Common causes of pneumonia in horses
Strep zoo, actinobacillus suis and A. Equuli, Staph aureus (less common), E.coli, klebsiella, pseudomonas, bordetella, clost perfringens, bacteriodes fragillus

step zoo invades what and is residential where?
common secondary invader after viral pneumonia
commensal in upper airway

actinobacillus suis and A. equuli are commensal where?
upper resp tract

Drug of choice for strep zoo

Procaine penicillin disadvantages
IM, long term, large volume needed, painful

procaine penicillin will drug test positive for how long?
14 days (because of procaine- local anesthetic)

signs of procaine penicillin toxicty
anxiety, extreme excitement, transient: occurs when injected in artery (aspirate!)

potassium penicillin route given and timing
IV ONLY (potassium is irritating), q 6hr

procaine penicillin dose and route
procaine penicillin IM (300,000 iu/ml)

Sodium penicillin- route and timing
IV or IM, q 6 hours (good blood levels)

sodium penicillin- disadvantages
expensive, limited availability

give horses oral penicillin?
no- poor bioavailability
– high doses may induce laminitis

Name horse oral potentiated sulfonamides
sulfadiazine and trimethoprim, sulfamethoxazole and trimethoprim

oral potentiated sulfas- disadvantages
less favorable MICs than penicillin for b strep organisms
-diarrhea at higher doses (>30mg/kg q 12 hours)
-toxicity with long term tp: diarrhea, bone marrow suppression (rare)
-may be irritating to veins (IV suspension)

Naxel Ceftiofur: advantages
better than penicillin for streptococci?
-Q 12 hour admin (vs q 6 hour penicillin)
-can be given IM

Naxel Ceftiofur: disadvantages
IM injections are irritating
IV use associated with diarrhea (can be severe)

Kohn’s preference for resp infection (strep zoo)
-start with pen, choose naxal if poor response to PPen and or KPen or if can’t use IV cath

Sodium ampicillin- advantages
-using sodium salt rather than trihydrate repository formulation, not irritating for IM, can be given IV
-give q 6 hours
-usually higher MIC for b strep than penicilin
***** allowed in competition horses ****

beta streptococcal organisms are usually resistant to….

Actinobacillus suis/ actinobacillus equuli gram neg or pos?

Actinobacillus suis/ actinobacillus equuli commonly associated with what kind of infections?
beta strep

Actinobacillus suis/ actinobacillus equuli susceptibility pattern
unpredictable, may be more sensitive to Gentocin than Amikacin

pneumonia: Actinobacillus suis/ actinobacillus equuli common tx
usually give penicillin with gentocin because beta strep organisms are often present

pneumonia: staph aureus tx
give penicillin with aminoglycosides, atleast initially
-usually not pen sensitive but some are
-ceftiofur, gentamicin, amikacin

Pneumonia: gram neg tx (E.coli, klebsiella, pseudomonas) tx
gentamycin, amikacin, ceftiofur, enrofloxacin (baytril)
– include penicillin

Baytril (enrofloxacin) spectrum
poor gram positive spectrum

Pleuropneumonia epidemiology
similar to pneumonia
-anaerobes if nasal discharge of pleural fluid has foul odor
– anaerobes may occur mixed infectrions with aerobes

Pleurpneumonia: clostridium spp tx
-metro (bitter, can give PO or per rectum)

Pleuropneumonia: bacteriodes fragilis tx
***** resistant to penicillin

chloramphenicol disadvantages
poor blood levels, cleared rapidly
-human health hazard: fatal aplastic anemia

where horses carry strangles
pharynx, LN, gutteral pouches for months, can be inapparent carrier and transmit the disease
-pulmonary parenchymal abscess can occur

strangles tx
-penicillin: drug of choice
– may need weeks of tp to get into LN, etc
-drain and flush abscess + penicillin gel
-Rifampin ($)

