The body protects itself with physical barriers and the immune system.
What Are Physical Barriers Against Infection?
Skin, fur, feathers, and scales, unless damaged by a cut or insect bite, protect the body from infectious organisms it contacts.
Mucous membranes (lining the mouth, nose, eyelids, airways, intestines, and reproductive tract) produce secretions (like tears, ear wax, mucus, and stomach acid) that help kill germs.
The flow of urine through the urinary tract washes away microorganisms from this tract.
How Does the Immune System Protect Against Infection?
The immune system uses white blood cells and antibodies to destroy organisms that get past physical barriers. During infection, the immune system produces cells and substances that target and kill specific microorganisms. Antibodies often form and attach to invading organisms, damaging them directly or marking them so white blood cells can destroy them.
Your pet's body has a number of different ways to help fight infection.
Blood
The body increases certain white blood cells—neutrophils and monocytes—to surround and destroy invading microorganisms. Eosinophils are another type of white blood cell that increases mainly with allergies and parasitic infections. White blood cells can be released quickly from the bone marrow to fight infection.
Inflammation
Injury or attack by microorganisms increases blood flow to the affected area, causing inflammation. Damaged tissues release substances that help bring white blood cells and other cells to the site to kill invading organisms and clean up and repair damage. Neutrophils attack microorganisms and release chemicals that keep inflammation going. Other substances trigger blood clotting to limit spread of the organism through the body.
Substances produced by cells in the immune system during inflammation often cause pain, fever, and stiffness. This immune response alerts the animal (and the owner and the vet) that there is a problem, usually by producing swelling, redness, or warmth in the area. (See Pain and Pain Management in Pets.)
Fever
Fever is a protective rise in body temperature that strengthens the body's defenses against invading microorganisms. Dogs' and cats' normal body temperature is around 101.5°F (38.6°C), a few degrees higher than that of people. Reptiles, like snakes and lizards, and amphibians, like frogs, are dependent on the environment to regulate their temperature, so they can't develop a true fever, though they might seek extra warmth when they are feeling ill.
An animal's brain controls body temperature. When fever sets in, the body feels chills, so it raises its temperature by decreasing blood flow to the skin and by shivering. This helps limit heat loss, and the fever helps fight the infection. Once the infection is cleared, the body cools by sweating (limited in many animals) and by sending blood to the skin to release heat.
Fever patterns during illness vary. Body temperature might rise and then return to normal each day or stay above normal for a long time. Very young or very old animals with severe infections may show low temperatures instead.
Common causes of fever:
infection
cancer
allergic reactions
immune system disorders
exercise, especially in hot conditions
certain drugs, including anesthesia drugs
damage to the part of the brain that controls body temperature
Most fevers are due to infections like pneumonia (infection in the lungs) and urinary tract infections. Your veterinarian might be able to identify the cause in an exam, but they also might need to do blood work, x‑rays, or urine testing to determine the cause. High white blood cell counts often suggest infection. The numbers of each type of white blood cell typically give your vet more clues about the cause of the fever.
The vet will also ask about symptoms, past illnesses, medications, and recent boarding or travel. Contact with other animals or certain environments could be important.
Blood and other fluid samples might be sent to a lab to grow and identify microorganisms. Blood tests can look for antibodies against specific organisms to identify the cause of an infection.
If an animal has a long‑lasting fever and no cause is found after extensive testing, the condition might be called “fever of unknown origin.” Possible causes include unusual infections, immune system disorders, cancers, or other disorders. More advanced testing, like ultrasound, CT scans, or MRI, and biopsies from organs like the liver or bone marrow, might be needed to help identify the cause of the fever.
What Kinds of Things Affect a Pet's Immune System?
Anything that weakens the body’s defenses can lead to infections, even from microorganisms that are normally harmless.
Very young animals are still developing their immune systems, and older or sick animals have decreasing immune system strength. Animals with large burns or scrapes from an injury like a car accident are at high risk, because damaged skin can let microorganisms into the body. Chemotherapy drugs and radiation therapy can weaken the immune system, predisposing animals to illness. Immune system disorders also greatly decrease a pet's ability to fight infections.
Animals with weakened defenses are prone to infections caused by organisms that rarely cause illness in healthy animals. They can also become more seriously ill from common infections that aren't usually harmful to animals with normal immune systems.
What Are the Best Ways to Prevent Infections in Pets?
Several strategies help prevent infections in animals:
Antibiotics are sometimes given to prevent infection, such as before or during surgery.
Vaccines “train” your pet's immune system to recognize specific bacteria or viruses so it can fight them off quickly if they encounter them again. Certain vaccines (like those to prevent rabies) are strongly recommended for many species that might be exposed to particular infections. Modern vaccines are generally effective and safe.
