Bovine mastitis treatment failure

Despite an appropriate choice of antimicrobial, treatment of mastitis may be unsuccessful. This article is an attempt to identify a number of key factors associated with failure of treatment of bovine mastitis and illustrate the current knowledge and author’s view on those factors linked to MCOs and the mammary gland.

The success of mastitis therapy is dependant on such factors as correct diagnosis, appropriateness of the route of administration, the drug selected, stage at which treatment is initiated, severity of udder pathology, supportive treatment, elimination of predisposing factors (Prescott 1987; du Preez 2000) and some factors relating to the mastitis-causing organisms’ (MCO) themselves. Many of the factors mentioned, such as diagnosis of mastitis and supportive treatment, are very interesting topics, but are out of the scope of this discussion.

Despite an appropriate choice of antimicrobial, treatment of mastitis may be unsuccessful (McKellar 1991). Insufficient contact of the antimicrobial with MCOs at the site of infection is a major cause of mastitis treatment failure (Serieys et al 2005). Current treatments of clinical mastitis during lactation are not very successful and cure rates are poor, especially in the case of Staphylococcus aureus which is responsible for chronic infections and huge economic losses (Gruet et al 2001). Estimate of microbial cure rate during lactation in case of Staph aureus mastitis is usually between 25 and 50% (Sol et al 2000). Antimicrobial resistance or resistance development of MCOs is obviously not the key to explaining the problem (Pengov 2003; Hoe and Ruegg 2005). The explanation for mastitis treatment failure should therefore be sought in terms of other factors, which also influence the outcome of the therapy (Pengov 2003). Whilst microbial resistance receives the most attention, more practical problems such as development of localised scar tissue in the udder and blockage of the milk ducts probably have a greater effect (du Preez 1988).

This article is an attempt to identify a number of key factors associated with failure of treatment of bovine mastitis and illustrate the current knowledge and author’s view on those factors linked to MCOs and the mammary gland.

There are four major groups of factors associated with bovine mastitis treatment failure:

  1. Management and iatrogenic factors,
  2. Drug factors,
  3. Mastitis-causing organism factors, and
  4. Mammary gland factors.

Management and iatrogenic factors

Many management and iatrogenic factors can be the reason for mastitis treatment failure. In the literature (du Preez 1988; Soback 1988; Tyler et al 1992; du Preez 2000; Prescott and Baggot 1993; Pyorala 2002b; Serieys et al 2005), the following causes are mentioned:

Inaccurate diagnosis

Delayed initial treatment

Inadequate supportive treatment

Duration of treatment

Improper dose

Improper route of administration

Partial or full insertion of intramammary cannula

Introducing new IMI through non-sterile IMM injectors



Achieving clinical but not microbiological cure

Drug factors

The common reasons for mastitis treatment failure associated with the drug factors are (du Preez 1988; Sandholm et al 1990; Daley et al 1992; Tyler et al 1992; Prescott and Baggot 1993; du Preez 2000; Erskine et al 2003; Serieys et al 2005):

Improper antimicrobial selection

Short half-life of the drug

Inadequate local tissue concentration

Side effects of the drug

Other factors that will lead to inactivation of the antimicrobial in vivo or in vitro

Low bio-availability

Weak passage of drug across the blood-milk barrier

High degree of milk and serum protein binding

Combined use of microbicidal and microbiostatic antimicrobials

Factors related to mastitis-causing organisms

Tissue invaders or intracellular location

Tissue invading organisms, such as coagulase-positive staphylococci, become walled off in the udder parenchyma by thick fibrous scar tissue, deep-seated abscesses or gain a refuge within the acid phagolysosomes of macrophages and neutrophils. Therefore, antimicrobials cannot reach the MCO and failure may occur even when the organisms are sensitive to the antimicrobial used. This may be partly due to the pH of the phagolysosome being around pH=4 leading to low metabolic activity of the MCO preventing the drug from being fully effective. In other words, penetrability of the mammary gland by an antimicrobial becomes essential for evaluation of the potential therapeutic value of any intramammary (IMM) preparation. Antimicrobials may penetrate these cells poorly and even when they gain access to the cell, may not distribute into phagolysosomes (Ziv 1980; du Preez 1988; Soback 1988; Francis 1989; Sandholm et al 1990; McKellar 1991; Daley et al 1992; Ziv 1992; du Preez 2000; Sol et al 2000; Erskine et al 2003). In chronic Staph aureus mastitis cases, development of localised scar tissue which does not have blood vessels is promoted, meaning that intramuscular and intravenous injections probably provide little benefit and pose difficult therapeutic problems (du Preez 1988; Erskine et al 1993; du Preez 2000; Erskine et al 2003). Therapy may kill the organisms that are not walled off, but at a later date, the organisms within the scar tissue can break out, multiply, cause additional damage to the udder secretory tissue and promote further formation of scar tissue. When antimicrobial treatment is administered, such MCOs may not come into contact with the drug and are therefore not killed (du Preez 1988).

