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Antimicrobials

Antimicrobials. CHAPTER 10-2 Dr. Dipa Brahmbhatt VMD MpH dbrahmbh@yahoo.com. Objectives. Mechanisms of action of antibiotics Adverse drug reactions and how to mitigate these Selection of antibiotics: Ab resistance, drug absorption, distribution, location of bacteria and drug elimination

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Antimicrobials

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  1. Antimicrobials CHAPTER 10-2 Dr. DipaBrahmbhatt VMD MpH dbrahmbh@yahoo.com

  2. Objectives • Mechanisms of action of antibiotics • Adverse drug reactions and how to mitigate these • Selection of antibiotics: Ab resistance, drug absorption, distribution, location of bacteria and drug elimination • Antifungal drugs: Advantages, Disadvantages and side effects

  3. Cell Wall Agents • Bacitracin • Disrupts the bacterial cell wall and is effective against gram-positive bacteria • Used topically (skin, mucous membranes, eyes) and as a feed additive • Nephrotoxic • Vancomycin • Bacteriocidal; effective against many gram-positive bacteria; used for resistant infections • Useful in treatment of Staphylococcus aureus

  4. Cell Membrane Agents • Polymyxin B • Works by attacking the cell membrane of bacteria (remember that animal cells have cell membranes too) • Is a narrow-spectrum, gram-positive antibiotic • Not absorbed when taken orally or applied topically • Used as an ointment or wet dressing • Often combined with neomycin and bacitracin = triple ABX ointment

  5. Protein Synthesis Agents • Aminoglycosides • Interfere with the production of protein in bacterial cells • Are a specialized group of antibiotics with a broad spectrum of activity, used for gram-negative bacteria. Pneumonia • Aerobic bacteria, bactericidal • Are not absorbed well from the GI tract, so are given parenterally • Suffix –micinor –mycin(but are not the only group to use these suffixes e.g. clindamycin, erthromycin) • T1/2 = 2-5 hours still SID for safety • Penicillin's (cell wall) will enhance their activity • Cross resistance not as common as penicillins

  6. Protein Synthesis Agents • Aminoglycosides • Pharmacokinetics • Hydrophilic mostly parenterally • Well absorbed in GI through neonates, haemorrhagic, necrotic intestine • Well absorbed locally if skin is denuded/ abraded • Parenterally > ECF: volume of distribution is larger in neonates/ young animals > low drug conc. Most diluted in ECF • Not good for brain/ eye infections systematically • Accumulate: bronchioles, kidneys, inner ear, cross placenta • Eliminated in Kidney • Not effective with cellular debrie.g pus, dirt, fecal material, anaerobic conditions: deep wounds, abscess

  7. Protein Synthesis Agents • Aminoglycosides • Side effects are nephrotoxicity and Ototoxicity: cats sensitive, vestibular signs; circling, head tilt, nystagmus • Increase dosage interval for safety • Monitor: BUN and serum creatine (70-75% kidney damage), urine sediment, urine SPG • Early signs: casts or increased protein - urine • Examples include gentamicin, neomycin, amikacin, streptomycin, kanamycin, netilmicin, tobramycin, and dihydrostreptomycin • Don’t use in pregnant animals, geriatric patients, dehydration, shock or kidney disease • NOT approved for use in food-producing animals. Granular casts

  8. Protein Synthesis Agents • Tetracyclines (Oral and parenteral forms) • Are a group of bacteriostatic antibiotics with a broad spectrum Ab. • Rickettsial agents: Treats Lyme disease (borreliosis), Ehrlichia - dogs, Hemobartonella – dogs/ cats, RMSF • Salmon poisoning – dogs • Mycoplasma pneumonia • Chlamydial infections: ocular infections – feline • Psittacosis – birds • Epiphora: canine? • Are recognized by –cyclinesuffix • Examples include • Older tetracyclines: hydrophilic - tetracycline, oxytetracycline • Newer tetracyclines: lipophilic - chlortetracycline, doxycycline, and minocycline. Longer t1/2, more broad spectrum, better penetration

