Marike Visser Companion birds include members of the orders Psittaciformes (e.g., parakeets, parrots, lories, cockatoos, and macaws), Passeriformes (e.g., canaries and finches), and Columbiformes (e.g., pigeons and doves). Psittacine birds are the most common pet birds; over 50 species are commonly seen in veterinary practice. Microbial diseases are common and use of antimicrobials is an important part of avian practice. Optimal treatment regimens can be developed if the principles of rational antimicrobial therapy are integrated with the unique behavioral and physiological characteristics of birds. The general approach to selecting an avian antimicrobial treatment regimen is similar to other species. However, the husbandry must be especially scrutinized to insure a healthy plane of nutrition and that any habitat corrections are implemented. The site and cause of infection should be identified and the minimal inhibitory concentrations (MIC) of potentially effective antimicrobial drugs determined. However, there are no CLSI breakpoints for pathogens of birds. Selection of the most appropriate drug will thus depend on the severity of illness, site of infection, pharmacokinetic and pharmacodynamic properties of the selected drugs (if available), and the routes of administration that can be accomplished by the owner or veterinary staff. Additional considerations are adverse drug effects, toxicity, and cost. The Federation of European Companion Animal Veterinary Associates (FECAVA) provides a flow diagram in multiple languages that can aid the clinician in assessing whether antimicrobial therapy is warranted (www.fecava.org/policies‐actions/guidelines/). A wide variety of primary and secondary bacterial pathogens have been identified in companion birds; however, some are more common than others. Often, poor husbandry and nutrition are contributing factors. In psittacine birds, Gram‐negative bacterial infections are most common, especially those caused by Escherichia coli, Klebsiella spp., and Pseudomonas aeruginosa. Other Gram‐negative bacteria include Enterobacter spp., Proteus, Citrobacter, and Serratia marcescens. Gram‐positive bacterial pathogens include Staphylococcus aureus, S. pseudintermedius, Clostridium spp., Streptococcus spp., and Enterococcus spp. Chlamydophila psittaci is the most important intracellular pathogen; Mycobacterium avium and M. genavense infections are occasionally seen. Anaerobes are relatively uncommon, although clostridial infections of the alimentary tract do occur. Similar pathogens are found in canaries and pigeons; Enterococcus faecalis is an important cause of respiratory disease in canaries and there is a higher incidence of Salmonella spp. and Streptococcus gallolyticus infections in pigeons. Mycotic infections are also important. Yeasts most commonly affect the alimentary tract and common pathogens include Candida albicans and Macrorhabdus ornithogaster. Hyphal fungi are important pathogens of the respiratory tract and, occasionally, the eye and skin. Aspergillus fumigatus and A. niger are the most common isolates; Mucor spp., Penicillium spp., Rhizopus spp., and Scedosporium spp. and other opportunist molds may rarely infect immunocompromised birds. Usually, there are predisposing factors that result in infection, particularly in parrot species, ranging from poor husbandry, aspiration of food or medicine, to underlying disease. In companion birds, septicemia and infections of the alimentary tract, respiratory tract, and liver are the most common sites of microbial infection. It is important to note that simply culturing a potential pathogen is not an indication for antimicrobial treatment. It is not unusual to culture small numbers of Gram‐negative bacteria or yeasts from the cloaca and choana of apparently healthy birds. Treatment may be indicated if the organism is present in large numbers and there are accompanying clinical signs. Physical exam findings, results of clinical laboratory tests, and a Gram stain of material from the suspected site of infection can help determine if a microbial infection is the cause of illness. To be effective, the pathogen must be susceptible to the drug at concentrations that are achievable in birds. Some pathogens have known susceptibility (e.g., Chlamydophila psittaci is invariably susceptible to doxycycline), but most will require susceptibility testing to guide therapy. Susceptibility tests reporting MIC values are quantitative and provide the most useful information to guide drug selection. Disk diffusion tests can be used, but it is important to recognize that the designations of susceptible, intermediate, and resistant may not correlate with treatment success in birds as there are no validated breakpoints. These designations are based on the achievable drug concentrations in humans (or in a limited number of animal species) and it may be difficult to achieve similar concentrations in birds. Chapter 2 discusses susceptibility testing. Companion birds often hide signs of disease and may present at an advanced stage of illness. If a bacterial infection is strongly suspected, it may be necessary to start empirical treatment before the results of culture and susceptibility tests are available. In companion birds, Gram‐negative bacterial infections are most common, especially those caused by E. coli, Klebsiella spp., and P. aeruginosa. Chlamydiosis most commonly occurs in birds recently obtained from commercial sources (e.g., pet stores, flea markets, and breeders). Salmonella is common in pigeons. If these organisms are suspected, a broad‐spectrum antimicrobial with excellent Gram‐negative activity is most appropriate for initiating empirical treatment; doxycycline is preferred if chlamydiosis is likely. The treatment plan can be modified once the bird is stable and results of laboratory testing are available. The frequency and route of administration are important considerations when choosing a dosage regimen. Most birds will need to be captured and restrained to deliver medication, so that treatment regimens with longer dosage intervals are preferred. In sick birds, a parenteral route of