38 Mycoplasmas
Microorganisms in the class Mollicutes, including mycoplasmas, are the smallest prokaryotic cells capable of self-replication. Because these pleomorphic organisms cannot synthesize peptidoglycan or its precursors, they do not possess rigid cell walls but have flexible triple-layered outer membranes. They are resistant to antibiotics such as penicillin which interfere with the synthesis of bacterial cell walls. They require enriched media for growth and although most mycoplasmas are facultative anaerobes, some grow optimally in an atmosphere of 5 to 10% CO2. Characteristically, microcolonies have an umbonate appearance when illuminated obliquely and a ‘fried-egg’ appearance in transmitted light.
Mycoplasmas are found on mucosal surfaces of the conjunctiva, nasal cavity, oropharynx, and intestinal and genital tracts of animals and humans. The haemotropic mycoplasmas are found on the surface of red blood cells. In general, they are host-specific and survive for short periods in the environment. The genera Mycoplasma and Ureaplasma contain animal pathogens. The major diseases associated with infection by Mycoplasma species are summarized in Table 38.1. Mycoplasma and Ureaplasma species increasingly associated with disease in animals are listed in Table 38.2.
Table 38.1 Mycoplasma species of veterinary significance, the disease conditions which they cause and their geographical distribution.
| Mycoplasma species | Hosts | Disease conditions | Geographical distribution |
| M. mycoides subsp. mycoides (small colony type) | Cattle | Contagious bovine pleuropneumonia | Endemic in parts of Africa; not currently reported from other continents |
| M. bovis | Cattle | Mastitis, pneumonia, arthritis | Worldwide |
| M. agalactiae | Sheep, goats | Contagious agalactia | Parts of Europe, northern Africa, western Asia |
| M. capricolum subsp. capripneumoniae | Goats | Contagious caprine pleuropneumonia | Northern and eastern Africa, Turkey, parts of Asia |
| M. capricolum subsp. capricolum | Sheep, goats | Septicaemia, mastitis, polyarthritis, pneumonia | Africa, Europe, Australia, USA |
| M. mycoides subsp. capri | Goats | Septicaemia, pleuropneumonia, arthritis, mastitis | Parts of Asia, Africa, Europe, Australia |
| M. hyopneumoniae | Pigs | Enzootic pneumonia | Worldwide |
| M. hyorhinis | Pigs (3–10 weeks of age) | Polyserositis | Worldwide |
| M. hyosynoviae | Pigs (10–30 weeks of age) | Polyarthritis | Worldwide |
| M. gallisepticum | Chickens | Chronic respiratory disease | Worldwide |
| Turkeys | Infectious sinusitis | Worldwide | |
| M. synoviae | Chickens, turkeys | Infectious synovitis | Worldwide |
| M. meleagridis | Turkeys | Airsacculitis, bone deformities, reduced hatchability and growth rate | Worldwide |
| M. haemofelis | Cats | Feline infectious anaemia | Worldwide |
Table 38.2 Mycoplasma and Ureaplasma species which are being increasingly associated with clinical conditions in domestic animals.
| Hosts | Pathogen | Clinical conditions |
| Cattle | Mycoplasma alkalescens | Mastitis |
| M. bovigenitalium | Seminal vesiculitis, vaginitis, mastitis | |
| M. bovirhinis | Mastitis | |
| M. bovoculi | Role in keratoconjunctivitis | |
| M. californicum | Mastitis | |
| M. canadense | Mastitis | |
| M. dispar | Pneumonia in calves | |
| M. leachii | Mastitis, polyarthritis, pneumonia | |
| Ureaplasma diversum | Vulvitis, infertility, abortion | |
| M. wenyonii | Mild anaemia | |
| Sheep, goats | M. conjunctivae | Keratoconjunctivitis |
| M. ovipneumoniae | Pneumonia | |
| M. ovis | Haemolytic anaemia, varying in severity | |
| Goats | M. putrefaciens | Mastitis, arthritis |
| Turkeys | M. iowae | Embryo mortality |
| Horses | M. felis | Pleuritis |
| M. equigenitalium | Implicated in abortion | |
| Cats | M. felis | Conjunctivitis, respiratory disease |
| M. gateae | Arthritis, tenosynovitis | |
| Dogs | M. cynos | Implicated in the kennel cough complex |
| M. haemocanis | Mild or subclinical anaemia; more severe signs in splenectomised animals | |
| Pigs | M. suis | Mild anaemia, poor growth rates |
Mycoplasmas are differentiated by their host specificity, colonial morphology, requirement for cholesterol and biochemical reactivity. For growth, these organisms require enriched media containing animal protein, a sterol component or adenine dinucleotide. Immunological tests, using specific antisera produced against each pathogenic species, are required for definitive identification. Growth inhibition tests in which filter paper discs impregnated with specific antisera are placed on an agar plate seeded with the mycoplasma are used for species identification. Fluorescent antibody staining of individual microcolonies can also be used for identification. Rapid plate agglutination tests are employed for screening poultry flocks and for the field diagnosis of contagious bovine pleuropneumonia. Many of the difficulties associated with culture and identification of mycoplasmas have been resolved with the advent of molecular techniques. PCR-based procedures can be used for the detection of organisms in clinical samples and for specific identification of cultured mycoplasmas.