Rifampin: rules to give
must give with another antimicrobial
-will penetrate strangle abscesses

pulmonary or pleural abscesses: whats in there?
-staph aureus (less common)
– gram negs, associated with strep sp.

treatment for gram negs in pulmonary or pleural abscesses
– pen and gent
– SMZ TMP, SMZ TMP plus pen

Rodococcus equi (intracellular organism) (2-4 mo old) tx
-erythromycin and rifampin
– azithromycin ($)
– clarithromycin + rifampin ($)- best (> azithromycin and rifampin and erythromycin and rifampin)

erythromycin use in adult horses
don’t do it, toxic –> diarrhea
**avoid macrolides in adult horses**

Bacterial causes of enterocolitits
-neoricketsia risticii
-clostridium difficile
-lawsonia intracellularis

Salmonella infection characteristics
-invasive organism, system dz
-chronic GI infection usually unresponsive to abx

Salmonella infection tx
Gent, amickacin ($$$)
– enrofloxacin: reduce fecal shedding?

Neoricketsia risticii dx
-obtain PCR or serologic dx

neoricketsia risticii
oxytet –> rapid resolution of enterocolitis in 2-4 days
-doxy PO after colitis resolves

Clostridium difficile- affects who?
foals and adults
-life threatening
-infection may be associated with administration of beta lactam antibiotics

C. difficile anaerobic or aerobic?

C. difficile tx
metro IV or per rectum (adults)

lawsonia intracellularis causes…
proliferative enteropathy

lawsonia intracellularis signs
-severe hypoproteinemia
-weanlings (adults-rare)

lawsonia intracellularis tx
-doxy (success at OSU)

Anterior enteritis: duodenitis prox jejunitis dx
culture nasogastric reflux
-c. perfringens

Anterior enteritis: duodenitis prox jejunitis tx
-potassium penicillin and gent IV
-metro per rectum
other tx important:
-NPO, NG tube to decompress stomach, IV fluids

Mesenteric abscess caused by
strep equi var equi, strep zoo, R. equi, salmonella

Mesenteric abscess: strep zoo
-long term tx

peritonitis etiology
usually idiopathic
-actinobacillus suis/ equuli
-mixed pop gram neg
-E. Coli
-fungal: blastomyces (poor px)

cholangitis: organisms
-ascending infection?
-enteric organisms
-most common cause is E.Coli
-may be associated with choleliths (poor px)

Cholangitis tx
-potentiated sulfas: long term oral use (practical and effective!)
– penicillin and gent IV ($): intial tx for sicker horses

cystitis dx
quantitative urine cultures (>10^4) of pathogen
-often associated with uroliths

cystitis organisms
-E.Coli and other gram negs

cystitis tx
SMZ-TMP q2-6 weeks

is pyelonephritis common in horses?

pyelonephritis organisms
same as cystits organisms (E coli and other gram negs)

pyelonephritis tx
acute: IV gent, amikacin, enrofloxacin (consider renal function)
-switch to oral drug like SMZ TMP if susceptible
-high urinary conc of drugs that are excreted in active form in urine can be effective when MIC is higher than desirable

septic joint or tendon sheath causes
-iatrogenic: staph aureus
-hematogenous distant site of infection (cath, pneumonia, wound)

septic joint or tendon sheath tx
-IV antibiotics: pen/gent, pen/amikacin
-local tp: lavage/debride, RLP, CRI in joint lavage

septic joint or tendon sheath: E.coli in foals tx
-Pen/gent IV
-pen/amikacin IV
if poor response…
-ceftazidime (fortaz), cefotaxime (claforan)

myositis caused by…
clostridium perfringens

myositis tx
**life threatening
-drain abscess
-fenestrate emphysematous areas
-pen IV
– oxytet IV
-doxy PO?

cellulitis caused by…
staph aureus

cellulitis signs
severe edema/inflam/infection soft tissues of extremities, particularly the hind limbs

Cellulitis tx
life threatening!
-IV antibiotics: pen/gent, ceftiofur
– obtain culture and sensitivity if possible

