Histidine-rich protein 2 (HRP2) is a water-soluble protein characterized by its high content of histidine and alanine amino acid repeats. This protein is produced by a single-celled organism and its structure consists of a large region of tandem repeats, making it highly polymorphic. The protein is synthesized and then exported into the host’s system where it can be found in circulation.
The Link Between HRP2 and Malaria
The significance of HRP2 stems from its connection to the deadliest species of malaria parasite, Plasmodium falciparum. During the blood-stage of an infection, this parasite produces and secretes large quantities of HRP2 into the bloodstream, reaching readily detectable concentrations. Because HRP2 is exclusively produced by P. falciparum, it functions as a specific biological marker for this parasite.
This specificity allows healthcare professionals to distinguish P. falciparum infections from those caused by other, less virulent Plasmodium species. The levels of HRP2 in the bloodstream also correlate with the severity of the malaria infection. Higher concentrations of the protein are associated with more severe forms of the disease, such as cerebral malaria.
Use in Rapid Diagnostic Tests
The presence of HRP2 in the blood of infected individuals forms the basis for most modern malaria rapid diagnostic tests (RDTs). These tests are simple to use and provide quick results, making them invaluable in areas without access to laboratory facilities. They often replace traditional microscopy as the primary method of diagnosis.
The mechanism of an HRP2-based RDT is comparable to a home pregnancy test. A small blood sample from a finger prick is applied to a test strip. The strip contains specific monoclonal antibodies that bind to the HRP2 protein. If present, HRP2 attaches to these antibodies as the blood moves along the strip, triggering a reaction that creates a colored line indicating a positive result.
The entire process takes only 15 to 20 minutes. The speed and simplicity of these tests allow for prompt diagnosis and the immediate start of treatment.
The widespread use of HRP2-based RDTs has transformed malaria case management in remote settings. By enabling local health workers to diagnose malaria accurately, these tests ensure that anti-malarial drugs are given to those who truly need them. This reduces the likelihood of severe illness and helps to prevent the unnecessary use of medication.
The Challenge of HRP2 Deletions
A challenge to the effectiveness of HRP2-based malaria testing has emerged in the form of genetic changes in the Plasmodium falciparum parasite. Some strains have undergone chromosomal rearrangements that result in the deletion of the gene responsible for producing HRP2, known as pfhrp2. This adaptation means the parasite can infect a person without producing the protein marker that tests are designed to find.
The immediate consequence of an infection with a pfhrp2-deleted parasite strain is a false-negative result on an HRP2-based RDT. The infected individual will test negative for malaria because the protein the test detects is absent from their blood. This leads to untreated infections, allowing the disease to progress and potentially cause severe complications.
Untreated individuals also contribute to the ongoing transmission of these harder-to-detect parasite strains within a community. The prevalence of pfhrp2 deletions is a growing concern in several regions, including parts of South America, Africa, and Asia, threatening malaria control and elimination efforts.
Diagnostic Alternatives and Solutions
To address the issue of pfhrp2 gene deletions, alternative diagnostic tools are being utilized. One such alternative is plasmodium lactate dehydrogenase (pLDH), an enzyme produced by all human-infecting Plasmodium species. RDTs that detect pLDH can identify a malaria infection even when the HRP2 protein is not present.
In addition to alternative RDTs, other diagnostic methods are available, though they often require more resources. Microscopy, the examination of a blood smear under a microscope, has long been considered the “gold standard” for malaria diagnosis. This method can detect all species of malaria parasites and determine the density of the infection, but it requires a trained microscopist and well-maintained equipment.
Molecular tests, such as polymerase chain reaction (PCR), offer the highest level of sensitivity and specificity. These tests can detect very low levels of parasite DNA in a blood sample and can identify specific strains, including those with pfhrp2 deletions. However, PCR testing is expensive and requires a sophisticated laboratory, limiting its use to surveillance and research settings rather than routine diagnosis in the field.