Antibiotics Commonly Used In Animal Shelters

Introduction to antibiotic usage in animal shelters

Antibiotic use is very important when medically indicated for the treatment of susceptible pathogens (usually bacterial.) However, in shelters, there are some major concerns about antibiotic use. First among the concerns is that fact that the most common diseases frequently observe in sheltered animals are viral in origin; Calicivirus, herpesvirus, and panleukopenia are major pathogens that threaten the health of sheltered cats, and in dogs parvovirus and distempervirus are diseases of great concern. All of these diseases are caused by viruses, none of which are susceptible to antibiotic therapy. In addition, widespread antibiotic use may reduce the animals' innate flora (colonization) resistance, actually worsening outbreaks of disease. Finally, widespread use of antibiotics may promote the evolution of resistant strains of bacteria. Animal shelters play a role in public health, and there is a lot of concern in the human medical profession about the veterinary profession's use of antibiotics.

The purpose of this primer is to review the rationale use of antibiotics in shelters, and to remind you of the mechanisms of action of some classes of antibiotics. It is important to note that by law antibiotics can only be prescribed to pet animals by a licensed veterinarian.

Documenting that an infection is there

You should only treat a dog or cat with antibiotics if you are pretty sure that an infection is present. The best way to determine if an infection is present is through a laboratory culture of an appropriate sample; for more information on which samples to submit please visit our diagnostic sampling page. Culturing has the additional advantages of confirming the identity of the bacteria and determining which antibiotics are appropriate choices for treatment. A temporary expedient while waiting for cultures is to look at a smear of a sample taken from the infected area under a microscope. This is important, because it helps you determine if there are several bacteria in a sample or if there are bacteria that are not growing. The next "best" indication of infection would be if the animal is sick in a way that suggests an infection: fever, elevated white blood cells, inflammation, x-rays or other tests with results that suggest certain types of infection. Finally, PCR and other DNA-based identification tests are great tools to use to determine if an infection is present (and can be useful for bacteria that cannot be grown in the lab) However, with this method you do not obtain an isolate for drug-testing and dead bacteria are often still PCR-positive.

Adverse effects of antibiotics

There are three main problems with using antibiotics. One is direct medical side-effects such as toxic effects (e.g. aminoglycoside antibiotics are toxic to kidney cells) or allergies (which can be life-threatening). The second is that your antibiotic could kill the normal flora and leave the patient more vulnerable to pathogens (often which are resistant to antibiotics). The last problem is that using antibiotics genetically selects for antibiotic resistance in your bacteria. It may develop if the bacteria has a genetic mutation, but this is an uncommon source of the problem. More commonly, other bacteria in the environment share genes that code for resistance on plasmids. If bacteria are spreading plasmids to other bacteria in the environment, they pose a risk to any animal the bacteria could infect (including humans). Whenever you use an antibiotic, you are applying artificial selection for resistance genes.

Minimizing adverse effects

You should only treat a dog or cat with antibiotics if you are pretty sure that an infection is present. The best proof of this is laboratory culture. This has the additional advantages of confirming the identity of the bacteria and allowing for antibiotic susceptibility testing. A temporary expedient while waiting for cultures is to look at a smear of the infected area under a microscope. This is important, because it helps you know if there are several bacteria in a sample or bacteria that aren't growing. The next "best" indication of infection would be if the animal is sick in a way that suggests an infection: fever, elevated white blood cells, inflammation, or x-rays or other tests with results that suggest certain types of infection. Finally, PCR and other DNA-based identification tests are great to tell if an infection is present (and can be useful for bacteria that can't be grown in the lab) but you never get an isolate for drug-testing and dead bacteria are often still PCR-positive.

More on drug resistance

Drug resistance and drug susceptibility occupy opposite ends of a spectrum. Along the spectrum are relative degrees of resistance and in many cases, if you can get more drug to an infected site, you could get a borderline resistant bacteria to die. So it is very important to use a drug to which the bacteria is relatively susceptible but also which will penetrate to the place where the infection is. The choice of drug for the bacteria to be susceptible is motivated either by culturing the bacteria in the laboratory and testing drugs to kill the bacteria (culture and susceptibility or C/S) or by knowing typical patterns of susceptibility, e.g. that most Pseudomonas bacteria are resistant to penicillin. If a cat is being treated repeatedly or for a prolonged period of time, it is helpful to get a C/S. The main reasons we don't always to C/S is because we know that almost all urinary tract infections respond very well to most broad-spectrum antibiotics (for example) or because we can't or won't pay for it.

Particularly good drugs for particular problems in shelter animals

Penicillins: Cidal, disrupt cell wall. Spectrum: broad except bacteria with specific resistance. Main use in cats: general use. Side-effects: allergy, fever, rash, loss of white blood cells, anemia, GI upset.

  • Amoxicillin: comparable to ampicillin but absorbed from gut better.
  • Ampicillin: better for some gram-negative bacteria than Pen G but not as good for anaerobes. Penicillin G, procaine pen G (long-acting injection)
  • Penicillin V: slightly less effective than Pen G but better absorbed when given orally.
  • Oxacillin: for penicillin-resistant Staphylococci.
  • Amoxicillin/clavulanate: overcomes much resistance, restores broad spectrum.
  • Ticarcillin: injectable, especially for severe Pseudomonas.

