Introduction

Mycoplasma and Ureaplasma belong to the class of Mollicutes, meaning soft skinned. Derived from gram-positive bacteria, they have undergone reductive evolution. They have a small genome and are the smallest self-replicating microorganisms at  0.1 μm in diameter. Their simplicity allowed the construction of a synthetic mycoplasma called, Mycoplasma laboratorium.

Below is the taxonomic position of more common mycoplasmas identified in animals. The genera in this group have recently undergone changes. You will note that in textbooks and many publications, most still will have the genus name Mycoplasma . The chart below is to orientate you only, don’t learn it. The haemotropic mycoplasmas will not be dealt with in this section, as they are vector-borne diseases. Thus they will be discussed in the Chapter on Vector-borne Bacterial Diseases.

Morphology of the mollicutes

1. Mollicutes lack a cell wall or outer cell membrane. This implies that they are fragile and susceptible to desiccation and other environmental stressors. They have to live on mucous membranes and use host-specific receptor sites to cling to the cells. Sometimes they will even enter cells and attach to the inner cell membrane. Therefore. most mollicutes are highly-host adapted and rarely cause disease.
2. The lack of a rigid outer membrane and a high proportion of sterols in their cell membranes allows them to change shape i.e. be plastic. This plasticity and gram-negative staining makes them hard to see using the light microscope. They are also able to glide over surfaces very much like an amoeba does.  Occasionally spherical or ring-like forms are observed in tissue samples.
3.  By not having a cell wall these bacteria can resist the action of the beta-lactam ring (penicillins, cephalosporins) and other wall active antibiotics.
4. Whilst being simple, these bacteria are able to switch their surface antigenicity by phase variation. This and their ability to survive intracellular is believed to be why they persist in their host causing chronic infections

Diseases caused by the mollicutes

Disease due to the mollicutes is often stress-induced, typically chronic and usually involves organ systems that have mucous membranes i.e. the respiratory and urogenital tracts. The most important diseases caused by the mollicutes are listed per target organ systems in the Tables below. Mycoplasmas often act synergistically with other bacterial or viral pathogens to produce more severe disease.

Respiratory disease

Respiratory pathogenic mycoplasmas adhere to and paralyse ciliated epithelium in the respiratory tract, allowing them to glide to the lower respiratory tract and pave the way for opportunistic pathogens.  For example, this occurs with Mycoplasmopsis bovis in the bovine respiratory disease complex and Mesomycoplasma ovipneumoniae in bronchopneumonia in sheep and goats. Other than the more virulent agents of CBPP and CCPP (see in the Table below), most mycoplasmal diseases of the respiratory tract are polymicrobial and stress-induced. In the USA Mesomycoplasma ovipneumoniae that has originated from domestic stock is causing high mortalities in Big horn sheep.

Many pathogenic mycoplasmas carry antigens in the cell membrane that are similar to the cell surface proteins of their hosts. Furthermore, they show phase variation in their surface antigens. This delays immune-recognition of these bacteria. The incubation period is 1 to 4 weeks with most of the lesions associated with an excessive inflammatory response. Furthermore, virulent mycoplasmas are able to produce proteases that induce cell apoptosis.

Polyarthritis

Mollicutes affecting the joints of animals are usually commensals within the respiratory tract where they cause either no or mild disease. In stressed animals they spread haematogenously and lodge in the joints causing chronic polyarthritis. In birds, Mycoplasmopsis synoviae may also be transmitted transovarially when the bacteria in the bloodstream are sequestered in the egg during its manufacture.

A list of the more common mycoplasmas causing arthritis are listed in the Table below.

Reproductive disease

Mycoplasmas carried on the urogenital mucosa (or respiratory mucosa) can be transmitted to other animals via urine droplets or by coitus. They usually cause superficial epithelial infections in all organs of the urogenital tract.

A list of the more common diseases are provided in the Table below.