Microbial dormancy and metabolic state

Mastitis-causing organisms are most susceptible to antimicrobials during their logarithmic growth phase. Non-multiplying organisms are not sensitive to most antimicrobials. It is generally accepted that all microbial populations contain some organisms that are not in the active growth phase, which therefore survive. Bacteria exposed to antimicrobial may be inhibited from growth and can remain so for some time after the termination of therapy (Soback 1988; Francis 1989; du Preez 2000; Erskine et al 2003). Low multiplication rates of organisms are seen within phagocytes. According to Prescott et al (1993) this is particularly true for Staph aureus infections.

"L" form of mastitis-causing organisms

Many antimicrobials, such as penicillins and cephalosporins, kill bacteria by preventing synthesis of new cell walls. Sometimes certain organisms develop an acapsular "L" form that is contained only in a cell membrane. Such L-forms are not susceptible to antimicrobials that attack the cell wall. The process of development of L-form is reversible. The transformation of the “normal” to L-form of the MCOs is encouraged by the: use of Antimicrobials, low level of complement, impaired activity of phagocytes, availability of strains and infective dose of causative organisms, degree of inflammation, and other unknown factors (du Preez 1988; Owens et al 1988; du Preez 2000; Erskine et al 2003).

Microbial mechanisms that overcome antimicrobial effects in milk

MCOs can escape from endogenous antimicrobial factors by capsule or slime formation, receptor-mediated absorption of host proteins into microbes, interference with phagocyte function, leukocidin production or production of enzymes capable of digesting antimicrobials. Other mechanisms include adherence of bacteria to tissue linings that results in avoidance of the wash-out effect of milking, upward flotation of microbes with cream, and an increase in microbial replication rate (Barrio et al 2000; du Preez 2000; Vasi et al 2000).

Mastitis-causing organisms that are short lived in the mammary gland, such as coliforms

The pathogenesis of coliform mastitis is different to that caused by Gram positive organisms, in that most of the clinical symptoms are associated with the acute inflammatory reaction of the mammary gland rather than the presence of coliforms per se. The clinical recognition of this form of mastitis is usually after initial peak microbial concentration and polymorphonuclear leucocytes (PMN) migration occur, leading to release of pro-inflammatory products during phagocytosis, or also when the microbial growth and numbers are in decline. Therefore, the antimicrobial therapy may be of secondary importance relative to immediate supportive treatment of endotoxic shock, but it remains an integral part of a therapeutic regimen for severe mastitis.

Drug tolerance and resistance

Widely varying results of the efficacy of antimicrobial treatments have been reported. Although the MCOs are initially sensitive to the antimicrobial agent, they may become resistant and a different agent must then be used (du Preez 1988; Soback 1988). Selecting the wrong and ineffective antimicrobial agent, such as penicillin to treat β-lactamase-producing Staph aureus or Bacteroides fragilis (du Preez 1988; Sandholm et al 1990; Malinowski et al 2002; Erskine et al 2003) can results in treatment failure. Antimicrobial resistance can be natural (intrinsic) or acquired and can be transmitted horizontally or vertically. Acquired resistance is more common. Biochemical resistance of MCO to antimicrobial agents may occur through natural selection, mutation, transformation, transduction or conjugation. Biological mechanisms of resistance can be summarised into the three categories: antimicrobial destruction or transformation, active efflux and receptor modification (Alanis 2005). The antimicrobial resistance, when dealing with MCOs, is associated with many testing issues - the achieved concentrations of given antimicrobial in the udder, what are real minimum inhibitory concentrations (MIC) for the common MCOs. At present most of MIC values are taken from human studies(Owens et al 1997), the basic physiology and pathophysiology of the udder during mastitis is not completely understood yet. Recent studies show that antimicrobial sensitivity testing for mastitis cases are not rewarding; for exantimicrobialple the studies of Hoe and Ruegg (2005) or Constable and Morin (2002). Earlier studies based on the laboratory results demonstrated a high antimicrobial resistance in MCO; for exantimicrobialple Rajala-Schultz et al (2004) found 39% of penicillin resistance in first calving cows and 26% in older cows. Recent studies comparing lab test results and field experience, found no differences in clinical outcome for cows with mild or moderate mastitis that could be attributed to differences in results of in vitro susceptibility testing (Hoe and Ruegg 2005).