  9. Protein Synthesis Agents • Tetracyclines: Don’t use with penicillins/ cephalosporins • Pharmacokinetics • Doxy/minocycline: lipophilic better oral absorption • Only 20% not absorbed with chelators, not significant • Penetrate brain, eye and prostate better than older drugs • Doxycycline uses: CNS signs related to Lyme disease/ excreted in GI hence ok to use in kidney cases. GIVE WITH FOOD • IV Doxycycline: DON’T GIVE TO HORSES, cardiac arrhythmias, collapse, death • Oxt/tetracycline: hydrophilic. Expired products > FANCONI syndrome (glycosuria) • Readily chelated (bound and ppt. out of solution) by minerals with divalent cations • Ca++, Mg ++ , Fe ++, Cu ++ • If given with milk products, antacids (Mg ++), Fe ++ supplementation, andidiarrheal: kaolin/ pectin, bismuth subsalicylate (Pepto-Bismal): chelate drug in GI and drug is not absorbed • Oxytetracycline: IM – LA 200 q 2-3 days • Tetracycline: PO – SE: superinfection, v/d, anorexia. Cat’s tolerate this less: fever, depression, abdominal pain • Excreted mostly by kidney than liver • Young animals: chelate Ca++ yellow, mottled teeth also combine with Ca++ and slow bone development

  10. Protein Synthesis Agents • Chloramphenicol: bacteriostatic • Is a broad-spectrum antibiotic that penetrates tissues and fluids well (including the prostate, eyes and CNS) and rickettsiae • Has toxic side effects (bone marrow depression: myelosuppression) that extremely limit use: aplastic anemia – human • In cats kidney function is essential to use this drug • Use caution when handling this product • Chloramphenicol is the only drug in this category • Also available in ophthalmic solution • Banned from use in food-producing animals. • Don’t use in pregnant animals and use with caution in neonate kittens • Concurrent use with phenobarbital and primidone will make them toxic in the body • Not considered a first-line drug

  11. Protein Synthesis Agents • Florfenicol (Nuflor®): Newer. Bacteriostatic • Is a synthetic, broad-spectrum antibiotic • Injectable solution • Used to treat bovine respiratory disease complex (shipping fever)and foot rot. • 2 injections: IM every 48 hrs. apart • Drug withdrawal: 28 days • Side effects include local tissue reaction (possible loss of tissue at slaughter), in appetence, decreased water consumption, and diarrhea • Florfenicol is the only drug in this category • Don’t use in breeding animals

  12. Protein Synthesis Agents • Macrolides • Interfere with the production of protein in bacterial cells • Are broad-spectrum antibiotics that have a large molecular structure. Don’t penetrate CNS. • Used to treat penicillin-resistant infections or in animals that have allergic reactions to penicillins • May cause stomach upset in animals • Erythromycin (oral or ointment): In foals – respiratory disease can get superinfectionhence add probiotics. Label – foal RhodococcusEqui. In adult horses and ruminants (oral): diarrhea • Tylosin (used mainly in livestock even though labeled by dogs/cats - can cause fatal diarrhea in horses) • Tilmicosin (Micotil: SQ: used to treat bovine respiratory disease – single injection). Can cause death IV and irritating IM. Toxic: horses, primates, swine, humans (farmers who inject themselves accidentally or in the eyes) • Azithromycin (Human drug: Zithromax): Mycoplasma - FRDC • SE: Intestinal cramping, abdominal pain, diarrhea, suprainfection(erythromycin and Azithromycin)

  13. Protein Synthesis Agents • Lincosamides: -cidal/static • Interfere with the production of protein in bacterial cells • Are narrow-spectrum, gram-positive aerobic cocci antibiotics e.g. Staphylococcus aureus • Side effects include GI problems • Veterinarians typically use erythromycin instead. • Examples • Clindamycin (Antirobe): Label – dogs. Anaerobic bacteria, deep pyoderma, abscess, dental infections, bite wounds, osteomyelitis • Pirlimycin (Pirsue): 36 hrs - milk; 28 days - meat • Lincomycin: Label – dogs, cats, swine, poultry • DON’T USE rabbits, hamsters, guinea pigs, horses, ruminants: severe GI effects and also death • SE: v/d and bloody diarrhea • Nursing kittens/ pups: DIARRHEA • With Kaopectate (antidiarrheal) cannot absorb drug: hence give lincomycin first than 2 hours later antidiarrheal.

  14. Nucleic Acid Agents • Fluoroquinolones/ Quinolones: -cidal • Are antibiotics with fluorine bound to the quinolone base, which increases the drug’s potency, spectrum of activity, and absorption • Disrupt DNA gyrase of bacteria • Are broad-spectrum antibiotics (gram + and gram -): Staphylococcus, Pseudomonas (better than Gentamicin), Klebsiella, Escheria coli, Salmonella spp. DON’T use for Streptococcus spp. Or anaerobes • –floxacin suffix • Examples • Enrofloxacin (Baytril): 1st . Label: dog/cat. 5mg/kg • Ciprofloxacin: Label – human. • Marbofloxacin (Zeniquin). Label: dog/cat • Orbifloxacin (Orbax). Label: dog/cat • Difloxacin (Dicural) • Sarafloxacin (Saraflox). Label – poultry, REMOVED • Nalidixic acid (older) and norfloxacin (human): not often used in vet medicine

