Infectious Diseases Affecting the Urinary System

Urinary tract infections occur in the urethra, bladder, prostate, and kidneys. When pathogens get past the natural defenses of the urinary tract, they can cause disease, which is generally characterized by the organ that is infected and the organism causing disease. Some infections are chronic, a disease with consistent symptoms that occur over months or years and will not be cleared without treatment. Some infections are acute, which refers to a disease with sudden onset that is quickly resolved by the immune system. Urethritis is the inflammation of the urethra due to infection. Escherichia coli, a species of gram-negative bacteria, is responsible for about 80-90% of urinary tract infections because it is present in the rectal area, and the gram-positive Staphylococcus saprophyticus is responsible for most of the rest. An infection of the bladder, referred to as cystitis, causes a burning sensation during urination, more frequent urination, feeling the need to urinate just after urinating, lower abdominal pain, feeling generally unwell, and blood in urine. In men, infections can reach the prostate, and the resulting infection is one cause of prostatitis, or prostate inflammation. An infection of the kidney, called pyelonephritis, can cause fever and pain near the kidneys. Both cystitis and pyelonephritis are typically acute. They only last a few days and are often easily treated with antibiotics such as penicillin. Urinary tract infections are far more frequent in women due primarily to proximity between the urethra and anus and having a shorter urethra. A much more serious infection of the urinary tract is caused by a spiral-shaped bacterial species. Leptospirosis, an infection of the bladder and kidneys caused by the bacterial genus Leptospira, can lead to kidney failure and jaundice. It spreads to other animals by traveling through the urinary tract suspended in urine. If left without treatment it can be life-threatening. While leptospirosis is very dangerous and extremely rare, it is not impossible for humans to come in contact with it. Family pets can be infected and pass on the disease to their owner. These animals may pick up the disease from coming into contact with soil or mud that was infected by wild animals, such as raccoons. Leptospirosis can be treated with courses of broad-spectrum antibiotics. Schistosomiasis, a urinary tract infection caused by parasitic flatworms, called schistosomes, is contracted when a person drinks contaminated water. Flatworms are common in Africa, South America and some Caribbean islands, and Asia. Typically, the causative parasite is Schistosoma haematobium, but it can also be S. mansoni or S. japonicum. Any area of freshwater that can support the intermediate host, a snail from the genus Bulinus, can be a source of infection. Humans become infected when they consume water or immerse in water that is contaminated by snails, the schistosome secondary host. The eggs are first released into the water through the feces or urine of an infected person. The eggs hatch, releasing miracidia—larva—which penetrate the snail tissue. Sporocytes in the snail release cercariae, free-swimming larva. The cercariae penetrate human skin and circulate in the bloodstream. They mature in the liver and then migrate to the rectum and bladder. Urinary schistosomiasis is typically a chronic infection, meaning the symptoms can be treated, but the person will carry the disease for ever. Schistosomes infect the veins surrounding the bladder, which can lead to blood in the urine. These worms are parasites, attaching to tissues and drinking the blood of their host. Tissue damage that occurs due to a long-term infection can lead to bladder cancer and kidney failure. Both leptospirosis and schistosomiasis can be diagnosed using polymerase chain reaction (PCR), a technique for rapidly producing many copies of a section of DNA. PCR can be used to test for the presence of specific pathogens, such as Leptospira, in infected humans by targeting specific DNA sequences in the pathogen's genome and producing millions of copies that can be visually detected with gel electrophoresis. Another common diagnostic technique is enzyme-linked immunosorbent assay (ELISA), a diagnostic test that uses antibodies to specifically bind to the proteins of pathogens, which are then identified by a color change.

Epidemiology is a science that investigates the cause, transmission, timing, and distribution of infectious disease episodes, with a focus on recognizing outbreaks, controlling those outbreaks, and treating the infected. The epidemiological study of urinary tract infections has revealed three primary routes by which urinary tract infections are most commonly spread: community-acquired, catheter-acquired, or through imbalanced microbiota.

The first route of infection is called community-acquired, where the pathogen is transferred to the urinary tract from another organ system. As the most common causative agent of urinary tract infections is Escherichia coli, the most common source of community-acquired UTIs is the gastrointestinal tract. Contamination of the urinary tract by gut microbes generally occurs when the genitals come into contact with fecal matter, which contains many of the same bacteria (E. coli) that live in the gut. This contamination can be prevented with regular hygiene and safe sex practices. For example, women wipe themselves from front to back, urethra before the anus, in order to prevent fecal bacteria from getting into the urethra. Women are also encouraged to urinate before and after sexual intercourse to clear the urethra of potential contaminants.

The second route of infection is via catheter, a medical instrument that is inserted into the urethra in order to drain the bladder artificially. Because the catheter travels all the way to the bladder and prevents urine flow through the urethra, bacteria attached to the surface of the catheter, or located in the urinary tract, may be transferred to the bladder. Some bacteria, such as E. faecalis, E. coli, S. epidermidis, and S. aureus, are uniquely adapted to colonize the interface between the catheter and the epithelial wall, building structures called biofilms that reduce the efficacy of antibiotic treatment. These infections can be prevented through frequent replacement and cleaning of the catheter to remove bacterial growth.

The third route of infection is through infection by organisms that are a part of the healthy microbiota. These normal microbes that inhabit the body change their behavior to cause infection under certain circumstances. The second most common causative agent of urinary tract infections is Staphylococcus saprophyticus, a gram-positive bacterium that is a common member of the microbial communities that naturally colonize parts of the urinary tract. S. saprophyticus shifts to pathogenesis when cells evade the natural defenses of the urinary tract and reach more vulnerable organs, such as the bladder. Streptococcus agalactiae is a gram-positive coccus-shaped bacterium that is commonly found colonizing both the large and small intestines and the genitourinary tract in healthy humans. Before the development of PCR amplification and DNA sequencing technologies, bacteria from similar species were differentiated based on other biochemical characteristics. One system designed to distinguish between streptococci was called the Lancefield system, which grouped them based on the content of their cell walls. S. agalactiae produces a rhamnose-glucosamine capsule surrounding its cells and so was designated a Group B streptococcus, which is how it is commonly referred to in research. This capsule is known as a virulence factor, a feature enabling a pathogen to cause disease by releasing toxins, invading host cells and tissues, suppressing the host's immune responses, or otherwise damaging the host. S. agalactiae also produces an enzyme called beta-hemolysin, which forms pores in the cell membranes of red blood cells, causing them to lyse, or break open.

Group B streptococcus is found colonizing as much as 30% of the human population, though it does not cause disease in most people. Group B streptococcus infections can be passed from pregnant mothers that to infants, in whom it can cause serious illness. A newborn is inoculated by bacteria in the vagina during birth or during the rupture of the membrane surrounding the infant prior to birth. However, the infant's immune system is not yet equipped to fight the infection. S. agalactiae is, therefore, the leading cause of infections in newborns and can be very dangerous. Group B streptococcus can quickly spread to the blood, causing sepsis.

Additionally, Group B streptococcal infections of the urinary tract of pregnant women can induce labor and cause preterm birth. Newborns are at higher levels of risk of developing infection if their mothers are colonized or infected, if they experience a period of labor longer than 18 hours, or if they are born prematurely. One common means of preventing infections is to screen pregnant mothers to see if they are naturally colonized by S. agalactiae. If the mother has the bacteria in their normal microbiota, it can be treated with antibiotics during labor.