Parasite is an organism that lives on or inside another organism (host) and benefits (e.g. by getting nutrients) from the host at its expense. Parasitic disease, also known as parasitosis, is an infectious disease caused or transmitted by a parasite. Depending on the nature of parasitic infections, specimens for laboratory diagnosis can be various ones. Some of them are:
In those parasitic infections where parasite itself in any stage of its development circulates in blood stream, examination of blood film forms one of the main procedures for specific diagnosis. E.g. In malaria, parasites are found inside red blood cells (RBCs). In Bancroftian and Malayan filariasis, microfilariae are found in the blood plasma.
- Serology: Used to detect antibodies when immune system is trying to fight off parasite or parasite antigens when the body is infected with a parasite.
- Blood smear: Used to look for parasites that are found in blood. It is done by placing a drop of blood on a microscope slide, then stained and examined under a microscope. Parasitic diseases such as filariasis, malaria etc. can be diagnosed.
Examination of stool forms an important part in diagnosis of intestinal parasitic infections and also for those helminthic parasites that localize in the biliary tract and discharge their eggs into the intestine. Antibiotics, laxatives, and antacids, anti-parasitic agents should not be used until stool sample has been collected as they can reduce the number of parasites enough to make observation of parasites in a stool sample difficult or impossible.
- In protozoan infections, either trophozoites (during active phase) or cystic forms (during chronic phase) may be detected. E.g. Amoebiasis, Giardiasis etc.
- In case of helminthic infections, adult worms, their eggs, or larvae are found in the stool. E.g. Ascariasis, Tapeworm Infections.
Stool can also be tested for proteins released by parasite or genetic materials from them.
When the parasite localizes in urinary tract, examination of the urine will be of help in establishing parasitological diagnosis.
E.g. In urinary Schistosomiasis, eggs of Schistosoma haematobium are found in urine. In chyluria caused by Wuchereria bancrofti, microfilariae are found in urine.
Examination of the sputum is useful in the following parasitic infections:
- In cases where habitat of parasite is in respiratory tract, as in Paragonimiasis, the eggs of Paragonimus westermani are found.
- In amoebic abscess of lung or in case of amoebic liver abscess bursting into lungs, trophozoites of Entamoeba histolytica are detected in sputum.
It varies with different parasitic infections. A biopsy may be done to obtain a sample of intestinal or other infected tissue; skin may be snipped E.g. spleen punctures in cases of kala-azar, muscle biopsy in Cysticercosis, Trichinelliasis, and Chagas disease, Skin snip for Onchocerciasis.
Urethral or vaginal discharge
It is mostly collected on suspecting of infection by Trichomonas vaginalis (Trichomoniasis), sexually transmitted infection that often produces no symptoms. In some cases, it may cause itching, redness, irritation, and an unusual discharge from genital area.
Indirect evidences or changes indicative of intestinal parasitic infections are:
Cytological changes in blood:
- Eosiniphilia often gives an indication of tissue invasion by helminthes.
- Reduction in white blood cell count is an indication of kala-azar.
- Anemia may be a feature of hookworm infestation and malaria.