EPM caused by what organism?
sarcocystis neurona

FDA approved drugs for EPM
marquis (ponazuril)
baycox (toltrazuril)
SMZ/Pyrimethamine (compound)

Meningoencephalomyelitis and brain abscess in foals: etiology
sepsis/exposure to strangles, salmonella agona

meningoencephalomyelitis in foals tx
-ceftazidime (fortaz)
-cefotaxime (claforan)
-good penetration in to CNS, $$$

Osteomyelitis of stylohyoid and petrous temporal bones signs
head tilt, ptosis, peripheral vestibular signs, otitis interna

Osteomyelitis of stylohyoid and petrous temporal bones dx

Osteomyelitis of stylohyoid and petrous temporal bones causative agent
usually unknown

Osteomyelitis of stylohyoid and petrous temporal bones tx
SMZ TMP empirically

sepsis in foals: multisystem dz epidemiology
-in Ohio
-24 hours old: actinobacillus equuli/suis
-other organisms: salmonella and others
-life threatening!!

sepsis in foals tx
-pen/amikacin IV
-high dose ceftiofur (10mg/kg q 6 hours)
-pen/gent: some more sensitive to gent than amikacin

Approved oral antimicrobials in horses

approved IM drugs in horses
procain pen G, ceftiofur, ampicillin

IV approved drugs in horses
ampicillin, sulfadimethoxine, trimethoprim/sulfadizine 48% suspension

intrauterine approved drugs in horses
amikacin, gent, ticarcillin

ionized drugs will have a ____ vol of dist

conc dependent antimicrobial concentration goal
Pconc of 10:1 (plasma conc : MIC)

conc dependent antimicrobials
aminoglycosides, fluoroqinolones, metro

the effect of inc Pconc with a TIME dependent drug
nothing, once above MIC, efficacy does not increase

time-dep antimicrobials
cephalosporins, chloramphenicol, macrolides, penicillins, sulfonamides, tetracyclines

do not give Pen G to horses this route
PO (acid labile)

Pen G is synergistic with

Pen G indications
upper and lower resp infections, lacerations, wounds, UTI’s

name the aminopenicillins
amipicillin, amoxicilin

inc spectrum of aminopen’s from pen G
more gram neg

aminopen’s resistance
E coli, salmonella, pasturerlla

aminopen’s: PK for neonates
PO for 1-2 weeks, IV or IM sodium ampicillin well absorbed

aminopenicillins high dist where?
synovial fluid

Pen G resistance
gram negs

amoxocillin use
streptococcal and non-b lactamase producing staph infections in FOALS a 8-12 hours

antipseudomonal pen’s synergystic with

Timentin indication (ticarcillin plus clavulanic acid)
intrauterine infections, may be useful in some septic foals

Cephalosporins commonlly used in equine med
ceftiofur, ceftazidime, cefotaxime

cephalosporin elimination
renal, unchanged in urine

cephalosporin indications
septicemia, resp dz, osteomyelitis

3rd gen cepalosporin resistance
enterococci and listeria

3rd gen cepalosporin good for reaching conc where?

Ceftiofur indications
-resp pathogens: streptococci, pasteurella, anaerobes
-label: tx strep zoo in resp infections

Ceftiofur use in neonatal foals
5-10 mg/kg q 6 hr
-tx gram neg enteric organisms with higher MIC’s than strep sp.

