Antibiotic Therapy in Reptiles

Transcription

1 Antibiotic Therapy in Reptiles Kevin Eatwell, BVSc (Hons), DZooMed (Reptilian), MRCVS, RCVS Recognized Specialist in Zoo and Wildlife Medicine Birch Heath Veterinary Clinic, Birch Heath Road Tarporley, Cheshire, CW6 9UU, United Kingdom A b s t r a c t : Antibiotics are the most commonly administered therapy to reptiles. A logical approach to therapy is important and this encompasses quantifying the need for therapy, followed by an appropriate choice o f agent. Antibiotics need to be given at an appropriate dose, frequency and route, based on current information available. It is important to keep patients at their optimal temperature (To) to maximize the effectiveness of therapy. There are three potential methods for obtaining an appropriate dosage. Firstly, an empirical dose based on experience with perceived effectiveness can be considered. Secondly, metabolic scaling can be used to estimate a likely effective dose. Thirdly, dose rates derived from pharmacokinetic studies should be consid ered when treating other similar reptiles. This review reports on the pharmacokinetic data that exists for antibiotics in reptiles and the spectrum o f activity for each agent. The effects o f the renal portal system are also briefly discussed. K e y w o r d s : re p tile s, a n tib io tic s, re n a l p o rta l sy s te m, p h a rm a c o k in e tic s. Antibiotics are the most commonly administered therapy to reptiles. A logical approach to therapy is important and this encompasses quantifying the need for therapy, followed by an appropriate choice o f agent. Antibiotics need to be given at an appropriate dose, frequency and route, based on current infor mation available. Clinical signs and presentation can immediately suggest a need for antibiotic treatment. Indications include obvious trauma with subsequent infection. However many other con ditions, such as respiratory signs, conjunctivitis or diarrhea can be caused by other disease processes. In an attempt to identify the cause culture samples can be taken, ideally prior to any antibiotic therapy. M ost o f these cultures will grow Gram negative bacteria with variable resistance patterns. The bacteria cultured may be potential pathogens and their signifi cance should be determined by the clinical presentation, type and num ber o f bacteria. In one study perform ed anaerobic bacteria were isolated in 54% o f samples that were cultured anaerobically (Stewart, 1990). Therefore, when treating rep tile patients, a drug that is also effective against anaerobes, should be considered. Most laboratories only perform aerobic cultures unless you specify otherwise. However the fact bacteria have grown is irrelevant without supportive evidence o f an inflammatory response or suitable clinical signs. In studies into the normal fecal flora o f reptiles, all isolates had been reported in normal animals (Beehler and Sauro, 1983, Jacobson, 1999). Thus other testing is required to confirm their significance. Cytology can confirm the pres en ce o f an o rg a n ism and an in fla m m a to ry re sp o n se confirm ing its pathogenicity. Presum ptive therapy can be started while waiting for culture results. A Gram stain can identify the organisms morphology and staining characteris tics and, in the event o f the culture failing to grow, the likely effective therapy can be given. Cytology can also identify organisms that may be difficult to culture. Histopathology of a lesion can also confirm pathogens, but sadly there is a delay prior to getting the result. Hematology can also be used to confirm an inflammatory response, with a leukocytosis and/or toxic activity being indicative. Heterophilia is typically asso ciated with an acute inflammatory response. Once the need for antibiotics has been confirmed then the choice o f agent is important. This may be governed by a cul ture results, but it is im portant to realize that not all the potential pathogens (or any o f them) may have grown at all. ANTIBIOTIC THERAPY The rate o f uptake and elimi nation o f pharmaceuticals is influenced by metabolic rate. In ectothermic animals body temperature can have an effect on the therapeutic levels achieved and the toxicity o f agents. It is important to keep patients at their optimal temperature (To) to maximize the effectiveness o f therapy. However a study in royal pythons, Python regius, kept at 25 C (77 F) and 37 C (99 F) showed no clinically significant pharmacokinetic dif ferences between these temperatures when given amikacin by intracardiac or intramuscular routes (Johnson, et al, 1997). S im ilar findings w ere reported in pine snakes, Pituophis melanoleucus catenifer. However at the higher temperatures the bacterial sensitivity was enhanced and the volume o f dis tribution increased in the pine snakes (Mader, et al, 1983). In gopher tortoises, Gopherus polyphemus, temperature did have an effect on the m etabolic rate and excretion o f amikacin (Caligiuri, et al, 1990). There are a num ber o f published form ularies available which provide appropriate dosages and many include the original references, However, other sources o