Mycoplasma Mastitis In Dairy Cattle: If Ignored, It Can Be a Costly Drain On the Milk Producer
Rubén N. González
Quality Milk Promotion Services
College of Veterinary Medicine, Cornell University
Ithaca, New York
Mycoplasma bovine mastitis is a highly contagious disease that results in milk loss and culling of infected animals. Frequently, this type of mastitis goes unrecognized in dairy herds and is spread in part by the sale of infected cows to unsuspecting buyers.
The disease was first reported in the United States in 1961 in the state of Connecticut (16). Mycoplasmas were subsequently responsible for epizooties of mastitis occurring in dairy herds in New York State. The causative agent, a Mycoplasma serologically identical to the Connecticut strain, was later designated M. bovis. Mycoplasma mastitis epizooties also occurred in California in the middle 1960's. Since that time, the disease has been diagnosed in the states of Oregon, Washington, Idaho, Alaska, Florida, Texas, Arizona, Arkansas, Hawaii, Pennsylvania, and more recently in Alabama (25), Wisconsin (33) and Ohio (18). In our laboratory, M. bovis has been isolated from herds in Maine, Vermont, New Jersey, Massachusetts, Maryland, Tennessee, Colorado, Michigan, Georgia, New Hampshire, Iowa and Puerto Rico. It seems that we only need to look for mycoplasmas to find them.
Mycoplasmas and mastitis
Initially, mycoplasmas were called pleuropneumonia-like-organisms (PPLO) in view of their similarity to the pleuropneumonia agent of cattle. The term PPLO is still used today for commercial media prepared for the isolation of mycoplasmas. The byproducts of growth and metabolism of mycoplasmas irritate the mammary gland tissue resulting in a marked inflammatory response characterized by acute swelling and agalactia. Several species of mycoplasma (M. bovis, M. californicum, M. canadense, M. bovigenitalium, M. alkalescens, M. arginini, M. bovihirnis, M. dispar, bovine group 7, and F-38) can cause mastitis in dairy cows (1,24,27). Disease detection at the herd level is usually made by isolation of mycoplasmas from either bulk tank milk or samples from cows with clinical mastitis.
Mycoplasma bovis is the most frequent and pathogenic bovine mycoplasma in the United States (15). It has been associated with a variety of bovine diseases such as abortion and low fertility, arthritis, keratoconjunctivitis, mastitis, pneumonia, and synovitis (4,17,23,28,29,31). Diseases caused by mycoplasmas are resistant to antimicrobial therapy although they are susceptible in vitro to several antibiotics (3,4). Mastitis caused by Mycoplasma may be subclinical, clinical or chronic. The affected cow lacks systemic signs of disease and continues to eat and drink normally.
Mastitis-causing mycoplasmas are commonly found in the mucous membranes of the respiratory and urogenital tracts of healthy cows. However, stresses such as calving, extreme temperature variations, transportation, disease or external trauma may allow the organisms to enter other body tissues or directly into the mammary gland resulting in clinical mastitis (4,21). Herds with and without mycoplasma mastitis may contain both young and mature asymptomatic carriers (4,21,23). The young are exposed to the various mycoplasmas during calving by direct contact with the urogenital tract, from nasal discharges of the dam, and in milk they receive from shedding animals (4,21). Mycoplasma may be shed in nasal discharges of calves and in vaginal discharges of heifers at the time of calving. Thus, it is extremely important to realize that even though a dairy is not currently experiencing mycoplasmal mastitis, the organisms are more than likely present within the herd and the possibility of a mastitis outbreak always exists.
Cows of all ages and at any stage of lactation are susceptible; however, cows in early lactation seem to suffer more severely because of the increased mammary gland edema that occurs (4). In lactating cows the characteristic signs of mycoplasma mastitis are (4,21):
1. An increase in severe clinical mastitis cases that resist treatment, but with little other effect on the cow.
2. The involvement of more than one mammary quarter, sometimes all four.
3. A marked drop in the milk production of affected cows.
4. Abnormal udder secretions that may vary from watery milk with a few clots to a colostrum-like material. Chronically infected cows may show a tannish secretion with sandy or flaky sediments that resembles cooked cereal in a whey-like fluid. Udder secretions may become purulent and last for several weeks.
Cows which continue lactating produce less milk than expected for the current lactation, usually with normal appearance but with high somatic cell counts. They may shed mycoplasma organisms intermittently for variable periods (4,21,23). Infected cows may return to their expected milk production in the same lactation, remain infected in the dry period, and increase their milk production in the following lactation while shedding M. bovis (10,12). Some cows may eliminate M. bovis mammary gland infections by themselves after the clinical episode, usually during the dry period (10,12). The variable duration of clinical signs and shedder status contributes to the difficulty in predicting the outcome of infected quarters and the determination of complete bacteriologic recovery. For this reason, cows diagnosed positive for Mycoplasma should probably be considered positive for life even though this may not be the case (21). The severity and the recovery from the infection may vary within herds and between herds, depending upon the species of Mycoplasma as well as the relative susceptibility of the cows. Dry cows are equally susceptible to infection by Mycoplasma but show little swelling or other signs until freshening, at which time a full blown clinical case results. Lameness due to an arthritis caused by the presence of Mycoplasma in the hocks and fetlocks of mastitic and non-mastitic cows is frequently seen in infected herds (4,11).
