Outstanding questions in P. vivax research
Can P. vivax cause severe or fatal malaria?
According to the CDC, severe malaria occurs when P. falciparum infections
are complicated by serious organ failures or abnormalities in the patient's blood or metabolism. The manifestations of severe malaria include:
- Cerebral malaria, with abnormal behavior, impairment of consciousness, seizures, coma, or other neurologic abnormalities
- Severe anemia due to hemolysis (destruction of the red blood cells)
- Hemoglobinuria (hemoglobin in the urine) due to hemolysis
- Pulmonary edema (fluid buildup in the lungs) or acute respiratory distress syndrome (ARDS), which may occur even after the parasite counts have decreased in response to treatment
- Abnormalities in blood coagulation and thrombocytopenia (decrease in blood platelets)
- Cardiovascular collapse and shock
- Acute kidney failure
- Hyperparasitemia, where more than 5% of the red blood cells are infected by malaria parasites
- Metabolic acidosis (excessive acidity in the blood and tissue fluids), often in association with hypoglycemia
- Hypoglycemia (low blood glucose). (Hypoglycemia may also occur in pregnant women with uncomplicated malaria, or after treatment with quinine.)
There have been scattered reports in the last 30 years of P. vivax causing cerebral malaria, thrombocytopenia, disseminated
intravascular coagulation (DIC), ARDS, and renal failure. Recently Prakash
et al. (2003) analysed blood smears of 577 Indian patients with acute renal failure
and found P. vivax parasites in 13 of them, with no microscopic evidence of P. falciparum co-infection. However, as in the majority of such reports, the diagnosis was made without PCR to rule out the possibility of a mixed infection with P. falciparum.
Choi et al. (2004) reported
blood smear and PCR confirmation of P. vivax infection of a South Korean soldier suffering from retinal haemmorrhage, a condition more commonly observed in P. falciparum malaria.
In 2005 Kochar et al.
reported 11 cases of severe P. vivax malaria in patients admitted to a malaria intensive care ward in Bikaner, India. Clinical presentations included
ARDS, cerebral anemia, renal failure, bleeding diathesis, and jaundice. Two of the patients died, and one among the recovered developed postmalarial psychosis.
A battery of diagnostic tests, including PCR, established infection with P. vivax and ruled out co-infection with P. falciparum.
The view that P. vivax may no longer be a
paradigm for uncomplicated malaria was argued by Picot (2006). Questions and ambiguities raised by a decade of reports of severe and complicated P. vivax malaria, tabulated in a recent review by Baird (2007), underscored the need for more study of the morbidity and mortality effects of chronic P. vivax exposure, inadequate therapy, relapse, and parasite genotype.
Does circumsporozoite protein variation affect P. vivax survival in mosquitoes?
(contributed by Dr. Deirdre Joy , Laboratory of Malaria and Vector Research, National Institutes of Health )
In southern Mexico P. vivax malaria
parasites carrying different circumsporozoite (CS) protein variants exhibit
differences in development success in two local mosquitoes, An. albimanus
and An. pseudopunctipennis [Gonzalez-Ceron
et al., 1999; Rodriguez
et al., 2000]. The geographical ranges of the two mosquitoes do
not overlap extensively, with An. pseudopunctipennis found primarily
at higher altitudes (above 600 m) [Manguin
et al. 1995] and An. albimanus at altitudes < 100m [Rubio-Palis
et al., 1997]. The two CS protein variants, termed VK210 and
VK247, differ in the amino acid composition of the central repetitive region of
the gene: ANGA(G/D)(N/D)QPG in VK247 and GDRA(D/A)GQPA in VK210 [Rosenberg
et al., 1989; Kain
et al., 1992]. In general, An. pseudopunctipennis is more
susceptible to VK247 variants and An. albimanus is more susceptible to
VK210 variants [Gonzalez-Ceron
et al., 1999]. However, subsequent studies of the fate of
parasites carrying the two CS variants revealed that the destruction of
parasites in the incompatible mosquito occurs prior to the expression of CS,
probably during the ookinete and/or early oocyst stages [Gonzalez-Ceron
et al., 2006; 2001].
