Development of Infection

Infections usually occur when microorganisms enter a pet's body and multiply. They typically start by attaching to body cells. They might remain in the area where they enter the body or spread to other parts of the body, depending on different factors.

For example, tetanus is caused by a poisonous substance, or toxin, produced by a particular type of bacteria that enters wounds. Some food poisoning is caused by toxins produced by staph bacteria that may be found in food. Many toxins attach specifically to certain target cells in the body and cause illness.

After invasion, microorganisms multiply. From there, one of three things can happen:

• The organisms may overwhelm the pet's defenses and cause illness.

• They may reach a balance with the immune system, continuing to live in the body, and cause chronic infection.

• The body may clear them, with or without treatment.

Disease‑causing microorganisms often have features that make them more harmful and more able to resist the body’s defenses. Some bacteria produce substances that destroy thebody's cells and help the bacteria spread. Others interfere with the body’s ability to resist infection. Some have a coating or capsule around them that protects them from being attacked by white blood cells. Certain bacteria resist substances in the blood or even produce chemicals that block the effects of antibiotics.

Different types of microorganisms can cause the same disease (for example, pneumonia can be caused by viruses, bacteria, or fungi). Identifying the specific type of organism causing disease helps guide treatment. There are different ways to help your veterinarian identify disease-causing organisms:

• Microscope: Sometimes microorganisms can be recognized under a microscope by their shape and the way they stain with certain dyes. If too few are present or they're too small, they can be missed.

• Culture: Once an organism is cultured (grown in a dish in a lab), it can be tested against multiple antibiotics to see which antibiotic works best to kill it. This is important because many microorganisms are resistant to the effects of antibiotics.

• Antibody testing: Some microorganisms are difficult to culture. They might be identified using blood or fluid tests that detect antibodies against these microorganisms.

• PCR: PCR tests detect pieces of a microorganism’s DNA (genetic material) in blood, urine, pus, or other body fluids or organs. This DNA is present only if the organism is there. PCR is usually used when a specific disease is suspected.