Diagnosis of mycoplasma infection in milk depends primarily upon microbiological culture of udder secretions. Speciation is accomplished by immunofluorescence or an indirect immunoperoxidase test. Although the reliability of the method is satisfactory, it is time-consuming, as 3 to 7 days of incubation may be necessary before plates receive a final reading; speciation can require several more days. Isolation of mycoplasma is usually made on petri plates containing modified Hayflick medium, of which there are several variations (6). Composite milk samples are usually streaked over one-half of a plate and bulk tank milk samples over an entire plate. Plates are examined for colonies under low power on a standard microscope or, more effectively, under a stereomicroscope or dissecting microscope. Growth may be seen after 3 days of incubation at 37 C in a moist 10% CO2 incubator or in a moist candle jar, but 5-7 days of incubation is needed for the full development of colonies. Incubation should proceed 7-10 days before plates are diagnosed as negative.
Large numbers of mycoplasmas are usually present in samples from clinical cases of mastitis, but only a few organisms may exist in samples from chronic or carrier cows (4,21,23). Culture of bulk tank milk is a useful procedure to determine the existence of mycoplasma infected cows in a herd (9,21); however, the method has its limitations (7). In 2 herds in which mycoplasma was isolated from bulk tank milk, Mycoplasma sp was isolated from 1 (0.33%) of 307 milking cows and from 2 (0.36%) of 548 milking cows (7). However, M. bovis was not isolated from the bulk tank milk in herds where 7 of 66 (10.61%) and 6 of 51 (11.76%) cows were infected with the organism (9). This could be attributable to various rates of mycoplasma shedding related to stages of infection in infected udders (7,8), the sensitivity of the bacteriologic procedure used to detect Mycoplasma, or to the fact that dairy farmers withhold abnormal milk from mastitic cows from the tank (9,21,32).
Enhanced growth in broth followed by culture on a mycoplasma agar medium has been suggested (24,35). Thurmond and co-workers (35) found that the combined use of direct inoculation and pre-enrichment yielded 70% more isolates of Mycoplasma from bovine milk, than the direct inoculation on mycoplasma medium plates alone. At our laboratory, the examination of 4,116 milk samples by both direct inoculation of milk on Hayflick agar plates and pre-enrichment in Hayflik broth produced only a 6% increase (13). Therefore, we agreed with Jasper (24) that the use of pre-enrichment did not increase isolation of Mycoplasma significantly, but increased the cost of diagnosis.
Most mycoplasmas isolated from bulk tank milk and cow milk samples are pathogenic but some may be Acholesplasma laidlawii, a common nonpathogenic saprophytic contaminant frequently found in the dairy environment and the teat skin (4,21). Therefore, speciation of Mycoplasma-like colonies is recommended. Digitonin inhibition of sterol metabolism by mycoplasma was reported as a practical and easy method to discriminate between isolates of Mycoplasma and A. laidlawii from milk (36)
All mycoplasmas are very sensitive to pH changes in the milk. Best recovery rates are achieved when fresh milk samples are plated soon after collection and delivery to the laboratory. Samples may be kept refrigerated for 3 days or frozen for longer periods before culturing on mycoplasma medium.
A herd suspected by a veterinarian of having mycoplasma mastitis, based on history and clinical signs, should be cultured in order to establish the nature of the infection. Mycoplasma infections are frequently complicated by common bacterial infections appearing concurrently.
Most transfer of mycoplasma infection within herds occurs at milking time by means of fomites such as milking machines, teat cups and hands (4,21,23). Many new herd infections occur from the introduction of replacements with infected udders (4,9,21). Treatment of mastitis provides a good opportunity for spread from cow to cow, and even from herd to herd if rigid sanitary precautions are not followed (4,21). However, an outbreak of mycoplasmal mastitis may occur in previously clean herds without introduction of animals or history of previous intramammary treatment (4,9,21).
Since M. bovis is widely found as a resident of the bovine respiratory tract of apparently normal cows, transfer from the lungs to the mammary gland by hematogenous or other routes has been postulated (23). Once an udder infection is established, rapid spread within a herd can occur by more routine methods for spreading mastitis. Hematogenous spread of M. bovis was demonstrated when the organism was recovered from viable fetuses and calves of cows with mastitis (30). Later, researchers isolated mycoplasma from the blood of calves within a week after intratracheal inoculation (34).
The prepuce and distal urethra were found to be sites of colonization by various mycoplasma and ureaplasma in the genital tract of apparently normal bulls (19,20). This results in infected semen and may be a way of dissemination of these organisms. Use of infected semen has been shown to result in lowered conception rates, increased services per conception and prolonged calving interval (26).