  15. Nucleic Acid Agents • Fluoroquinolones/ Quinolones: -cidal • Pharmacokinetics • Effectively absorbed from GI tract dogs/cats • Uses: severe skin infections (pyoderma), respiratory tract, urinary tract, prostate infections • Extralabel use in horses: caution in foals • Avoid with antacids and sucralfate or give 4 hours later • They can exacerbate seizures • SE: • bubble-like cartilage lesions in growing dogs (X5 times higher dosage): contraindicated in small/medium size dogs of 2-8 months age Large breeds: 12 months; giant breeds: 18 months • Crystalluria • Quinolone-induced blindness in cats (>20mg/kg). • Indiscriminate use may result in bacterial resistance: Staphylococcus and Pseudomonas spp. Hence reserve for severe infections • Can’t use in food animals

  16. Antimetabolites • Sulfonamides and Potentiated sulfonamides: -static/cidal • Are broad-spectrum antibiotics that inhibit the synthesis of folic acid (needed for the growth of many bacteria) • Enteric forms or systemic forms • Examples; Bactericidal when potentiated with trimethoprim or ormetoprim; gram + organisms: Streptococci, Staphylococci, Nocardia, Coccidia, Toxoplasma and Chlamydia • sulfadimethoxine/ormetoprim (Primor) • sulfadiazine/trimethoprim (Tribrissen) • sulfadimethoxale/trimethoprim (human drug: Septra) • Sulfachlorpyridazine(livestock and poultry) • Sulfasalazine (Azulfidine: IBD). Enteric. More antiinflammatory effect esp in colon as aminosalicyclic acid. Caution in cat

  17. Antimetabolites • Sulfonamides and Potentiated sulfonamides: -static/cidal • Pharmacokinetics • Absorbed in monogastric GI tract • Mostly lipophilic: prostate, pleura, CSF, ocular, UTI • Don’t use in pregnant/ lactating animals • Excreted: kidney • SE: crystalluria (esp. older drugs), KCS (dry eye): can be irreversible, and skin rashes (most common): pruritus, hives, swelling – face esp. Doberman pinschers, thrombocytopenia, leukopenia, anemia, cats: profuse salivation • Precipitate in kidneys of animals that are dehydrated or have acidic urine; Adequate water intake = very important! • Antibiotic resistance

  18. Miscellaneous Agents • Nitrofurans • Are broad-spectrum antibiotics that include furazolidone, nitrofurantoin (Furadantin), and nitrofurantoin • Used to treat wounds (topically) and urinary tract infections (not 1st choice) • Filtered unchanged through kidneys • Carcinogenic residues in animal tissues • Nitroimiazoles • Have antibacterial and antiprotozoal activity; work by disrupting DNA and nucleic acid synthesis • An example is metronidazole, which is considered by some the drug of choice for canine diarrhea

  19. Metronidazole (Flagyl®) is drug of choice for canine diarrhea • Disrupt syn. of DNA and nucleic acids. Works with anaerobic bacteria • Used to treat Giardia, Entamoebahistolytica, Balantidiumand Trichomonasinfections, deep puncture wounds • Also used for amoebiasis and anaerobic bacteria • Oral or intravascular administration • No approved veterinary form of metronidazole (used off-label) • Do not use in pregnant animals • SE: Can cause neurologic signs: vestibular signs, tremors, seizures with oversose/ long periods of time

  20. Miscellaneous Agents • Rifampin: -cidal/static • Disrupts RNA synthesis by inhibiting RNA polymerase • Is broad-spectrum; used in conjunction with other antibiotics (usually erythromycin) for Corynebacteriumequi (Rhodococcusequi), Staphylococcus infections, Fungi?: Histoplasma, Aspergillus, Blastomyceswith amphotericin B • May impart a reddish color to urine, tears, sweat, and saliva. • Increases metabolism of: propranolol, quinidine, chloramphenicol, diazepam, zolazepam(Telazol), phenobarbital, pentobarbital, prednisone and dexmathasone

  21. References • Romich, J.A. Pharmacology for Veterinary Technicians, 2nd edition. 2010. • Bill, R.L. Clinical Pharmacology and Therapeutics for the Veterinary Technician, 3rd edition. 2006. • http://ahdc.vet.cornell.edu/clinpath/modules/ua-rout/castssed.htm

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