Small Animal Antibiotic Therapy
Antibiotic therapy in small animals is generally similar to that seen for humans.  Some specific instances where antibiotic therapy would be appropriate include canine borreliosis (lyme disease, which may also occur in cats, horses, or cows, although less frequently), characterised by lameness, lymphadenopathy, and secondary anorexia and fatigue; renal failure in cats leading to urinary tract infection, or infected wounds of any small animal.  Other infections, less common, in dogs include canine ehrlichiosis which is also obtained through ticks (it resembles Rocky Mountain spotted fever producing blood dyscrasias, lymphadenopathy, anorexia, loss of stamina, depression -- treatment of choice is doxycycline) and canine leishmaniasis which produces cachexia, alopecia, hepatomegaly, and splenomegaly (the treatment of choice is antimony, which can be toxic).
 

Specific Classes of Antibiotics with relevant small animal problems
Penicillins -- inhibit bacterial cell wall synthesis
Guinea pigs, chinchillas, birds, snakes, and turtles are sensitive to the local anæsthetic actions of procaine Pen G -- it should not be used in these species.  High dose or chronic use of penicillins may cause ataxia, tachypnœa, dyspnœa, œdema, and tachycardia in dogs.  Princillin® is a small animal formulation of ampicillin.

Cephalosporins -- inhibit bacterial cell wall synthesis
Chronic use of cephaloridine specifically can cause anæmias in cats.  Cefadroxil (Cefa-tabs®) is commonly used in canines.  High doses (100 mg/Kg/day) of ceftriaxone (Rocephin®) may cause a "sludge" to build up in the bile of dogs.

Aminoglycosides -- inhibit bacterial ribosomal protein synthesis
Cats are especially sensitive to the ototoxicity of the aminoglycosides.  Additionally, caution should be used with aminoglycoside use in working dogs, due to potential hearing impairment.

Neomycin in combination with isopropamide and prochlorperazine (Neodarbazine®) is used in dogs for GIT infections.

Tetracyclines -- inhibit bacterial ribosomal protein synthesis
Same effects as in humans.  Cats typically have a low tolerance to oral tetracyclines (especially tetracycline and oxytetracycline), with the drug producing colic, fever, hair loss, and depression.  Chlortetracycline (Aureomycin®, CTC-50®, Pfichlor®) is often used in catteries where individual treatment is impractical.  In these instances, the powder is place in food (50 mg/day/cat) for one month for the treatment of conjunctivitis.

Doxycycline (human Vibramycin®) is especially useful for the treatment of canine ehrlichiosis and borreliosis.
Tetracycline (Panmycin®) is available as a 100 mg/ml solution in 15-30 ml dropper bottles for use in small animals.  Tetracycline is also available in combination with novobiocin (Albaplex®) for use in dogs.

Chloramphenicol -- inhibit bacterial ribosomal protein synthesis
Same effects as in humans.  Viceton® tablets are available for the treatment of GIT infections in dogs.

Macrolides and similar antibiotics -- inhibit bacterial ribosomal protein synthesis
Same effects as in humans.  The pseudomembranous colitis and overall GI effects of clindamycin (Antirobe®) may cause death in rabbits, hamsters, and Guinea pigs.  It is sometimes used to treat toxoplasmosis in cats.

Sulphonamides -- PABA antagonists to decrease DNA function
The use of pyrimethamine in small animals may cause anorexia, malaise, vomiting, depression, and blood dyscrasias.  The combination of trimethroprim/sulphadiazine or sulphamethoxasole may cause eye damage and hypersensitivity (especially Dobermans) in dogs.  Additionally, in dogs, the sulphonamides may cause follicular thyroid hyperplasia.  They may also retard wound healing if used topically.  A commonly used sulpha combination in veterinary small animal practice is sulphadimethoxine/ormetoprim (Primor®).

Fluoroquinolones -- inhibit DNA function
Same effects as in humans.  Enrofloxacin (Baytril®) and other fluoroquinolones are contraindicated in the treatment of young (< 8 months) dogs, due to articular cartilage changes leading to joint destruction.

Metronidazole -- inhibits bacterial protein function
Used primarily for the treatment of giardiasis in small animals.  In dogs, metronidazole may cause neurotoxicity including tremors, muscle spasms, ataxia, and convulsions.

