Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

CHAPTER 8 ANTIBACTERIAL DRUGS
Table 8.3 Suggested administration of oral antibacterial
drugs in dogs in relation to feeding a
Category
Better when fasting
Drug absorption may
be impaired by
ingesta.
Fasting means no food
for at least 1–2 h
before and 1–2 h
after dosing
Better with food
Drug availability is
improved, or
gastrointestinal
upsets are reduced
by ingesta
No restriction
needed
Drugs or drug groups
Azithromycin
Cefradine
Most erythromycin preparations
Most fluoroquinolones b
Lincomycin
Most penicillins#
Rifampicin
Most sulfonamides
Most tetracyclines
Cefadroxil
Chloramphenicol palmitate (in cats)
Doxycycline *
Ibafloxacin
Metronidazole *
Nitrofurantoin *
Cefalexin
Chloramphenicol capsules and tablets
Chloramphenicol palmitate (in dogs)
Clarithromycin
Clindamycin
Hetacillin
Spiramycin
a
Data from human studies except: cefadroxil, cefalexin,
chloramphenicol, clarithromycin, enrofloxacin, ibafloxacin,
penicillins.
b
Enrofloxacin availability is reduced by ingesta in dogs. Effects of
ingesta on fluoroquinolones are generally mild but absorption
may be delayed slightly; avoid dairy foods.
# Effect on amoxicillin is mild.
* Food may reduce gut irritation without hindering absorption
importantly.
or subcutaneous (SC) administration may be equally
satisfactory in these instances. The intravenous (IV)
route should be considered if maximum plasma drug
concentrations are desired immediately after dosing,
as with life-threatening infections, or in shocked or
hypotensive patients where poor tissue perfusion
may impede drug absorption after administration by
other routes.
Dosage and frequency of administration
Dosing regimens for commonly used antibacterial drugs
are suggested in Table 8.4 and are based on the mode
of action of each drug (concentration or time dependent),
the susceptibility of the target pathogen and the
pharmacokinetics of the drug. The ideal regime may
vary with the case, depending on the susceptibility of
the pathogen, tissue penetration of the drug and degree
of immunocompetence of the patient. Smaller doses may
suffice in lower urinary tract infections using drugs
excreted in high concentration in urine but larger or
more frequent doses may be required for relatively resistant
pathogens or infections in areas where drug penetration
is poor.
Assessment and duration of therapy
In acute infections, it is usually evident within 2–3 days
whether treatment is having the desired effect. An inadequate
response should prompt re-evaluation of the
diagnosis and treatment. If underdosing or poor tissue
penetration is suspected, an increased dose rate and/or
frequency of administration of the same drug might be
appropriate. Otherwise, selection of a different drug is
warranted. For the majority of uncomplicated and acute
infections in dogs and cats, treatment for 4–5 days up
to a week seems adequate.
With chronic infections, it may take longer to determine
whether treatment is being effective and prolonged
administration is usually required. This can be explained
by existing tissue damage, impaired blood supply and
compromised local immunity. Treatment for 4–6 weeks
or more is generally required when bacterial infections
are chronic. Similarly prolonged treatment is also
advised for pyoderma, prostatitis, pyelonephritis, recurrent
lower urinary tract infections, septic arthritis,
osteomyelitis, septicemia, pneumonia, bacterial endocarditis
and antibiotic-responsive diarrhea.
Combination antibacterial therapy
Combination antibacterial therapy is only indicated in
certain specific circumstances (except sulfonamidetrimethoprim
and β-lactam plus β-lactamase inhibitor).
Bacteriostatic and bactericidal drugs should not generally
be used in combination. The disadvantages of combination
chemotherapy include increased cost and
increased risk of toxicity.
The primary indications for using combinations
include:
● mixed bacterial infections
● to delay emergence of resistance in certain specific
circumstances
● severe infections where the etiology is unknown
(ensure selected drugs are compatible)
● life-threatening infections prior to availability of
susceptibility data
● unusual pathogens, including Mycobacterium,
Rhodococcus and fungi.
Examples of antibacterial combinations sometimes used
in small animal practice include: a fluoroquinolone with
metronidazole (capable of penetrating difficult body
barriers and efficacy against Gram-negative aerobes,
obligate anaerobes, penicillinase-producing Staphylococcus,
most Gram-positive aerobes except Streptococ-
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