Eye infections

Like other infectious causes of keratoconjunctivitis, mollicutes are transmitted by direct contact with eye and nasal secretions of infected animals. Mycoplasmal keratoconjunctivitis should always be suspected when an animal fails to respond to treatment with the beta-lactam drugs, bacitracin and potentiated sulphonamides.

general diagnosis and control of mollicute infections

Although there are many diseases that are attributed to mollicutes, the principles of diagnosis are essentially the same.

diagnosis

Samples. For culture or qPCR, the samples of choice are fresh tissue or swabs of lesions

Culture. Although some mycoplasmas will grow as fine beta-haemolytic colonies on blood agar after 3 to 5 days of incubation, most prefer a sterol-rich medium where they produce minute colonies with a fried-egg appearance that has a central dense area surrounded by a thin film. The colonies are best viewed when magnified 10 to 40 times (use a dissecting microscope). The haemotropic mycoplasmas are usually grown in blood. Once cultured the mycoplasmas can be identified using immunological, PCR or chemical assays. They can also be tested for antimicrobial susceptibility.

qPCR. The preservation of specimens for culture as well as the difficulty of growing mycoplasmas has led to the qPCR-based assays being preferred.

ELISA and other immunological assays. The ELISA, IFAT and immunoperoxidase test has been used to detect the mycoplasmas in samples. However, they only perform well when there are high numbers of mycoplasmas in the sample.

Serology. Best as a herd or flock test that can be used to determine whether a specific mycoplasma has infected the group. These tests are usually used to monitor the disease-free status of herds or flocks. High antibody titres in 20% or more of a group of animals tested imply that infection is active.

general control of molliculte infections in animals

Whether or not to implement control measures depends on the disease status, disease manifestation and disease prevalence as well as the available resources.

Biosecurity and Sanitation practices. As most of the economically important diseases are contagious, one of the best ways to reduce the spread of mycoplasmas is to isolate infected animals and then to treat or in the case of notifiable diseases – euthanise them. Good farm biosecurity is considered to be highly successful in preventing introduction of disease. The same is true for biosecurity surrounding the importation of animals or their products. Additionally, it may be important to control any wild animals that could carry mycoplasmas e.g. Wild psittacines carrying Mycoplasmoides gallisepticum.

 “Stamping out” or depop-repop. For the economically important diseases/agents such as CBPP; CCPP; M. gallisepticum; and M. hyopneumoniae. Since mycoplasmas are susceptible to environmental destruction and are host-specific, a “Stamping out” policy whereby all animals of the affected species on the farm are euthanased will be successful. Disinfection of the premises and then restocking with animals tested to be free of the relevant mycoplasma.

Vaccination. Vaccination is carried out for economically import mycoplasmas where the disease is endemic i.e. the farm or region will live with the disease.  Mainly live modified vaccines are administered to young animals to reduce the severity of disease and reduce the spread of disease. Vaccines have been developed for the poultry and pig mycoplasmas. The live-attenuated T44 MmmSC vaccine is used in cattle in CBPP endemic areas.

Antibiotic therapy. Mass in-feed or in-water treatment with either tetracyclines or macrolides in livestock or fluoroquinolones in non-food animals will reduce the bacterial load and assist in recovery. However, none will remove carriage of these mycoplasmas. Resistance to macrolides is common where this antibiotic is used.

Mollicute infections in cattle

Whilst a large number of mycoplasmas can infect cattle, it is only Mycoplasmopsis bovis and Mycoplasma mycoides ssp. mycoides SC that cause serious disease.

Mycoplasmopsis bovis

Mycoplasmopsis bovis is a significant economic disease of the dairy and feedlot industries in Australia and other countries where it is a cause of chronic pneumonia,  polyarthritis, nervous disease and keratoconjunctivitis in young animals and mastitis and abortion in cows. New Zealand has taken large and very expensive measures to eradicate it from the dairy herds. It contributes to the Bovine Respiratory Disease Complex that is discussed in detail in the Chapter on The Pasteurellaceae and Infectious Respiratory Disease.

The disease in animals is often precipitated by stress and other concurrent infections. M. bovis glides to the lungs, paralysing the cilia as it moves. It causes a marked inflammatory response as noted by areas of caseonecrosis of the lungs.

Ureaplasma diversum

Ureaplasma diversum is a urea hydolysing mollicute, causes a venereal infection of the external genitalia of bulls and cows that can spread deeper into the reproductive tract causing infertility and abortions in cows. When newly introduced into a breeding herd from introduced animals, it will spread rapidly causing a massive drop in the number of live calves born.

