Avoiding Salmonella Infection

Salmonella, also called salmonellosis, is a foodborne illness named for the group of bacteria that cause it.

Salmonella causes diarrhea and gastroenteritis, and, rarely, typhoid fever. It is often spread through contaminated food or water. Raw or improperly prepared or stored foods such as beef, poultry, produce, raw milk, and eggs can become contaminated with the salmonella bacteria. All foods, including vegetables, can potentially become contaminated. Contaminated foods usually look and smell normal. Cooking kills the bacteria.

The amount of salmonella bacteria needed to make a person ill depends on the type of bacteria, the number of bacteria ingested, and the strength of a person’s defense systems. Acid secreted by the stomach to digest food is usually the body's first defense against salmonella. Bacteria that survive the stomach acid invade the walls of the small intestine and trigger a reaction by the immune system, the second line of defense. It takes a million or more bacteria to make someone in good health fall ill. A smaller number of bacteria can make a person ill, but it may take a longer time for the illness to develop. Usually, contaminated food contains more bacteria than contaminated water. Cheese and milk, antacids, H2 antagonists, and antibiotics can decrease the amount of stomach acid, making it easier for the salmonella bacteria to make it out of the stomach to the intestine, where they can multiply. Chronic medical conditions such as lupus, diabetes, cancer, or HIV make a person more vulnerable to salmonella. Young children and elderly adults, both of whom have weakened immune systems, also are at risk.

In severe salmonella infections, or in people with weakened immune systems, the bacteria can spread from the intestine to the bloodstream, a condition called bacteremia. This can cause an infection of blood vessels, heart (endocarditis), bones (osteomyelitis) or joints (septic arthritis).


Some people can become infected with salmonella but not fall ill. They become carriers of the disease--able to pass it on to others without having symptoms of salmonella. Animals also can be carriers of salmonella. In an example reported in 2002 in the New England Journal of Medicine, a blood donor unwittingly passed on salmonella to two people. He had been infected by his snake. Neither the man nor the snake had symptoms of illness.

Snakes aren't the only animals that can harbor salmonella. Lizards, turtles, chicks and ducklings also are especially likely to pass salmonella to people. Dogs, cats, wild or pet birds, wild or pet rodents, horses and farm animals also can infect people. Salmonella has been found in the saliva of dogs and cats, so the bacteria can be transmitted by licking. Some pet foods made from animal products, such as pig ears, may be a source of salmonella for both dogs and the people who handle the foods.

Typhoid fever

Typhoid fever, which is also caused by salmonella bacteria, affects about 400 Americans each year, according to the Centers for Disease Control and Prevention (CDC). Most of these infections are acquired during international travel; typhoid fever is common in the developing world. Salmonella Typhi, the strain that causes typhoid fever, infects only humans. It can be transmitted by contaminated food or water. After recovering from typhoid fever, a few people become carriers. Antibiotics will cure both people who are carriers and those who have symptoms of infection.


Symptoms of gastroenteritis caused by salmonella usually begin six hours to three days after you are infected. These are the most common symptoms:

  • Loose stool or watery diarrhea that usually does not have blood; this usually lasts three to seven days

  • Fever of 100 to 102 degrees F that begins within two days of infection and usually lasts two days; a fever that lasts a week or more and increases daily suggests typhoid fever

  • Abdominal cramps

  • Chills

  • Headache

These symptoms, along with possible nausea, loss of appetite and vomiting, usually last for four to seven days.

Symptoms of typhoid fever usually begin five to 21 days after you are infected. You may have all the symptoms of gastroenteritis and appear to get well before the persistent, high fever begins. This fever usually begins a week after exposure and increases for four to five days. In addition to fever, these are other symptoms of typhoid fever:

  • Tiredness or weakness

  • Lack of appetite

  • Muscle and joint aches

  • Cough

  • Sore throat

  • Headache

A rash also usually appears on the chest. Called a rose spot rash, it is made up of clusters of slightly raised, irregularly shaped pink spots that turn white if pressed with your finger.


To make a diagnosis, your health care provider will take a stool sample. Several samples may be necessary, however, because bacteria often are not present in the first sample. If the bacteria have spread from the intestine to the blood or other parts of the body, samples of blood or tissue may be needed. The stool or blood samples can identify if you an active infection or if you are a carrier.


If you have uncomplicated gastroenteritis, your health care provider probably will not prescribe an antibiotic, because your body’s defenses should be able to cure the infection. Antibiotics won't shorten the length of time you have symptoms and may increase the time you can carry the bacteria and pass them to others. If you have severe disease, are at risk for the bacteria to spread to the blood or joints, or you have typhoid, you will be treated with antibiotics.

Salmonella bacteria may have natural defenses against antibiotics. Some salmonella strains have developed resistance to antibiotics, limiting the choices of medication available for treatment.

Preventing foodborne illness

Foods from animals are frequently contaminated with salmonella. Produce such as spinach can become contaminated by animal or human feces. Cross-contamination of foods can occur when uncontaminated food is prepared on the same surfaces or using the same utensils as contaminated foods. Hands, cutting boards, counters, and knives and other utensils should be washed thoroughly after handling uncooked foods. Hands should be washed before handling any food, and between handling different food items. People who have a salmonella infection should not prepare food or pour water for others until a health care provider clears them from carrying the salmonella bacterium.

One of the most important ways to control salmonella is by thoroughly cooking food.

The National Institute of Allergy and Infectious Diseases recommends the following measures to help prevent salmonella infection:

  • Don't drink unpasteurized milk.

  • Don't eat foods containing raw eggs, such as homemade caesar salad dressing, cookie dough, and hollandaise sauce, or drink homemade eggnog made with raw eggs.

  • Handle raw eggs carefully. Keep eggs refrigerated. Throw away cracked or dirty eggs.

  • Cook eggs thoroughly. Salmonella has been found in the ovaries of some chickens; in these cases, the bacteria can contaminate the eggs before the shells have been formed.

  • Cook poultry products to an internal temperature of 170 degrees F for breast meat and 180 degrees F for thigh meat.

  • Wash thoroughly with soap and hot water all food preparation surfaces and utensils that have come in contact with raw poultry, beef, or raw eggs.

  • Wash hands immediately after handling raw poultry or raw eggs.

  • Wash hands immediately after handling reptiles or having contact with pet feces.

  • If you have a weakened immune system, be extra cautious when visiting farms and touching farm animals, including animals at petting zoos.

Preventing typhoid

If you are traveling to an area where typhoid is common, get immunized. Two types of typhoid vaccine are available: an oral form and an injected form. Both need to be given over a two-week period. The oral form is effective for five years, the injected form for two years.

Watching what you eat and drink when you travel is as important as being vaccinated. This is because the vaccines are not completely effective. The CDC recommends these precautions when traveling:

  • Drink only bottled water, or boil water for one minute before you drink it. Bottled carbonated water is safer than bottled non-carbonated water.

  • Don't drink beverages over ice, unless you know that the ice has been made from bottled or boiled water. Don't eat popsicles and flavored ices.

  • Make sure any foods you eat have been cooked thoroughly. One clue is that they are still hot and steaming.

  • If you eat raw vegetables or fruits, choose only those that can be peeled. Wash your hands first and peel them yourself.

  • Don't eat food or drink beverages from street vendors.