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Pet Owner Version

Development of Infection


Manuals Staff

Reviewed/Revised Feb 2020 | Modified Nov 2022
Topic Resources

Infectious diseases are usually caused by microorganisms that invade the body and multiply. Invasion by most microorganisms begins when they adhere to cells in the body. Whether the microorganism remains near the invasion site or spreads to other sites depends on many factors, including whether it produces toxins, enzymes, or other substances. For example, Clostridium tetani in an infected wound produces a toxin that causes tetanus. Food poisoning is caused by toxins produced by staphylococcal organisms that are outside the body. Most toxins contain components that bind specifically with molecules on certain cells (target cells).

After invading the body, microorganisms start to multiply. One of 3 things can then happen: the microorganisms can continue to multiply and overwhelm the body’s defenses, resulting in illness; a state of balance can be achieved, resulting in a chronic infection; or the body—with or without treatment—can destroy and eliminate the invading microorganism.

Many microorganisms that cause disease have properties that increase the severity of the disease and help them resist the body’s defense mechanisms. For example, some bacteria produce enzymes that break down tissue, allowing the infection to spread faster. Other microorganisms have ways of blocking the body’s defense mechanisms, such as by interfering with production of antibodies or T cells (a type of white blood cell). Others have outer coats (capsules) that resist being ingested by white blood cells. Some bacteria resist being destroyed by substances circulating in the bloodstream. Some even produce substances that counter the effects of antibiotics.

Identifying an Infectious Organism

Many different microorganisms can cause a given condition (for example, pneumonia can be caused by viruses, bacteria, or fungi). It is usually important to know what specific microorganism is causing an illness because the treatment is different for each organism.

There are many ways to identify microorganisms. Taking a sample from the site of infection and examining it under the microscope is often the most rapid method of identifying microorganisms. Sometimes, microorganisms can be recognized by characteristic shapes and colors. However, if the microorganisms are too few or too small to see under the microscope, they may not be found.

Another method of identifying an infectious organism is to grow it in the laboratory so that additional chemical tests can be done. The process of growing the organism is called a culture. Many microorganisms can be grown this way. Cultured microorganisms can also be tested for their susceptibility to various antibiotics, which can help determine what drug to use to treat an infected animal. This testing is important because microorganisms are constantly developing resistance to antibiotics that were previously effective.

Some microorganisms are very difficult to culture. These infections can be identified by finding antibodies to the microorganisms in the infected animal’s blood or other body fluids (serology tests). Antibody-based tests are used to identify many infections, but they are not always reliable. Antibodies may stay in the body for many years after an infection has cleared, so a positive test result does not always indicate an active infection, but it does indicate previous exposure. New tests, such as the polymerase chain reaction (PCR), can identify pieces of the genetic material (DNA) of the microorganism, which are present only when the organism is present. These tests are usually performed only when a particular disease is already suspected.

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