Complications of Salmonella Infection
Blood infection, irritable-bowel syndrome, and reactive arthritis are complications that impact a small percentage of people with Salmonella infection.
In approximately 5% of non-typhoidal infections, patients develop bacteremia. [5, 8] In a small proportion of those cases, the bacteria can cause a focal infection, where it becomes localized in a tissue and causes an abscess, arthritis, endocarditis, or other severe illness. Infants, the elderly, and immune-compromised persons are at greater risk for bacteremia or invasive disease. [5, 23] Additionally, infection caused by antimicrobial-resistant non-typhoidal Salmonella serotypes appears to be more likely to cause bloodstream infections. [9, 23]
Overall, approximately 20% of cases each year require hospitalization, 5% of cases have an invasive infection, and one-half of 1% die. Infections in infants and in people 65 years of age or older are much more likely to require hospitalization or result in death.  There is some evidence that Salmonella infections increase the risk of developing digestive disorders, including irritable bowel syndrome. [10, 35]
Although most persons that become ill with diarrhea caused by Salmonella recover without any further problems, a small number of persons develop a complication often referred to as Reactive Arthritis. The terminology used to describe this type of complication has changed over time. The term “Reiter Syndrome” was used for many years, but has now fallen into disfavor. The precise proportion of persons that develop reactive arthritis following a Salmonella infection is unknown, with estimates ranging from 2% to 15%.  Symptoms of reactive arthritis include inflammation (swelling, redness, heat, and pain) of the joints, the genitourinary tract (reproductive and urinary organs), or the eyes.
More specifically, symptoms of reactive arthritis include pain and swelling in the knees, ankles, feet and heels. It may also affect wrists, fingers, other joints, or the lower back. Tendonitis (inflammation of the tendons) or enthesitis (inflammation where tendons attach to the bone) can occur. Other symptoms may include prostatitis, cervicitis, urethritis (inflammation of the prostate gland, cervix or urethra), conjunctivitis (inflammation of the membrane lining the eyelid) or uveitis (inflammation of the inner eye). Ulcers and skin rashes are less common.  Symptoms can range from mild to severe.
One study showed that on average, symptoms developed 18 days after infection. A small proportion of those persons (15%) had sought medical care for their symptoms, and two thirds of persons with reactive arthritis were still experiencing symptoms 6 months later.  Although most cases recover within a few months, some continue to experience complications for years. Treatment focuses on relieving the symptoms.
There are a lot of gaps in our knowledge surrounding this complication. Since there is no specific test for reactive arthritis, doctors rely on signs and symptoms of the patient in order to make the diagnosis. However, there are no clearly defined criteria or set of symptoms used to diagnose this condition. The role of genetics is also unclear. It is thought that the presence of a gene called human leukocyte antigen (HLA)-B27 predisposes a person to develop reactive arthritis, along with other autoimmune diseases; however, several studies have shown that many persons that develop reactive arthritis lack this genetic factor.