Bacterial urinary tract infections (UTIs) typically result when normal skin and GI tract flora ascend the urinary tract and overcome the normal urinary tract defenses that prevent colonization. Bacterial UTI is the most common infectious disease of dogs, affecting 14% of all dogs.
Although UTIs are uncommon in young cats, the incidence of UTI is much higher in older cats, which may be more susceptible to infection because of diminished host defenses secondary to aging or concomitant disease (eg, diabetes mellitus, renal failure, or hyperthyroidism). Approximately two-thirds of older cats with UTIs also have some extent of chronic kidney disease.
Bacterial UTIs in ruminants are associated with catheterization or parturition in females and, as both cause and consequence, with urolithiasis in males.
In horses, UTIs are uncommon and typically are associated with bladder paralysis, urolithiasis, or urethral damage.
Many animals with UTI do not display clinical signs. Consequences of bacterial UTI can be major if the infection goes undiagnosed and untreated.
Colonization of any part of the urinary tract with bacteria increases susceptibility to infection in other parts of the urinary tract and body. Some consequences of undiagnosed UTI include infertility, urinary incontinence, diskospondylitis, pyelonephritis, and renal failure. Septicemia can result from UTI in immunocompromised patients.
In intact males, UTI frequently extends to the prostate gland or other accessory sex glands. The blood-prostate barrier makes it difficult to eradicate bacteria from the prostate gland, potentially resulting in reinfection of the urinary tract after appropriate treatment, systemic bacteremia, infection of other parts of the reproductive tract, or local infection within the prostate and eventual abscess formation.
Large, retrospective studies have documented the most common species of uropathogens in dogs and cats. Escherichia coli is the single most common pathogen in both acute and recurrent UTIs. The other common pathogens include Staphylococcus, Proteus, Streptococcus, Klebsiella, and Pseudomonas spp. In UTIs in horses, E coli, Streptococcus, and Enterococcus spp predominate, whereas Corynebacterium renale and E coli are the most common pathogens in ruminants. In immunocompromised animals, funguria from Candida spp may occur.
With chronic UTI due to highly resistant bacteria, treatment options are limited.
Bacteriologic culture of urine (with concomitant antimicrobial susceptibility testing) is the gold standard for diagnosis of UTI. Indications include clinical signs, visualization of bacteria during urine sediment examination, evidence of pyuria, dilute urine (urine specific gravity < 1.013), and immunosuppressive conditions.
Antimicrobial Treatment of Bacterial UTIs in Animals
Antimicrobials are the cornerstone of UTI treatment, and many animals with recurring UTIs are managed empirically with repeated courses. This approach fails if the underlying pathophysiologic process predisposing the animal to the UTI is not addressed; it also encourages the emergence of resistant bacteria.
Considerations for choice of antimicrobial include the pharmacokinetics and pharmacodynamics of the drug, potential adverse effects, ease of administration, and cost.
Urine concentrations of antimicrobials are more important than serum concentrations during treatment of sporadic bacterial cystitis (simple UTI); however, results of susceptibility testing typically reflect achievable serum concentrations. In general, urine concentrations exceed serum concentrations if the antimicrobial is excreted in an active form in the urine.
If the urine concentration is ≥ 4 times the minimum inhibitory concentration (MIC), the drug will most likely be successful in treating UTI that is due to that pathogen. Despite a susceptibility testing result of R (resistant) for amoxicillin in the treatment of sporadic cystitis due to E coli or Staphylococcus pseudintermedius in dogs or cats, the extremely high urine concentrations attained make amoxicillin a first choice for treatment. Similarly, injectable penicillin G is efficacious as a first-line treatment of simple UTI due to E coli in horses, cattle, and small ruminants.
Pharmacokinetic and pharmacodynamic integration should be considered in determining the appropriate dosage regimen. For beta-lactam antimicrobials, there is a significant correlation between the time that the drug concentration is greater than the MIC (T > MIC) in serum, urine, or renal tissue and the effect in either urine or urinary tract tissue.
The importance of T > MIC may explain the poor efficacy results of beta-lactam antimicrobials in the treatment of UTI, which are not administered frequently enough. Although the labeled dose of amoxicillin for dogs and cats is sufficient, the labeled frequency (every 12 hours) may be less effective than dosing every 8 hours.
