Today is the #WorldMalariaDay2021. In this international awareness day – recurring the 25th of April each year – we commemorate the global efforts and gains we made against malaria. A lot of progress has been achieved, and millions of lives have been saved since the year 2000. However, malaria still remains a big killer and more needs to be done: according to the latest world malaria report, in 2019 there were 229 million infections globally and more than 400,000 people died from it.
I decided to celebrate this day a bit differently. I am gonna talk about some incredible fossils which helped scientists to shed light on the origin of malaria. Malaria parasites of the genus Plasmodium are transmitted by various mosquitoes. Besides the species causing the disease in people, other plasmodia infect monkeys, rodents, birds and even reptiles. Understanding when these pathogens originated in the distant past is absolutely fascinating, but it is not an easy task since these organisms rarely leave fossils behind. However, that is not the case if they were “lucky” enough to get trapped inside tree resin.
Parasites frozen in time
Tree resin fossilizes turning into what we know and appreciate today as amber. Amber quite often contains a variety of both plant and animal material, which is basically preserved and frozen in time. Insects are a very common occurrence in amber – ants, wasps, fleas and mosquitoes just to name a few – and it may have happened that the animal was infected with parasites at the time.
The fascinating discovery was published in 2005 by George Poinar of the Oregon State University. The scientist found a female of a previously undescribed mosquito – which was named Culex malariager – trapped in Dominican amber (Figure 1A). Scientists do not entirely agree on the dating of this type of amber, but the general consensus places it between 15 and 45 million years. The most interesting thing, though, is what was found in the insect abdomen: several developmental stages of an organism closely resembling a parasite currently infecting birds (more precisely, Plasmodium juxtanucleare). In particular, the fossil contained both the parasite oocysts (the rounded structures developing in the mosquito gut) and the sporozoites (the motile stages which move into the insect salivary glands, and are responsible for the transmission to the vertebrate host) (Figure 1B). This prehistoric pathogen, the oldest member of the group ever discovered, has been baptized Plasmodium dominicana.
The same year George Poinar published another similar discovery, this time from Myanmar amber dated at least 100 million years: a female of a biting midge containing oocysts of another ancient malaria parasite, which was named Paleohaemoproteus burmacis (Figure 1C). Extant parasites of the genus Haemoproteus are related to plasmodia, but they infect only birds, reptiles and amphibians and are transmitted by biting midges, horseflies and louse flies.
How did malaria evolve?
These incredibly well-preserved fossils can give us some insights on whether the progenitor of malaria parasites originated in vertebrate animals (and then adapted to insects) or vice versa (from insects to vertebrates). Scientists are still debating about this issue, but they tend to consider the latter scenario more plausible.
In fact, malaria organisms have many morphological and functional features in common with gregarines. Gregarines are microbes which are considered the most primitive members of the phylum Apicomplexa, a large group of parasites which includes organisms like Plasmodium, Haemoproteus, and Toxoplasma as well. Gregarines parasitize a wide array of invertebrates, and they have been found in all the major malaria vectors. Fossilized gregarines have also been found in mosquitoes trapped in amber.
These incredible fossils show us that malaria parasites have been infecting animals for a very long time, well before the appearance of our species. The World Malaria Day is here to remind us how much we still need to do to free humanity from this scourge. To find more information about this important day, you can click here.
World malaria report 2020: 20 years of global progress and challenges. Geneva: World Health Organization; 2020. Licence: CC BY-NC-SA 3.0 IGO.
Poinar G Jr. What Fossils Reveal About the Protozoa Progenitors, Geographic Provinces, and Early Hosts of Malarial Organisms. Am. Entomol. 2016 Spring; 62(1):22–25. doi: 10.1093/ae/tmw006.
Poinar G Jr. Plasmodium dominicana n. sp. (Plasmodiidae: Haemospororida) from Tertiary Dominican amber. Syst Parasitol. 2005 May;61(1):47-52. doi: 10.1007/s11230-004-6354-6.
Poinar G Jr, Telford SR Jr. Paleohaemoproteus burmacis gen. n., sp. n. (Haemospororida: Plasmodiidae) from an Early Cretaceous biting midge (Diptera: Ceratopogonidae). Parasitology. 2005 Jul;131(Pt 1):79-84. doi: 10.1017/s0031182005007298.