Why malarial parasite usually not found in peripheral blood

Pic: Malaria distribution

Pic: Malaria transmission

It is a common problem that malarial parasite is usually not found in peripheral blood in most suspected cases of malaria. The causes of this failure are many. It is important that there are many causes of febrile illnesses coming with shivering and going off with sweating like in malaria.

Among these:

1) Misdiagnosis: It is documented-

a.Tuberculosis, particularly extra pulmonary cases may be confused with malaria because of its single rise of temperature at the evening time. Particularly in younger patients the bout of fever may be so heightened that it starts with shivering and goes with sweating. In tuberculosis history is longer, weight loss and anorexia almost present in all cases but headache is rare which is common in malaria.

b. UTI (urinary tract infection)- fever in UTI case usually appears with shivering and goes with sweating but it occurs in multiple times in a day. There should be no symptoms of burning or frequency micturition when it affects the upper part of urinary tract. In this case (upper UTI) lower abdominal or back pain is common. Simple routine urine examination excludes the diagnosis.

c. Viral infection:- Viral infection is the commonest cause of febrile illnesses. Its onset is usually acute, seasonal and similar history of other members or neighbours. Most people in our country uses few doses of paracetamol before seeking advice from physician. Physician finds history of shivering and sweating in these cases but it is due to effect of paracetamol. When the action of paracetamol goes off fever comes again with shivering and when it acts fever goes with sweating.

d. Meningitis : It is a case of medical emergency. High rise of fever with headache is must so can be confused with malaria.

2. Use of some anibiotics can also prevent appearence MP in peripheral blood- like fluoroquinolones (e.g, ciprofloxacin), cotrimoxazole, tetracycline etc.

3. Collection of blood: After excluding the above causes of fever if diagnosis goes in favour of malaria, still malarial parasite may not be found in peripheral blood due to

a. Blood examined at early stage of malarial infection - Particularly in first week of illness number of infected RBC may be so low that MP can not be seen even after searching for longer period.

b. Time of collection- This is very critical that when blood should be collected for MP. Many physicians advice to collect blood at the height of temperature. But this is a wrong idea. Fever in malaria synchronizes with rupture of infected RBC releasing hemozoin, a febrile toxin. So at the height of temperature there should not be any intact infected RBC containing MP for demonstration in peripheral blood unless double cycles or multiple cycles on malarial infection are running which also not very rare. So, blood should be collected 1 to 2 hours or more before the onset of fever. When a fever goes off blood should to be collected at least 10 hours after an attack of fever because, this time is needed to develop a ring form of malarial parasite inside red cells.