Merck Manual

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Pet Owner Version

Infections Caused by Bacteria


Manuals Staff

Reviewed/Revised Feb 2020 | Modified Nov 2022
Topic Resources

Bacteria are microscopic, single-celled organisms. Millions of different kinds of bacteria live throughout the world. Some live in the environment, and others live on the skin, in the airways, in the mouth, and in the digestive and urinary tracts of animals and people. Some bacteria always cause disease, some cause disease under certain conditions, and many bacteria do not cause disease.

Bacteria shapes

Bacteria shapes

Bacteria commonly enter the bloodstream, but usually only a small number of bacteria do so at one time, and no signs develop. In addition, most bacteria that enter the bloodstream are rapidly removed by white blood cells White Blood Cells of Dogs The function of white blood cells (also called leukocytes) is to defend the body against infection. There are 2 main types of white blood cells formed in the bone marrow: phagocytes and lymphocytes... read more . Sometimes, however, there are too many bacteria to be removed easily, and an infection develops. An infection that is widespread throughout the bloodstream is called sepsis.

Bacteremia and Sepsis

Bacteremia is the presence of bacteria in the bloodstream; sepsis, which is also called septicemia, is illness resulting from the persistence of microorganisms or their toxins in the bloodstream.

Temporary bacteremia may occur during dental procedures, because bacteria living on the gums around the teeth are freed and enter the bloodstream. Bacteria can also enter the bloodstream from the intestine, but they are rapidly removed when the blood passes through the liver. These transient events are usually not serious.

Sepsis is less common than bacteremia. Sepsis can develop when there is an infection somewhere in the body, such as the lungs, abdomen, or urinary tract. Sepsis can also occur when surgery is performed on an infected area or on a part of the body where bacteria normally live, such as the intestine. The presence of a foreign object, such as an intravenous line or a drainage tube, may increase the risk of sepsis, and the chance of sepsis increases the longer the object is left in place. Sepsis is more likely to develop in animals with a suppressed immune system or other immune disorder. Rarely, nonbacterial infections can cause sepsis.

The circulating bacteria may settle in sites throughout the body if treatment is not started quickly. An infection can develop in the tissues surrounding the brain (meningitis), the sac around the heart (pericarditis), the bones (osteomyelitis), the joints (infectious arthritis), and other areas. Certain types of bacteria, such as staphylococci, can result in accumulations of pus that form abscesses in the organs they infect.

Signs of Sepsis

Because the body is usually able to clear small numbers of bacteria quickly, temporary bacteremia rarely causes signs. When sepsis does develop, signs include shaking, fever, weakness, confusion, lack of appetite, vomiting, and diarrhea. Other signs can also be present depending on the type and location of the initial infection.

Diagnosis of Sepsis

A sudden high fever in an animal that has an infection may indicate sepsis. Bacteria in the bloodstream are usually difficult to find by simply examining the blood under a microscope. Several blood samples may be needed to make the diagnosis; the samples are sent to a laboratory for culture, a procedure to try to grow the bacteria for specific identification. Blood cultures can take several days. Unfortunately, bacteria may not always grow in a blood culture, particularly if the animal is taking antibiotics. Cultures of other fluids and substances (such as urine, cerebrospinal fluid, and wound tissue) may also be analyzed for the presence of bacteria.

Treatment and Outlook for Sepsis

Bacteremia caused by surgery or dental procedures usually does not require treatment. However, animals at risk of developing serious infections (such as those with heart valve disease or a weakened immune system) may be given antibiotics to prevent bacteria and sepsis before undergoing such procedures.

Sepsis is very serious and can lead to septic shock. Antibiotic treatment should begin immediately—even if test results confirming the diagnosis are not yet available. A delay in starting antibiotic treatment greatly decreases the animal’s chances of survival. Initially, one or more antibiotics are selected based on the bacteria most likely to be present, which depends on where the infection started. Often, 2 or 3 antibiotics that are effective against a broad range of bacteria are given together to increase the chances of killing the bacteria before the identity of the infectious agent is known. Later, when the test results become available, the antibiotic may need to be changed to one that is most effective against the specific bacteria causing the infection. Surgery may sometimes be needed to eliminate the source of the infection.

Septic Shock

Septic shock is a life-threatening condition caused by an infection in the bloodstream (sepsis or blood poisoning) in which blood pressure falls dangerously low and many organs malfunction because of inadequate blood flow. Animals with low white blood cell counts or a chronic disease are at increased risk of developing septic shock.

Septic shock is caused by substances produced by the immune system to fight an infection (cytokines) and by toxins produced by some bacteria. These substances cause the blood vessels to widen, or dilate, which results in a drop in blood pressure. Blood flow to vital organs, particularly the kidneys and brain, is reduced even though the body tries to compensate by increasing both the heart rate and the volume of blood pumped. Eventually, the toxins and the increased work of pumping weaken the heart, which reduces blood flow even more. The walls of the blood vessels may leak, which allows fluid to escape from the bloodstream into tissues, causing swelling. In the lungs, this leakage and swelling causes difficulty breathing.

Signs of Septic Shock

Early signs of septic shock can include disorientation, a shaking chill, a rapid rise in temperature, warm skin, a rapid pulse, and rapid breathing or panting. Urinary output decreases. Tissues with poor blood flow release excess lactic acid into the bloodstream. When the blood becomes more acidic, many different organs malfunction. In later stages, body temperature often falls below normal.

Diagnosis of Septic Shock

As septic shock worsens, internal organs continue to fail. For example, the kidneys can greatly decrease or stop producing urine, allowing metabolic waste products (such as urea nitrogen) to accumulate in the blood. Problems in the lungs can lead to difficulty breathing and a reduced level of oxygen in the blood. The heart can fail, resulting in fluid retention and tissue swelling. Additionally, blood clots may form inside blood vessels.

Confirming a diagnosis of septic shock may require sophisticated analysis of several blood samples. Results of such analyses usually identify high or low levels of white blood cells, a decreased oxygen level, fewer platelets than normal, excess lactic acid, and increased levels of metabolic waste products. An electrocardiogram (ECG) may show irregularities in heart rhythm, indicating inadequate blood supply to the heart. Blood cultures are performed to identify the infecting organisms. Because there are causes of shock other than sepsis, additional tests may be needed.

Treatment and Outlook

As soon as signs of septic shock are apparent, large amounts of fluid are given intravenously to increase blood pressure. Drugs are given to increase blood flow to the brain, heart, and other organs. Oxygen is also administered.

After blood samples have been taken for laboratory cultures, high dosages of antibiotics are given intravenously. As in sepsis, 2 or 3 antibiotics may be given together to increase the chances of killing the bacteria. Once the specific bacteria causing the infection have been identified, the antibiotic may need to be changed.

Surgery may be needed to drain abscesses or to remove any dead tissue, such as gangrenous tissue of the intestine. Despite all efforts, many animals with septic shock do not survive.

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