How Many Bacteria Does It Take To Infect A Pig’s Bladder? By George Charbonneau from South West Ontario Veterinary Services

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Urinary tract infection (UTI) is one of the most common bacterial infections worldwide. Cystitis is an inflammation of the urinary bladder. This infection is often the result of an ascending bacterial infection. The condition is often subclinical in pigs but in more severe cases it can be associated with loss of appetite, frequent urination, vulval discharge, and fever. When sows are observed at slaughter it is not unusual to find that 15 % of sows are affected by cystitis. The incidence can be much higher in some herds.  In most cases opportunistic bacteria that inhabit the vagina and urethra will ascend along the urethra into the urinary bladder. If these pesky critters do try to swim upstream into the bladder they can encounter enough downstream urine flow to make that impossible. When sows are not drinking enough water the bacteria can more easily swim upstream. These bacteria can originate in the intestinal or reproductive tract of the sows. The organisms may have been gradually increasing in the sows environment when there is suboptimal hygiene. Escherichia coli is the predominant bacterial species associated cystitis and it accounts for about  70% of cystitis cases. If not diagnosed and treated, chronic cystitis can increase the number of stillborns, preweaning mortality, and reduce pregnancy rate and litter size at next breeding.

When discussing cystitis with farm staff it is not unusual to observe a more sympathetic response from women. Urinary tract infection (UTI) will affect approximately 50% of women at some point during their lifetime. In women the predominant bacterial agent is also Uropathogenic E coli (UPEC) . Unfortunately, in many women the infection can reoccur within 2 to 6 months and may trouble them for long periods of their lives. Because this disease is so common in people researchers are always looking for an animal model that can be used to study UTI. Often a mouse model is used but it does not seem to correlate well with how the disease works in people. It has been harder to study early bladder colonization and the early innate defence mechanisms elicited to prevent this. These Danish researchers wanted to test out a model of urinary tract infection in pigs. Pig are naturally susceptible to UTI and have greater similarity to the physiology and anatomy of the human urinary tract than traditional rodent UTI models. In this study the pig model was used to investigate the minimal infectious inoculum of uropathogenic Escherichia coli that could initiate a bladder infection in a pig. The researchers used a 5 minute installation of 100 ml of a challenge culture containing 100 c.f.u. /ml . After 5 minutes of sitting in the bladder almost all of the inoculum was pulled back out. The researchers could not produce an exact number of bacteria remaining in the bladder but suggested that the number was likely only 1 to 3% (i.e. 10–30 bacteria) or less of the initial bacteria that were infused.

The researchers found the following:

  • as few as 30 Uropathogenic E coli organisms were able to adhere to the bladder surface and in some but not all of the animals were able to progress into a UTI with associated inflammation.
  • as a positive control the researchers infused the environmental bacteria P. protegens at a much much higher challenge level but these bacteria were not able to colonize the bladder even after increasing the contact time to 1 hour. was unable to colonize the porcine bladder despite inoculation at a considerably higher

Take Home Messages

  • after many years of relating the importance of preventing urinary tract infections in sows to the situation in people it appears that there is some good evidence that pigs and people are very similar in their susceptibility to urinary tract infections
  • the number of bacteria required to establish a urinary tract infection in pigs is exquisitely small. It is no wonder that this is such a common problem in sows and that attention to detail on factors such as water flow and hygiene are critical for minimizing the impact on production.

Submitted by George Charbonneau, DVM

Reference: Kristian Stærk , Morten Østergaard Andersen , Thomas Emil Andersen  Uropathogenic Escherichia coli can cause cystitis at extremely low inocula in a pig model  J Med Microbiol. 2022 Apr;71(4). doi: 10.1099/jmm.0.001537.