f information may simply provide a dose with no reference and this can be misleading. There are three potential methods for obtaining an appro priate dosage. Firstly, an empirical dose based on experience with perceived effectiveness can be considered. This relies on anecdotal information on perceived effectiveness and safety. Secondly, metabolic scaling can be used to estimate a likely effective dose. Thirdly, pharm acokinetic studies are being increasingly perform ed in reptiles in common species and these dose rates should be considered when treating other similar reptiles. Comparing these results to metabolic scaling may be helpful in finalizing a dose. However extrapolation of either method does not account for species specific variation in how xenobiotics are handled. If a pharmacokinetic study exists for one species it may also be wise to consider meta Volume 17, No. 2,2007

2 bolic scaling the dose and frequency based on the size o f the animal presented. In one study performed with enrofloxacin in green iguanas, Iguana iguana, metabolic scaling failed to produce an accurate dosing frequency although the dose did compare favorably with that determined by pharmacokinetic methods (Maxwell and Jacobson, 1999). The frequency o f dosing depends on how the antibiotic exerts its effects on bacteria. Some antibiotic classes such as the aminoglycosides and fluoroquinones lend themselves well to pulse therapy and bactericidal activity is dependant on obtaining a high peak serum level up to three to five times the MIC for the aminoglycosides and eight times the MIC for flu oro quinolones (Young, et al, 1997, M itchell, 2006). In addition a post antibiotic effect (PAE) can lead to increased bacterial kill even when the antibiotic concentration has fall en below the M IC (Fuursted, 1997). Essentially the PAE allows for more infrequent dosing compared to the regime calculated from pharmacokinetic studies. Other antibiotics, such as the penicillins, require a steady state plasma level to exert their effect over time. As a result pulse therapy should prove to be effective with the anminoglycosides and reduce the risk o f nephrotoxicity. AMINOGLYCOSIDES Aminoglycosides are bactericidal and act on bacterial ribosomes preventing protein production. Side effects include nephrotoxicity (Johnson, et al, 1997) and ototoxicity (Bagger-Sjoback and Wersall, 1976). They are effective against Gram negative aerobic infections and are not absorbed from the gastrointestinal tract. They can be used in' com bination with other antibiotics (such as penicillins) to control Gram-positive infections. The risk o f nephrotoxicity, in humans, is based on the antibiotic remaining above a cer tain threshold in the serum (Holz, et al, 1997, Johnson, et al, 1997). This level m ay w ell be below the M IC for m any pathogens (Holz, et al, 1997). Less frequent dosing may need to be considered where pharm acokinetic studies have not been performed to reduce the risk o f nephrotoxicity (Johnson, et al, 1997). M any veterinarians w ill provide appropriate fluid therapy to reptiles prior and during treatm ent w ith am inoglycosides in an attem pt to m inim ize the risks o f nephrotoxicity. The use o f these agents with cephalosporins or penicillins leads to synergism due to increased cell perme ability to the am inoglycosides and m ay help to m aintain effectiveness in this situation (Johnson, et al, 1997). Clinical studies have demonstrated antagonism between gentamicin and carbenicillin and for this reason some authors have rec ommended injecting antibiotics on differing days (R iff and Jackson, 1972). This interaction is specific to only these two agents so there is no reason not to give other agents on the same day. To improve effectiveness further a concentration of three to five times the MIC o f the organism at the site o f infection is recommended for the aminoglycosides (Mitchell, 2006). G entam icin pharm acokinetics in pine snakes has been determined and a dose o f 2.5 mg/kg every 3 d was a safe regime, but despite this, accumulation was likely to occur with repeat dosing (Bush, et al, 1976). In Burmese pythons, Python molurus bivitattus, and blood pythons, Python curtus, similar findings were reported (Wagner, et al, 1989, Hilf, et al, 1991). The safest dose reported in blood pythons was a loading dose o f 2.5 mg/kg IM followed by 1.5 mg/kg every 4 d (Hilf, et al, 1991). In pine snakes a dose o f 2.5 mg/kg IM every 3 d was the only regime tested that maintained ade quate levels without overt risk o f nephrotoxicity (Bush, et al, Volume 17, No. 2, ). Pharmacokinetic studies have also been performed in red eared terrapins, Trachemys scripta elegans, and a dose of 2.5 mg/kg every 3 d was recommended (Holz, et al, 1997). This is markedly different from previous recommendations of 10 mg/kg IM every 2 d in red eared terrapins and western painted turtles, Chrysemys picta bellii, (Bush, et al, 1977). In another study doses o f 6-10 mg/kg IM every 2-5 d was sug gested for red eared terrapins (Raphael, et al, 1985). Am ikacin is safer than gentam icin, although some iso lates m ay be re sista n t to am ikacin but y et