Clinical Mastitis - The Merck Veterinary Manual

Clinical Mastitis

When the balance between host defenses and invading pathogens attracts a marked inflammatory response, clinical signs become apparent. Infections from any pathogen can be clinical or subclinical, depending on the duration of infection, host immune status, and pathogen virulence. The control of clinical mastitis usually focuses on the prevention and elimination of pathogens that arise from an environmental reservoir. Thus, the epidemiology and prevention of clinical mastitis is similar to previously discussed concepts regarding the control of subclinical mastitis.

Epidemiology:

Except for outbreaks of Mycoplasma , clinical mastitis in most dairy herds is caused by environmental pathogens. In addition, many clinical mastitis cases are transient, especially those that are initial episodes for a cow and quarter. Thus, from an epidemiologic perspective, assessment of clinical mastitis is based on incidence and not prevalence. The standard methods of monitoring subclinical mastitis, routine somatic cell counts (SCC) and culture of cows with elevated SCC, are poor indicators of clinical mastitis. Cows with high SCC caused by chronic infections may occasionally display clinical mastitis, although it is usually mild. However, cows with low SCC are also prone to develop clinical mastitis, especially those cases with an acute onset. Herds with low SCC may actually have a higher incidence of clinical cases caused by environmental organisms (30-50 cases/100 cows/yr) than herds with higher SCC. Similarly, routine culture of milk samples from a cow with low SCC is a poor indicator of the probability of developing clinical mastitis, especially if the culture yielded no organisms. Thus, records that indicate the incidence of clinical cases, and data from each case that may determine risk factors (eg, season, age, stage of lactation, and previous episodes) should be recorded as part of a mastitis control program. Milk samples should be collected from affected quarters and, when feasible, antibacterial susceptibility testing performed. For well managed herds in which mastitis caused by contagious pathogens has been controlled, a goal for the incidence of clinical mastitis should be 2 cases/100 cows milking/mo. Severe mastitis cases should be in the range of 1-2 cases/100 cows milking/yr. Typically, 10-40% of milk samples collected from clinical mastitis cases yield no organisms on culture. However, of the samples that do yield organisms, 90-95% of the isolated bacteria include a wide variety of streptococci, staphylococci, or coliforms. If this is not the case, especially if a single pathogen such as a noncoliform gram-negative rod or a fungus predominates, a point source of infection should be considered.

Severe Clinical Mastitis:

Coliforms (lactose-fermenting gram-negative rods of the family Enterobacteriaceae) are the most common cause of this form of mastitis. Following infection, coliform numbers in milk increase rapidly, often attaining peak bacterial concentrations within a few hours. A subsequent decline (rapid in most cases but may take several days in truly severe mastitis) in bacterial concentration follows neutrophil migration into the gland. The majority of coliform infections are cleared from the gland with few or mild clinical signs. However, if bacterial concentrations are elevated enough to elicit an acute inflammatory response, systemic involvement is a frequent consequence. Coliform-related mastitis results in a higher incidence of cow death or agalactia-related culling (30-40%) than mastitis caused by other pathogens (2%). Prognosis for cases of Klebsiella infection should be particularly guarded, as cows are twice as likely to be culled or die than those infected by other coliforms. Thus, primary therapy for severe clinical mastitis should be directed against coliform organisms, although secondary consideration must be given for causative agents. Supportive care, including fluids, is usually indicated, and in the case of coliform mastitis, may be the most beneficial component of the therapeutic regimen. Antibacterial therapy is ideally based on identification of the causative pathogen; however, this is not attainable for some hours after initial case recognition. In addition, most antibacterial therapeutic regimens currently used for severe clinical mastitis in the USA are not approved by the FDA.

Many inflammatory and systemic changes seen in severe coliform mastitis result from the effects of release of lipopolysaccharide (LPS) endotoxin from the bacteria. By the time therapy is initiated, maximal release of LPS has likely occurred. Thus, the primary therapeutic concern is the treatment of endotoxin-induced shock with fluids, electrolytes, and anti-inflammatory drugs. IV fluids are preferred as the initial method of administration. If isotonic saline is administered, 30-40 L are necessary over a 4-hr period, which can be difficult under farm conditions. A practical alternative is 2 L of 7% NaCl (hypersaline) administered IV. This allows rapid fluid uptake from the body compartment into the circulation. Cows should then be offered free choice water to drink, and if at least 10 gal. is not consumed, 5-7 gal. should be pumped into the rumen. Many cows with endotoxic shock are marginally hypocalcemic, thus 500 mL calcium borogluconate should be administered SC (to avoid potential complications that could arise from IV administration). Alternatively, rapid absorption calcium gels, designed for periparturient hypocalcemia, can be given. If the cow remains in shock, continued fluid therapy should be administered PO or IV as isotonic, not hypertonic, fluids.