Further, in Columbia
the opposite pattern was observed, with VK210 parasites more prevalent in An.
et al., 2001].
Microsatellite typing of P. vivax in southern Mexico has revealed population structure and restricted gene flow in
an area of less than 100 square kilometers [Joy et al. 2008]. Re-analysis of laboratory mosquito feeding experiments has
confirmed that these populations differ in their ability to infect An. albimanus and An. pseudopunctipennis,
suggesting that parasite population structure results from the adaptation of malaria parasites to their vectors.
Underlying genetic structure appears to have led to a spurious association between CS variants and mosquito infectivity.
Therefore, despite the observed association between CS repeat variant and mosquito compatibility, it seems unlikely that
CS is directly involved.
What is the incidence of P. vivax malaria in Africa, and are Duffy-negative humans ever infected?
The Duffy antigen gene encodes a cytokine receptor expressed on the surface of red blood cells (RBCs). It is also required for P. vivax merozoite entry into the RBC.
Most people born in Central and West Africa are homozygous for the FYnull allele of the gene which contains a single nucleotide substitution in its promotor that suppresses
its expression in RBCs. As such, it is assumed that these 'Duffy-negative' individuals cannot be infected by P. vivax, and indeed vivax malaria is considered to be very rare in equatorial
Africa. Sporadic reports of vivax malaria there have generally been attributed to rare, local occurences of Duffy-positive phenotypes, even though this has not been verified.
On the contrary, Ryan et al. (2006)
recently reported tentative molecular, immunological and cytometric evidence of P. vivax transmission among Duffy-negative individuals of the Luo ethnic group and anophiline mosquitoes in Kenya. The authors note that the number of Duffy-negative humans previously tested for P. vivax susceptibility is small, and further suggest
that the true extent of P. vivax in equatorial Africa may have been obscured by its confusion with the microscopically similar species P. ovale. They propose three explanations for the seeming paradox of P. vivax in Duffy-negative individuals:
Explanation (1) seems unlikely given the use of multiple vivax-specific markers -- including a monoclonal antibody against CSP variant
VK247 and PCR amplification of SSU rRNA and the MSP-1 gene -- though their success varied considerably, perhaps due to the extremely low P. vivax densities in their subjects.
Explanation (2) is plausible by analogy to P. knowlesi and P. yoelii, which can exploit receptors other than Duffy on monkey and murine RBCs, respectively. Moreover, CSP variants, as noted in the previous section, may mark parasite populations that are biologically distinct in ways unrelated to CSP function; if so, this could be the basis for the putative ability of a P. vivax subpopulation to use receptors other than Duffy -- including novel receptors proposed in (3), whose discovery awaits more extensive blood typing of the Luo.
- the parasite is not really P. vivax and thus does not require Duffy protein for erythrocyte invasion
- the parasite is a variant of P. vivax that has evolved to use a known receptor other than Duffy
- the population unders study has a unique RBC surface proteins that can substitute for Duffy protein function in merozoite invasion
Whither the 'vivax-like' malaria parasite?
The term 'vivax-like' has been used in the malaria literature to describe 1) a disease similar to vivax malaria, but more fatal (e.g. Garnham 1988) and 2)
a putative fifth human-infecting species of Plasmodium whose existence is posited solely from the discovery in 1993 of a 'new' circumsporozoite
protein (CSP) variant allele in vivax-infected human blood samples from Papua New Guinea
et al., 1993a). The sequence of the 'vivax-like' CSP gene was found to be different from that of the two known types of P. vivax CSP but
identical to the CSP gene of the monkey parasite Plasmodium simiovale, raising the possibility of host-switching. P. vivax-like CSP
was detected in samples from Brazil, Madagascar, and Indonesia by one group
(Qari et al., 1993b) but a second group was unable to detect it in any samples from Central America, South America, Africa, Southeast Asia, or India
et al., 1994). To date, apart from CSP, no other protein or DNA sequences attributable to the putative fifth malaria parasite has been identified in