Malaria Parasite count

Malaria parasite count

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Malaria parasite count provides information on severity of infection and the response of treatment. It provide information on malaria burden which is necessary for prognostic purpose and help in monitoring the disease and is  vital for proper management of patient.

This may also help to assess response to malaria vaccine or drugs in clinical trials.

 Introduction

Malaria is caused by a blood parasite of the plasmodium genus. 4 species are known to cause malaria commonly. These are

  • P. vivax
  • P. falciparum
  • P. ovale and
  • P. malariae

Malaria parasite count is performed on P. falciparum, P. vivax, P. ovale, P. malariae – asexual stage.

Normally, gametocytes are not counted, but if present- it should be reported.

Malaria parasite count is performed on thick film, but if thick film is not available, count should be done on thin film. If count is > 100 parasite in each field, then thin film counting should be done.

Materials and equipment needed

  • Microscope with 10x, 40x, 100x (oil) objective and 10x eyepiece.
  • Multiple or 2 key tally counter. One count Malaria parasite and other count WBC.
  • Giemsa stained blood slide.
  • Immersion oil
  • Lens paper
  • Pen, pencil, calculator

Procedure

From thick Film

  • Place the slide under microscope, find Malaria parasite of all species asexual form (Except gametocyte).
  • Start from top left area; find a area where good number of parasite and WBC are present. Start counting.
  • Use multiple key counters to count Malaria parasite and WBC separately.
  • See the entire slide from left to right, then right to left in other filed. As shown in the figure.
  • Note and report counting.
  • Stop counting after 200-500 WBC count
    • If Malaria parasite is >/= 100 in 200 WBC, stop counting
    • If Malaria parasite is </= 99 in 500 WBC, stop counting.
  • Count all Malaria parasite and WBC in the final field, even if WBC count exceeds 500.
  • Calculate the parasite density on the basis of patients actual WBC count.

Say, total count is 8000/ micro litre.

The parasite/micro litre of blood will be = No. of Parasite counted x 8000 ÷ No. of WBC counted

From thin Film

  • Place the slide under microscope, find Malaria parasite of all species asexual form (Except gametocyte).
  • Start from top left area; find an area where good number of parasite and WBC are present. Start counting.
Counting Direction
Counting Direction
  • Use multiple key counters to count Malaria Parasitized RBC and RBC separately.
  • See the entire slide from left to right, then right to left in other filed. As shown in the figure.
  • Select an area (under 100x), a typical area contain about 250 RBC.
  • Stop counting after 20 field, 5000 RBC. Count all RBC even if it exceeds 250 in number.
  • Note and report counting.
  • Calculate the parasite density on the basis of patients actual RBC count.

Say, total RBC count is 5 000 000/micro litre.

The parasite/micro litre of blood will be = No. of Parasitized RBC  x 5 000 000 ÷ 20 field x 250 RBCs

% parasitemia = (parasitized RBCs/total RBCs) × 100

If the parasitemia is high (e.g., > 10%) examine 500 RBCs; if it is low (e.g., <1%) examine 2,000 RBCs (or more)

Please Note

  • Thus 4-5% or more parasitemia, or more than 100000 parasite/microliter are commonly regarded as indicator of risk of severe malaria.
  • Parasite load estimation is also an objective measures in response to treatment- an aid to clinicians for decision making in high endemic zone.
  • It is also helpful in clinical trials of antimalarial drugs.
  • Determination of “No Parasites Found” (NPF): For malaria diagnosis, WHO recommends that at least 100 fields, each containing approximately 20 WBCs, be screened before calling a thick smear negative.  Assuming an average WBC count of 8,000 per microliter of blood, this gives a threshold of sensitivity of 4 parasites per microliter of blood
  • In nonimmune patients, symptomatic malaria can occur at lower parasite densities
  • Gametocyte count is important in Falciparum Malaria
  • Fluorescent dyes can be used counting Parasite.  Slide are stained with acridine orange and examined with either a fluorescence microscope or a light microscope adapted with an interference filter system.
  • In the Quantitative Buffy Coat (QBC®; Becton Dickinson) method gives a good sensitivity for detection of malaria parasites, and has also been applied to other parasites such as trypanosomes, microfilaria and Babesia spp.

Alternate Method

There is some other procedure, by which Malaria Parasite count can be given as +, ++, +++, or +++ counting 100 field in oil immersion. But method described by above by calculating RBC or WBC count is the standard procedure recommended by W.H.O.

Alternate Method count
Alternate Method count

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