The urinary tract is one of the most common sites for infection particularly in women. Between 10 and 20 percent of women will, at some point in their life, suffer from a urinary tract infection UTI and a significant number suffer from recurrent infections. Most infections are short-lived, and cause little more than discomfort, however, severe infections can result in the loss of kidney function and have serious long term effects. In females a distinction is made between cystitis, infection of the bladder, urethritis, infection of the urethra and vaginitis, infection of the vagina although symptoms may overlap.
What causes it?
UTI are most often caused by bacteria, but they can be caused by bacteria, viruses, fungi or protozoa.
Organisms responsible for UTI:
Most Common Causes of UTI
- E. coli 80%
- other Gram negatives e.g. Klebsiella, Enterobacter, Serratia, P. aeruginosa 7%
- P. mirabilis 6%
- Coagulase-negative staphylococci 4%
- other Gram positives e.g. Staph. epidermis, Staph. aureus, Enterococcus faecalis 3%
- Inpatients (Nosocomial Infection)*
- E. coli 40%
- other Gram negatives 25%
- other Gram positives 16%
- P. mirabilis 11%
- Candida 5%
- Coagulase-negative staphylococci 3%
*The organisms causing UTI in hospitals are often resistant to multiple antibiotics
How do you become infected?
a) Outside --> Urethra --> Bladder --> Kidney
b) Blood --> Kidney --> Bladder
There are two possible routes for bacterial infection of the urinary tract
a) The ascending route
b) Haematogenous spread
The ascending route is by far the most common way in which a UTI is acquired. The most common cause of infection by this route is the Gram-negative rod E. coli. Other members of the Entereobacteriaceae are also implicated, Proteus mirabilis is often associated with urinary stones (calculi) because it produces a potent urease which acts on urea to produce ammonia, rendering the urine alkaline. Klebsiella, Enterobacter, Serratia species and Pseudomonas aeruginosa are more frequently associated with hospital acquired UTI because their resistance to antibiotics favours their selection.
Less commonly bacteria infecting the blood may infect the urinary tract, infecting the kidneys first, this is haematogenous spread. When this occurs species like Staph. aureus, S. typhii and M. tuberculosis may be found.
Viral UTI appear to be rare, although certain viruses may be recovered from the urine in the absence of urinary tract disease. The Human polyoma viruses JC and BK enter the body via the respiratory tract, spread through the body and infect the epithelial cells of the kidney tubules and ureters. They establish latency with persistence of the viral genome but not infectious virus. About one third of healthy people have polyoma virus DNA sequences in their kidneys. Reactivation of the virus may occur in normal pregnancy and in immunocompromised patients, with large amounts of virus present in the urine. Cytomegalovirus can cause asymptomatic shedding of large titres of virus in congenitally infected infants. In contrast to the asymptomatic shedding of virus, some serotypes of adenovirus have been implicated as a cause of haemorrhagic cystitis.
For the rest of this write-up I shall concentrate on bacterial UTI as this is most common, and can be diagnosed and treated with relative ease.
Symptoms of UTI
The symptoms brought on by UTI can include:
The first step towards confirming a diagnosis of UTI is to obtain a sample of urine. For most patients this will involve collecting a mid-stream urine sample (MSU). To collect an MSU the patient urinates normally, after 5-10 seconds begins sampling (switches aim from toilet to cup). The reason for doing this is to provide a sample clear of the normal microbial flora found in the lower urethra. For patients with a catheter in situ it is possible to take a sample from the catheter although this shouldn't be taken from the drainage bag. Obtaining a sample from an infant is tricky without a catheter in place. It is possible to use a hyperdermic needle and syringe to aspirate a small volume of urine directly from the bladder, though this itself could lead to infection of the urinary tract with skin flora.
Once a sample has been obtained, it should first be dip tested for the presence of blood cells, both erythrocytes and leukocytes, nitrites and protein. It should then be sent of to the microbiology lab for culturing and antibiotic susceptibility tests.
Treating Urinary Tract Infections
Treating an uncomplicated bacterial UTI is often done with either a single dose, or a 3-4 day course of an oral antibacterial agent. The commonly prescribed agents are shown below, the agent of choice should be selected based on the results of susceptibility tests, however, this is not always the case and the doctor often make a best-guess prescription based on knowledge of the likely pathogens and their antibiotic susceptibilities. After the course of treatment is complete a sample should be obtained so that the eradication of the pathogen can be verified.
Oral antibacterials for UTI
Complicated UTI, such as pyelonephritis, should be treated with systemic antibacterials to which the organism is known to be susceptible until symptoms subside. This can then be replaced with oral therapy. The length of the treatment course is likely to be 10 days, but it may take longer for the kidneys to be sterilised.
Prevention of UTI
- Urinate before and after sexual intercourse
- Wash your hands after going to the toilet
- Drink plenty of water
- Urinate frequently
- Women should wipe front to back
A study published in the British Medical Journal, has gone some way to confirming what has been believed for some time. The randomised study (BMJ 2001 Jun 30;322(7302):1571), tested whether drinking cranberry juice has any effect on the number of episodes of UTI in women who had had recurrent UTIs. It found that at 8 months whilst 36% of the women in the control group had had a recurrence of infection, only 16% in the cranberry juice group had suffered another episode of UTI. Other papers have postulated that compounds called proanthrocyanins present in the cranberry juice affect the ability of bacteria to bind to tissue in the urinary tract. If bacterial attachment is lessened then the bacteria can be washed out of the urinary tract with the normal flow of urine.
Medical Microbiology. Mimms et al. Mosby 1994
Biology of Microorganisms. Brock et al. Prentice Hall 1994
Randomised trial of cranberry-lingonberry juice and Lactobacillus GG drink for the prevention of urinary tract infections in women. Kontiokari T, Sundqvist K, Nuutinen M, Pokka T, Koskela M, Uhari M. BMJ 2001 Jun 30;322(7302):1571
Family Practice Notebook (fpnotebook.com)