Denagard Performance – Ileitis in vitro studies

In vitro studies show that L. intracellularis, the cause of ileitis, is sensitive to Denagard.
calendar icon 14 February 2012
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Key points

  • Ileitis, a disease caused by the pathogen Lawsonia intracellularis, remains a widespread and costly problem in pigs worldwide.
  • In vitro tests with Lawsonia intracellularis strains isolated in Europe and United States demonstrate their high susceptibility to tiamulin.
  • Tiamulin, the active ingredient in Denagard, is the most active antimicrobial inhibiting the intracellular and extracellular activity of Lawsonia intracellularis.
  • These in vitro test results are consistent with the clinical response to Denagard observed in the field and in many in vivo studies in the case of Lawsonia-based infections.

Ulrich Klein
International Technical Services Manager


Ileitis, also known as proliferative enteropathy, is a disease that affects growing and finishing pigs. It is caused by the obligate intracellular bacterium Lawsonia intracellularis and is a costly disease because it results in poor appetite, poor growth and sometimes scours. Ileitis can also be present in an acute form called proliferative haemorrhagic enteropathy, which usually strikes older pigs and can lead to acute intestinal haemorrhage and sudden death. Sows can have subclinical L. intracellularis and be carriers of the pathogen1.

The incidence and severity of ileitis is thought to have increased worldwide, although this apparent change in pattern may represent a switch from the chronic to more acute form of the disease1.

Until 1995, little research had been conducted or published regarding antibiotic sensitivity to L. intracellularis, which hindered effective treatment of ileitis in the field. Ileitis was treated with a wide range of antibiotics and recommendations for treatment were not based on in vitro or in vivo susceptibility testing2.

When in vitro studies were conducted in the 1990s by Dr Steven McOrist and colleagues, they showed that tiamulin, the active ingredient in Denagard and a type of pleuromutilin antibiotic, was highly active against L. intracellularis. These study results are confirmed by new data generated with European and US strains of Lawsonia intracellularis.

Denagard (tiamulin) concentrates inside cells where L. intracellularis lives, as the pathogen’ name implies.

In vitro Studies

Development of an in vitro cell culture system necessary for the growth of L. intracellularis made it possible to test antibiotic sensitivity to the pathogen. In a study, Dr McOrist and Dr Connie J. Gebhart, assessed the sensitivity of several antibiotics to L. intracellularis by determining the minimum inhibitory concentration (MIC) for each; the investigators also determined the minimum bactericidal concentration (MBC) for several selected antibiotics3. The MIC was defined as the lowest concentration that prevented multiplication of 99% of L. intracellularis isolates. The MBC was the lowest concentration where use of a ‘pulse’ antibiotic treatment stopped the growth of L. intracellularis.

Up to three isolates of L. intracellularis were used to test each antibiotic. Both intracellular and extracellular testing was conducted. Results for four of the antibiotics tested appear in Table 1 and highlight tiamulin’s low MIC of 4mcg per ml.

In addition to a low MIC, tiamulin also had a MBC of <2mcg/ml, compared to 4mcg/ml for tylosin.

Table 1. MICs (mcg/ml) of various antibiotics for L. intracellularis, the cause of ileitis
Antibiotic No. of strains tested MIC (mcg/ml)
Intracellular activity assay
MIC (mcg/ml)
Extracellular activity assay
Tylosin tartrate 3 64 64
Lincomycin HCI 2 32 32
Spectinomycin 1 32 32
Tiamulin 3 4 4

Penicillin and fluoroquinolones had low MIC and MBC values in the study but are not widely recommended for ileitis treatment in swine. In addition, antibiotics such as penicillin are used in humans, and their use in swine raises concerns about the development of antibiotic resistance in people. Denagard is not used in humans.

McOrist and Gebhart note that although a few clinical reports suggested that bacitracin or aminoglycosides such as neomycin were useful in the treatment of ileitis, their MIC and other data would not support their use for L. intracellularis.

In 1998, McOrist and colleagues reported on additional in vitro testing of tiamulin, which confirmed the drug`s MIC was again 4mcg per ml when tested against L. intracellularis4.

In a new study, Dr S. Wattanaphansak and Dr C. Gebhart determined minimum inhibitory concentrations for several antibiotics against 10 L. intracellularis strains from the United States (n=6) and Europe (n=4). All antimicrobials were tested in a range of different concentrations (0.25 to 128µg per ml), which makes a titration of low and high MICs possible.

The MIC of each antimicrobial against L. intracellularis was identified as the lowest concentration that inhibited 99 per cent of L. intracellularis proliferation as indicated by heavily infected cells. Cells were considered in the study to be heavily infected cells if the number of L. intracellularis proliferated inside the cell was greater than 30 bacteria per cell compared to the antimicrobial-free control.

Table 2. Intracellular and extracellular MIC ranges for L. intracellularis strains from the United States and Europe5
Antimicrobial agent US L. intracellularis isolates
European L. intracellularis isolates
MIC (mcg/ml)
MIC (mcg/ml)
MIC (mcg/ml)
MIC (mcg/ml)
Chlortetracycline 4-64 32-64 0.25-16 16-64
Lincomycin 16->128 >128 8-64 32->128
Tylosin 0.25-32 1->128 0.5-2 2-16
Tiamulin 0.125-0.5 1-32 0.125 1-4

Intracellular and extracellular MIC assays for L. intracellularis were performed to mimic the real infectious situation in which L. intracellularis are exposed to antimicrobials before and after invasion into the pig small intestinal cells.

The results of four antimicrobials which are commonly used for the treatment and control of ileitis are summarized in Table 2.

The intracellular MICs for all antimicrobials were lower than the extracellular MICs.

Tiamulin was the most active antimicrobial inhibiting the intracellular activity of all L. intracellularis isolates at <0.5mcg per ml. The extracellular activity results indicate the highest sensitivity to tiamulin and lower MIC ranges for tiamulin than the other antimicrobials tested. An interesting observation from this study was that the intracellular and extracellular MICs for the European L. intracellularis strains tended to be lower than the US isolates. The authors conclude that each L. intracellularis strain has a different antimicrobial susceptibility pattern.


In vitro studies show that Denagard has low MIC values when tested against L. intracellularis, the cause of ileitis in swine. Not surprisingly, in vivo studies have also shown that Denagard is effective against L. intracellularis. Denagard works by disrupting bacterial ribosome activity and inhibiting the protein formation.


  1. McOrist S. et al. 2003. Global patterns of porcine proliferative enteropathy. The Pig Journal. 51:26-35.
  2. McOrist S. et al. 1995. Antimicrobial susceptibility of ileal symbiont intracellularis isolated from pigs with proliferative enteropathy. Journal of Clinical Microbiology. 33 (5): 1314-1317.
  3. McOrist S, and C. Gebhart. 1995. In vitro testing of antimicrobial agents for proliferative enteropathy (ileitis). Swine Health and Production, July and August 1995, p146-149.
  4. McOrist S. et al. 1998. In vitro and in-life studies of efficacy of valnemulin for proliferative enteropathy (ileitis). 15th International Pig Veterinary Society Congress, Birmingham, England.
  5. Wattanaphansak S., D. Rau, R. Singer and C. Gebhart. 2007. In vitro activity of six antimicrobial agents against ten Lawsonia intracellularis isolates from the United States and Europe. 3rd Asian Pig Veterinary Society Congress, Wuhan, China.

Further Reading

- Find out more information on ileitis by clicking here.

February 2012
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