sen sitiv e to gentamicin (Jacobson 1999). Other authors consider amikacin to have a greater spectrum o f activity (Klingenberg, 1996). Pharmacokinetic studies o f amikacin have been performed in pine snakes and a loading dose o f 5 mg/kg IM was recom m ended follow ed by 2.5 m g/kg every 3 d (M ader, et al, 1985). In royal pythons a single dose 3.5 mg/kg IM was sug gested due to the prolonged half life compared to the pine snakes. A larger dose o f 12 m g/kg m ay be required for Pseudomonas sp., (Johnson, et al, 1997). An osmotic pump has been used to provide a slow release of amikacin in com snakes, Elaphe gutatta gutatta, after a loading dose was given IM, it failed to achieve therapeutic levels but no toxicity was reported and a constant serum level was reported for the 12 day study (Sykes, et al, 2006). Pharmacokinetic studies have been performed in gopher tortoises. A dose o f 5 mg/kg IM every 2 d was recommended. Temperature effects were noted in this species (Caligiuri, et al, 1990). In American alligators, Alligator mississippiens, a dose o f 2.25 mg/kg IM every 3 4 d was determined (Jacobson, et al, 1988). CEPHALOSPORINS Ceftazidime is a third generation cephalosporin with excellent anti-pseudomonal activity. It is bactericidal and kills aerobic and anaerobic b acteria by inhibiting m ucopeptide synthesis in bacterial cell w alls (Klingenberg, 1996). It is useful as a primary agent or along side am inoglycosides or fluoroquinolones to extend the spectrum o f cover. They are widely distributed both into bone and the CNS if meningitis is present. It is excreted by renal tubular secretion and glomerular filtration. Anaerobic bacterial resistance is low (Lawrence, et al, 1984). This is available, in the United Kingdom, as an injection only. Once reconstituted it only maintains potency for 24 hr at room tem perature. Refrigerated injections last for seven days. Frozen in je c tio n s re m a in p o te n t fo r th re e to fo u r m o n th s (K lingenberg, 1996). Only one pharm acokinetic study in squamates has been performed. A dose o f 20 mg/kg IM every 3 d was effective in snakes (Lawrence, et al, 1984). Only one pharmacokinetic study in a chelonian has been performed. A dose o f 20 mg/kg IM every 3 d was effective in loggerhead sea tu rtle s, C a retta ca re tta, (S tam p er, et al, 1999). P h a rm a c o k in e tic stu d ie s in to c e fo p e ra z o n e in teg u s, Tupinambis teguixin, suggested a dose o f 125 mg/kg IM every 24 hr and in false water cobras, Hydrodynastes gigas, 100 mg/kg IM every 4 d (Klingenberg, 1996). Synergism has been noted between bactericidal antibiotics such as gentamicin and ceftazidime. This is because antibi otics that act on the cell wall (such as the penicillins and cephalosporins) increase the cell permeability to other antibi otics that act within the bacterial cells. CHLORAMPHENICOL Pharmacokinetic studies with chloramphenicol have been performed. These studies demon strated the variability in dosing regim es between sim ilar species. Sixteen species o f snakes were dosed with 50 mg/kg 43

3 IM o f chloramphenicol. The snakes required repeat dosing every 12 hr to 3 d to maintain plasma levels (Clarke, et al, 1985). Another study suggested 40 mg/kg IM every 24 hr for pine snakes (Bush, et al, 1976). Oral chloramphenicol failed to reach therapeutic dosages at a dose o f 12 mg/kg in pine snakes (Bush, et al, 1976). M ore recently florfenicol has become available. These drugs are bacteriostatic to bacterici dal depending on serum concentrations and are effective against aerobes, anaerobes, Gram-positive and Gram-negative infections (Klingenberg, 1996). Mycoplasma are also suscep tible but in recent studies, Mycoplasma alligatoris and M. iguanae had higher M ICs com pared to other antibiotics (Hemlick, et al, 2002, Westfall, et al, 2006). They work by inhibiting protein synthesis. Only one study using florfenicol has been performed in reptiles in loggerhead sea turtles. An IM or IV dose o f 30 mg/kg failed to achieve levels above the MIC for most pathogens and could not be detected after 12 hr (Stamper, et al, 2002). FLUOROQUINOLONES Enrofloxacin is bactericidal and effective against aerobic bacteria and Mycoplasma sp. In recent studies M. alligatoris and M. iguanae had low MICs (Hemlick, et al, 2002, Westfall, et al, 2006). Enrofloxacin was the only antibiotic to have bactericidal effects on M igua nae (Westfall, et al, 2006). It acts by inhibiting DNA gyrase and resistance is currently less likely, com pared to other antibiotic classes, and occurs via mutation o f the bacteria. It has good activity against Pseudomonas sp. They are metabo lized by hepatic biotransformation and oral administration leads to a first pass effect (Mitchell, 2006). Thankfully the hepatic extraction ratio is low (Holz, et al, 1997). Side effects include cartilage damage in growing animals and central ner vous system signs in humans (Mitchell, 2006). Excitation and d ia rrh e a h as b ee n re p o rte d in G a la p ag o s to rto is e s, Geochelone elephantopus nigra, (Casares and Enders, 1996). Muscle damage due to the alkaline ph (11) o f the injectable enrofloxacin, 2.27 