Malaria Metrics
Malaria is often deadly or debilitating, so there has been a long-standing interest in measuring it [1–3]. This is a brief overview of the metrics we use to measure and understand malaria. It is presented roughly chronologically. Please follow the links in the text or sidebar for a longer introduction.
Malaria Epidemiology
There were methods to measure malaria in populations before we knew what caused it. In 1948, 32 years before Laveran’s discovery, malaria was defined by symptoms and context. The first metric to measure malaria in populations was based on a symptom: Dempster used the fraction of a population with a palpably enlarged spleen to measure malaria in populations [4]. He called it the spleen rate.
After Laveran, methods were developed to identify parasites through light microscopy [5]. Light microscopy was more specific than splenomegaly, and it became the standard for diagnosing malaria in clinical settings.
Parasite Rate
Malaria prevalence in populations is the fraction of blood samples from a population that test positive for malaria. After the spleen rate, it was called the malaria rate or malaria parasite rate or parasite rate.
In the years that followed his discovery in [6], Ross devoted himself to the Prevention of Malaria [7]. He developed mathematical models to understand malaria prevalence and use it as a quantitative basis for malaria control [7,8].
The main focus of these essays is Plasmodium falciparum. Parasite rate could describe infection with any of the malaria parasites that infect humans, so we stipulate that we mean the PfPR (or Plasmodium falciparum parasite rate).
Infection Duration
To understand malaria transmission in populations, he needed a basis for understanding the prevalence. Prevalence is affected by exposure and infection and infection duration In Ross’s models, he assumed infections would clear at some rate, \(r,\) and some attention focused on measuring it. How long does a malaria infection last?
Force of Infection
Starting in 1915, Ross sought to develop general theory for epidemics, which he called a priori pathometry [9–12]. An important concept was the happenings rate, or the hazard rate for infection in a susceptible population. Today, we would call the force of infection (FoI). In honor of Ross, we still use \(h\) to denote the FoI.
The basic model, sometimes called the catalytic conversion model [13], has been used as a basic starting point for estimating the force of infection [14]. If \(r\) is the waiting time to clear an infection, then the dynamics of prevalence (\(x\)) are described by: \[\frac{dx}{dt} = h (1-x) - rx.\] The equation focused some attention on the clearance rate or equivalently the duration of an infection.
Multiplicity of Infection
Seroconversion Rate
Disease
Malaria is a disease of humans that is caused by infection with malaria parasites. If we’re going to measure malaria, and not just infection, we need to discuss the concept of disease.
Mosquitoes
After Ross, several metrics were developed to study mosquitoes. In 1905, Ross wrote a paper identifying the important role for mosquito movement in malaria control [15].
Entomological Inoculation Rates
Field studies were devised to measure exposure to the bites of infectious mosquitoes [16]. One of those statistics, now called the entomological inoculation rate, is endorsed by the WHO [17].
Vector Bionomic Parameters
In 1952, Macdonald wrote a paper on the sporzoite rate, which was a synthesis of decades of research [18]. The paper named parameters. Later that same year, Macdonald presented a formula for \(R_0,\) called the basic reproductive number (or rate or ratio) [19]. In 1964, Garrett-Jones isolated the mosquito-specfici parts of \(R_0\) and called it vectorial capacity. After Macdonald, there was an increased interest in measuring mosquito survival and other bionomic parameters. An important concept was parity [20].
Surveillance Metrics
During the Global Malaria Eradication Programme (1955-1969), there was a need to identify places where transmission was still occurring and then verify that malaria had been eliminated. For this, attention turned to clinical surveillance.