Defenses Against Infection

Reviewed/Revised Feb 2020

Physical barriers and the immune system defend an animal’s body against organisms that can cause disease. Physical barriers include outer coverings, such as skin, fur, feathers, and scales. Additional barriers include the mucous membranes, tears, ear wax, mucus, and stomach acid. Also, the normal flow of urine washes out microorganisms that have entered the urinary tract. The immune system relies on white blood cells and antibodies to identify and eliminate organisms that manage to pass through the physical barriers.

Physical Barriers

Unless damaged—by injury, insect bite, or burn, for example—an animal’s outer coverings usually prevent invasion by microorganisms. Other effective physical barriers are the mucous membranes, such as the linings of the mouth, nose, and eyelids. Typically, mucous membranes are coated with secretions that fight microorganisms. For example, the mucous membranes of the eyes are bathed in tears, which contain an enzyme that attacks bacteria and helps protect the eyes from infection.

The airways filter out particles that are present in the air that is breathed in. The walls of the nasal passages and airways are coated with mucus. Microorganisms in the air become stuck to the mucus, which is coughed up or blown out the nose. Mucus removal is aided by the coordinated beating of tiny hairlike projections, called cilia, that line the air passages. The cilia sweep the mucus up the airways, away from the lungs.

The digestive tract has a series of effective barriers, including stomach acid, pancreatic enzymes, bile, and intestinal secretions. Additionally, there are beneficial microorganisms (referred to as resident flora or the intestinal microbiota) that reside in the intestines and support the immune system in resisting infections. The contractions of the intestine and the normal shedding of cells that line the intestine help remove harmful microorganisms.

In the urinary tract, urine passes through the urethra (a tubular structure) as it leaves the body. In adult males, the long length of the urethra generally prevents bacteria from passing through it to reach the bladder. In females, the urethra is shorter, and bacteria outside the body can more easily pass into the bladder. The flushing effect as the bladder empties is an active defense mechanism in both sexes. The vagina is protected from harmful microorganisms by its normally acidic environment.

The Blood

Another way in which the body defends against infection is by increasing the number of certain types of white blood cells (neutrophils and monocytes) that surround and destroy invading microorganisms. The numbers of neutrophils and monocytes can increase within several hours, because reserve pools of white blood cells can be released quickly from the bone marrow. The number of neutrophils increases first. If an infection persists, the number of monocytes increases. The number of eosinophils, another type of white blood cell, increases in allergic reactions and many parasitic infections but usually not in bacterial infections.


Any injury, including an invasion by microorganisms, results in a complex reaction in the affected area, called inflammation. Inflammation develops as a result of many different conditions and begins with the release of different substances from the damaged tissue. It then directs the body’s defenses to attack and kill any invading organisms, dispose of dead and damaged tissue, and begin the process of repair. However, inflammation alone may not be able to overcome large numbers of microorganisms.

During inflammation the blood supply also increases. This can be seen in an infected area near the surface of the body, which becomes red and warm. The walls of blood vessels become more porous, allowing fluid and white blood cells to pass into the affected tissue. The increase in fluid causes the inflamed tissue to swell. The white blood cells (neutrophils) attack the invading microorganisms and release substances that continue the process of inflammation. Other substances trigger clotting in the tiny blood vessels in the inflamed area, which inhibits the spread of the microorganisms and their toxins. Many of the substances released during inflammation stimulate the nerves, causing pain, which can be hard to recognize in animals ( see Introduction to Pain Management). Reactions to the substances released during inflammation include the fever and muscle stiffness that commonly accompany infection.

Immune Response

When an infection develops, the immune system responds by producing several substances and agents that are designed to attack the specific invading microorganisms. For example, the immune system can create killer T cells (a type of white blood cell) that recognize and kill the invading microorganism. In some cases, the immune system can also produce antibodies that are specific to the invading microorganism. Antibodies attach to and immobilize microorganisms—killing them outright or helping the neutrophils target and kill them.


Body temperature increases as a protective response to infection and injury. The resulting fever enhances the body’s defense mechanisms. Normal body temperature varies among species. Dogs and cats, for instance, have an average body temperature around 101.5° F, which is higher than that of people. Reptiles and amphibians are cold‑blooded and do not maintain a set body temperature. Thus, they cannot develop a true fever as a response to infection. (They sometimes seek out unusually warm locations, developing what is known as behavioral fever, to perform the same function.)

The hypothalamus, a part of the brain, regulates body temperature. When the thermostat of the hypothalamus is “reset,” a fever results. The body increases its temperature by shunting blood from the skin surface to the interior of the body, thus reducing heat loss. Shivering (chills) may occur to increase heat production through muscle contraction. The body’s efforts to conserve and produce heat continue until blood reaches the hypothalamus at the new, higher temperature, which is then maintained. Later, when the thermostat is set back to its normal level, the body eliminates excess heat through sweating (minimal or absent in some animals) and shunting blood to the skin.

Fever may follow a pattern: sometimes temperature peaks every day and then returns to normal. Alternatively, the temperature may vary but does not return to normal. Some animals (such as the very old and the very young) may experience a drop in temperature as a response to severe infection.

Common Causes of Fever

  • Infection

  • Cancer

  • Allergic reactions

  • Autoimmune diseases

  • Excessive exercise, especially in hot weather

  • Certain drugs, including anesthetics

  • Damage to the hypothalamus (the part of the brain that controls temperature), such as from a brain injury or tumor

Usually, fever is caused by an infection (for example, pneumonia or a urinary tract infection) that can often be diagnosed with a history, physical examination, and occasionally a few tests, such as a chest X‑ray or a urinalysis. However, fever can also result from inflammation, cancer, or an allergic reaction. Sometimes, the cause cannot be easily determined. If fever continues for several days and has no obvious cause, a more detailed investigation is required. Potential causes of such a fever include infections, diseases caused by antibodies against the animal’s own tissues (autoimmune diseases), and an undetected cancer (especially leukemia or lymphoma).

Your veterinarian will ask about your pet’s present and previous signs and diseases, any medications your pet is currently receiving, possible exposure to infections (such as being boarded), and recent travel. The pattern of the fever usually does not help with the diagnosis. A history of exposure to certain materials or interaction with other animals is also important.

A thorough physical examination will be done to look for a source of infection or evidence of disease. Blood and other body fluids may be sent to a laboratory for culture, which is an attempt to grow the microorganism outside the body so that it can be identified. Other blood tests may be done to detect antibodies against specific microorganisms. Increases in the white blood cell count usually indicate infection. The differential count (the proportion of different types of white blood cells) gives further clues. An increase in neutrophils, for example, suggests bacterial infection. An increase in eosinophils suggests the presence of parasites, such as roundworms or heartworms.

When an animal has a sustained fever for several weeks and extensive investigation does not reveal a cause, a veterinarian may refer to it as a fever of unknown origin. In such cases, the cause may be an unusual chronic infection or something other than infection, such as a connective tissue disease, cancer, or some other disease. Advanced diagnostic imaging techniques (for example, ultrasound, computed tomography [CT], or magnetic resonance imaging [MRI]) may help determine a diagnosis. A biopsy specimen from the liver, bone marrow, or another suspected site may be needed for examination.

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