mg/ml, is well known (Mitchell, 2006). Dilution o f samples in saline or administration intracoelomically (in sea turtles) has been suggested to avoid these local tissue effects and apparently shows similar absorption to the IM route (Mader, et al, 2002). In Burmese pythons, star tor toises, G eochelone elegans, and red eared terrapins, the production o f ciprofloxacin via hepatic biotransformation has been reported (Raphael, et al, 1994, Young, et al, 1997, James, et al, 2003). In the burmese pythons ciprofloxacin had a longer h a lf life (Young, et al, 1997). Ciprofloxacin has greater anti-pseudomonal activity and many isolates resistant to enrofloxacin are sensitive to ciprofloxacin (Klingenberg, 1996). Some pharmacokinetic studies have not differentiated between enrofloxacin and ciprofloxacin due to test methodol ogy as high performance liquid chromatography is able to distinguish m etabolites but m icrobiologic assays do not (Raphael, et al, 1994). P h arm aco k in etic studies o f en ro flo x acin in B urm ese pythons calculated a 10 mg/kg loading dose, then 5 mg/kg IM every 2 d. For resistant Pseudomonas sp 10 mg/kg IM every 2 d was required (Young, et al, 1997). Pharmacokinetic stud ies in reticulated pythons, Python reticulates, showed a dose o f 6.6 mg/kg every 24 hr or 11 mg/kg every 2 d IM to be a p p ro p ria te (K lin g e n b e rg and B ack n e r, 1992). O ral ciprofloxacin has been assessed in reticulated pythons and a d o se o f 2.5 m g /k g ev e ry 2-3 d w as su g g e ste d, oral enro flo x acin also show ed absorption (K lingenberg and Backner, 1991, Klingenberg and Backner 1992). In savannah monitors, Varanus exanthematicus, pharmacokinetic studies elucidated a dose o f 10 mg/kg IM every 5 d. Oral therapy at 10 mg/kg took six hours to get over the MIC but continued to in c re a s e o v e r th e 36 h r o f th e study. C o n v e rsio n to ciprofloxacin was minimal in this species (Hungerford, et al, 1997). A dose o f 5 mg/kg IM in green iguanas maintained therapeutic levels for 16 hr. Oral therapy at 5 mg/kg PO every 36 hr lead to marked variability in pharmacokinetic parame ters due to variable absorption (M axw ell and Jacobson, 1997). In one study 10 mg/kg PO every 24 hr for 10 d was effective at elim inating sensitive Salmonella', how ever in another this was not the case (Mitchell, et al, 2001, Burnham, et al, 2002). In chelonians studies have been performed in H erm ann s tortoises, Testudo hermanni, and a dose o f 10 mg/kg every 24 hr IM was determined (Sporle, et al, 1991). Pharmacokinetic studies in star tortoises determined a dose of 5 mg/kg IM every 12 hr for Pseudomonas and Citrobacter but every 24 hr for o ther isolates. S ignifican t levels of ciprofloxacin w ere detected in this study (Raphael, et al, 1994). Gopher tortoises dem onstrated a considerably pro longed half life and a dose o f 5 mg/kg IM every 24 hr to 2 d was determined (Prezant, et al, 1994). Red eared terrapins were administered both PO and IM doses o f enrofloxacin; 5 m g/kg IM every 24 hr and 10 m g/kg PO every 2 d were appropriate dosages determined. Ciprofloxacin was identified during the study co n firm in g hep atic b io tran sfo rm atio n (James, et al, 2003). In box turtles, Terrepene sp., a dose of 5 mg/kg IM every 4-5 d was suggested (Jacobson, 1999). In loggerhead sea turtles oral dosing at 20 mg/kg every 7d was recommended, but this took a day to reach maximum serum concentrations (Jacobson, et al, 2005). American alligators dosed at 5 mg/kg IV every 36 hr m aintained blood levels beyond the MIC for sensitive organisms but for more resis tant species higher doses or m ore frequent administration w ould be required. M inim al levels o f ciprofloxacin were detected and it remained below MIC levels at all times. Oral therapy at this dose was not expected to achieve the MIC but it is likely it was still being absorbed at the end o f the four day study (Helmick, et al, 2004). M arbofloxacin is another fluoroquinolone that has more recently been used in exotic animals. It has an acidic ph and is excreted via the kidneys and one pharmacokinetic study has been undertaken in royal pythons. Snakes were given the drug IV and PO. A dose o f 10 mg/kg PO every 2 d was rec ommended, however, serum levels were comparable by either route after 24 hr (Coke, et al, 2006). Pulse therapy works well and efficacy is determined by obtaining a plasma level eight times or more than the MIC (Young, et al, 1997). There is a significant post antibiotic effect (PAE) that acts to inhibit bacteria even when levels drop below the MIC. This effect can last up to fifteen hours and depends on bacterial species, drug concentration and effective time above the MIC (Fuursted, 1997). MACROLIDES Macrolides have been recently investigat ed as they have activity against both aerobic and anaerobic bacteria, but also Mycoplasma sp. M. alligatoris isolated in a recent study had low MICs against both tilmicosin and tylosin (Hemlick, et al, 2002). M iguanae had a low MIC against tylosin (W estfall, et al, 2006). Their effectiveness against Gram-negative pathogens is much reduced. They are bacte riostatic but high doses can become bactericidal. They are m etabolized by the liver and cleared by the kidneys. In a study o f anaerobic bacteria isolated from reptiles 95% were Volume 17, No. 2,2007

