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2. Specific pharmacokinetic (PK) behaviour: • Distribution to the entire gastrointestinal tract; •


• A sustained activity at the site of infection. 1. Favourable MICs


Lawsonia intracellularis susceptibility testing Limited data on the in vitro sensitivity of L.i. strains to anti- biotics are available. This is due to the difficulty of isolating and maintaining L.i. from faecal or intestine samples and the need to use complicated cell culture systems to evaluate antimicrobial activity in vitro. The in vitro sensitivities of L.i. strains from important pig production countries are shown in Table 1. The MICs evaluate the L.i. antimicrobial susceptibility in four different important pig production regions worldwide. The intracellular MICs were determined to measure the effect of the antibiotics on L.i. when the intracellular organisms were inside the enterocytes. The intracellular MIC results show that tiamulin is the most active compound against the test- ed L.i. isolates. Tylosin shows intermediate activity and linco- mycin is the least active compound with high MICs. High MIC variation was determined for chlortetracycline.


Brachyspira hyodysenteriae susceptibility testing Sensitivity tests with B.h. strains are conducted routinely worldwide in several institutes with experience on Brachyspira culturing, isolation and MIC testing. In Table 2, results of MIC investigations on the use of broth microdilution method (one study use of agar dilution) are summarised. In the majority of the countries, low MIC values and high susceptibility to tiamu-


Intracellular absorption/high concentration in crypts and epithelial cells of large intestine; and


Table 1 – Intracellular MIC endpoints (in µg/ml) for different antimicrobials against L. intracellu- laris strains from USA, Denmark, UK, Brazil, Thai- land and Korea. The bacteria were tested twice.


L.i. strain US No. 1 L.i. strain US No. 2 L.i. strain US No. 3 L.i. strain US No. 4 L.i. strain US No. 5 L.i. strain US No. 6 L.i. strain DK


L.i. strain UK No. 1 L.i. strain UK No. 2 L.i. strain UK No. 3 L.i. strain Brazil No. 1 L.i. strain Brazil No. 2 L.i. strain Thailand No. 1 L.i. strain Thailand No. 2 L.i. strain Thailand No. 3 L.i. strain Korea No. 1 L.i. strain Korea No. 2


Lincomycin 128 128 16


128 128 128


16-32 16 8


8-64


>128 >128 >128 >128 >128 >128 16


Tiamulin 0.125


0.125-0.5 0.125 0.125 0.125 0.125 0.125 0.125 0.125 0.125 0.125 1-2 1 1


0.125-0.25 2


0.25-0.5


Tylosin 2-8


8-32


0.25-0.5 4-8 4 2


0.5-1 1 1


0.5-2 2-8 2


8-16 4 2


0.25 0.25-0.5


lin was found. In contrast, most Brachyspira isolates show low sus- ceptibility to lincomycin and tylosin with constantly high MICs. B.h. isolates displayed heterogeneous susceptibility to tylvalosin with moderately high MICs.


2. Pharmacokinetics Once the MIC against L.i. and B.h. strains is determined it is neces- sary to relate the MIC data to the PK characteristic of the antibiot-


▶PIG PROGRESS | Volume 36, No. 3, 2020 19


Chlortetracycline 4-8


4-16


16-32 64


0.125 64


0.25 0.5


8-16 2-8


32-64 8-16


64-128 >128 32 8


2-4


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