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Plasmodium vivax

Diagnostic Form:

Whenever possible, specimens should be collected before treatment is initiated. When malaria is suspected, blood smears should be obtained and examined without delay. Since the parasitemia may fluctuate, multiple smears might be needed. These can be taken at 8 to 12 hour intervals for 2 to 3 days.

The diagnostic form of the plasmodium, found in the peripheral blood, can appear in four developmental forms (ring, trophozoite, schizont, and gametocyte). A peripheral blood smear (thin and thick) is stained with Giemsa stain. It is the characteristic appearance of these developmental forms in the peripheral blood smear as well as the morphololgical changes that they impart on the red blood cells that will aide in the definitive diagnosis of Plasmodium vivax.

The ring forms do not impart any enlargement to the red blood cells and occasionally fine Schüffner’s dots can be seen. The parasite has a large cytoplasm with occasional pseudopods and a large chromatin dot.

The trophozoite, schizont and gametocyte forms cause enlargement of the red blood cells and fine Schüffner’s dots can be seen. 12 to 24 merozoites can be seen in the schizont.

The table below provides more detailed morphological characteristics that are imparted on infected red blood cells by each developmental stage of the parasite. The morphological appearance of the parasite in each developmental stage is also listed.

 

FORM
APPEARANCE OF RED BLOOD CELLS
APPEARANCE OF PARASITE
RING

Normal to 1-1/4 X, round; occasionally fine Schüffner’s dots; multiple infection of RBC not uncommon

Large cytoplasm with occasional pseudopods; large chromatin dot

TROPHOZOITE

Enlarged 1-1/2–2 X; may be distorted; fine Schüffner’s dots

large ameboid cytoplasm; large chromatin; fine, yellowish-brown pigment

SCHIZONT

enlarged 1-1/2–2 X; may be distorted; fine Schüffner’s dots

large, may almost fill RBC; mature = 12-24 merozoites; yellowish-brown, coalesced pigment

GAMETOCYTE

enlarged 1-1/2–2 X; may be distorted; fine Schüffner’s dots

round to oval; compact; may almost fill RBC; chromatin compact, eccentric (macrogametocyte) or diffuse (micro- gametocyte); scattered brown pigment

 

schizonts schizont schizont merozoites

 

p vivax ring and gametocyte gametocyte gametocyte p vivax ring form ring form1 ring form 2

shell operculum knob interior

 

Geographic Prevalence:

Central and South America, India and SouthEast Asia. 70-90% of cases in most of Asia and South America. 50-60% of cases in SouthEast Asia and the Western Pacific.

Disease It Causes And The Clinical Symptoms:

As with all infectious diseases, a clinical history of the patient is sometimes the most important part of the process in making a correct diagnosis. This is especially true of the malarial diseases, which can present with a myriad of symptoms that are rather non-specific in many cases, and can many times lead to a misdiagnosis in the absence of a complete clinical history. The presence of a history of travel or residence in a disease endemic area should alert the clinician to the possibility of malarial disease. This is especially important because untreated malaria can progress to severe forms that can rapidly lead to death in less than 24 hours.

Symptoms that are frequently seen in malarial disease include fever and chills, which can be accompanied by headache, myalgias, arthralgias, weakness, vomiting, and diarrhea. Clinical features such as splenomegaly, thrombocytopenia, hypoglycemia, diarrhea, pulmonary or renal disfunction, and anemia can also be present. Many of the symptoms will vary depending the infecting malarial species and even more importantly the immune status of the individual and/or the level of parasitemia present.

The initial paroxysm is moderate to severe. Plasmodium vivax fever patterns are tertian. Meaning that fever paroxysms occur every 48 hours. There are three stages to a paroxysm (cold stage - hot stage - and sweating stage). The cold stage (lasting 15-60 minutes) includes a feeling of intense cold with an elevated body temperature, and vigorous shaking. The hot stage (lasting 2-6 hours) consists of intense heat and an intense headache. The paroxysm ends with a sweating stage (lasting 2-4 hours) where there is profuse sweating, a break in the elevated body temperature, and an exhausted weak state that will usually lead to sleeping. When the patient wakes up there is no ill effect other than exhaustion.

The maximum infection duration for P. vivax is 5-8 years.

 

Location In The Host:

Plasmodium vivax infects reticulocytes. A relapse from persistent liver forms is seen in Plasmodium vivax infections.

 

Primary Vector:

Female Anopheles mosquito.

 

Life Cycle:

 

malaria

 

 

Test Recommended For Detection/Diagnosis:

A thick and thin smear of peripheral blood should be obtained for Giemsa staining and microscopic analysis by a competent microbiologist or hematologist. Thick smears aide in detecting low numbers of parasites while the thin smears aide in morphological observations. The level of parasitemia can be determined by both the thick and thin smear.

The blood specimen can be obtained either by fingerstick or venipuncture.

It's very important that two thick and two thin smears be prepared as soon as possible after the blood has been collected. Any delay in the preparation of the smears will cause changes in the morphology and staining characteristics of the parasite.

Specimens are stained with Giemsa stain. The use of the Giemsa stain with optimal pH will allow for the demonstration of Schuffner's dots. It is important to be aware that Wright or Wright-Giemsa stains will not allow for the demonstration of Schuffner's dots.

Antigen detection tests and PCR amplication tests are available that may make the diagnosis quicker and are valuable when the diagnosis could constitute a medical emergency. However, these tests are not readily available in most routine clinical laboratories and therefore the smear observation may have to suffice.

 

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