4 sensitive to clindamycin (Stewart, 1990). Clarithromycin is an oral macrolide and pharmacokinetics in desert tortoises, Gopherus agassizii, has been determined. A dose o f 15 mg/kg PO every 24 hr has been suggested. However, long term steady state levels may be achieved by dosing every two to three days (W imsatt, et al, 1999). A zithrom ycin has been investigated in royal pythons. Both IM and PO routes were considered. Ten mg/kg PO every 2-3 d was recommended (Coke, et al, 2003). Other agents in use include tylosin at 5 mg/kg IM every 24 hr, although no pharmacokinetic study has been performed (Jacobson, 1999). NITROIMIDAZOLES The mode o f action o f nitroimidazoles is unknown but is believed to inhibit DNA synthesis. Over 50% o f oral medication is absorbed. Toxic effects effect prim arily the liver, CNS and gastrointestinal tract. Species th a t are su sce p tib le to to x ic ity in clu d e In d ig o snakes, Drymarchon corais, and kingsnakes, Lampropeltis sp., when doses o v er 100 m g/kg have been used. Toxic effects in Uracoan rattlesnakes, Crotalus sp., occurred with doses o f 40 mg/kg. Antibacterial dosages vary from m g/kg PO. Antiprotozoal dosages are much higher (Mitchell, 2006). A study into anaerobic bacteria in reptiles demonstrated 100% sensitivity to metronidazole (Stewart, 1990). Metronidazole can be used in com bination with other agents designed to tre a t G ra m -n e g a tiv e a e ro b e s su ch as e n ro flo x a c in. Pharmacokinetic studies have been performed in rat snakes, Elaphe obsoleta quadrivittata, and green iguanas and a dose o f 20 mg/kg PO every 2 d was recommended for sensitive anaerobes (Kolmsetter, et al, 1997, Kolmsetter, et al, 1998). A study in com snakes showed a dose o f 50 mg/kg PO every 2 d to be effective against sensitive anaerobic bacteria and proto zoa (Bodri, et al, 2001). PENICILLINS Semi-synthetic penicillins such as carb e n ic illin, tic a r c illin and p ip e ra c illin h av e in c re a se d antipseudomonal activity over traditional penicillins. These act by inhibiting bacterial cell wall synthesis but beta lacta m ase re sistan c e is po ssib le. T hese req u ire stead y state pharmacokinetics and maintaining the serum level above the MIC for an extended time period is important in their effec tiveness. In snakes carbenicillin dosing was partly determined and a dose o f 400 mg/kg IM every 24 hr was tentatively sug gested (Lawrence, et al, 1984). However a more recent study demonstrated that 200 mg/kg was sufficient when injected either into the cranial or caudal aspect o f carpet pythons, Moreilia spilota, (Holz, et al, 2002). Carbenicillin has been used at 400 m g/kg in tortoises (Law rence, et al, 1984). However, a pharmacokinetic study determined that a dose of 115 mg/kg every 24 hr to be appropriate in red eared terrap ins. In red eared terrapins recirculation o f the drug from the bladder extended its activity (Holz, et al, 1997.) This was also identified in Hermann s and spur thighed tortoises, Testudo graeca. 400 mg/kg IM every 2 d was considered an appropri ate dosing regime in these species (Lawrence et al, 1984). Ticarcillin has been assessed in loggerhead sea turtles and 50 mg/kg IM every 24 hr or 100 mg/kg IM every 2 d was recom mended (Manire, et al, 2005). Piperacillin dosing has been pharmacokinetically determined in blood pythons, 100 mg/kg IM every 2 d w as recom m ended (W agner, et al, 1989). However a previous dose determined was 100 mg/kg every 24 hr (Hilf, et al, 1991). A pharmacokinetic study o f ampicillin in H erm ann s tortoises dem onstrated a dose o f 50 mg/kg IM every 12 hr to be effective (Sporle, et al, 1991). Volume 17, No. 2,2007 SU L PH O N A M ID E S P otentiated sulphonam ides are broad spectrum antibiotics and are bactericidal. They affect bacterial folate synthesis. No pharmacokinetic studies have been performed in reptiles. An empirical dose o f parenteral trimethoprim-sulphadiazine has been suggested at 30 mg/kg, the first two doses being 24 hr apart followed by treatment every 2 d (Jacobson, 1999). T E T R A C Y C L IN E S - C h la m y d o p h ila sp., and M ycoplasm a sp., are becom ing increasingly recognized pathogens in reptiles (Jacobson, et al, 1989, Hemlick, et al, 2002). The tetracyclines are bacteriostatic but do have the advantage o f being truly broad spectrum, covering aerobic and anaerobic pathogens and both M ycoplasm a sp., and Chlamydophila sp. In recent studies M. alligatoris and M iguanae had low MICs against oxytetracycline and doxycyc lin e (H e m lic k, e t a l, , W e stfall, e t a l, ). Tetracyclines act by binding to the ribosomes and inhibiting protein synthesis. They undergo enterohepatic recirculation in some m ammalian species and can have a long duration o f activity. Their action is time dependant and a steady state above the MIC is required. Local irritation leads to slow release o f the drugs from the injection site. They are excreted by glomerular filtration (Helmick, et al, 