Different methods are used to diagnose parasitic diseases. Diagnosis is particularly important because these diseases can often have similar symptoms with other diseases and be consequently misdiagnosed. A big part of the diagnosis process is to look at the past history of the patient, for example where he travelled or lived before in the case of immigrants. This patient’s history helps to narrow the scope of the tests to perform in order to get the good diagnosis, because, with the exception of direct visualisation by the naked eye or by microscope, the different test available are usually specific to only one or few parasite species. Furthermore, it is very important to get an accurate diagnostic in order to avoid delay and receive the best available treatment.
Visualisation of parasites is a straightforward diagnostic method. Helminths parasites, being easily visible with the naked eye, can indeed be found in faeces or vomit of infected individuals. In that case, it is usually easy to distinguish the responsible parasite according to its specific morphological characteristics. However, it is better to diagnose this type of infection before this step where the parasite load is usually very important. It is noteworthy that protozoan parasites can usually be seen under the microscope, but are often too small to be formally identified without any doubt. Parasite’s eggs or cysts present in the faeces of an infected person can also be visualised using a microscope. While testing for the presence of eggs or cysts in faeces, it is normal to test three or more different samples from different days, as they could not be always present in the faeces. If it is not possible to find parasite’s eggs or cysts in the faeces, a colonoscopy, consisting of the insertion of a camera by the rectum, could be used to visualise the inside of the intestine and potentially detect parasites or specific damages caused by parasites.
Biological materials, such as blood, urine, phlegm or faeces, can be analysed in laboratory to find evidences of parasitic infection. Biopsy of suspected infected tissues or organs, like skin or intestine, could also be used. These evidences can be either the presence of specific antigens, which correspond to proteins released by the parasite, or genetic material such as DNA from the parasite. Live parasites extracted from these various biological materials can also be cultivated in laboratory.
Antibody testing can also be performed using patient’s blood sample. Antibodies are release by the immune system of an infected person in order to combat the infectious disease. As antibodies are very specific to one particular type of microorganism, testing which antibodies are released by the patient’s immune system is a good way to find which parasite infects him. Blood sample from patient can also be visualised using the microscope to detect the presence of few protozoan parasites, such as Plasmodium and Babesia.
Finally, other diagnostic tools, such as X-ray and different types of scan, can be used to indirectly diagnose parasitic diseases by looking at the specific damages that they did to the inside organs. As such, they can also be used to assess the gravity of the parasitic infection.
Finally, some parasite species present a high resistance profile to antiparasitic drugs, meaning that some drugs that were supposed to kill them are now inactive against them. In such cases, it can be appropriate to screen the parasite involved in the infection for drug resistance, especially if the infected person has visited a region where resistance is highly prevalent. This test is made by putting the cultivated parasites from the biological sample in presence of different drugs to determine if they are able to survive it or not. This test is very useful in treatment, because it allows the patient to be cured by taking the right drug able to kill the parasite infecting him immediately, instead of trying several unsuccessful drugs.
Sources: Centers for Disease Control and Prevention, Merck Manual