Cefpodoxime generation

cefpodoxime in neonatal foals: use and dose
Q 8 hrs for organisms with higher MICs: E.Coli, Salmonella
Q 12 hours for organisms with lower MICs: strep, kelbsiella, pasteurella

cefodoxime resistance
pseudomonas, enterococcus, rhodococcus

does cefpodoxime penetrate the CSF?

aminoglycosides MOA
bind to 20S ribosomal subunit (protein synthesis inhib)

aminoglycoside spectrum
most gram neg, some staph and strep

aminoglycoside with inc spectrum of activity
amikacin (and less nephrotoxic than gent)

aminoglycoside route of admin

aminoglycoside indications
septicemia, pneumonia

aminoglycoside dosing
want trough <2ug/ml to dec risk of nephrotox SID dosing, keep Cmax at 10x MIC, check renal function

Chloramphenicol MOA
binds to 50S ribosomonal subunit

Chloramphenicol spectrum

chloramphenicol tox to humans
aplastic anemia (fatal)

chloramphenicol indications
resp, abscess, corneal infections, osteomyelitis

Potentiated sulfa MOA
inhibit bacterial folic acid synthesis by inhibiting PABA incorporation

potentiated sulfa resistance
bacteriodes, pseudomonas, enterococci

potentiated sulfa adverse reactions
non-regen anemia, congenital defects in foals born to dams treated long term

potentiated sulfa indications
resp infections, wounds, EMP

Tetracyclines MOA
bind to 30S ribosomal subunit

tetracycline spectrum
gram +, gram -, few anaerobes, some ehrlicia, protozoa

doxy route

oxytet route

tetracycline adverse effects
renal tubular necrosis when administered to ill foals, cardiovasc collapse if given IV rapidly, IV doxy –> CV collapse and death in ADULTS!!!!***** NEVER GIVE DOXY IV TO ADULT HORSE

tetracycline indications
PHF, lawsonia, FLD in neonates

most lipophilic tetracycline
doxy, without feed before and shortly after administration

doxy dose
10 PO q 12 hours for organisms with MIC < 0.25 (gram pos microbes), anaplasma phagocytophilum, neoricketsia risticii, lawsonia intracellularis

Fluoroquinolones: name the one most commonly used
Enrofloxacin (baytril)

fluoroquinolones MOA
inhibit DNA gyrase

fluoroquinolone routes of admin
well-tolerated IV
irritating IM
no PO formulation for horses (must be COMPOUNDED)

floroquinolones adverse effects
non0inflam, arthropathy related to cartilage damage in foals
*** do not administer to horses under 2 years old!!

fluoroquinolones indications
gram neg infections, MRSA, staph aureus, lymphangitis, cellulitis

enforflox: time or conc dependent
conc (high dose, SID)
-7.5mg/kg PO or diluted, slowly IV q 24 hours

Macrolides MOA
bind to 50S ribosomonal subunit

macrolides concentrated in …

macrolide dist
high Vd, high conc attained in bronchoalveolar and pulmonary epithelial cells

name the macrolides
clarithromycin, azithromycin, erythromycin

macrolide adverse effects
erythromycin can cause hyperthermia
colitis in dams exposed via treated foal
diarrhea in foals

rifampin MOA
inhibits RNA polymerase

Rifampin spectrum
Grm +, esp staph aureus, rodo equi, corynebacterium psedoTb

rifampin resistance
develops rapidly when used alone via chromosomal mutation, ALWAYS use in combo tp

rifampin indications
R equi, internal abscess or severe infection with C. speudotb, strep equi with another drug

Metro MOA
damage to bacterial DNA

Metronidazole spectrum
anaerobes, many protozoa

metro good vol of dist where?
bone, abscesses, CNS tissue

Metro indications
anaerobic infections, usedin combo tp for some pneumonias, C. diff colitis

high Vd drug categories
chloramphenicol, tetracyclines, fluoroquinolones, macrolides, rifampin, metro

low Vd drug categories
Pen G, Amino Pen’s, Cephalosporins, aminoglycosides

moderate Vd
potentiated sulfa

renal elimination
Pen G, Aminopen’s, cephalosporins, aminoglycosides

Chloramphenicol elim
liver, kidneys, hepatic conj

potentiated sulfa elim
liver, urine, bile, milk, sweat

tetracycline elim
renal, biliary, GI

rifampin elim
bile, delayed renal

macrolide elim

fluoroquin elim
partially metabolized to cipro, primarily renal elim

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