2004). In Hermann s tortoises a dosing regime for doxycycline o f 50 mg/kg IM followed by 25 mg/kg every 3 d was determined (Sporle, et al, 1991). In loggerhead sea turtles a pharm acokinetically derived dosage for oxytetracycline o f 41 mg/kg IM as a loading dose followed by 21 mg/kg every 3 d for sensitive organisms. If higher MIC is required the dose can be doubled (Harms, et al, 2004). American alligators given lomg/kg o f a long acting oxytetracycline preparation either IM or IV maintained thera peutic levels throughout the whole o f the 8 day study and a dose o f 10 mg/kg every 5 d by either route was recommended (Helmick, et al, 2004). THE RENAL PORTAL SYSTEM The renal portal system can influence drug pharmacokinetics for those items that are eliminated by tubular secretion. Blood from the tail and the hind limbs o f chelonians may pass via the kidney tubules before entering the circulation. Any remaining blood from the hind limbs passes to the liver prior to entering the circulation. In one study perform ed in red eared terrapins most blood from the caudal body bypassed the renal portal system and went directly to the liver (Holz, et al, 1997). In green iguanas all blood from the tail entered the renal portal system and m ost blo o d from the hind lim bs b y p assed the k idneys (Benson and Forrest, 1999). There may be significant first pass effect for hepatically biotransformed antibiotics, such as enrofloxacin, in these circum stances. Despite some drugs entering the renal portal system there are direct connections between the afferent and efferent veins in red eared terrapins and snapping turtles, Chelydra serpentia, and so the renal tubules can be bypassed within the renal tissue itself (Holz, et al, 1997). Reptiles cannot produce hypertonic urine because they lack a loop o f Henle. In order to reduce water loss the urine output m ust decrease. To achieve this, the rate o f glomerular filtration must decrease. This is achieved by vaso constriction o f the afferent glomerular arterioles and hence perfusion o f the nephrons is reduced. It is believed that the renal portal system is an evolutionary mechanism that is acti v a te d d u rin g p e rio d s o f d ro u g h t (an d h en ce red u ced glomerular filtration rate) to preserve tubular function by pro 45

5 viding an alternative blood supply (Holz, et al, 1997). In the absence o f a glomerular filtrate, the tubule collapses and secretion stops as there is no fluid to transport material down the tubule (Holz, et al, 1997). It is possible drugs excreted in this way would have a prolonged half life in dehydrated animals despite flowing via the renal portal system (Holz, et al, 1997). The use of sequential blood iohexol concentrations can be used to establish the GFR o f an individual anim al (Hernandez-Divers, et al, 2005). The renal portal system may therefore be active in sick or traumatized reptiles and this should be noted. In red eared terrapins a valve was identified that determ ined blood flow (Holz, et al, 1997). In birds parasympathetic stimulation closes the valve and preserves renal tubular function by diverting blood to the renal portal system. Sympathetic stimulation opens the valve in birds and blood flows to the liver (Rennick and Gandia, 1954). It is unknown if similar effects occur in reptiles. No such valve was identified in green iguanas (Benson and Forrest, 1999). Studies have been performed to determine the renal portal valve s influence upon therapy. There has been a study perform ed in carp et p ythons to determ ine its effects on pharmacokinetics of carbenicillin. This drug is excreted by the renal tubules and was injected in the epaxial muscles of the snakes. There were no significant pharmacokinetic differences between cranially and caudally placed injections. Pharm acokinetics o f gentam icin (w hich is excreted by glomerular filtration) were assessed in eastern box turtles, Terrapene Carolina Carolina, when given by intramuscular injection into the fore and hind limbs. As expected no clinically significant difference was noted between these two groups. What was of concern was the dose used of 3 mg/kg was higher than necessary for this species and had a half life of 44 hr (Beck, et al, 1995). A further study compared gentamicin (excreted by glomerular filtration) and carbenicillin (excreted by tubular secretion) pharmacokinetics in red eared terrapins injected intram uscularly into the front and hind limbs. There were no significant pharmacokinetic differences between cranial and caudally placed injections. There were lower serum concentrations of carbenicillin after injection into the hind limbs but despite this there was no major alteration on the recommended dosage regime (Holz, et al, 1997). Other drugs such as ceftazidime where bacteria may be highly susceptible to this agent (and hence have low MICs) over 50% of the drug may need to be excreted by the renal portal system b efo re a clin ically sig n ifican t effect is noted (Lawrence, et al, 1984, Hilf, et al, 1991). Understanding the renal portal system and how it handles different types of antibiotics allows for the use of antibiotics