Glucocorticoids are particularly helpful in cases of mastitis caused by endotoxin-producing coliforms. They should be administered early in the course of disease for maximal efficacy. Administration of dexamethasone (30 mg, IM) to dairy cows immediately following introduction of E coli into the mammary gland has been reported to reduce mammary gland swelling and inhibition of rumen motility. Isoflupredone (10-20 mg, IM) has also been shown to reduce local mammary swelling. Cattle are sensitive to glucocorticoid-induced immune suppression; however, it is unlikely that one-time administration of a glucocorticoid will adversely affect cows with endotoxin-induced severe clinical mastitis. Temporary suppression of inflammation as manifested by reduced neutrophil migration may well be beneficial. Care should be exercised in administering these drugs to pregnant animals; however, severe clinical mastitis in and of itself may cause pregnancy loss in cattle.

There is little published research on the use of glucocorticoids for mastitis caused by gram-positive bacteria. It is reasonable to expect that gram-positive infections would be less likely to benefit from the anti-inflammatory activities of glucocorticoids and may even be adversely affected. Intramammary glucocorticoid administration to reduce local inflammation, without affecting the migration of neutrophils into the gland, is an attractive therapeutic option. Although products that combine antibacterial and glucocorticoid drugs for intramammary administration exist in Europe, it is not clear whether clinical benefit is gained when compared with antibacterial therapy alone. As a general guideline, glucocorticoid treatment should be reserved for severe cases of gram-negative mastitis, with a single dose administered early in the disease course.

NSAID are widely used for the treatment of acute mastitis. Aspirin, flunixin meglumine, flurbiprofen, carprofen, ibuprofen, and ketoprofen have been studied as treatments for experimental coliform mastitis or endotoxin-induced mastitis. Orally administered aspirin should be used with caution in acute coliform mastitis because it may lead to severe rumen atony. If used, the Food Animal Residue Avoidance Databank (FARAD) recommends a milk and slaughter withdrawal interval of 24 hr to reduce the risk of Reye’s syndrome in children. Dipyrone has been studied and widely used as a treatment for acute mastitis. However, the use of dipyrone in food animals is specifically prohibited by the FDA and it is no longer available in the USA. Phenylbutazone has also been studied and widely used as a treatment for acute mastitis. However, the FDA and FARAD strongly discourage its use in food animals. The tolerance level for phenylbutazone is zero and detection of any concentration is an illegal residue. In addition, extended withdrawal intervals make phenylbutazone a poor economic choice.

Treatment with ketoprofen improved recovery of cows with acute clinical mastitis in a blinded, placebo-controlled study. Although ketoprofen is available as a veterinary product for use in horses, has a high therapeutic index, has favorable pharmacokinetics for use in lactating dairy cattle, and is approved for use in cattle in France, it is not currently labeled for food animal use in the USA. FARAD recommends withdrawal intervals of 7 days for slaughter and 24 hr for milk, with IV or IM administration, for dosages up to 3.3 mg/kg, sid, for up to 3 days.

Flunixin meglumine is labeled for beef and nonlactating dairy cattle with bovine respiratory disease. It is the only NSAID labeled for use in cattle in the USA and is therefore the most logical choice for treating clinical mastitis. In field studies, increased survival and improved milk production have not been demonstrated following treatment of clinical acute mastitis with flunixin meglumine at a dosage of 1.1 mg/kg. However, in studies of experimental mastitis, this drug reduced the severity of clinical signs such as fever, depression, heart and respiratory rates, and udder pain. FARAD recommends withdrawal intervals of 4 days for slaughter and 72 hr for milk when used as specified. As with the glucocorticoids, NSAID may provide symptomatic relief and promote well-being. Administration early in the course of the infection is likely to increase clinical benefit.

Antibacterial therapy may be of secondary importance relative to immediate supportive treatment of endotoxic shock, but it remains an integral part of a therapeutic regimen. Occasionally, coliform infections do result in chronic mastitis. Research suggests that bacteremia may occur in >40% of severe coliform cases. In addition, numerous other pathogens including gram-positive cocci cause severe clinical mastitis, which can be difficult to distinguish from cases caused by coliforms at initial presentation.

Selection of an appropriate antibacterial for severe coliform mastitis depends primarily on the susceptibility of the organism to the selected drug and the ability to maintain effective concentrations at the primary pharmacologic target (which, in the case of coliform mastitis, is the plasma compartment of the cow).

