What is malaria and its symptoms?

‘Plasmodium falciparum is the name of one of the greatest enemies of world health: malaria. We analyze this disease in-depth, how it is spread, how it is fought, and why there is still no vaccine.

Malaria is the most important parasitic disease of humans. It is caused by various species of a genus of protozoa called Plasmodium transmitted from one human host to another through the bite of female mosquitoes of the genus Anopheles. There are between 200 and 300 million cases worldwide and around one million deaths annually, 90% in sub-Saharan Africa and children under five years of age.

Plasmodium falciparum is the parasite that causes almost all cases of severe and complicated malaria. Malaria is one of the leading causes of infant mortality, and in countries where it is still endemic; it is one of the significant factors contributing to underdevelopment.

Furthermore, and especially in sub-Saharan Africa, it is one of the leading causes of morbidity in the form of chronic anemia in children and pregnant women, abortions, low birth weight, and neurological squeal, including psychomotor retardation. All this, together with absenteeism from school and work during febrile crises in countries where there are no social protection systems, makes malaria closely linked to poverty.

In hyper endemic areas, as a consequence of repeated infectious bites from Anopheles mosquitoes, adults develop a state of immunity. Individuals are chronically infected and only occasionally experience malaria episodes. This state of partial exemption is incomplete since it does not prevent new infections but protects against the development of clinical severe manifestations. But that semi-immunity is lost when people leave endemic areas, as is the case with immigrants.

Young children and pregnant women, the most vulnerable to infection

Children under five and pregnant women are at the highest risk for severe and complicated malaria and, therefore, die. This is so because young children do not yet have an adequate immune response against the parasite, and in pregnant women, this immune response is temporarily weakened.

These should be added people with HIV / AIDS infection and travelers from countries where the disease does not exist (who do not have any immunity) and who visit endemic areas. Mainly unique travelers such as immigrants ( and within this group their children born in non-endemic areas) who return to their countries of origin to visit their relatives and friends ( VFR: visit friends and relatives ) who have a much higher risk of acquiring the infection than a standard tourist traveler.

How Malaria Is Spread

There are just over 20 species of plasmodia that affect primates, and, until recently, it was thought that only four were the species that affected man, but lately, a fifth species have been described. The four classic species are Plasmodium falciparum, P. malariae, P. vivax, and P. ovale. The last species implicated in malaria transmission is P. knowlesi, which was first described as a natural infection in man in 1965 in the USA in a traveler returning from Malaysia. Since then, there have been several communications of human cases about this new species. Its human-to-human transmission through the vector is now well established in various areas of Southeast Asia.

The malaria parasite is generally transmitted by the bite of infected mosquitoes of the genus Anopheles (known as vector transmission). There are other transmission routes, although they are much less frequent: the vertical route ( mother to child, during pregnancy ) and the transfusion (blood transfusion, laboratory accident, syringes in intravenous drug addicts). These types of transmissions are usually more common in many sub-Saharan African countries, where many women experience episodes of malaria during pregnancy that exacerbate the anemia and requiring blood transfusions many times from donors with malaria infection (in most of these countries, transfusions are very unsafe and donors are only tested for HIV ).

In the case of vector transmission, at the bite’s time, the infected female mosquito anopheline inoculates the parasites into the man. Male mosquitoes do not feed on blood but plants, so they are not involved in malaria transmission. The parasites that the female Anopheles inoculates go to localize to the liver. They undergo a series of transformations until they pass into the blood and infect the red blood cells or red blood cells.

Malaria parasite symptoms and relapses

While they are in the liver, they do not produce any symptoms ( incubation period ), appearing when they reach the blood. The behavior of parasites in the liver is different according to the species of plasmodia. In P. vivax and P. ovale, only a part of the parasites found in the liver pass into the blood, the rest remaining in the liver in dormant form for months or years, hence their name hypnozoites.

Hypnozoites can pass into the blood and produce symptoms again at any given time, which can vary from a few weeks to five years. This phenomenon that occurs in P. vivax and P. ovale is known as relapse.

Relapses do not occur in the other species since they do not have hypnozoites and, when the parasites found in the liver pass into the blood, they do so all at the same time. If symptoms recur in these species, it is due to a failure in treatment and is known as “recrudescence,” occurring mainly in cases of malaria P. falciparum.

Treatment failure in the latter species may be due to the use of an inappropriate drug, to the fact that it has been administered at insufficient doses, or that it has not been well absorbed.

The third cause of reappearance of symptoms is that another infected mosquito bites us. In this case, it is a new infection or “reinfection” due to a parasite other than the primary infection. Relapses due to the existence of hypnozoites and reinfections are those that have given rise to widespread false belief. On the other hand, that once a person acquires malaria, they already have it for life.