URINARY TRACT INFECTION
The urinary tract infection is one of the most common ailments in small animal practice yet many pet owners are confused about the medical approach. Some common questions we hear are:
The kidneys make urine every moment of the day. The urine is moved down the ureters and into the bladder. The urinary bladder is a muscular little bag which stores the urine until we are ready to get rid of it. The bladder must be able to expand for filling and contract down for emptying and respond to voluntary control.
The bladder is a sterile area of the body which means that bacteria do not normally reside there. When bacteria (or any other organisms for that matter) gain entry and establish growth in the bladder, infection has occurred and symptoms can result. People with bladder infections typically report a burning sensation during urination. With pets we see some of the following signs:
Sometimes there are no symptoms at all so it is important to periodically screen patients at risk
The external genital area where urine is expelled is teeming with bacteria. Bladder infection results when bacteria from the lower tract climb into the bladder, defeating the natural defense mechanisms of the system (forward urine flow, the bladder lining, inhospitable urine chemicals etc.). Bladder infection is not contagious
TESTING FOR BLADDER INFECTION
There are many tests that can be performed on a urine sample and many people get confused about what information different tests provide.
The urinalysis is an important part of any database of laboratory tests. It is an important screening tool regardless of whether or not an infection is suspected. The urinalysis examines chemical properties of the urine sample such as the pH, specific gravity (a measure of concentration), and amount of protein or other biochemicals present. It also includes a visual inspection of the urine sediment to look for crystals, cells, or bacteria. This test often precedes the culture or lets the doctor know that a culture is in order. Indications that a culture of a urine sample should be done based on urinalysis findings include:
URINE CULTURE (AND SENSITIVITY)
This is the only test that can confirm a urinary tract infection. In this test, the urine is spun rapidly in a centrifuge to separate out the solids from the liquid. The solid part, called the sediment, is transferred to a specialized container and incubated for bacterial growth.
Growth of bacteria on the culture plate confirms bacteria in the urine. In addition, a culture at a reference laboratory identifies the species of the bacteria, estimates the concentration of bacteria (how many are growing), and includes sensitivity testing (which antibiotics will be effective.) Knowing the concentration of bacteria in the sample helps determine if the bacteria cultured might represent contaminants from the lower urinary tract or bacteria that are transient and not truly colonizing the bladder. Similarly, knowing the species of bacteria also helps determine if the bacteria grown are known to cause disease or likely to be innocent bystanders. The antibiotic profile tells us what antibiotics will work against the infection. There is, after all, no point in prescribing the wrong antibiotic. Clearly, the culture is a valuable test when infection is suspected.
Urine culture results require at least a couple of days as bacteria require at least this long to grow.
A free catch sample is obtained by catching urine mid-air as it is passed. The sample may be contaminated by the bacteria of the lower urinary tract but will not be contaminated by the floor or other environmental surface.
With the catheter method, a small tube is passed into the bladder and the sample is withdrawn. This is not the most comfortable method for the patient though the procedure is fairly quick. Potentially, bacteria can be introduced into the bladder accidentally with the catheter so this represents a drawback though fortunately, this is a rare occurrence (assuming the catheter is only for urine sample collection and not placed for longer term urine collection). The sample obtained is unlikely to be contaminated and should represent urine as it exits the bladder.
The ideal collection method is cystocentesis: a needle tap directly into the bladder. In this way, an uncontaminated sample is collected directly from the bladder. Sometimes a little blood is enters the sample during the needle stick but for culture purposes, the sample can be considered pristine.
TREATMENT FOR SIMPLE INFECTION
A simple bladder infection is usually easily treated with 7 - 14 days of antibiotics. The patient’s symptoms usually resolve quickly, within the first 2 days of treatment, though the entire course of treatment should be given. Inadequate treatment leads to infection recurrence and possibly future bacterial resistance.
Ideally, approximately 7 days after the last antibiotic dose, a new sample is cultured to be sure the infection is gone. If the infection has not cleared or if a new infection has developed, there is usually a reason why. Alternatively, a post-treatment urinalysis instead of culture (or in addition to culture) can be done to check for evidence of white blood cells.
NOT SO SIMPLE INFECTIONS
There are several special situations concerning urinary tract infections:
KIDNEY INFECTION (PYELONEPHRITIS)
If the patient’s immune system is not ideal, the infection in the bladder may ascend into the kidneys where it can cause kidney failure and a more serious infection. There is currently no good test to determine whether or not a kidney is infected though there might be hints on the lab work (urinary tract infection in combination with fever, elevated white blood cell count, pain in the area of the kidneys). Ultrasound can help and there are special radiographic studies that can help as well. If infection in the kidney is suspected, the length of the antibiotic course increases to 4-6 weeks.
Stones in the bladder can cause infection and infection can cause stones. We have created a special library center on bladder stones for more information.
In embryonic life, urine is removed from the body via the umbilical cord. A structure called the “urachus” exits the top of the bladder and enters the umbilical cord so that urine can be dumped into the mother’s bloodstream for removal by her kidneys. After birth, the urachus degenerates but sometimes a small nipple-like protrusion exists on the top of the bladder. This section can protect a bladder infection in which case recheck cultures will reveal the same organism over and over until the urachal diverticulum is surgically removed.
Bladder tumors, with or without infection, often create symptoms similar to those of a severe bladder infection. The tip off to look for a tumor is that infection and/or symptoms do not clear up with an appropriate antibiotic course, urine is bloody, and there are no bladder stones on radiographs. The most common bladder tumor is the “transitional cell carcinoma”. Our library section on this topic includes more information on how to detect this tumor; often ultrasound is needed to image the inside of the urinary bladder.
The unneutered male dog has a special risk: prostate infection. The prostate gland is located at the neck of the bladder and, due to its glandular nature, infection in the bladder readily spreads to the prostate where the special crypts and crannies are particularly protective to the infection. It is nearly impossible to clear the prostate of the infection without neutering.
Sometimes when an infection simply cannot seem to be cleared up, the reason is a vaginal stricture. A vaginal stricture is a small narrowing in the vagina, creating a ledge for bacteria to colonize. If a female dog's UTI seems stubborn against antibiotics that the culture indicates should be effective, a vaginal exam may be warranted. A stricture can generally be broken down by the veterinarian's finger though some dogs find this painful and sedation may be needed.
Most urinary tract infections are straightforward and require only a relatively short antibiotic course for clearance.
Page last updated: 6/27/2021