Mammary gland factors

Poorer uneven distribution and physical obstruction

In all cases of mastitis, oedema and inflammatory products to a certain extent obstruct the diffusion of antimicrobials by compression or blockage of the milk duct system, (Ziv 1980; du Preez 1988; Soback 1988; Francis 1989; Sandholm et al 1990; Ziv 1992; du Preez 2000; Malinowski et al 2002; Erskine et al 2003), as does extensive necrosis (Prescott 1993) of the affected area of the gland and abscess formation (du Preez 1988; Sandholm et al 1990; McKellar 1991; Daley et al 1992; Erskine et al 2003). The diffusion of antimicrobial solutions throughout the gland is impaired and for this reason it is often very difficult to bring antimicrobials into contact with MCO, particularly by IMM route. A systemic administration may overcome these problems. On the other hand, approximately 97% of IMM delivered drugs stay in the udder and are void in the milk, whilst systemic treatment results with only 3% product becoming IMM, with the rest void mostly in urine after systemic dispersal (du Preez 1988; Soback 1988; Sandholm et al 1990; du Preez 2000) (Ziv 1980; Hillerton and Berry 2005). Many cases of mastitis are thus resistant to treatment even when the MCOs are fully sensitive to the antimicrobial used. Frequent milking at 1 to 2 h intervals is recommended to remove toxins, debris, MCOs, other inflammatory products and for the maintenance of milk duct patency. There are some additional complications in animals with mastitis if they have scarring or abscessation in the udder as this can serve as reservoir or foci of infection (Daley et al 1992).

Udder tissue necrosis

Mastitis which causes udder tissue necrosis leads to a poor blood supply of the affected areas and consequently a decreased redox potential that favours anaerobic MCOs. There is no effective passage of drugs into necrotic avascular udder tissue.

Teat canal infection (TCI)

Standard methods of antimicrobial administration into a mastitic quarter or for dry cow therapy do not necessarily eliminate TCI. TCI serves as a potential source of organisms for infection of the mammary gland parenchyma. After antimicrobial treatment the existing TCI may be a source of a new IMI.


Trauma predisposes the quarter to infection or re-infection. Tissues lining the teat duct are very delicate and any un-natural manipulation of this structure, such as cannula insertion, may jeopardise its antimicrobial function. The solution to this problem is partial insertion of the cannula. In quarters infected at drying-off, treatment efficacy with partial insertion of infusion cannula into the teat canal is higher compared to full insertion of infusion cannula into the teat cistern.

Adverse effects of drugs

In addition to their action on MCOs, drugs may exert direct effects on the phagocytic efficacy of PMN (Francis 1989; Daley et al 1992; Pyorala 2002a). The oxidative burst activity of bovine PMN can be altered after IMM treatment, due to direct effect of the antimicrobials or formulated excipients. Research has found that cloxacillin has no effect and enrofloxacin increases PMN activity. Conversely, neomycin, lincomycin, dihydrostreptomycin, doxycycline, oxytetracycline, danofloxacin, penicillin, ceftiofur, spiramycin, erythromycin and chloramphenicol reduce the oxidative burst activity of bovine PMN (Hoeben et al 1998; Paape et al 2003). These effects can be influenced by the vehicle (Francis 1989; Daley et al 1992). Because intracellular MCOs are not sensitive to antimicrobial action, dysfunctional PMNs may serve as a constant reservoir of protected MCO and thereby lead to a relapsed infection.


The mammary gland tissue could be sensitive and irritated by the active component of the drug or additives, such as vehicle or thickeners. If the drug causes irritation then inflammatory process will be exacerbated.


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Kiro R.  Petrovski

Kiro R. Petrovski
3 articles

Senior research officer, Institute of Veterinary, Animal and Biomedical Sciences, Massey University, New Zealand

A native of the Republic of Macedonia in Europe, Kiro R Petrovski began working with dairy cows at age 11. With a lifelong interest in animals and animal health, he completed his primary, secondary and tertiary education in Macedonia, then graduated from the Veterinary Faculty Skopje in 1997. Following graduation, he worked in a mixed animal practice and a dairy farm in Macedonia until mid 1999 when he moved to New Zealand. He is currently working at theSchool of Animal and Veterinary Sciences at the University of Adelaide. 

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