Bennett and Jasper (2) found a high percentage of young calves fed M. bovis-infected milk to have respiratory infections (19.8-47.4%). Many of these infections persisted for at least 9 months. Calves from herds without mycoplasma mastitis had low rates of M. bovis infection (3.9-8.7%) for up to 5 months. Lateral transmission of respiratory infection between calves occurs (11). It may be airborne (11,20) and persist until the first calving (30). In some small New York herds in which calves, heifers, and cows were kept in the same barn, mastitis cases started within a month after calves were diagnosed as having pneumonia and inflammation of the joints (9,11).
In California, the risk of large herds (>350 cows) having a mycoplasma-positive bulk tank sample was 15 times greater than that of small herds (<350 cows) (32). The reason for this was thought to be a combination of several poorly understood management factors commonly found in larger California herds (32). In contrast, size was not a risk factor in affected New York herds having 30 to 400 milking cows (9).
In New York, the highest frequency of clinical mastitis due to mycoplasma was found to occur during the winter, starting late in the fall, peaking in January, and decreasing by mid-spring (9). A similar seasonal variation of Mycoplasma mastitis was also observed in California and was attributed to improper ventilation in the barns (22).
Quality Milk Promotion Services investigated 140 herds with mycoplasma mastitis problems between January 1989 and December 1995. In almost all the herds, mycoplasmas were introduced when replacements (virgin heifers, pregnant heifers or cows) were purchased and commingled with the existing herd without quarantine and bacteriological testing (14). In the states of New York and Pennsylvania, we have frequently seen that purchased heifers were the origin of severe mycoplasma mastitis in previously mycoplasma free herds, the heifers showing clinical mastitis immediately after calving (9,11).
Lateral transmission of the infection was frequently observed in poorly ventilated barns and airborne transmission among animals of different ages was suspected (9,11). In several farms where calves, heifers and cows shared the same barn, clinical mastitis in lactating cows was associated with exposure to calves, heifers and cows with signs of respiratory disease (11). On other farms, cases of clinical mastitis started after calves were diagnosed as having pneumonia and arthritis (14).
In some areas in New York State where mycoplasma mastitis appears to be endemic and where epizooties occurred between 1976 and 1980, we speculated that the disease could have been transmitted between adjacent farms by people who were in contact with infected milk, such as milkers, herd owners, milk plant truck drivers, and veterinarians (9). Contaminated equipment, treatment devices, clothing, sampling meters or any type of improperly cleaned material could have served as fomite vehicles (4).
There is no treatment for mycoplasma mastitis. Control of the disease relies on identification of infected cows by culture of composite or quarter milk samples from all milking and dry cows in the herd (4,21). All cases of clinical mastitis should also be cultured as well as all animals at freshening, including heifers. Mycoplasma-infected cows must be segregated and milked last or with a separate milking unit from those used on uninfected cows to minimize the risk of infection for other cows (12,21). Recently, the first spontaneous and complete recovery from M. bovis mastitis was reported (10,12). Cows infected with other mycoplasma may recover and stop shedding during the same lactation (4,21,23). Slaughter of all infected cows is indicated when a few animals in the herd are infected. The exact mode of handling will vary from dairy to dairy based upon the owner's attitude, facilities, number of infected animals, the level of milk production and reproductive status of carrier animals, and the availability of replacements (4,21). The use of rubber or plastic gloves and disinfection of gloved hands between cows is advised when milking or treating cows in a mycoplasma infected herd (4,21,23). Single treatment devices are recommended if treatment for any other type of mastitis is necessary in herds known or suspected to have mycoplasma mastitis.
In large herds, culture of bulk tank milk samples collected after milking each production group may be used as a method to locate groups in which mycoplasma infected cows exist. Then, individual composite milk samples can be used to identify the infected cows in those production groups.
Weekly monitoring of bulk tank milk to detect the presence of mycoplasmas should be encouraged to monitor the success of control procedures. This monitoring should continue until pregnant heifers and all cows that were dry during the mycoplasma mastitis outbreak have calved.
Spread of mycoplasma can be greatly reduced by good milking procedures. Premilking teat disinfection before applying teat cups and postmilking teat dipping should be used. At the Quality Milk Promotion Services, we favor the use of iodine products (0.5 to 1%) during mycoplasma bovine mastitis outbreaks. The use of backflushing for disinfection of milking units between cows has been emphasized (4,32,33). However, the installation of this system is usually expensive, and in our experience has minimal effect in reducing infections. Furthermore, teat dipping should also be used before and after intramammary treatment of nonlactating or lactating cows for organisms different than Mycoplasma.
Great care should be used when purchasing cows and heifers. Milk from all replacements should be cultured for Mycoplasma as well as for Streptococcus agalactiae and Staphylococcus aureus before allowing replacements to commingle with the herd (13,14,21). When herds are purchased, it is a good policy to culture all suspected mastitic cows as well as the bulk tank. All actions should be based upon the understanding of the highly contagious nature, slow recovery rates and the ineffectiveness of treatment of mycoplasma infections (21).
With informed dairy farmers and alert veterinarians, most herd infections can be prevented or diagnosed early (21).
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