A parasite can sound like something out of a science-fiction movie: A foreign creature enters your body, usually through food, water, or an insect bite, causing anything from minor symptoms to life-threatening illness. Though diseases caused by parasites are far more common in underdeveloped countries, which may have unclean water and poor pest control, they still affect millions of Americans each year. At the Yale Medicine Laboratory Medicine Department, these diseases are diagnosed using the most up-to-date testing methods.

Malaria, an infection of the red blood cells that is spread through mosquitoes, is one of the most common fatal diseases in the world. Yale Medicine physicians may detect it among patients who travel.

There are two tests the lab medicine department uses to test for malaria. One quickly detects malaria proteins in the blood through a standard blood sample. The second type is a blood smear, which scientists examine under a microscope. The smear can show more information, such as which species of malaria parasite is infecting the patient, and can help determine the best course of treatment.

Toxoplasma is another parasite often investigated by the Yale Medicine lab. Toxoplasma is carried by an estimated 60 million Americans, but rarely causes problems unless the patient is pregnant or has a compromised immune system.

“You usually get this parasite from eating undercooked meat or from cleaning a cat’s litter box, since it is transmitted in their stool,” says Yale Medicine pathologist Sheldon Campbell, MD, PhD.

A patient who is being tested for gastrointestinal parasites must collect a few stool samples in a preservative vial, which is then sent to the lab. For common parasites, a rapid test looks for parasite proteins. For other parasites, the stool is examined under the microscope.

These kinds of parasitic diseases are not very common in the U.S. They may be found in patients who have immigrated from another country or have traveled internationally, and children who have been adopted from foreign countries, Dr. Campbell says.

“The advantage of looking at the sample under a microscope is that instead of just testing for one or two things, we can look for a whole host of other parasites that might appear in the stool,” says Dr. Campbell. Stool tests are complex, requiring different preparations and microscopies, and may take several days to complete.

While parasites can occasionally be seen in urine or other body fluids, the parasites most commonly found in this part of the world are detected through blood or stool samples. One notable exception is a sexually transmitted parasite called Trichomonas vaginalis, which is very common and very curable. It is diagnosed through DNA testing on a vaginal swab.

Before being tested for a parasitic disease, the most important thing you can do is give your healthcare provider a good history of where you’ve traveled, what kind of food, water, and insects you were exposed to, and what you might have done to protect yourself from insect bites, Dr. Campbell says. This will enable your provider to order the right tests for you.

While not as common in the U.S., parasitic diseases are a major public health concern in the rest of the world. Researchers at the Yale School of Public Health are doing important research into innovative, non-medical ways of controlling parasites, Dr. Campbell says. This includes basic parasite biology, vaccines, novel treatments and cutting-edge methods of manipulating the insects that spread the diseases.