Aminoglycosides: Cidal, inhibit protein synthesis. Spectrum: gram-negative, some gram-positive bacteria. Main use in cats: severe or resistant gram-negative bacteria, must be injected (except neomycin). Side-effects: severe  kidney damage, hearing loss, facial swelling, nerve damage.

Amikacin, Gentamycin, Neomycin: Not given injectably because of severe kidney damage, not absorbed into bloodstream if eaten/used as enema/applied to skin but will treat bacteria in GI tract, on skin.

Cephalosporins: Cidal, disrupt cell wall. Spectrum, first-generation: gram-positive bacteria, anaerobic bacteria.  Spectrum, third generation: excellent gram-negative, some gram-positive. Main use in cats: skin disease,  susceptible infections. Side-effects: allergy (cross-reactions with penicillins), diarrhea

  • Cefadroxil (Cefa-tabs): 1st gen.
  • Cephalexin (Keflex): 1st gen.
  • Cephalothin (Keflin): 1st gen, injectable.alothin (Keflin): 1st gen, injectable.
  • Ceftiofur (Naxcel): 3rd generation, injectable.

Tetracyclines: Static, inhibit protein synthesis. Spectrum: broad-spectrum (but many resistant bacteria), rickettsias, other bacteria that live in cells. Main use in cats: hemobartonella, susceptible bacteria. Side-effects: GI upset, enamel discoloration, liver/kidney disease, hair loss, photosensitivity, esophageal stricture from doxycyline unless given as liquid.

  • Doxycycline: fewer doses/day, different target organs, reduced likelihood of enamel discoloration.

Clindamycin (Antirobe): Static or cidal, disrupts protein synthesis. Spectrum: gram-positive bacteria, anaerobic  bacteria. Main use in cats: dental disease, abscesses, diarrhea. Side-effects: GI upset.

Erythromycin: Mostly static, inhibits protein synthesis of bacteria. Spectrum: gram-positive bacteria, rickettsias, Chlamydophila, et al. Main use in cats: Giardia, anaerobes, diarrhea. Side-effects: neurological problems, white blood cell reduction, liver damage, blood in urine, vomiting and diarrhea.

Tylosin (tylan): Static, inhibits protein synthesis of bacteria. Spectrum: variable. Main use in cats: diarrhea, inflammatory bowel disease. Side-effects: few.

Enrofloxacin (baytril): Cidal, inhibits DNA gyrase, synthesis. Spectrum: gram-negative bacteria, Brucella,  Chlamydophila, Staph, Mycoplasma. Main use in cats: various infections resistant to other antibiotics. Side-effects: cartilage damage in young animals, urine crystals, GI disease.

Sulfa drugs: Static, interferes with enzyme systems essential to normal metabolic and growth patterns. Side effects: nausea, vomiting, fever, anemia, leukopenia and irritation of the liver or kidneys.

  • Trimethoprim sulfa (diazine or methoxazole)(TMS, SMX-TMP et al.): sulfa is static, combo is cidal, block thymidine (an important enzyme) in the bacteria. Spectrum: broad, many bacteria, some protozoa. Main use in cats: many infections. Side-effects: dry eye, liver disease, GI disease, anemia, allergy.

Metronidazole (Flagyl): cidal, disrupts DNA synthesis? Spectrum: anaerobic bacteria, some protozoa (Giardia, amoebas). Main use in cats: Giardia, anaerobes, diarrhea. Side-effects: neurological problems, white blood cell reduction, liver damage, blood in urine, vomiting and diarrhea.

Rifampin (Rifadin or Rimactane): cidal or static, inhibits RNA polymerase. Spectrum: intracellular bacteria  (Mycobacteria, Staph, Rhodococcus, Chlamydophila et al.), some fungi, some viruses. Main use in cats: with   anti-  fungals in brain fungal disease. Side-effects: discolored tears and urine, GI upset, liver damage (very high doses).

Commonly used antibiotics in animal shelters

Antibiotic susceptibility patterns for positive bordetella cultures submitted to the Veterinary Medical Teaching Hospital (VMTH) at Davis
  Shelter samples *VMTH Samples  
(including all shelter samples)
 
Antibiotic Dogs Cats Dogs and Cats VACCINES
Amoxicillin/clavamox 81.2%(16) 29.1%(55) 59%(78) 100%(3)
Ampicillin 28.6%(14) 3.6%(56) 7.6%(79) 0%(3)
Timentin 85.7%(14) 66.7%(57) 71.1%(76) 100%(3)
Amikacin 100%(7) 82.5%(57) 85%(80) 100%(3)
Gentamicin 100%(17) 87.7%(57.7) 90%(80) 100%(3)
Chloramphenicol 68.8%(16) 80.4%(51) 75.3%(73) 100%(3)
Enrofloxacin 87.5%(16) 93%(57) 90.5%(74) 100%(3)
Tetracycline 100%(16) 98.3%(58) 98.8%(80) 100%(3)
Trimethoprim Sulfa 70.1%(17) 94.7%(57) 87.5%(80) 100%(3)

*Only a small number of positive Bordetella cultures are listed here. We are currently working on a comprehensive listing of all Bordetella positive cultures tested for sensitivity in the VMTH

Decision-tree for choosing and using antibiotics in an animal shelter

As always, re-access patients regularly to determine if antibiotic chosen is effective. Duration of illness and clinical impression of an antibiotic's effect should be tracked for each animal to determine what antibiotics are most successful.