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Bovine mastitis therapy and why it fails
J H du Preez
a
INTRODUCTION
Mastitis can occur at any stage of a cow’s
productive life. Microbiological investiga
tions of clinical bovine mastitis reveal a
causative agent in 75–95% of cases. There
is good evidence that microorganisms are
involved in almost all cases
36,40,49
. At least
137 infectious causes of bovine mastitis
are known, but the majority of cases are
caused by only a few common bacterial
pathogens, namely staphylococci, strep-
tococci, coliforms and
Arcanobacterium
pyogenes.
For antibacterial mastitis therapy to be
successful, the active drug must attain
and maintain concentrations exceeding
the minimum inhibitory concentration
(MIC) at the focus of infection for long
enough to break the production and
toxin-producing cycle of the causative
pathogen
3
. This may be prevented by a
number of factors that include udder pa
thology and poor selection of anti
microbials, and is also influenced by the
route of administration. Therapy may
have poor results owing to tissue damage
and introduction of new infections dur
ing treatment, and/or failure to eliminate
the management factors that predispose
to mastitis. Although there is an underly
ing assumption that the primary goal of
antibacterial mastitis therapy is to kill bac
teria and that the normal udder is sterile,
usually the best that can be achieved is
temporary reduction or suppression of
the bacterial population to allow the host
to overcome the infection. The mecha
nisms to cleanse the udder are evidently
poorly developed in some cows, as re
lapses and re-infections commonly follow
antimicrobial therapy. Udder infections
tend to be dynamic, and stress may con-
tribute to udder infections becoming clin-
ically apparent
48,49
.
The purpose of this article is to provide
an update on bovine mastitis therapy and
reasons for its failure.
PRINCIPLES OF MASTITIS THERAPY
The success of mastitis therapy depends
on correct diagnosis, appropriateness of
the route of administration and the drug
selected, stage at which treatment is initi
ated, severity of udder pathology, sup
portive treatment, and elimination of
predisposing factors. There is no standard
treatment for mastitis, but it is advocated
that the clinical forms should be treated
according to the severity of the udder in
flammation
12
.
Bovine mastitis is most commonly
treated by intramammary infusion of
drugs
29,64
. This is the route of choice in
subclinical, mild or moderately severe
mastitis, and is used as an adjunct to
parenteral administration in severe mas
titis. Some clinicians prefer intra
mammary administration, without
systemic administration, even in severe
disease
40
. For effective intramammary
treatment, drugs should distribute
throughout the udder and be rapidly ab
sorbed into the general blood circulation
(Table 1). Significantly better results can
be obtained when the drug is adminis
tered intracisternally in 1
of 0.5 % glu
cose solution, rather than in 50 m
saline
23
.
The disadvantage of local application of
antimicrobials is the slow and uneven dis
tribution of certain drugs in the infected
udder (Table 1). In acute, severe disease,
distribution through the udder may be
impaired by inflammation or blockage of
milk ducts by debris. Parenteral adminis
tration may overcome these problems, al
though it is usual to administer agents
concurrently by the intramammary
route
17
. Severely inflamed udders should
be milked out frequently, with the aid of
oxytocin if necessary. In peracute or acute
clinical mastitis cases with systemic signs,
combined systemic and intramammary
treatment with compatible antibiotics,
supplemented with supportive therapy,
is recommended
64
.
It is always desirable to treat infectious
mastitis according to the antimicrobial
drug sensitivity pattern of the pathogens
and clinical experience
40
. The basic rule in
selecting the drug is to opt for one with as
narrow a spectrum as possible, to focus
treatment on a specific pathogen and
minimise side-effects. Because it takes
time to do sensitivity determinations,
broad-spectrum antibiotics must be given
initially for practical reasons, based on
knowledge of the pharmacokinetic prop-
erties of the drugs and the formula-
tion
9,29,32,65,66,67
. In general, narrow-spec-
trum antibiotics are bacteriocidal and
those with a broad spectrum are bacterio
static
58
.
Response to treatment increases with
persistence of the antibiotic in the udder.
The concentration of the drug used must
at least exceed the MIC-value for the
pathogen, but preferably also the MBC-
value (minimum bacteriocidal concentra
tion) in the udder. If bacteriostatic drugs
are used, the need to maintain high con
centrations in the target organ is in
creased.
Successful intravenous or intramuscu
lar mastitis therapy depends on effective
passage of the drug from blood into milk
to reach foci of infection, which is largely
governed by lipid solubility, degree of
ionisation (dependent on the dissocia
tion constant (pKa)), and the extent of
protein-binding of the drug with
plasma
44,45,53
, since drugs cross the
blood–milk barrier by passive diffusion
30
.
Only the unbound or free drug can dif
fuse through the blood–milk barrier and
exert pharmacological or antimicrobial
0038-2809 Jl S.Afr.vet.Ass. (2000) 71(3): 201–208
201
a
Technology Transfer Division, ARC - Onderstepoort
Veterinary Institute, Private Bag X05, Onderstepoort,
0110 South Africa

Treatment of bovine mastitis depends on the cause, the clinical manifestation and the antibiotic susceptibility of the agent. Mastitis therapy is commonly unsuccessful owing to pathological changes that occur in the udder parenchyma as a result of the inflammatory reaction to mastitogenic bacteria, pharmacokinetic properties of antimicrobial mastitis drugs, mastitogenic bacterial and related factors, and poor animal husbandry and veterinary interventions.