More frequent dosing affects client compliance in administering the drug. The importance of client compliance makes daily-dosed drugs (eg, fluoroquinolones, cefpodoxime) or long-acting injectables attractive. Highly protein-bound beta-lactams (eg, cefovecin, ceftiofur) provide 14 days of treatment after a single injection.
Because their bactericidal effect is concentration dependent, the efficacy of fluoroquinolones and aminoglycosides correlates best to ratios of the area under the concentration-time curve (AUC) to the MIC (AUC:MIC ratios). In a murine model, treatment with gentamicin and a fluoroquinolone markedly lowers bacterial counts, compared to treatment with beta-lactam antimicrobials, indicating that the rapid killing of bacteria is important in the treatment of UTI.
For dogs, antimicrobials should be administered just before bedtime or in conjunction with confining the dog, to maintain high urine concentrations in the bladder for the longest possible time.
Penicillins
Amoxicillin and Ampicillin
Amoxicillin and ampicillin are bactericidal and relatively nontoxic, with a spectrum of antibacterial activity greater than that of penicillin G. They have excellent activity against staphylococci, streptococci, enterococci, and Proteus, and they may achieve urinary concentrations high enough to be effective against E coli and Klebsiella. Pseudomonas and Enterobacter are resistant.
In dogs and cats, amoxicillin is more bioavailable (better absorbed from the GI tract) than is ampicillin, so its dosage is lower. Absorption of ampicillin is also affected by feeding, so successful treatment may be easier to achieve with amoxicillin. Injectable amoxicillin trihydrate is approved for use in cattle and swine and can be administered to small ruminants.
As penicillins, amoxicillin and ampicillin are weak acids with a low volume of distribution. Therefore, they do not achieve therapeutic concentrations in prostatic fluid.
Amoxicillin-Clavulanic Acid
Amoxicillin-clavulanic acid is administered orally in dogs and cats. Clavulanic acid potentiates amoxicillin's spectrum of activity against gram-negative bacteria. Clavulanic acid irreversibly binds to beta-lactamases, enabling the amoxicillin fraction to interact with the bacterial pathogen. This combination usually has excellent bactericidal activity against beta-lactamase–producing staphylococci, E coli, and Klebsiella. Pseudomonas and Enterobacter remain resistant.
However, clavulanic acid undergoes some hepatic metabolism and excretion, so the antimicrobial activity in urine may be due primarily to the high concentrations of amoxicillin achieved in urine. It is not clear that amoxicillin-clavulanic acid is more efficacious for sporadic bacterial cystitis than is amoxicillin, and most treatment guidelines suggest amoxicillin as a first-line treatment for UTIs in dogs and cats.
Cephalosporins
Cephalosporins have greater stability to beta-lactamases than do penicillins, so they have greater activity against staphylococci and gram-negative bacteria. They have excellent activity against Staphylococcus spp, Streptococcus spp, E coli, Proteus, and Klebsiella. Pseudomonas, enterococci, and Enterobacter are resistant.
The use of cephalosporins predisposes patients to enterococcal infections, including vancomycin-resistant clones. Vomiting and other GI signs may occur in dogs and cats treated with cephalosporins.
Cefadroxil and Cephalexin
Cefadroxil and cephalexin are first-generation cephalosporins. Cefadroxil is a veterinary-labeled suspension or tablet product. Cephalexin is available in both human and veterinary formulations as tablets, or suspension products.
Like the penicillins, cefadroxil and cephalexin are bactericidal, acidic drugs with a low volume of distribution, and they are relatively nontoxic.
Cefovecin
Cefovecin is an injectable, third-generation cephalosporin approved for the treatment of dogs with UTI due to E coli or Proteus. In cats, it is approved only for skin infections; however, it may be used in an extra-label manner for UTIs. Because of the large extent of protein binding, SC administration results in concentrations that are effective for 14 days, making this an attractive treatment choice for fractious patients.
Cefpodoxime
Cefpodoxime is an oral, third-generation cephalosporin approved for use in dogs for skin infections (wounds and abscesses); however, it is used in an extra-label manner to treat canine UTI. Cefpodoxime has a relatively long half-life in dogs because of the large extent of protein binding, and is dosed every 24 hours.
Ceftiofur
Ceftiofur is an injectable third-generation cephalosporin approved for respiratory disease in horses, sheep, swine, and cattle and for the treatment of canine UTI due to E coli and Proteus. Ceftiofur has pharmacokinetic properties very different from those of other cephalosporins. Like cefovecin and cefpodoxime, it is highly protein bound and slowly eliminated.
After injection, ceftiofur is immediately metabolized to desfuroylceftiofur, which has antimicrobial activity different from that of the parent compound. Desfuroylceftiofur is as effective as ceftiofur against E coli (MIC 4 mcg/mL); however, it is much less effective against Staphylococcus spp, and its activity against Proteus varies (MIC 0.5–16 mcg/mL).
Because of the instability of desfuroylceftiofur, microbiology services use a ceftiofur disk when performing susceptibility testing, so a false expectation of therapeutic efficacy may result for some pathogens. Pseudomonas, enterococci, and Enterobacter spp are resistant to ceftiofur and desfuroylceftiofur.
Ceftiofur is associated with a duration- and dose-related thrombocytopenia (see Platelet Disorders) and anemia in dogs, which would not be expected with the recommended dosage regimen.
Chloramphenicol
Chloramphenicol has a high volume of distribution, and high tissue concentrations can be achieved, including in the prostate of male dogs and cats. The drug is active against a wide range of gram-positive and many gram-negative bacteria, against which it is usually bacteriostatic. Chloramphenicol is typically active against enterococci, staphylococci, streptococci, E coli, Klebsiella, and Proteus. Pseudomonas spp are resistant.
North American isolates of methicillin-resistant Staphylococcus aureus and Staphylococcus pseudintermedius are typically susceptible. Even though chloramphenicol is well known for causing idiosyncratic (non-dose-dependent) anemia in humans and dose-dependent bone marrow suppression in animals, its use in both human and veterinary medicine is increasing because of resistance to other antimicrobial drugs. It is banned for use in food animals in most countries.
Fluoroquinolones
The fluoroquinolones are bactericidal, amphoteric drugs. They possess acidic and basic properties; however, they are very lipid soluble at physiologic pH (pH 6–8), so they have a high volume of distribution.
All fluoroquinolone drugs usually have excellent activity against staphylococci and gram-negative bacteria; however, their activity against streptococci and enterococci may vary. The treatment advantage of these drugs is their gram-negative antimicrobial activity and large extent of lipid solubility.
The use of fluoroquinolones should be reserved for UTIs that are due to gram-negative bacteria (especially Pseudomonas spp) or to uropathogenic E coli (UPEC) that are potentially intracellular in location. Their excellent penetration into the prostate gland and activity in abscesses also make them good candidates for treating UTIs in intact male dogs.
Enrofloxacin, orbifloxacin, and marbofloxacin are all fluoroquinolones approved to treat UTIs in dogs. Although all three of these drugs are used in cats, only some are approved for this use. In cats, enrofloxacin should be used at dosages of < 5 mg/kg every 24 hours, if at all, because of retinal toxicity caused by a genetic polymorphism. Their extra-label use in food animals may be restricted in some countries.
Pradofloxacin is approved for use in both dogs and cats in Europe and Canada. In the US, however, it is approved for use only in cats, because of a small incidence of thrombocytopenias occurring in treated dogs. Large animal injectable fluoroquinolone formulations are available for the treatment of respiratory tract infections in food animals; however, extra-label drug use in the US is strictly prohibited.
Ciprofloxacin is the most commonly used human fluoroquinolone, and it may be less expensive than veterinary products approved for use in very large dogs. Oral bioavailability is markedly lower in dogs than in humans and requires higher dosages. However, pharmacokinetic differences in oral absorption in other animals may result in inefficacy.
Because they are concentration-dependent killers with a long postantimicrobial effect, the fluoroquinolones are efficacious with once-daily, high-dose treatment for a short time period. The newest fluoroquinolone for dogs and cats, pradofloxacin, requires two genetic mutations for resistance, so MICs for Enterobacteriaceae are lower than those of other fluoroquinolones, and it is hoped that pradofloxacin will be less selective for antimicrobial resistance.
The fluoroquinolones should be avoided for chronic, low-dose treatment because this approach encourages the development of bacterial resistance—often to multiple drugs. Cases that involve Pseudomonas spp and UPEC should be carefully investigated for underlying disease processes. Once Pseudomonas spp and E coli become resistant to fluoroquinolones, there are no other convenient treatment options.
Aminoglycosides
Aminoglycosides (eg, gentamicin, amikacin) very large, polar (water-soluble) molecules. They have a low volume of distribution and do not penetrate the blood-prostate barrier. They are not absorbed orally and must be administered by SC, IM, or IV injection.
Aminoglycosides have a spectrum of activity similar to that of fluoroquinolones. However, their use against UTI is limited because of the necessity of parenteral injections and the risk of toxicosis with anything but short-term use.
Like fluoroquinolones, aminoglycosides have concentration-dependent bactericidal activity with a long postadministration effect. Therefore, once-daily treatment of short duration is effective and minimizes the risk of nephrotoxic effects.
Aminoglycosides can be considered for in-hospital or outpatient treatment of UTI due to fluoroquinolone-resistant pathogens; however, the importance of identifying and correcting underlying disease must be emphasized. Renal accumulation means that prolonged withdrawal periods are required if these drugs are used in food animals.
Nitrofurantoin
Nitrofurantoin is a human product available as tablets, capsules, and a pediatric suspension. It is not commonly used in veterinary medicine. It is typically used only to treat UTI in humans, because it has a low volume of distribution and effective concentrations are attained only in urine.
Considered a carcinogen, nitrofurantoin is banned for use in food-producing animals in some countries. However, its use in small animals is increasing with the rising rates of antimicrobial resistance to veterinary antimicrobials.
Nitrofurantoin is used to treat infections due to E coli, enterococci, staphylococci, Klebsiella spp, and Enterobacter spp. It is increasingly indicated for treatment of UTIs due to multidrug-resistant bacteria, which are otherwise difficult to treat with conventional veterinary antimicrobial agents.
The pharmacokinetics and adverse-effect profile of nitrofurantoin have not been investigated in dogs, cats, or horses, and the need for multiple daily doses makes it inconvenient for owners.
Tetracyclines
Tetracyclines are bacteriostatic, amphoteric drugs with a high volume of distribution. They are broad-spectrum antimicrobials.
Because of plasmid-mediated resistance, susceptibility varies in staphylococci, enterococci, Enterobacter, E coli, Klebsiella, and Proteus. In most tissues, Pseudomonas spp are resistant. However, tetracyclines are excreted unchanged in urine, so high concentrations in the urine may result in treatment efficacy.
Doxycycline is a highly lipid-soluble tetracycline better tolerated in cats than other tetracyclines are. It reaches therapeutic concentrations in the prostate, so it may be useful for some UTIs. Doxycycline may also be effective to treat methicillin-resistant staphylococcal UTIs in small animals.
If doxycycline is administered in capsule or tablet form, it is critical that the dog or cat eat a small amount of food or drink afterward to ensure passage into the stomach. If capsules remain in the esophagus, severe local necrosis with subsequent esophageal stricture can occur.
Long-acting and short-acting injectable oxytetracycline formulations may be useful in large animals.
Trimethoprim-Sulfonamides
Trimethoprim-sulfonamides (TMP-sulfas) are combinations of two dissimilar drugs that act synergistically on different steps in the bacterial folic acid pathway. Trimethoprim is a bacteriostatic, basic drug with a high volume of distribution and a short elimination half-life. The sulfonamides are bacteriostatic, acidic drugs with a medium volume of distribution and long half-lives (ranging from 6 to > 24 hours).
TMP-sulfas are formulated in a 1:5 ratio of TMP to sulfa, although the optimal bactericidal concentration is a ratio of 1:20 TMP:sulfa. Microbiology services use the 1:20 ratio in susceptibility testing; however, the widely varying pharmacokinetic properties of this drug combination make it difficult to determine a therapeutic regimen that achieves the 1:20 ratio at the infection site.
Although the TMP-sulfa combination does penetrate the blood-prostate barrier, sulfa drugs are ineffective in purulent material because of freely available para-aminobenzoic acid from dead neutrophils. The TMP-sulfa combination is synergistic and bactericidal against staphylococci, streptococci, E coli, and Proteus. Its activity against enterococci and Klebsiella varies, and Pseudomonas is resistant.
TMP-sulfas are associated with various adverse effects, and chronic low-dose treatment may result in bone marrow suppression and keratoconjunctivitis sicca in dogs. Injectable formulations of TMP-sulfas are available for humans, horses, and food animals in some countries.
Ancillary Treatment of Bacterial UTIs in Animals
Various ancillary treatments have been used in the management of bacterial UTIs in animals.
Cranberries contain proanthocyanidins, which can inhibit the adhesion of E coli to the uroepithelium by interfering with bacterial fimbriae. Although cranberry extract has been marketed for use in animals, evidence to support its use in the management of bacterial UTIs in animals is lacking.
Phenazopyridine is used as a urinary tract analgesic in humans. Phenazopyridine is contraindicated in cats because of dose-related methemoglobinemia and oxidant injury to erythrocytes, resulting in fatal Heinz body hemolytic anemia. Scientific information on dosage, efficacy, and safety in other animals is lacking.
Urinary tract antiseptics (eg, methenamine) are sometimes used adjunctively in the management of bacterial UTIs in humans; however, their effectiveness has not been substantiated in animals.
Although pharmacological treatment to support the glycoprotein lining of the transitional epithelium of the urinary tract (eg, chondroitin sulfate, polysulfated glycosaminoglycans) in the management of bacterial UTIs in animals is reasonable, data to support specific recommendations are lacking.
Dosage Regimens for Sporadic Bacterial Cystitis in Animals
Sporadic bacterial cystitis (simple urinary tract infection) is common in dogs; it is due to a temporary break in host defenses, is associated with clinical signs of lower urinary tract inflammation, responds quickly to appropriate treatment, and does not readily recur (recurrent cystitis is usually defined as ≥ 3 episodes of UTI in the previous 12 months). Treatment with NSAIDs may help ameliorate clinical signs.
Because most antimicrobials achieve high concentrations in the urinary tract tissues and urine, most cases of sporadic bacterial cystitis are onetime infections that respond well to appropriate treatment.
Treatment for a sporadic UTI may be empirical, based on knowledge of the commonly isolated pathogens and their typical susceptibility to antimicrobials. In dogs, the antimicrobial should be administered last thing at night to ensure that the bladder contains urine with a high antimicrobial concentration for as long as possible.
For all species, the duration of treatment for UTI is controversial. Although animals are commonly treated with antimicrobial drugs for 10–14 days, shorter-duration antimicrobial regimens are routinely prescribed in human patients, including single-dose fluoroquinolone treatment.
A clinical comparison between 3 days of a once-daily high dose of enrofloxacin and 2 weeks of twice-daily amoxicillin-clavulanic acid showed equivalence in the treatment of sporadic bacterial cystitis in dogs (1). However, additional studies are needed to determine the optimal dosage regimens for different classes of antimicrobials, and it is inappropriate to use fluoroquinolones as a first-line treatment for sporadic bacterial cystitis.
If clinical signs resolve, posttreatment urinalysis or urine culture is not recommended for cases of sporadic cystitis.
Management of Recurrent Bacterial Cystitis in Animals
Recurrent bacterial cystitis is uncommon in large animals but frequently affects small animals. In dogs and cats, if UTIs occur only once or twice yearly, each episode may be treated as sporadic bacterial cystitis.
In dogs, recurrent UTIs are often due to different strains or species of bacteria. These reinfections are attributed to reinoculation of the urinary tract by GI flora in a host with deficient immune defense mechanisms. Bacteriologic culture of urine and antimicrobial susceptibility testing are still indicated.
Usually, recurrent UTIs are due to an identifiable underlying cause (eg, endocrinopathy, immunosuppressive drug treatment). The underlying disease process must be addressed whenever possible.
Ideally, a clinical cure is achieved with minimal adverse effects. A complete microbiological cure (ie, continually normal results of culture) is desirable but often not achievable.
Because of concerns about antimicrobial resistance, chronic antimicrobial treatment is no longer routinely recommended for patients with recurrent UTIs—even those with comorbidities, such as diabetes mellitus. The current recommendation is short-duration treatment (3–5 days) without follow-up culture.
For longer treatments, urine culture should be considered after 5–7 days of treatment. If results of culture are positive during appropriate treatment, issues of client compliance or underlying disease that is preventing bacterial eradication (eg, urolithiasis) should be investigated. If culture results are normal and clinical signs have resolved, discontinuation of treatment may be considered.
If clinical signs have resolved, posttreatment cultures are no longer recommended. The presence of bacteria after treatment in the absence of clinical signs is considered subclinical bacteriuria (see below).
Management of Relapsing UTIs in Animals
Relapsing UTIs may be due to underlying disease (eg, urolithiasis, pyelonephritis) that allows the recurrence of clinical signs because the original bacteria were never completely eliminated despite appropriate treatment or because uropathogens with enhanced intrinsic virulence are present.
Bacterial virulence factors enhance colonization of the urinary epithelium and development of UTIs. Strains of uropathogenic E coli (UPEC) have multiple virulence mechanisms that enable them to invade, survive, and multiply within the uroepithelium. UPEC strains are responsible for > 90% of UTIs and are often found among the fecal flora of the same host. The sequestration of UPEC within the bladder uroepithelium presents a challenge in the treatment of human and veterinary patients. Relapses are often suspected when the same species of pathogen is repeatedly found on culture, especially if the susceptibility pattern is the same.
For relapsing UTIs due to UPEC, it is important to ensure that adequate antimicrobial concentrations are achieved in the urine and bladder uroepithelium. Unfortunately, the antimicrobials that achieve therapeutic intracellular concentrations are limited. They include fluoroquinolones, tetracyclines, and chloramphenicol.
Subclinical Bacteriuria in Animals
Subclinical bacteriuria is not uncommon. It occurs at rates of up to 12% in otherwise healthy dogs and up to 30% in dogs with comorbidities such as diabetes mellitus or chronic kidney disease, or that are being treated with immunosuppressive drugs. Despite fears of secondary complications, there is little evidence that subclinical bacteriuria increases the risk of clinical UTI or other infectious complications in dogs or cats.
In human medicine, it is standard practice not to treat subclinical bacteriuria, even in compromised patients (eg, patients with diabetes or hyperadrenocorticism). Treatment may eliminate the bacteriuria in the short term; however, recolonization is common, and it is associated with increasing antimicrobial resistance. Therefore, if a dog or cat has no clinical signs of UTI, it is reasonable not to treat with antimicrobials or at least to restrict the treatment to a short duration (eg, 5 days).
Even isolation of a multidrug-resistant pathogen does not necessarily mandate treatment. In patients unable to show clinical signs of UTI (eg, patients with spinal cord injury, immunosuppressed patients), the veterinarian must make a clinical judgment about whether to treat.
Antimicrobial Resistance in Uropathogens
Acquired resistance to antimicrobials by uropathogens is of great concern in both human and veterinary medicine. Multidrug resistance in uropathogens is increasingly encountered, particularly with regard to infections in dogs and cats. Extended-spectrum beta-lactamase genes are increasingly identified in E coli isolates from companion animals.
Increases in the occurrence of fluoroquinolone-resistant E coli in dogs have been widely reported. Because the mechanism of resistance to fluoroquinolones frequently involves efflux pumps, it also conveys multidrug resistance. Fluoroquinolone resistance is also increasing in other uropathogens, including enterococci, Proteus mirabilis, and Staphylococcus pseudintermedius isolates. Methicillin-resistant staphylococci have been identified in cases of canine UTI.
There is increasing evidence that animals are an important reservoir of antimicrobial-resistant bacteria that cause infections in humans. Enterococci isolated from canine UTIs have been associated with several resistant phenotypes, with most resistant to ≥ 3 antimicrobials. An Enterococcus faecium isolate was found to be highly resistant to vancomycin and gentamicin. Sequence analysis suggested that this resistance was due to gene exchange between human and canine enterococci (2).
The use of last-resort human antimicrobials in veterinary patients with resistant infections is controversial. Vancomycin, imipenem-cilastatin, meropenem, fosfomycin, quinupristin-dalfopristin, and tigecycline should not be used routinely in the treatment of UTIs in animals. Nonantimicrobial control of infection should be considered whenever feasible.
References
Westropp JL, Sykes JE, Irom S, et al. Evaluation of the efficacy and safety of high dose short duration enrofloxacin treatment regimen for uncomplicated urinary tract infections in dogs. J Vet Intern Med. 2012;26(3):506-512. doi:10.1111/j.1939-1676.2012.00914.x
Pomba C, Rantala M, Greko C, et al. Public health risk of antimicrobial resistance transfer from companion animals. J Antimicrob Chemother. 2017;72(4):957-968. doi:10.1093/jac/dkw481
