Plasmodium vivax is also called “benign tertian malaria” and is the most commonly acquired of the malaria due to its wide geographic distribution.
Disease / Pathogenesis
Plasmodium vivax causes paroxysms that are tertian (paroxysms that typically recur every 48 hours or every third day, reckoning the day of the paroxysm as the first). The paroxysms can be debilitating to the infected person. Though characterized as a benign malaria, P. vivax can cause serious and fatal disease and thus the name can be considered a misnomer.
The parasite targets the young red blood cells of the reticuloendothelial system much like Plasmodium ovale., The parasitemia is generally <2% and thus with each paroxysm there is <2% of red blood cell destruction. Generally not as acute as the >5% seen in Plasmodium falciparum. Splenic removal of parasitized cells and red blood cells that are compromised accounts for splenomegaly. Waste products from the destruction of red blood cells and antigenic immunologic response of the host also contribute to the inflammatory response.
Infected red blood cells have a propensity for sticking to each other and to the walls of capillaries. This can lead to the obstruction of blood vessels and deprive tissues/organs of oxygen. Neurological involvement can lead to loss of consciousness, or death. Jaundice is possible due to liver involvement. Kidney involvement can lead to kidney failure. More complications of malaria can also be hypoglycemia and low blood pressures caused by cardiovascular collapse.
Plasmodium vivax (much like Plasmodium ovale) has the unique ability to relapse years later after presumed successful treatment due to the ability of the organism to transform into hypnozoites in liver cells. These hypnozoites are dormant parasites that can reactivate years later to cause disease in previously treated or resolved infections. Many times persons with symptoms of malaria, in the absence of recent exposure or travel to an endemic area, will have had a past history of infection with Plasmodium vivax or ovale. A careful collection of the patient’s history will alert to the physician of such a possible relapse.
Location in the Host
The parasite is located in the red blood cells … specifically the young red blood cells that it preferentially infects. The parasite is also found in its dormant state as hypnozoites in liver cells where it can be reactivated years later provided the right conditions occur (such as immunosuppression or other factors).
Plasmodium vivax is found in temperate regions as well as tropical and subtropical areas. Because of its wide distribution, Plasmodium vivax is the most geographically diverse/commonly acquired Plasmodium.
Morphology & Diagnosis
The blood smear detection of the parasites is still considered the gold standard for detection. Plasmodium vivax infect young red blood cells . Because red blood cells infected are young they can be slightly enlarged (up to 1 1/2x to 2x) in size or normal in size. The infected red blood cells, much like those of Plasmodium ovale, can have Schuffner’s dots visible in them when stained with Giemsa stain. The pH of the stain is critical for the visibility of Schuffner’s dots in the parasites (pH 7.2).
Trophozoites and ring forms in the red blood cells tend to be described as amoeboid. The ring cytoplasm is delicate with a well discernible chromatin dot.
Schizonts will have between 12-24 merozoites present whereas Plasmodium ovale will have between 6-14 merozoites.
Blood smears prepared just prior to paroxysms stained with Geimsa stain are still the gold standard as mentioned previously. Stains prepared are thin and thick smears. Thin smears are utilized mainly for morphologic differentiation/identification purposes while the thick smear (which is a concentrated smear that has the red blood cells lysed) is for measuring the volume of parasitemia present or detect low levels of parasitemia that may not be otherwise detected in the thin smear).
New rapid methods have been developed that offer quick screening methods for the detection of the most common malaria (P. falciparum, malaria, ovale, vivax). An example is the BinaxNow test. A point of care test that is an immunochromatographic assay for the qualitative detection that is available for use and is practical and easy to use in the field. It will detect Plasmodium antigens circulating in human venous and capillary EDTA whole blood of individuals with signs and symptoms of malarial infection. It is mainly useful in differentiating a malarial infection from Plasmodium falciparum and the other less virulent forms. Negative results and or detection of other Plasmodium spp., other than Plasmodium falciparum, will require confirmation/morphologic differentiation/identification via smear preparations.