Miscellaneous Antibiotics Used in Small Animals
Nitrofurantoin and Methenamine are used in the treatment of urinary tract infections in small animals.

Anthrax (Bacillus anthracis) or Charbon -- Recent cases have been reported in South Dakota, Nebraska, Arkansas, Mississippi, Louisiana, Texas, and California.  Infection is characterised by fever, CNS excitation followed by CNS depression, staggering, trembling, cardiorespiratory collapse, and death within 2-3 days (or possibly sooner).  Upon death, rigor mortis is rare or incomplete.  Necropsy indicated extensive hæmorrhaging and clotting.  Treatment includes penicillin or oxytetracycline.  Additionally, there is a live bacterial vaccine for anthrax.

Brucellosis (Brucella abortus) -- Infection may cause abortion in females and orchitis in males.  Infection animals may often demonstrate normal gestation after an initial abortion.  No effective treatment exists but a vaccination is available to prevent infection.  If animals are infected, generally the entire herd is tested and infected animals must be slaughtered to prevent further infection.  In man, brucellosis presents as undulant fever (treatment with tetracycline and either streptomycin or gentamicin).

Tuberculosis (Mycobacterium bovis) -- In cattle this presents as a pulmonary infection while in chickens it is primarily a gastrointestinal infection.  Unlike human tuberculosis, treatment in cattle is rarely effective (isoniazid is the most commonly employed agent) and the infected animals must be slaughtered.

Tularemia (Francisella tularensis) -- Primarily a disease of sheep, vectors of tularemia include ticks and the deer fly.  A 1-10 day incubation period is followed by fever, lethargy, stiffness, tachycardia, tachypnœa, and coughing progressing to diarrhœa.  Death may occur within hours or days of the onset of signs and symptoms.  Treatment is streptomycin, gentamicin, or the tetracyclines.

Lumpy Jaw (Actinomycosis) -- An infection of cattle, characterised by swelling of the maxilla and mandible causing loose teeth, decreased mastication, and ultimately emaciation.  Treatment is penicillin.

Clostridial infections --

Red Water Disease (Port Wine Urine) -- Primarily occurs in cattle, causing liver damage and dark red coloured urine.  Treatment is penicillin.

Black Leg -- Occurs especially in beef cattle, causes muscle swelling with the muscle appearing dark-red or black, spongy, with a sweetish odour, and infiltrated with small bubbles.  There is no effective treatment.  Vaccination may prevent infection.

Clostridium perfringens -- Lamb dysentery, struck (sheep), scours (pig) -- a gastrointestinal infection causing diarrhœa, abdominal pain, and convulsions.  Prognosis is variable with death occurring within a few hours in some animals while others may show a complete recovery.  The best treatment is vaccination of pregnant dams to prevent infection.
 

Erysipelas -- Primarily an infection of swine, it may cause acute septicæmia, skin lesions (redness to diamond-shaped lesions), chronic arthritis, vegetative endocarditis, and fever (104-108oF).  Additionally, the animals may exhibit behavioural signs such as walking stiffly on their toes, lying on their sternum, and squealing when handled.  A vaccine does exist, however if infection occurs, penicillin is the treatment of choice.

Listeriosis (Circling Disease) -- This is primarily a winter-spring disease of sheep, cattle, goats, and pigs.  It comprises an encephalitis with death within 4-48 hours in sheep and goats and within 4-14 days in cattle.  The infected animals will typically crowd into corners, push their head against fences/walls/trees, walk or stumble in a circle (always in the same direction), and facial paralysis (causing them to appear blind and also causing a nasal discharge).  The drug of choice is penicillin, however erythromycin, chloramphenicol, and the tetracyclines are also effective.

Equine Ehrlichiosis -- This is a non-contagious disease of late fall, winter, and early spring, occurring primarily in Northern California, Colorado, Illinois, Florida, Arkansas, Washington, and Pennsylvania.  It is characterised by fever, CNS depression, limb œdema, ataxia, and regional vasculitis (primarily in the legs).  Treatment is oxytetracycline.

Glanders (Pseudomonas) -- a respiratory infection of horses (the cutaneous infection is referred to as "farcy"), the treatment of choice is sulphadiazine.

