Manual of basic techniques for a health laboratory - libdoc.who.int

182 Manual of basic techniques for a health laboratoryAfter procedure (i) or (ii), use a simple mathematical formula, multiplying thenumber of parasites by 8000 and then dividing this figure by the number ofleukocytes (200 or 500). The result is the number of parasites/ml of blood. It isnormal practice to count all the species present and to count and record separatelythe gametocytes of P. falciparum and the asexual parasites. This is particularlyimportant when monitoring the response to antimalarial drugs that areactive against the schizont stage, which would not be expected to have anyeffects on gametocytes.number of parasites counted number of leukocytes= number of parasites ml of bloodExample:If 200 leukocytes are counted:( 50 parasites ¥ 8000 200 leukocytes )= 2000 parasites ml of bloodIf 500 leukocytes are counted:( 5 parasites ¥ 8000 500 leukocytes )= 80 parasites ml of blood2. A simpler method of counting parasites in thick blood films is to use the plussystem. This system is less satisfactory, however, and should be used only whenit is not possible to carry out the more acceptable count of parasites/ml of blood.In this system, a code of between one and four plus signs is used:+ = 1–10 parasites per 100 thick film fields++ = 11–100 parasites per 100 thick film fields+++ = 1–10 parasites per single thick film field++++ = more than 10 parasites per single thick film field.Remember: For proper identification and reliable parasite counting, use clean slidesand well-made and well-stained thick films.Note: Patients with very high parasite densities (over 10 parasites per thick filmfield) require urgent treatment. Therefore, if you find a high parasite density, statethe result clearly in your report and send it immediately to the patient’s physician.Reporting resultsIf the result of the examination of the stained blood films is positive, specify:— the species of parasite found;— the stage of development of the parasite;— the parasite density.Blood films containing P. ovale and P. vivax may contain few parasites and thereforetake more time to examine under the microscope. However, it is necessary to differentiatethe two species, since they may reappear in the blood without reinfection.Patients infected with P. ovale or P. vivax require additional treatment to eradicatethe liver stages of these parasites.A patient can harbour more than one species of malaria parasite at the same time(e.g. P. falciparum and P. malariae or P. falciparum and P. vivax).If the result is negative, report as “no parasites found”.4.7.3 Trypanosoma spp.Trypanosomiasis is caused by infection with parasitic protozoa of the genusTrypanosoma. It occurs in southern and western Africa, where it is known assleeping sickness or African trypanosomiasis, and in Central and South America,where it is called Chagas disease.