Malaria and its Transmission

Abstract

Malaria is a life threatening illness transmitted to humans through a particular species of mosquitoes. The female Anopheles mosquito is responsible for carrying the malaria parasite, and transferring it to humans via its bites. An approximate of 30 to 40 species of Anopheles mosquitoes are capable of transmitting malaria - their transmission efficiency is dependent upon a number of factors. This disease predominantly occurs in tropical regions but is fortunately, both preventable and curable. This article aims to provide a comprehensive insight into its method of transmission - this includes the transmission through mosquito bites, organ transplantations, sharing of contaminated needles, blood transfusions and from a mother to her child before or after birth.

Introduction 

Marked by intermittent episodes of fever and chills, anemia, splenomegaly i.e. the enlargement of the spleen, and potentially life threatening complications, malaria is a severe recurring infection in humans. It is caused by unicellular parasites belonging to the genus Plasmodium[1]. The Centers for Disease Control and Prevention (CDC) define a parasite as an organism that resides within or on a host organism, deriving its nourishment from the host, and is often to the detriment of the host’s well being[3]. The parasites causing malaria are transmitted to humans through the bites of the Anopheles mosquitoes.

Malaria in humans is attributed to six different species of the Plasmodium genus. They are the following[4]: 

• Plasmodium falciparum: This species is the most prevalent and lethal form of malaria, accounting for the majority of malaria-related fatalities. Patients suffering from severe cases of falciparum malaria may experience complications such as liver and kidney failure, convulsions, and coma. It is predominantly found in Africa[5].

• Plasmodium vivax: This species is the most frequently encountered type of malaria outside the sub-Saharan African region[2].

• Plasmodium malariae: This species is distributed globally and is known to cause what is referred to as "benign malaria," which is significantly less severe than the forms caused by P. falciparum or P. vivax[4].

• Plasmodium ovale curtisi: Generally associated with milder illness, this species has the ability to remain dormant in the liver for extended periods, potentially leading to a resurgence of symptoms months or even years later[5].

• Plasmodium ovale wallikeri: This species has a preference for young red blood cells, known as reticulocytes.

• Plasmodium knowlesi: Although primarily a zoonotic pathogen, it poses a significant risk to human health in various regions of Southeast Asia[4].

While malaria can manifest in temperate regions, it is prevalent in tropical and subtropical areas. In numerous parts of sub-Saharan Africa, entire communities experience persistent infections upon the entry of the parasite. Additionally, malaria is prevalent in Central America, the northern regions of South America as well as in South and Southeast Asia. Cases have also been reported in countries adjacent to the Mediterranean sea, the Middle East and in East Asia. Moreover, in European countries, North America, and the developed nations of East Asia, malaria is carried by travellers who enter from the aforementioned affected regions[1].

 In 2023, it was estimated that there were 263 million cases of malaria globally, and of those, 597,000 were fatalities. The African Region, as defined by the World Health Organisation (WHO), bears an excessive proportion of the global malaria burden. In particular, the region accounted for a staggering 94% of all malaria cases, totalling 246 million, and 95% of deaths caused by malaria. This figure amounted to 596,000. It is important to note that children under the age of 5 represented approximately 76% of all malaria-related deaths within this region[2].

Transmission 

The transmission of malaria can occur in a number of different ways, either through mosquito bites, blood transfusions from an infected person, organ transplantation, or by the sharing of contaminated needles[8].

1. Mosquito-to-human Transmission

Mosquito-to-human malaria transmission occurs when sporozoites from the salivary gland of the mosquito are injected into the skin when it feeds on the blood of the host. Parasites then pass to the liver where they replicate; each sporozoite can yield many thousands of merozoites which go on to cause patent infection. When the parasite matures, it leaves the liver and infects red blood cells. This is when symptoms typically develop.

Relatively little is known about the population dynamics of malaria between the bite of the infected mosquito and patency. It is currently assumed that the probability of mosquito-to-human transmission is determined simply by the presence of salivary gland sporozoites and not the total number of parasites. For example, malaria transmission intensity and the human force of infection are measured using the entomological inoculation rate (EIR) - this represents the average number of infectious mosquito bites per person per year [1]. EIR is calculated by multiplying the human biting rate by the proportion of mosquitoes with salivary gland sporozoites and therefore does not explicitly consider how heavily infected the mosquitoes are[6].

2. Transmission Through Blood Transfusions

Transfusion-transmitted malaria (TTM) represents one of the earliest documented cases of infection associated with blood transfusions. This condition arises from the inadvertent transfer of the Plasmodium parasite from an asymptomatic donor harbouring the parasite to the individual who is receiving the blood. TTM poses a considerable threat, especially in areas where malaria is not endemic. Research has demonstrated that Plasmodium parasites have the capability of remaining viable in blood and plasma that is stored at a temperature of 4°C for up to 18 days. This increases the risk of transmission[7].