Mycoplasmas adhere to host cells, an attribute essential for pathogenicity. This close contact, which facilitates toxic damage to host cells by soluble factors such as hydrogen peroxide produced by the pathogen, often occurs on mucosal surfaces. Variation in surface proteins is an important virulence attribute of Mycoplasma species as it allows the organism to rapidly adapt to the host environment and to evade the developing immune response. Mechanisms facilitating horizontal gene transfer, together with a high mutation rate, also contribute to the ability of mycoplasmas to rapidly evolve and adapt to their host environment. Some Mycoplasma species have the ability to produce biofilm, which may contribute to their persistence in some animals despite treatment with antimicrobial agents. In addition, the recently demonstrated ability of some mycoplasmas to survive intracellularly may account for their persistence in particular tissues. Factors such as extremes of age, stress and intercurrent infections may predispose to tissue invasion. In some instances, mycoplasmas may exacerbate disease initiated by other pathogens, particularly in the respiratory tract. Mycoplasmal infections cause respiratory diseases of major economic importance in farm animals, especially in ruminants, pigs and poultry.
Contagious bovine pleuropneumonia, a severe contagious disease of cattle, is caused by M. mycoides subsp. mycoides. The disease, which is transmitted by aerosols, requires close contact with clinically affected animals or asymptomatic carriers. Although spread of infection may be slow, the mortality rate may be high. The acute form of the disease is characterized by sudden onset of high fever, anorexia, depression, accelerated respiration and coughing. At postmortem, pneumonic lungs have a marbled appearance. The disease can be confirmed by isolation and definitive identification of the pathogen by serology and by molecular techniques. In countries where the disease is exotic, slaughter of affected and in-contact cattle is mandatory. In endemic regions, control strategies are based on prohibiting movement of suspect animals, mandatory quarantine and the elimination of carrier animals by serological testing and slaughter. Annual vaccination with attenuated vaccines is carried out in endemic areas but although it decreases the severity of clinical signs, it does not prevent infection.
Contagious caprine pleuropneumonia, caused by M. capricolum subsp. capripneumoniae, is a disease in goats similar to contagious bovine pleuropneumonia of cattle. The disease, which is highly contagious, is transmitted by aerosols. Pleuropneumonia in goats can also be caused by M. mycoides subsp. capri and differentiation of isolates from those of M. capricolum subsp. capripneumoniae can be carried out using PCR-based procedures. Vaccination gives good protection against M. capricolum subsp. capripneumoniae.
Strains of M. bovis, which is worldwide in distribution, can cause severe pneumonia in calves in the absence of other respiratory pathogens and can exacerbate respiratory disease caused by Pasteurella and Mannheimia species. Mycoplasma bovis is associated with chronic respiratory disease and there is convincing evidence that the organism plays a central role in the development of lesions in caseonecrotic bronchopneumonia frequently observed in natural outbreaks of M. bovis infection and disease. This organism is also associated with mastitis and polyarthritis. Treatment and control of respiratory disease is based on management practices and antimicrobial therapy, although response to treatment is frequently poor in chronic disease.
Enzootic pneumonia of pigs, caused by M. hyopneumoniae, is an economically important disease which occurs worldwide in intensively reared pigs. Poor ventilation, overcrowding and temperature fluctuations may precipitate an outbreak. Clinical, epidemiological and pathological findings are usually indicative of the presence of the condition. Although antimicrobial drugs such as tylosin or tiamulin are used therapeutically, prevention and control are primarily based on the development of specific-pathogen-free herds. Vaccines are also available.
Mycoplasmas are important pathogens of poultry and with some mycoplasma species, such as M. gallisepticum and M. meleagridis, egg transmission is an important means of spread. Aerosol transmission also occurs and is the principal means of spread for M. synoviae. Disease control measures include vaccination, antimicrobial prophylaxis and development of specific-pathogen-free flocks.
Haemotropic mycoplasmas cause infections in a wide variety of hosts. Although disease is usually mild or subclinical, reduced productivity in farm animals can cause significant economic loss. Pathogenicity appears to be associated with direct damage to erythrocytes through adhesion and invasion by the organism and through immune-mediated lysis of red blood cells. Some haemotropic mycoplasmas also target vascular endothelium and may contribute to vascular thrombosis and haemorrhagic diathesis. Feline infectious anaemia, which is caused by M. haemofelis, occurs worldwide and is considered to be one of the most clinically significant diseases caused by haemotropic mycoplasmas. Disease is comparatively common in free-roaming tom cats between one and three years of age and transmission through bite-wounds or by biting arthropods has been suggested. Recovered cats may remain asymptomatic carriers. The acute form of the disease presents with fever, anaemia, depression, weakness and, occasionally, jaundice. Mycoplasma haemofelis may be demonstrated on the surface of erythrocytes in Giemsa-stained blood smears but PCR analysis is currently the preferred method for diagnosis of infection with haemotropic mycoplasmas. Doxycycline therapy initiated early and continued for up to 21 days is effective for treatment of clinical signs but may not eliminate infection.