Mastitis -- Infection of the mammary glands and/or teats is usually first detected by watery milk with a few flecks or large yellow clots.  The appearance of the milk may progress to brownish with fine flakes of clots.  If the infection is allowed to progress, the mammary gland may permanently loose its productiveness.  Mastitis may present as either wet (in lactating cattle) or dry (non-lactating cattle).  Typically, dry mastitis responds to treatment better than wet mastitis.  The treatment of choice is penicillin, although other drugs are also effective (including erythromycin and sulpha combinations).  The drug may be administered either systemically (erythromycin is better distributed to the milk that penicillin with systemic dosing) or directly into the teat with a specially-adapted syringe.  In dry mastitis, milk out the affected quarter, clean and disinfect the teat, infuse the medication, close and massage the teat and udder.  It wet mastitis, the above procedure is employed after each milking for at least three treatments.

Vaginitis -- Infection of the vagina may commonly occur in large animals.  Often, the antibiotic is infused directly into the vagina by a procedure that is very similar to gravity enemas.  Systemic dosing is also employed.

Aminoglycosides -- inhibit the 30S subunit of bacterial ribosomes.
Amikacin (Amiglyde-V®) -- often given intrauterine for PID, it is available in 48 ml vials (250 mg/ml).  It has a 2 yr stability at room temperature (if the solution has discoloured to a pale yellow it is still acceptable for use).

Gentamicin (Garacin®) -- available as a swine soluble powder, pig pump oral solution, or injection, withdrawal times range from 3 to 10 to 40 days.

Neomycin -- This drug is extremely ototoxic and nephrotoxic.  It is used primarily for gastrointestinal infections (it has very limited from the GI tract).  Biosol® is available as oral solution or drops and as a feed additive.  Neomycin is also used in combination with sulphamethazine, vitamins A and D, and niacin (Sulkamycin S®, Methpyrin®, Neo-175®).
 

Aminocyclitols -- Inhibit the 30S subunit of ribosomes
Apramycin (Apralan®) -- used for swine bacterial enteritis, the soluble powder may be placed in water (375 mg/gallon of drinking water).  A medicated feed is also available (75 G/lb of feed).  Withdrawal time for apramycin is 28 days.

Spectinomycin -- Used primarily in swine for white scours (Spectam®, Scour-Halt®)

Macrolides -- Inhibit the 50S subunit of ribosomes.
Erythromycin -- The use of erythromycin should be avoided in adult horses (in adult cattle and foals it may cause diarrhœa, however in adult horses it may cause life-threatening GI effects).
Erythromycin is specially formulated for wet (Erythro-36®) or dry (Erythro-dry®) mastitis.  Withdrawal times are 36 hrs for milk and 14 days for slaughter.
Tilmicosin (Micotil®) -- parenteral only, this drug is used primarily for respiratory Pasturella infections, since it is highly concentrated in the lungs.  It may produce cardiovascular toxicity and should be administered slowly (rapid administration will increase the cardiac effects).

Tylosin (Tylan®) -- Tylosin may be used in small animal colitis, but is more often used in large (non-lactating) animal enteritis.  It is also used in fowl in a soluble form (to be added to water).

Lincosamides
Lincomycin -- Similar to clindamycin in its actions and chemistry, lincomycin is commonly used in porcine infections.  It is available as a soluble powder (40 Gm with 16 Gm of activity) packs that are added to 64 gallons of drinking water.

Pirlimycin -- Used primarily for wet mastitis, Pirsue® gel in a 50 mg/10 ml teat syringe is administered directly into the teat.  Milk withdrawal time is 36 hrs.

Penicillins -- Inhibit transpeptidase and interact with penicillin-binding proteins
Amoxicillin -- available in 62.5 mg/syringe for wet mastitis (Amoxi-Mast®) and standard dosage forms (Robamax-V® and Amoxi-Bol®).

Ampicillin -- Polyflex® injectable and Amp-Equine® injectable for numerous types of infections.

Hetacillin -- Hetacin-K® is available for oral or intramammary (wet mastitis) administration.

Ticarcillin -- Ticillin® is available as a sterile powder for intrauterine infusion.