Contagious bovine pleuropneumonia (CBPP) 

Contagious bovine pleuropneumonia is a contagious chronic lower respiratory tract disease of cattle caused by Mycoplasma mycoides subsp. mycoides small colony – bovine biotype (MmmSC). In the past this disease was widely distributed, including in Australia, however, due to massive eradication campaigns, CBPP is now limited to Africa (see map below). CBPP is a Notifiable Exotic Disease in Australia.

This highly contagious disease spreads geographically when infected, carrier cattle (recovered animals or those incubating the disease) are moved or change hands coming into contact with susceptible cattle. In Africa, there are still a large group of pastoralists who will seasonally move their cattle to better pastures and to markets. At night, animals from different owners may be penned together. Cattle are highly valued and are a measure of wealth. Thus they are used as a bridal price and are stolen. The long incubation period and insidious onset of this chronic disease means that it is usually only detected when large numbers of animals are already infected. The other problem is that this disease can clinically mimic the bovine respiratory disease complex as well as haemorrhagic septicaemia. So animals may be empirically treated with antibiotics that decrease the severity of disease, but not eliminate the mollicute. This is what happened in Portugal in the early 2000s.

Transmission of MmmSC is via close contact with respiratory secretions where the inhaled bacteria glide down the respiratory tract into the alveoli of the lungs. The immune system is delayed in its response as the bacterium is surrounded with a capsule and its surface antigens have a similar molecular structure to bovine pneumocytes. Severe inflammation occurs when the numbers of colonising bacteria have become high. Thus the pathology in the lungs are associated with inflammation.

If you want to find out more about CBPP – look at this YouTube video made during an outbreak of CBPP in North-West Botswana.

The lungs of highly susceptible cattle develop lobular bronchopneumonia which is characterised by the lobules of the lung showing different stages of the pneumonic process. The lymphatic ducts block with inflammatory cells resulting in the interlobular spaces becoming prominent. A profuse fibrinous pleuritis develops with up to 6 litres of fluid filling the thoracic cavity. The inflammatory fluid forms yellow clots and will eventually reorganise to fibrous tissue. Animals with extensive lung lesions have a mucoid to purulent nasal discharge, struggle to breathe, lag behind the healthy cattle and become very thin. The lung lesions in recovered cattle become sequested from the healthy tissue and some of these lesions continue to harbour viable MmmSC. Calves may develop a polyarthritis.

Whilst there are several ways to diagnose CBPP, the best way is to carry out a MmmSC-DNA-specific qPCR. Serology can be done in countries or regions where the disease is exotic. The serological test of choice is the indirect ELISA.

A Disease Stamping out (Eradication) programme is used in countries free of CBPP = Culling of all infected herds, irrespective of the individual animal disease status and quarantining and testing of neighbouring herds. However, this process is expensive and not very popular in countries where cattle are highly valued. Thus in endemic countries, vaccination with the live vaccine and antibiotic therapy of infected cattle are used. Endemic countries should also endeavour to restrict the extent of infected regions by the use of veterinary cordons and regular monitoring. Where possible a serological monitoring zone should be set up surrounding a disease zone. In this zone, cattle are not vaccinated.

CONTAGIOUS CAPRINE PLEUROPNEUMONIA (ccpp)

CCPP is a contagious respiratory disease of goats and some small wild ruminants caused by Mycoplasma capricolum subsp. capripneumoniae. Clinical signs and pathology in goats, sometimes sheep, resemble CBPP in cattle. Affected animals have difficulty in breathing and lose weight. A marked unilateral or bilateral pleural effusion is present. CCPP has never been reported in Australia and is a Notifiable disease. CCPP is present in parts of Africa, especially East Africa, the Middle East, former Soviet Union countries and Asia. The latest outbreak is in Mongolia (December 2024). Vaccination can be carried out in endemic areas.  If ever introduced into Australia, a quarantine, animal movement control and “stamping-out” will be carried out under State veterinary oversight.

mycoplasmosis of pigs

Chronic respiratory disease caused by Mesomycoplasma hyopneumoniae has been dealt with as a differential for Actinobacillus pleuropneumoniae infections in the Chapter on the Pasteurellaceae and infectious respiratory disease. This disease is endemic in all pig-rearing areas of the world and is the most important infectious agent causing economic losses. It has a high morbidity but low mortality rate. It is most common in pigs older than 8 weeks of age or grower pigs. Pigs older than 10 months of age are generally immune or resistant to infection.