In one study, IM gentamicin was not more effective in preventing agalactia or death resulting from severe coliform mastitis, or in improving other clinical outcomes, than IM erythromycin or no systemic antibacterials. Cows experimentally challenged with E coli and given 500 mg of intramammary gentamicin bid did not have lower peak bacterial concentrations in milk, duration of infection, convalescent SCC or serum albumin concentrations in milk, or rectal temperatures as compared with untreated challenged cows. In additional, gentamicin readily diffused through the milk-blood barrier, resulting in drug residues in the kidney for >6 mo. With increased interest and sophistication of drug residue testing among regulatory agencies, practitioners should carefully consider the 30-45 day half-life for clearance in bovine kidneys and extended milk withdrawal periods following use of aminoglycosides.

Oxytetracycline (11 mg/kg, IV, sid) improved outcome of cows with clinical coliform mastitis (not necessarily severe) as compared with cows that did not receive systemic antibacterials. Ceftiofur sodium (2.2 mg/kg, IM, sid) decreased the mortality and cull rates of cows with severe coliform mastitis. This drug distributes poorly to the mammary gland, supporting the emphasis on treating the cow rather than the mammary gland because of the risk of septicemia.

Intramammary infusion of commercial products that have good activity against gram-positive organisms should be administered to any cow with severe clinical mastitis. This treatment is not likely to affect the outcome of a case caused by coliforms, but may provide some benefit for cases caused by gram-positive cocci. The need for antibacterial therapy in cows with grossly abnormal milk, but with improved appetite, attitude, and milk production should be evaluated critically. Unnecessary extension of therapy in these instances results in increased discarded milk expense for the dairy producer and risk of antibacterials in marketed milk.

Mild Clinical Mastitis:

No microorganisms are isolated from 10-40% of bacteriologic cultures of milk samples collected from cows with clinical mastitis. Many mild mastitis cases (nearly 80%) are coliform intramammary infections that resolve before treatment is necessary. In addition, numerous mild clinical mastitis cases are temporary setbacks in the balance between pathogen and host defenses that occurs in more chronic intramammary infections. A “no antibiotic” approach to mild clinical mastitis cases avoids costs of discarded milk and residue risks that are inherent in antibacterial therapy. Although therapeutic and spontaneous cure rates for mild clinical mastitis cases caused by coliforms are similar, information on the efficacy of treatment for mild clinical mastitis cases caused by other pathogens is not extensive. Use of antibacterials for the treatment of clinical cases caused by streptococci has been reported to achieve bacterial cures in 60-65% of cases. However, a comparison of cure rate in treated vs. untreated cows was not reported in many studies. In a study of 3 California dairies, bacteriologic cure assessed at 4 and 20 days after treatment with amoxicillin, cephapirin, or oxytocin (no antibacterial) did not differ for mild clinical mastitis cases caused by streptococci and coliforms. Although milk production and survival did not differ, the rates of both relapses and recurring cases were higher in untreated cows, especially among streptococcal cases. A Colorado dairy study reported similar results, along with an increase in the incidence of clinical mastitis, prevalence of intramammary infections, and herd SCC associated with streptococcal infections following adoption of a no-antibiotic approach to clinical mastitis.

Common sense and individual herd history should determine the course of therapy for mild clinical mastitis cases in dairy herds. For initial occurrences of clinical episodes in any affected quarter, especially those caused by streptococci, use of approved commercial intramammary infusions is likely to be the best option. Assessment of success should be based on bacteriologic cure, but therapeutic success can be monitored more practically by improvement in days of marketed milk. If mastitis recurs regularly in affected quarters in the absence of systemic signs, repeated treatment of what now has become chronic intramammary infection is not warranted. Any moderate increase in cure rates gained by extensive parenteral therapy will not likely overcome the expense of discarded milk and other related treatment costs. If standard regimens achieve less than desired results, it would be better to extend therapy for a prolonged period, rather than to change to other antibacterial drugs or increase the amount of each dose. Alternatively, therapy of mild cases can be withheld until initial bacteriology is performed. If initial culture yields no organisms or coliforms, therapy is not initiated, whereas if gram-positive cocci are isolated, therapy is administered.

Unusual Pathogens:

Pseudomonas aeruginosa may cause outbreaks of clinical mastitis. Generally, a persistent infection occurs, which may be characterized by intermittent acute or subacute exacerbations. The organism is found in soil-water environments common to dairy farms. Herd infections have been reported after extensive exposure to contaminated wash water ( particularly wash hoses), teat cup liners, or intramammary treatments administered by milkers. Failure to use aseptic techniques for udder therapy or use of contaminated milking equipment may lead to establishment of P aeruginosa infections within the mammary glands. Severe peracute mastitis with toxemia and high mortality may follow immediately in some cows, while subclinical infections may occur in others. The organism has persisted in a gland for as long as 5 lactations, but spontaneous recovery may occur. Other than supportive care for severe episodes, therapy is of little value. Culling is recommended for cows.

