carrier cat, the potential for reactivation of infection exists, which can result in clinical disease, although this seems to be quite rare once haemoplasma infection has become established beyond acute infection.
These depend upon factors such as the haemoplasma species/strain involved, stage of infection and whether concurrent diseases or infections (e.g., FeLV, FIV) are present. Common clinical signs seen in acutely ill cats include pallor, lethargy, anorexia, weight loss, dehydration, pyrexia, and splenomegaly (extramedullary haematopoiesis). Icterus is uncommon unless severe acute haemolysis occurs.
The natural route of transmission of haemoplasma infection between cats in the field has not yet been determined. The cat flea, Ctenocephalides felis, has been implicated in transmission, but conclusive evidence for its role is lacking, although haemoplasma DNA has been found in fleas and ticks collected from cats. The clustered geographical distribution of haemoplasma infection in some studies supports the role of an arthropod vector in haemoplasma transmission. Swiss studies have found that subcutaneous inoculation of 'Ca. M. turicensis'-containing blood resulted in infection transmission, whereas the same inoculation method using 'Ca. M. turicensis'-containing saliva did not. This suggests that transmission by social contact (saliva via mutual grooming, etc.) is less likely than by aggressive interaction (blood transmission during a cat bite incident).3 Vertical transmission of feline haemoplasmas has not been definitively demonstrated but has been suggested for other haemoplasma species. Fresh blood transfusions can transmit infection, so blood donors should always be screened for haemoplasma infection.4
Haemoplasmas are currently non-culturable in vitro. Recently a number of haemoplasmas have undergone whole genome sequencing, including feline species.5,6 This has highlighted the limited metabolic capabilities of these pathogens (glucose is their only energy source) to direct future in vitro cultivation attempts.
Diagnosis of haemoplasma infection used to rely on cytological examination of a Romanowsky-stained blood smear with organisms appearing on the surface of RBCs as rounded bodies. However, cytology has poor sensitivity and specificity.4 Sensitivity is a particular issue (only 0–37.5%), whereas specificity is usually higher (84–98%), although these figures are based upon board-certified clinical pathologists examining and interpreting blood smears. If reliable blood smear interpretation analysis is available, the cytological detection of organisms during acute infection can be useful as a bench-side and immediate diagnostic test. However, in the author's experience, many cases diagnosed as being haemoplasma-infected on cytology by veterinarians have been false positives, with stain precipitate, Howell-Jolly bodies, and artefacts due to slow blood smear drying being the most common reasons for error. Additionally, cytology cannot differentiate between haemoplasma species.
Polymerase Chain Reaction (PCR)
PCR is commonly used for diagnosing haemoplasma infection, being more sensitive and specific than cytology when properly designed. The feline haemoplasma PCR assays currently available are based on the 16S rRNA gene. Both non-quantitative conventional PCR (cPCR) and real-time quantitative PCR (qPCR) assays exist, but qPCRs are increasingly used. These allow quantification of haemoplasma DNA in the sample being analysed (usually a defined volume of blood) so that the course of haemoplasma infection and response to treatment can be monitored. qPCRs have also allowed us to document the in vivo kinetics of haemoplasma infection; cats infected with M. haemofelis can show very large fluctuations in blood organism numbers, especially in the first few weeks postinfection. Up to 1/3 of M. haemofelis-infected cats continue to show these fluctuations for several months. This should be considered when interpreting qPCR results because if such fluctuations occur in naturally infected cats, changes in organism number cannot always be deemed to be a consequence of antibiotic treatment. In contrast, 'Ca. M. haemominutum'- and 'Ca. M. turicensis'-infected cats show very little fluctuation in organism numbers in the blood over time. The reasons for the marked fluctuations in blood M. haemofelis organism numbers is not known; no evidence of sequestration of M. haemofelis organisms in the spleen and liver at times of cyclical low organism numbers has been found.7 Antigenic variation may mediate the fluctuations; indeed a very large portion (62%) of the M. haemofelis genome encodes a set of uncharacterized hypothetical proteins arranged in series of paralogous repeats and these could mediate antigenic variation through differing expression of haemoplasma surface proteins over time, thus enabling M. haemofelis to evade the host's immune response.5
Genomic studies have led to the development of an ELISA based on recombinant M. haemofelis DnaK,8 which has found that antibody levels are maximal 2–4 weeks post-M. haemofelis experimental infection; thus, antibody levels may help differentiate acute from chronic M. haemofelis infection. Additionally, a similar assay has documented seropositivity in PCR-negative cats following 'Ca. M. turicensis' infection, suggesting that serology may be more sensitive than PCR.9 Further work on serology in naturally infected cats is required before their use can be recommended.