Enzootic swine pneumonia or mycoplasmal pneumonia of swine is one of the largest causes of erosive disease in commercial pigs globally. It also predisposes pigs to other bacterial respiratory tract infections. Farms that have eradicated this disease have observed large increases in profit both due to the more rapid growth of pigs as well as the decreased need for antibiotic treatment and vaccination.

Diagnosis 

1. Clinical signs. Mycoplasmal pneumonia has an insidious onset with the primary clinical sign being a non-productive persistent cough 3 weeks after infection.

2. Abbatoir lung scoring, where the lungs are graded according to the percentage of lung affected and the presence (1) or absence (0) of lung scars. Note that enzootic pneumonia is more likely to occur in the cranioventral portions of the lungs, whereas Actinobacillus pleuropneumoniae in the dorsocaudal areas of the lung.

3. Detection in lung tissue or respiratory secretions of Mesomycoplasma hyopneumoniae antigen using immunohistochemical techniques: IFT, IP or DNA using a qPCR. Bacterial cultures can be done, but require specialised media and very fresh samples.

Control of respiratory mycoplasmas in pigs

1. The best way is to eradicate the disease by removing carrier pigs by a complete depopulation, cleaning up the piggery and repopulating it with specific disease free pigs. However, in this system it is critical that introduction of the agent is prevented by a very strict biosecurity protocol. This type of control has been difficult to maintain in pig dense areas. Swiss depopulation or partial depopulation has become popular where all susceptible pigs less than 10 months of age are removed and the less susceptible pigs that are older than 10 months of age are vaccinated. For 3 weeks no on-site farrowing occurs and all older pigs are treated with mycoplasma effective antibiotics.
2. The presence of disease can be reduced by medicated early weaning, where the piglets are weaned early, treated with antibiotics and placed in a system of segreated age rearing where age groups are not mixed and pens are thoroughly cleaned and disinfected between batches of pigs.
3. Vaccination of piglets before weaning and again 2 – 3 weeks later. Ingelvac MycoFLEX^® Injection inativated M. hyopneumoniae vaccine and Respisure ONE (Zoetis). The latter vaccine can be used in piglets from 1 day of age.
4. Antibiotic therapy of affected pigs usually with oxytetracycline or tilmicosin.

Avian Mycoplasmosis

The most clinically important mollicutes in birds are:

• Mycoplasmoides gallisepticum (MG), which affects a number of bird species including chickens, turkeys, gamebirds and pigeons causing respiratory disease, conjunctivitis and decreased egg production; Incubation period is 4 to 4 days. Bird in lay are most severely affected.
• Mycoplsamopsis synoviae (MS), which affects chickens and turkeys causing respiratory disease, arthritis, breast blisters and loss of weight; and,
• Mycoplasmopsis meleagridis (MM) is a venereal disease, which only affects turkeys causing mortalities and decreased egg production and hatchability.

All the mycoplasmas can be transmitted vertically via the eggs in 30% of cases and horizontally by respiratory droplets. Recovered birds often remain carriers of the agents. Like all mycoplasma infections, disease is multifactorial where overcrowding, stress, concurrent viral and bacterial infections and poor weather conditions  Mycoplasma respiratory diseases are similar to  infectious coryza caused by Avibacterium paragallinarum and fowl cholera caused by Pasteurella multocida.  (See chapter on The Pasteurellaceae and Infectious Respiratory Disease). Thus a laboratory diagnosis is essential. This can done by serology (rapid latex agglutination test), culture of the bacterium with identification using either qPCR or immunofluorescence test, and direct qPCR on affected tissue.

Infections can be treated with antibiotics such as doxycycline and the macrolides: tylosin or tilmicosin. They will reduce the severity of disease, but not eliminate the mycoplasma. The disease is prevented by the breeding of disease-free animals and maintaining closed flocks with excellent biosecurity. Wild birds can also be affected by these agents and transmit. So they should be kept away from domestic birds. In endemic flocks, vaccines can be used.

VaxSafe MS and Vaxsafe MG are both temperature sensitive live vaccines that are administered once to birds 6 to 14 weeks of age by eye drops. Thus don’t administer antibiotics that are effective against mycoplasmas to birds for one week. If antibiotics are required, the beta-lactam drugs can be used as they don’t inhibit mycoplasmas.

END OF CHAPTER