Cephalosporins -- Mechanism of action similar to penicillins.
Ceftiofur (Naxcel®) -- A third generation cephalosporin used for bovine shipping fever (a Pasturella infection).  The dose is 1.1-2.2 mg/Kg intramuscularly once daily for 3-5 days.  There are no withdrawal times when used as labelled.

Cephalexin (Ceporex®) is available as an oil-based suspension for injection in large animal practice.

Cephapirin -- Used in mastitis.  Cefa-Lak® (200 mg/10 ml) is used for wet mastitis with milk and slaughter withdrawal times of 96 hrs.  Animals should not be milked at all for 12 hrs after dosing.  Treatment may be repeated, if needed.  Cefa-Dri® (300 mg/10 ml) is used for dry mastitis with a milk withdrawal time of 72 hrs post-calving and a slaughter time of 42 days.  It should not be administered if the animal is  30 days ante-calving.

Tetracyclines -- bind to and inhibit the 30S subunit of ribosomes
Oxytetracycline (Terramycin®, Liquamycin®, Oxyvet®, Oxyject®, Oxytet®) is used extensively in large animal practice.  It may be purchased without a prescription at many co-ops or pharmacies.  It is administered parenterally or orally as tablets and in food or water.  It is available as both tablets (scours tablets) and powder for reconstitution as well as solution for injection.  Oxytetracycline is approved for non-lactating cattle and other species.  Slaughter withdrawal times are 15-28 days for cattle, 20-26 days for swine, and 5 days for fowl.

Tetracycline (Polyotic® and Panmycin® Bolus) is available as bolus tablets or soluble powder for reconstitution in large animals.

Clioquinol (Rheaform® Bolus) -- This drug acts as an antibiotic, antifungal, and amœbocidal agent by interferring with DNA function (possibly by intercalation with DNA).  It is used primarily in the treatment of chronic unresponsive diarrhœa in horses.

Sulphonamides -- PABA antagonist that interfere with DNA function

Sulphachlorpyridazine (Vetisulid®) -- used primarily for the treatment of diarrhœa and respiratory tract infections.  Available as bolus, powder, and suspension, for use in swine and sheep.

Sulphadiazine/Trimethoprim (Tribrissen®) is available as tablets, suspension, paste (equine), and injectable.  Used for numerous infections including mastitis.

Sulphadimethoxine (Alban®, Bactrovet®) is available as tablets, suspensions, boluses, soluble powders, and concentrated solutions (for addition to water).  It is used primarily for the treatment of upper respiratory tract infections and calf diphtheria.

Miscellaneous Large Animal Antibiotics
Furazolidone (Furoxone®) -- inhibits bacterial enzymatic systems, it is the second drug of choice for enteritis.  It may also be used in the treatment of vaginitis and urinary tract infections.  It may cause head tremors, ataxia, and convulsions in small calves (especially Jerseys).  The use of furazolidone is contraindicated in any animal intended as a food source, since it may be carcinogenic.

Nitrofurantoin -- acts in a manner similar to furazolidone, it is used in horses for UTI and vaginitis and is available as an equine infusion.

Metronidazole -- forms a reactive intermediary that interferes with bacterial and protozoal proteins.  Although not approved, metronidazole is used in the treatment of anærobic infections in horses.

Novobiocin -- A narrow spectrum antibiotic with primarily Gram positive activity.  It produces GI side effects.  DryGard® (400 mg/10 ml suspension, should be shaken well prior to use) is used in the treatment of dry mastitis.  Special Formula 17900-Forte® is a combination of novobiocin and penicillin G for use in wet mastitis (milk withdrawal time is 72 hrs).  Albamix® is a formulation of novobiocin for fowl, to be administered as a food/water additive.  Slaughter time in fowl is 4 days.

Rifampin -- inhibits RNA polymerase to decrease RNA/DNA function.  Used primarily in combination with erythromycin for the treatment of Rhodococcus equi infections in foals.

Tiamulin -- binds to the 50S subunit to inhibit bacterial protein synthesis.  Used primarily in the treatment of swine respiratory tract infections (Hæmophilus) and swine dysentery (Treponema).
Tiamulin is available as soluble packets and as a food premix (Denagard®).  Slaughter times range from 3-7 days (dose-dependent).  Animals may exhibit weight gain following therapy.  Other, less common, side effects include redness of skin.  Toxicity of tiamulin generally presents as CNS depression and salivation.