in reptiles in either the front or rear half of the body with little concern for the renal portal system, for most of the drugs we are considering. Only drugs that are excreted by the renal tubules (such as some penicillins) or are nephrotoxic (such as the aminoglycosides) should be injected only in the front half o f a reptile, if practically possible. However hepatically biotransformed antibiotics (such as the fluoroquinolones) may suffer a first pass effect if they were to bypass the renal portal system, thankfully the fluoroquinolones do not have a high hepatic extraction ratio and have similar blood levels whether given orally or parenterally (Holz, et al, 1997). S u m m a r y Pharmacokinetic studies regarding antibiotic usage in reptiles are being increasing published. As more data becomes available, this information should be incorporated into existing dosing regimes used in clinical cas.es and enhancing the therapeutic effectiveness of the patients that we see. Ancillary tests to confirm both the need for antibiotic therapy and to select appropriate agents, are of critical importance to reduce the risk of bacterial resistance. A summary of dosing regimes are listed in table 1. Volume 17, No. 2,2007

6 Table 1. Effective pharmacokinetically derived dosing regimes of antibiotics in reptiles Antibiotic Dose Route Interval Species Reference Gentamicin Amikacin 2.5 mg/kg 2.5 mg/kg loading IM 3 days Pine snake, Pituophis melanoleucus catenifer Bush, et al, 1976 dose then 1.5 mg/kg IM 4 days Blood pythons, Python curtus Hilf, et al, mg/kg IM 3 days Hotz, et al, mg/kg IM 2 days Bush, et al, mg/kg IM 2-5 days Raphael, et al, mg/kg IM 2-5 days Western painted turtles, Chrysemys picta bellii Bush, et al, mg/kg IM 3-4 days American alligator, Alligator mississippiens Jacobson, et al, mg/kg loading dose then 2.5 mg/kg IM 3 days Pine snake, Pituophis melanoleucus catenifer Mader, et al, mg/kg, 12 mg/kg for Pseudomonas sp. IM Single dose only Royal pythons, Python regius Johnson, et al, mg/kg IM 2 days Gopher tortoises, Gopherus polyphemus Caligiuri, et al, mg/kg IM 3-4 days American alligator, Alligator mississippiens Jacobson, et al, 1988 Ceftazidime 20 mg/kg IM 3 days Snakes Lawrence, et al, mg/kg IM 3 days Loggerhead sea turtle, Caretta caretta Stamper, et al, 1999 Cefoperazone 125 mg/kg IM 24 hours Tegus, Tupinambis teguixin Klingenberg, mg/kg IM 4 days False water cobras, Hydrodynastes gigas Klingenberg, 1996 Chloramphenicol 50 mg/kg IM 12 hours - 3 days Snakes Clarke, et al, 1985 Enrofloxacin 40 mg/kg IM 24 hours Pine snake, Pituophis melanoleucus catenifer Bush, et al, mg/kg loading dose then 5 mg/kg. For Pseudomonas sp 10 mg/kg. IM 2 days Burmese pytnon, Python molurus bivitattus Young, et al, mg/kg IM 2 days Reticulated python, Python reticulates Klingenberg and Backner, mg/kg IM 5 days Savannah monitor, Varanus exanthematicus Hungerford, et al, mg/kg IM Not reported Green iguanas, Iguana iguana Maxwell and Jacobson, mg/kg IM 24 hours Hermann's tortoise, Testudo hermanni Sporle, et al, mg/kg IM hours Star tortoise, Geochelone elegans Raphael, et al, mg/kg IM 24 hours - 2 days Gopher tortoises, Gopherus polyphemus Prezant, et al, mg/kg IM 24 hours 10 mg/kg PO 2 days 5 mg/kg IM 5 days Box turtle, Terrepene sp Jacobson, mg/kg PO 7 days Loggerhead sea turtle, Caretta caretta Jacobson, et al, mg/kg IV 36 hours American alligator, Alligator mississippiens Helmick, et al, 2004 Marbofloxacin 10 mg/kg PO 2 days Royal python, Python regius Coke, et al, 2006 Clarithromycin 15 mg/kg PO 24 hours Desert tortoise, Gopherus agassizii Wimsatt, et al, 1999 Azithromycin 10 mg/kg PO 2-3 days Royal python, Python regius Coke, et al, mg/kg PO 2 days Rat snake, Elaphe obsoleta Kolmsetter, et al, mg/kg PO 2 days Green iguanas, Iguana iguana Kolmsetter, et al, mg/kg PO 2 days Corn snakes, Elaphe gutatta gutatta Bodri, et al, mg/kg IM 24 hours Snakes Lawrence, et al, mg/kg IM 24 hours Carpet python, Moreilia spilota Holz, et al, mg/kg IM 24 hours Red eared terrapins, Trachemys scripta elegans Holz, eta/, mg/kg IM 24 hours Hermann's, Testudo hermanni and Spur thighed tortoise, Testudo graeca Lawrence, et al, Ticarcillin 50 mg/kg IM 24 hours Loggerhead sea turtle, Caretta caretta Manire, et al, mg/kg IM 2 days Wagner, et al, mg/kg IM 24 hours Hilf, et al, 1991 Ampicillin 50 mg/kg IM 12 hours Hermann's tortoise, Testudo hermanni Sporle, et al, 1991 Doxycycline 50 mg/kg loading dose then 25 mg/kg IM 3 days Hermann's tortoise, Testudo hermanni Sporle, et al, mg/kg loading dose then 21 mg/kg IM 3 days Loggerhead sea turtles, Caretta caretta Harms, et al, mg/kg (long acitng preparation) IM 5 days American alligator, Alligator mississippiens Helmick, et al, 2004 Volume 17, No. 2,

