Parasites are organisms that live on or inside a host organism and derive nutrients at the expense of the host. Parasites can cause a variety of symptoms and health problems, so it is important to determine whether an infection is present. There are a few key ways a doctor can check for parasites.
Medical History and Physical Exam
The first step a doctor will take is to get a detailed medical history and perform a full physical exam. They will ask about any recent travel, exposure to unsanitary conditions, contact with animals, dietary habits, and symptoms. Common symptoms of a parasitic infection include:
- Digestive issues like diarrhea, cramping, bloating, gas, nausea, or vomiting
- Skin irritation such as a rash or itching
- Coughing or wheezing
- Fever and chills
- Fatigue or muscle aches
- Weight loss or inadequate weight gain
- Grinding teeth during sleep
During the physical exam, the doctor will check for signs of infection like enlarged organs or nodes, fluid buildup, skin lesions, or abdominal tenderness. Information from the history and physical allows the doctor to identify any clinical features that may indicate a parasitic infection.
Blood Tests
Blood tests check for the presence of parasites or antibodies created by the immune system to fight an infection. Common blood tests include:
- Complete blood count (CBC) – Checks for anemia, low platelet or white blood cell count which can occur with parasites.
- ESR – An elevated erythrocyte sedimentation rate indicates inflammation which may be caused by parasites.
- Serology – Measures antibodies against specific parasites like Toxoplasma gondii or Taenia solium.
- Enzyme immunoassay – Detects antigen compounds derived from parasites like Giardia lamblia or Entamoeba histolytica.
- Molecular assays – Use PCR to detect parasitic DNA in the blood such as Babesia or Trypanosoma cruzi.
These blood tests allow detection of many common parasites and help distinguish between different parasitic infections with overlapping symptoms.
Stool Examination
Checking stool samples under a microscope allows direct visualization of parasites in the gastrointestinal tract. Samples are typically evaluated for:
- Ova and parasites – Parasite eggs, larvae or cysts can be detected under the microscope. May require examination of multiple stool specimens as shedding is intermittent.
- Antigen testing – Stool antigens derived from parasites like Giardia or Cryptosporidium can be detected even if no intact parasites are visualized.
- PCR – Stool PCR allows detection of parasitic DNA from samples. Very sensitive for parasites like Entamoeba histolytica or Cyclospora cayetanensis.
Stool microscopy and antigen testing is useful for diagnosing intestinal protozoa and helminth infections. Samples should be evaluated in the context of a patient’s symptoms and travel history.
Imaging Studies
Imaging like endoscopy, ultrasound, CT or MRI scan may be utilized if parasites are causing symptoms in specific organs. Examples include:
- Endoscopy – Allows direct visualization of parasites in the gastrointestinal tract. Useful for infections like Cryptosporidium or Dientamoeba fragilis.
- Ultrasound – Can detect enlarged lymph nodes, lesions or cysts in organs caused by parasites like Echinococcus or Toxoplasma gondii.
- CT/MRI – Provides detailed images to assess organ damage or abscesses due to parasites. Especially helpful for tissue parasites like schistosomiasis.
Imaging studies provide visualization of tissue invasion, cysts, or damage caused by certain parasitic infections. Results must be combined with other testing to identify the specific parasite involved.
Microscopic Examination of Tissue or Fluid Samples
If parasites are suspected of infecting a certain tissue or body fluid, a sample may be directly examined under a microscope. Skin snips, lymph node aspirates, bone marrow samples or CSF can be evaluated for the presence of parasite larvae, eggs or protozoa. This allows detection of parasites like:
- Microfilaria in skin snips from Onchocerciasis.
- Leishmania in bone marrow or lymph nodes.
- Trypanosoma brucei in lymph node or CSF.
- Malaria parasites in blood smears.
Microscopy of tissue and fluid samples is an important diagnostic tool when a suspicion exists of parasites residing in that anatomic location. It allows visualization of parasites that may not be detectable in stool samples.
Biopsy
Biopsy involves surgical removal of a small piece of tissue for microscopic examination and additional testing. Biopsies may be used to confirm parasitic diseases such as:
- Intestinal biopsies – Can provide diagnosis of organisms including Cryptosporidium, Cyclospora, Giardia, or Helicobacter pylori.
- Skin or muscle biopsies – Useful for visualization of parasites like Onchocerca, Loa loa, cysticercosis or schistosomiasis.
- Liver or lymph node biopsies – Helpful for diagnosis of visceral leishmaniasis or toxoplasmosis.
Biopsy provides a definitive diagnosis when parasites are suspected but undetected on non-invasive testing. Histological findings can distinguish between different parasitic infections. The risks and costs of biopsy must be weighed against the likelihood of parasitism based on clinical presentation.
Culture Techniques
Growing parasites in culture medium can allow detection of viable organisms as well as assessment of antimicrobial sensitivities. Examples include:
- Stool culture – Can identify pathogenic bacteria and be positive for protozoa like Cryptosporidium and Cyclospora when microscopy is negative.
- Blood or bone marrow culture – May grow vector-borne parasites like Trypanosoma or Leishmania species.
- Dermal culture – Involves inoculating sterile dermalsnippets with specialized medium to grow larvae of parasites like Onchocerca.
Culture techniques are not routinely used for diagnosis of parasites. They are more often utilized for research applications. Culture allows detection of viable organisms and further characterization through analysis of growth patterns and molecular studies.
Molecular Methods
Sensitive molecular techniques like PCR allow detection of parasitic DNA or RNA in patient samples. Molecular assays include:
- PCR – Polymerase chain reaction amplifies target parasitic DNA allowing detection of protozoa, helminths, or microsporidia.
- LAMP – Loop mediated isothermal amplification can detect parasite DNA with similar sensitivity to PCR.
- Microarrays – Permits simultaneous detection of multiple parasite targets and differentiation of species.
- NGS – Next generation and metagenomic sequencing characterize all microbial sequences in a complex sample including parasites.
Molecular diagnostics provide rapid, sensitive detection of parasites with low burden or intermittent shedding. PCR and NGS continue to enhance diagnosis of parasitic diseases as costs decrease and accessibility improves.
Conclusion
Doctors utilize a wide array of techniques to determine if a patient has a parasitic infection. Initial evaluation includes a medical history, physical exam, blood tests and stool microscopy or antigen testing. If parasites are suspected in a particular location, imaging studies, tissue biopsy or culture may be employed. PCR and advanced molecular assays also aid diagnosis through detection of parasite nucleic acids. The doctor chooses the appropriate diagnostics based on the patient’s clinical presentation, travel and exposure history. A step-wise approach often combines multiple methods to definitively diagnose the presence and identity of medically important parasites. Increased access globally to sensitive diagnostics promises to improve management of parasitic diseases that impact human health.