administration should be used to rapidly establish effective drug concentrations. Once a bird is clinically stable, it may be relinquished to the owner’s care to complete antimicrobial therapy. Birds can be difficult to medicate, and the procedure is often stressful for both the bird and owner. If oral medication is used, low‐volume, palatable drug formulations can aid treatment success. Some avian veterinarians favor use of IM injection because bird restraint and drug delivery may be easier with this route. Additional pros and cons of different routes of administration are discussed below. Regardless of the treatment regimen, it is useful to check compliance and offer assistance after a few days of treatment. Choosing the dose can be challenging because drug formularies often list a wide range of recommended dosages. This is partly because there are sparse data on the pharmacokinetics of antibiotics in many species of psittacine birds. Many dosage regimens are empirically derived or extrapolated from other species. Table 35.1 provides suggested doses for selected commonly used antimicrobial drugs. However, even doses based on pharmacokinetic studies often represent only a single‐dose study in a limited number of individuals of a single species. Therefore, all treated birds should be monitored carefully since safety and efficacy have not been investigated for widespread use of many of the drug dosages listed. Basic pharmacodynamic principles should be considered when evaluating which dose to use. Drugs showing time‐dependent efficacy (e.g., beta‐lactams, macrolides, tetracyclines, and trimethoprim‐sulfonamides) must be dosed frequently enough to maintain plasma concentrations above the target MIC for most of the dosing interval. Birds rapidly excrete most beta‐lactam drugs, so penicillins and cephalosporins should be dosed at least 3–4 times daily unless pharmacokinetic data demonstrate that less frequent administration is adequate. Cephalosporins that show prolonged activity in other species (e.g., cefovecin in dogs) may have short activity in birds, especially if high protein binding is necessary for the prolonged half‐life (Thuesen et al., 2009). Concentration‐dependent antimicrobials (e.g., fluoroquinolones and aminoglycosides) can probably be dosed once daily if high peak concentrations and large area under the curve values are achieved. Since these values may depend on the route of administration, parenteral routes may be required to achieve the desired concentration for resistant organisms. Controlled studies involving large numbers of different avian species are lacking, so veterinarians should carefully monitor treatment efficacy and potential toxicity. This is especially important when using drugs with a narrow therapeutic range or treating an unfamiliar species. Relevant chapters in this book should be consulted on specific antimicrobials and their potential adverse effects and contraindications. Using broad‐spectrum antimicrobials may impact normal intestinal microflora. With the advent of advanced genomic sequencing, the microbiome has been found to be significantly different between species, even within the same class. Fecal samples from lovebirds and cockatiels have high numbers of Mycoplasma in presumed healthy birds, suggesting this may be a commensal organism. Pooled fecal samples from budgerigars, cockatiels, and domestic canaries report that Lactobacillaceae and Firmicutes (Clostridia and Bacilli class) are abundant (Garcia‐Mazcorro et al., 2017). Studies in chickens have reported abundant Lactobacillus in the crop, gizzard and small intestine and large intestine (Proszkowiec‐Weglarz, 2022). The use of a quality probiotic containing Lactobaccilus may help mitigate the impact of orally administered antibiotics. Differences in anatomy and physiology may alter drug pharmacology in birds as compared to mammals. For example, granuloma formation is a common avian response to infection by many microbial agents. Granuloma formation can inhibit drug penetration so that surgical debridement, use of lipophilic drugs, and prolonged treatment may be needed for treatment success. Table 35.1 Conventional dosage regimens for antimicrobial drugs in companion birds.a
35
Antimicrobial Therapy in Companion Birds
Establishing the Cause and Site of Infection
Choosing an AntimicrobialRegimen
Anatomical and Physiological Considerations
Drugs
Dose (mg/kg)
Interval (h)
Route
Studyb/species
Referencec
Comments
Penicillins
Ampicillin sodium
150
12–24
IM
PK/pigeons
1
Gram‐positive bacteria only.
50 –100
6–8
IM
PK/Amazon parrot
2
Ampicillin trihydrate
125–175
4–8
PO
PK/pigeons
1
Gram‐positives only. IM bioavailability 57% so double the IV dose.
150–200
4–8
PO
PK/Amazon parrots
2
Amoxicillin sodium
250
12–24
IM
PK/pigeons
1
Gram‐positives only.
Amoxicillin trihydrate
100
12–24
PO
PK/pigeons
1
Gram‐positives only.
150–175
4–8
PO
Empirical/psittacines
1
Amoxicillin + clavulanic acid
125–250
8
PO
PK/collared doves
3
Gram‐positives only.
100/25
8–12
PO
PK/collared doves
3
60–120
8–12
IM
PK/collared doves
3
125
8
PO
PK/blue‐fronted Amazon parrots
4
Piperacillin/tazobactam
100
8–12
IM, IV
PK/bsittacines
5
Ticarcillin
200
2–4
IM
PK/blue‐fronted Amazon parrots
6
Cephalosporins
Cephalothin
100
6
IM
PK/pigeon
7
Cephalexin
35–50
6
PO
PK/pigeon
7
Cefadroxil
100
12
PO
Psittacines, pigeons
8
Ceftiofur sodium free
10
4
IM
PK/cockatiels
9
10
8
IM
PK/orange‐winged Amazon parrots
9
Ceftiofur crystalline‐free acid
50
96
IM
PK/ring‐necked doves
10
Cefotaxime
75–100
4–8
IM
PK/blue‐fronted Amazon parrots
11
Ceftazidime
50–100
4–8
IM
11
Ceftriaxone
75–100
4–8
IM
PK/blue‐fronted Amazon parrots
11
Aminoglycosides
Amikacin
15–40
24
IM, IV
PK/cockatiels, blue‐fronted Amazon parrots
12, 13
Preferred aminoglycoside; potentially nephrotoxic.
Gentamicin
2.5–10
24
IM
PK/cockatiels, scarlet macaws, rose‐breasted cockatoos
12
Nephrotoxic.
Tobramycin
2.5–10
24
IM
Empirical
Empirical, based on gentamicin studies; used for Pseudomonas aeruginosa.
Fluoroquinolones
Enrofloxacin
7.5–15
12–24
IM
PK/African gray parrots
14
IM injection causes muscle irritation.
7.5–15
12–24
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