A number of antibiotics, notably tetracycylines (primarily doxycycline) and fluoroquinolones (e.g., marbofloxacin, pradofloxacin), are effective in reducing haemoplasma organism numbers in experimental studies, as well as improving clinical signs and haematological abnormalities. Most studies are with M. haemofelis. Doxycycline (10 mg/kg daily PO) is often used as first-line treatment for 2 weeks, with longer (6 weeks) treatment courses recommended by some to help eliminate infection, although no antibiotic treatment regime has yet been reported that consistently and predictably clears haemoplasma infection. One study10 suggested that pradofloxacin (at both the standard 5 mg/kg daily PO dose and a higher 10 mg/kg daily PO dose) may be more effective at clearing M. haemofelis than doxycycline. Another found that 'Ca. M. haemominutum' infection only temporarily responded to 4 weeks of marbofloxacin treatment (2 mg/kg daily PO), with no evidence of infection elimination.11 Variability in response to treatment in different studies may be due to the different haemoplasma species, strains, host factors, and/or route of administration.
Corticosteroids have been recommended as adjunct treatment for haemoplasmosis, to treat any immune-mediated component of anaemia, although their efficacy has not yet been proven. As mentioned above, in our experience, Coombs'-positive anaemic cats respond to antibiotic treatment and supportive care alone without the need for corticosteroids. Indeed, immunosuppressive doses of corticosteroids have been used experimentally to exacerbate haemoplasma infection, so their routine use is not advised.
This should include correction of dehydration with fluid therapy, and blood transfusion or treatment with an oxygen-carrying haemoglobin compound, if available, if the anaemia is severe.
Lessons from Cats for Humans
Human infections with swine haemoplasmas have been reported in China, but the descriptions are not very detailed; M. haemofelis DNA has been reported in a human with concurrent Bartonella henselae infection.12 We have recently published13 a human case report documenting acute haemolysis and pyrexia during infection with a novel haemoplasma species. The patient responded to doxycycline and moxifloxacin, with qPCR used to monitor response to treatment. Haemoplasma species may impact human and animal health, and future studies should evaluate zoonotic potential and transmission of these emerging pathogens.
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2. George JW, Rideout BA, Griffey SM, Pedersen NC. Effect of preexisting FeLV infection or FeLV and feline immunodeficiency virus coinfection on pathogenicity of the small variant of Haemobartonella felis in cats. American Journal of Veterinary Research. 2002;63:1172–1178.
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4. Tasker S. Haemotropic mycoplasmas: what's the real significance in cats? Journal of Feline Medicine and Surgery. 2010;12:369–381.
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6. Barker EN, Darby AC, Helps CR, et al. Genome sequence for 'Candidatus Mycoplasma haemominutum,' a low-pathogenicity hemoplasma species. Journal of Bacteriology. 2012;194:905–906.
7. Tasker S, Peters IR, Day MJ, et al. Distribution of feline haemoplasmas in blood and tissue following experimental infection. Microbial Pathogenesis. 2009;47:334–340.
8. Barker EN, Helps CR, Heesom KJ, et al. Detection of humoral response using a recombinant heat shock protein 70, DnaK, of Mycoplasma haemofelis in experimentally and naturally hemoplasma-infected cats. Clinical and Vaccine Immunology. 2010;17:1926–1932.
9. Novacco M, Wolf-Jackel G, Riond B, Hofmann-Lehmann R. Humoral immune response to a recombinant hemoplasma antigen in experimental 'Candidatus Mycoplasma turicensis' infection. Veterinary Microbiology. 2012;157:464–470.
10. Dowers KL, Tasker S, Radecki SV, Lappin MR. Use of pradofloxacin to treat experimentally induced Mycoplasma hemofelis infection in cats. American Journal of Veterinary Research. 2009;70:105–111.
11. Tasker S, Caney SMA, Day MJ, et al. Effect of chronic FIV infection, and efficacy of marbofloxacin treatment, on 'Candidatus Mycoplasma haemominutum' infection. Microbes and Infection. 2006;8:653–661.
12. Santos AP, Santos RP, Biondo AW, et al. Hemoplasma infection in HIV-positive patient, Brazil. Emerging Infectious Diseases. 2008;14:1922–1924.
13. Steer JA, Tasker S, Barker EN, et al. A novel hemotropic Mycoplasma (hemoplasma) in a patient with hemolytic anemia and pyrexia. Clinical Infectious Diseases. 2011;53:e147–151.