Arcanobacterium (Actinomyces) pyogenes is common in suppurative processes of cattle and pigs and produces a characteristic mastitis in heifers and dry cows. It is occasionally seen in mastitis of lactating udders after teat injury, and it may be a secondary invader. The inflammation is typified by the formation of profuse, foul-smelling, purulent exudate. Mastitis due to A pyogenes is common among dry cows and heifers that are pastured during the summer months on fields and that have access to ponds or wet areas. The vector for animal-to-animal spread is the fly Hydrotaea irritans . Control of infections is by limiting the ability to stand udder-deep in water and by controlling flies. Preventive treatment of heifers and dry cows in susceptible areas with long-acting penicillin preparations has been effective in reducing infections. Therapy is rarely successful, and the infected quarter is usually lost to production. Infected cows may be systemically ill, and cows with abscesses usually should be slaughtered. (See also actinomycosis, Actinomycosis: Introduction .)

Mycoplasma spp can cause a severe form of mastitis that may spread rapidly through a herd with serious consequences. M bovis is the most common cause. Other significant species include M californicum , M canadense , and M bovigenitalium . Onset is rapid, and the source of infection is believed to be endogenous after outbreaks of respiratory disease in heifers or cows. The disease is often seen in herds undergoing expansion in which animals from outside sources have been added. Some or all quarters become involved. Loss of production is often dramatic, and the secretion is soon replaced by a serous or purulent exudate. Initially, a characteristic fine granular or flaky sediment may be seen in the material removed from infected glands. Despite the severe local effects on udder tissue, cows usually do not manifest signs of systemic involvement. The infection may persist through the dry period. Because there is no satisfactory treatment, affected cows should be segregated at least for that lactation, or for their lifetimes. Identification of infected cows can be difficult because of the frequent propensity of infected cows to become asymptomatic carriers. Routine screening of the bulk tank and milk strings may help identify the presence of infected cows. However, culture of the mammary secretion of cows with clinical mastitis is the most reliable surveillance method. If cows continue to display clinical mastitis or systemic signs, they should be culled. Sanitary measures should be strictly enforced, especially at milking or during treatment. Milk from Mycoplasma -infected cows should not be fed to calves as this may result in respiratory and inner ear infections. Milk replacer, rather than discarded milk from mastitis cows, should be fed to calves in herds with Mycoplasma .

Nocardia asteroides causes a destructive mastitis characterized by acute onset, high temperature, anorexia, rapid wasting, and marked swelling of the udder. Response in the udder is typical of a granulomatous inflammation and leads to extensive fibrosis and formation of palpable nodules. Herd histories suggest that infection of the udder may be associated with failure to ensure asepsis in intramammary treatment of the common forms of mastitis. Slaughter is recommended for infected cows.

Serratia mastitis may arise from contamination of milk hoses, teat dips, water supply, or other equipment used in the milking process. The organism is resistant to disinfectants. Cows with this form of mastitis that continue to display clinical signs should be culled.

Mastitis due to various yeasts has appeared in dairy herds, especially after the use of penicillin in association with prolonged repetitive use of antibiotic infusions in individual cows. Yeasts grow well in the presence of penicillin and some other antibiotics; they may be introduced during udder infusions of antibiotics, multiply, and cause mastitis. Yet, heifers that have never received intramammary infusions may develop yeast mastitis. Signs may be severe, with a fever followed either by spontaneous recovery in ~2 wk or, more rarely, by a chronic destructive mastitis. Other yeast infections cause minimal inflammation and are self-limiting. If mastitis due to yeast is suspected, antibiotic therapy should be stopped immediately. Yeast or other mastitis infections can be reduced if the tip of the plastic infusion tube is only partially (rather than completely) inserted through the teat canal during intramammary therapy.

A chronic, indurative mastitis similar to that caused by the tubercle bacillus has been reported to be caused by acid-fast Mycobacterium spp derived from the soil, such as M fortuitum , M smegmatis , M vaccae , and M phlei , when such organisms are introduced into the gland along with antibiotics (especially penicillin) in oil or ointment vehicles. The oil apparently enhances the invasiveness of these organisms, and such therapy is contraindicated. These organisms otherwise tend to be saprophytic and to disappear from infected quarters, at least by the next lactation. In the meantime, mastitis is usually moderate. Distinct outbreaks do occur and several have been reported, especially with M fortuitum and M smegmatis .

Prevention:

Bacterins that utilize core-antigen technology based on J5 mutant Escherichia coli can be helpful in reducing the incidence and severity of clinical mastitis caused by coliforms. Vaccination programs using these bacterins should minimally include multiple administration during the dry period to reduce the incidence of clinical coliform mastitis that is frequently associated with early lactation.