- 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
- 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.
- Clinical signs
- 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
- 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
- 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
- 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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- Presented at the 1996 National Mastitis Council Regional
Meeting; Published in the 1996 National Mastitis Council Regional Meeting
Proceedings, pg. 37.
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