3. Organ Transplantations 

Although malaria is widespread internationally, the transmission of the parasites through solid organ transplantation from donors is infrequent. Nonetheless, this case serves as evidence that such transmission can occur. A patient, aged 15, suffered brain death due to spontaneous subarachnoid hemorrhage. The patient had no significant medical history; a parent had indicated that the patient had resided in a malaria-endemic area. Pre-donation blood tests for the donor returned negative results for the malaria antigen. Subsequently, double lungs, two kidneys and a liver were successfully retrieved and transplanted.

 However, 17 days post kidney transplant, one recipient was admitted to the hospital with a fever and malaise. Blood analysis reports revealed the presence of malaria parasites - they were confirmed as Plasmodium ovale through nucleic acid testing (NAT). Following this, the donor’s serum was re-evaluated using NAT; this detected Plasmodium falciparum DNA. Ten days later, both the liver recipient and the second kidney recipient exhibited fever and tested positive for Plasmodium ovale parasitemia via NAT. Hence, this case is proof that organ transplantation can serve as a pathway for malaria transmission[9].

4. Sharing of Contaminated Needles

Malaria transmission through needle sharing among intravenous drug users (IVDUs) is an uncommon occurrence. In the summer of 2000, researchers conducted a study to investigate a febrile illness affecting IVDUs at a camp near Ahvaz, a city located in southwestern Iran. The study involved all 98 male participants, with an average age of 25 years. Data was collected through a questionnaire, and peripheral blood smears were analyzed for the presence of Plasmodium using Wright and Giemsa staining techniques. Those identified as positive for Plasmodium received an initial treatment of 600 mg of chloroquine on the first day, followed by a 300 mg dose six hours later, and additional 300 mg doses on the second and third days.

Out of the participants, 27 individuals (27.6%) tested positive for Plasmodium falciparum, all of whom had engaged in needle and syringe sharing shortly before the onset of their illness. Notably, one individual had been transferred to the camp from Bandar Abbas two weeks prior. The camp environment was devoid of anopheline mosquitoes, and no such mosquitoes were found within a one-kilometer radius. Furthermore, there had been no prior reports of malaria transmission in this camp or among the surrounding population. This evidence suggests that malaria can indeed be transmitted through the sharing of contaminated needles[10].

5. Congenital Malaria

Congenital malaria is an uncommon yet potentially life-threatening condition that arises when a mother positive for Plasmodium parasites transmits them to her fetus during pregnancy or childbirth. It is characterised by the detection of asexual malarial parasites in an infant’s bloodstream within the first week of life[12]; an alternative description defines it as the identification of malarial parasites in the peripheral blood smear of a newborn aged between 24 hours and 7 days. Clinically evident congenital malaria is not common in regions where malaria is endemic and maternal antibody levels are found elevated. 

Typically, symptoms manifest between 10 to 30 days after the delivery. The prevalent clinical manifestations include fever, anemia, and splenomegaly. This is observed in approximately 80% of the cases. Additional signs and symptoms may encompass jaundice, diarrhea and inadequate feeding. In some circumstances, drowsiness and cyanosis etc may also be observed. The diagnosis is often overlooked. Infants with asymptomatic parasitemia at birth may either resolve this condition spontaneously or develop symptoms during the later stages of the neonatal period[11]. 

Conclusion 

Malaria is a lethal disease that has the potential for eradication. This necessitates a multifaceted approach at the national, regional and international levels. The prevention of parasite transmission is crucial, and can be significantly achieved by promoting practices such as refraining from sharing contaminated needles and utilising mosquito repellents. With the implementation of appropriate measures, eradication could be a reality by 2050. The advantages of eliminating malaria will greatly surpass the associated costs, leading to saved lives, diminishing poverty, and enhanced equity.

References

1. https://www.britannica.com/science/malaria 

2. https://www.who.int/news-room/fact-sheets/detail/malaria#:~:text=Malaria%20is%20a%20life%2Dthreatening,spread%20from%20person%20to%20person. 

3. https://www.cdc.gov/parasites/about/index.html 

4. https://pmc.ncbi.nlm.nih.gov/articles/PMC5627438/ - Species and genotype diversity of Plasmodium in malaria patients from Gabon analysed by next generation sequencing By Albert Lalremruata 1,2,#, Sankarganesh Jeyaraj 1,3,#, Thomas Engleitner 1,4, Fanny Joanny 1, Annika Lang 1, Sabine Bélard 5,6, Ghyslain Mombo-Ngoma 7, Michael Ramharter 1,2,7,8, Peter G Kremsner 1,2,7, Benjamin Mordmüller 1,2,7,✉, Jana Held 1,2,7

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9. https://journals.lww.com/transplantjournal/fulltext/2024/09001/242_4__donor_derived_transmission_of_malaria__.137.aspx#:~:text=Introduction%3A%20Despite%20the%20prevalence%20of,within%20the%20preceding%203%20years. - 242.4: Donor-derived transmission of malaria – Implications for donor screening. By O’Leary, Michael1; Ana, Domazetovska2; Lee, Rogan4; