Excellent Susceptibility of E. coli Strains from Europe and Thailand to ApravetĀ® and PK/PD Relationships

Ulrich Klein

Colibacilliosis infections caused by various Escherichia coli patho- and virotypes pose a major challenge for pig farms worldwide.

Figure 1a. E. coli neonatal diarrhoea

Figure 1b. E. coli post-weaning diarrhoea

Figure 1c. Pure culture of haemolytic E. coli on blood agar

 

Monitoring of antimicrobial susceptibility trends over time is an important component of antimicrobial stewardship to ensure long-term antimicrobial efficacy. Susceptibility tests to determine the minimum inhibitory concentration (MIC) of antimicrobials are conducted worldwide according to validated and internationally recommended methods (Clinical Laboratory Standards Institute (CLSI), 2018). 

The antibiotic selected for E. coli therapy must reach sufficient concentrations in the small intestine. The evaluation of the isolated E. coli strain's antimicrobial susceptibility and the use of the basic pharmacokinetic information is fundamental for a correct therapy.

 

Strengths of Apravet® and aminoglycosides

  • Apravet® is the treatment alternative to antibiotics (zinc oxide, colistin, enrofloxacin) with use restrictions
  • Apravet® works in a concentration dependent way (bactericial effect)
  • Aminoglycosides are not active in anaerobic conditions (major activity in small intestine)
  • Gastro-enteric enzymes can alter the aminoglycosides (Apravet® is more resistant to this than neomycin)

 

The apramycin MIC results from European (total of 238 VetPath strains) and Thailand (total of 311 strains) E. coli strains are presented and put in relation to the pharmacokinetics of Apravet® (apramycin). The European MIC data were generated by the broth microdilution method (BMM), and the Thailand MIC data using the agar dilution method (AD). The MIC data are put into relationship to Apravet® pharmacokinetic data.

 

Activities and characteristics of Apravet®

Apravet® belongs to the aminoglycoside group of antibiotics, which are poorly absorbed by the gastrointestinal tract. The antibacterial action of Apravet® is directed primarily against aerobic, Gram-negative bacteria like E. coli

In a pharmacokinetic study, Apravet® was used at an infeed treatment dose (8mg/kg body weight, 21 consecutive days of treatment) and the apramycin concentration in the blood, small and large intestines were determined during treatment and 12, 24, 36 and 48 hours after the treatment had stopped. 

 

Europe and Thailand apramycin MIC results

The collection of E. coli strains isolated from EU and Thai diarrhoea cases provided the opportunity to identify differences between EU countries and Thailand with regard to apramycin sensitivity.

No differences in the susceptibility patterns to apramycin were found between the E. coli isolates from the seven European countries involved in the study. As indicated in Figure 2, 94% of the EU strains had a low apramycin MIC (between 1 to 16µg/ml). Only a small number of EU isolates (6%) showed MICs between 256-1024µg/ml. The results verify the very high sensitivity of EU E. coli strains to apramycin.

 

Figure 2. Susceptibility of European E. coli strains to apramycin (Apravet®, VetPath strains isolated 2019-2020)

 

Consistent MIC50 values were found. The MIC90 values and MIC ranges indicate a trend on bi-modal MIC distribution for countries like Denmark, Spain and the United Kingdom as shown in Table 1.

 

Table 1. MIC50, MIC90 and MIC ranges (µg/ml) of apramycin (Apravet®) for E. coli strains from different European countries

 

The distribution of apramycin (Apravet®) MICs of respective E. coli isolates from Thailand are shown in Figure 3. The Thai MICs are unimodally distributed. 

Low apramycin (Apravet®) MICs (4 to 16µg/ml) were determined for the Thai E. coli strains generated in three different regions. Only a small number of Thai isolates (6%) showed MICs between 64-512µg/ml.

 

Figure 3. Susceptibility of Thai E. coli strains to apramycin (Apravet®, strains isolated in 2020-2021)

 

The MIC50 and MIC90 values, as shown in Table 2, were consistent indicating no variation between the Thai regions involved.

 

Table 2. MIC50, MIC90 and MIC ranges (µg/ml) of apramycin (Apravet®) for E. coli strains from different regions of Thailand

 

The pharmacokinetic (PK) and pharmacodynamic (PD) relationships of Apravet® (apramycin) determine its clinical efficacy, which is of great importance for the swine veterinarian. 

Figure 4 summarizes the pharmacodynamic (MIC) data generated in Europe and Thailand and relates those to the results of an Apravet® pharmacokinetic study.

 

Figure 4. PK/PD relationships for E. coli strains from Europe and Thailand following feed administration of Apravet® at a therapeutic dose (8mg apramycin sulphate/kg body weight)

 

The apramycin concentrations achieved in the upper and mid small intestine (duodenum and jejunum) which are relevant for E. coli infections at a treatment dose exceeds the MICs of 94% of the tested European E. coli strains (n=223) in comparison to 6% (n=15) with MICs higher than the small intestine concentration achieved. This means a substantial amount of the Apravet® dose is available in the small intestine and can provide a therapeutic effect. 

From the Thai strains, 6% (18 from 311 isolates) of the isolates had a higher MIC versus the small intestine concentration achieved. The small intestine peak apramycin concentration achieved at treatment dose (45µg/g) and 12 hours after medication cessation (33.2µg/g) were high, exceeding the determined MIC values of 94% of the European and Thai strains.

 

Conclusions from the European and Thai MIC data and Apravet® pharmacokinetics

According to the guidelines for the prudent use of antimicrobials in veterinary medicine (EU 2015/C 299/04), the described data justify the use of Apravet® for E. coli treatment in Europe and Thailand.

 

References are available on request