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In Fowler ME and Miller RC (eds): Zoo and Wild Animal Medi cine Current Therapy 4. WB Saunders Co, Philadelphia, PA: James SB, Calle PP, Raphael BL, Papich MG, Breheny J, Cook RA Comparison of injectable versus oral enrofloxacin pharmacokinetics in red eared slider turtles {Trachemys scripta elegans). JHMS, 13(1):5 10. Jacobson ER, Brown MP, Chung M, Vliet K, Swift R Serum concentration and disposition kinetics of gentamicin and amikacin in juvenile American alligators. J Zoo Wild Med, 19(4): Jacobson ER, Gaskin JM, Mansell J Chlamydial infection in p u ff adders {B itis arietans). J Zoo W ild M ed, 20 (4): Jacobson ER, Gronwall R, Maxwell L, Merrit K, Harman G Plasma concentrations of enrofloxacin after single dose oral administration in loggerhead sea turtles {Caretta caretta). J Zoo Wild Med, 36 (4): Johnson JH, Jensen JM, Brumbaugh GW, Boothe DM Amikacin pharmacokinetics and the effects of ambient temper ature on the dosage regimen in ball pythons {Python regius). 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8 Klingenberg RJ, Backner B The use of ciprofloxacin, a new antibiotic in snakes. Proc 5th International Symposium of Captive Propagation and H usbandry o f R eptiles and Amphibians, Klingenberg RJ, Backner B The use of enrofloxacin in reticulated pythons (Python reticulatus), a preliminary study. Proceedings of the 16th International Symposium of Captive Propagation and Husbandry of Reptiles and Amphibians. K olm setter CM, Frazier D, Cox S, Ram say EC Metronidazole pharmacokinetics in yellow rat snakes (Elaphe obsoleta quadrivitatta). JHMS, 11(2):4 8. K olm setter CM, Frazier D, Cox S, Ram say EC Pharmacokinetics of metronidazole in the green iguana (Iguana iguana). BARAV, 8:4 7. Lawrence K, Palmer GH, Needham JR Use of carbenicillin in two species of tortoise (Testudo graeca and Testudo hermanni). Res Vet Sci, 40: Lawrence K, Muggleton PW, Needham JR Preliminary study on the use o f ceftazidim e, a broad spectrum cephalosporin antibiotic, in snakes. Res Vet Sci, 36: Lawrence K, Needham JR, Palmer GH, Lewis JC A preliminary study on the use of carbenicillin in snakes. J Vet Pharm Thera, 7: Mader DR, Conzelman GM, Baggot JD Effects of ambient temperature on the half life and dosage regimen of amikacin in the Gopher snake. JAVMA, 187(11): Mader DR, Schaff S, Moretti R, Rose C Intracoelomic catheters in sea turtles. Proc ARAV, Manire CA, Hunter RP, Koch DE, Bryd L, Rhinehart HL Pharmacokinetics of ticarcillin in the loggerhead sea turtle (Caretta caretta) after single intravenous and intramuscular injections. J Zoo Wild Med, 36(1): Maxwell LK, Jacobson ER Preliminary single dose pharmacokinetics o f enrofloxacin after oral and intramuscular administration in green iguanas (Iguana iguana). Proc AAZV, 25. M axw ell LK, Jacobson ER M etabolic scaling of enrofloxacin in green iguanas (Iguana iguana). Proc AAZV, M itchell MA T herapeutics. In M ader DR (ed): Reptile Medicine and Surgery, Saunders Elsevier, Missouri, M itchell MA, Shane SM, Nevarez J, Pesti D, Sanchez S, Wolley RE, Ritchie B Establishing a Salmonella free iguana, Iguana iguana, model using enrofloxacin. Proc ARAV, Prezant RM, Isaza R, Jacobson ER Plasma concentrations and disposition kinetics of enrofloxacin in gopher tortoises ( Gopherus polyphem us). J Zoo Wild Med, 25 (1): Raphael BL, Clark CH, Hudson R Plasma concentrations of gentamicin in turtles. J Zoo Ani Med, 16: Raphael BL, Papich MG, Cook RA Pharmacokinetics of enrofloxacin after a single intramuscular injection in Indian star tortoises (Geochelone elegans). J Zoo Wild Med, 25(1): Rennick BR, Gandia H Pharm acology o f smooth m uscle valve in renal portal circulation of birds. Proceedings of the Society for Experimental Biology and Medicine, 85: Riff LJ, Jackson GG Laboratory and clinical conditions for gentamicin inactivation by carbenicillin. Arch Intern Med, 130: Sporle H, Gobel T, Schlidger B Blood levels of some anti-infectives in the H erm ann s tortoise (Testudo hermanni). Proc 4th International Colloquium on Pathology on Medicine of Reptiles and Amphibians. Stamper MA, Papich MG, Lewbart GA, May SB, Plummer DD, Stoskopf MK Pharmacokinetics of florfenicol in loggerhead sea turtles (Caretta caretta) after single intravenous and intram uscular injections. J Zoo Wild Med, 34(1):3 8. Stamper MA, Papich MG, Lewbart G.A, May SB, Plummer DD, Stoskopf MK Pharmacokinetics of ceftazidime in loggerhead seas turtles (Caretta caretta) after single intravenous and intramuscular injections. J Zoo Wild Med, 30(l): Stewart JS Anaerobic infections in reptiles. J Zoo Wild Med, 21(1): Sykes JM, Ramsay EC, Schumacher J, Daniel GB, Cox S, Papich MG Evaluation o f an implanted osmotic pump for delivery of amikacin to com snakes (Elaphe guttata gutatta). J Zoo Wild Med, (37)3: W agner RA, H ilf M, Swanson D, Yu VL Pharmacokinetics of gentamicin and piperacillin in blood pythons: new dosing regimen. Proc AAZV, 153. W estfall ME, Demcovitz DL, Plourde DR, Rotstein DS, Brown DR In vitro antibiotic susceptibility of Mycoplasma iguanae proposed species nova isolated from vertebral lesions of green iguanas (Iguana iguana). J Zoo Wild Med, 37(2): Wimsatt J, Johnson J, Mangone BA, Tothill A, Childs JM, Peloquin CA Clarithromycin pharmacokinetics in the desert tortoise (Gopherus agassizii). J Zoo Wild Med, 30 (l): Young LA, Schumacher J, Papich MG, Jacobson ER Disposition of enrofloxacin and its metabolite ciprofloxacin after intramuscular injection in juvenile Burmese pythons (Python molurus bivitattus). J Zoo Wild Med, 28(l): Volume 17, No. 2,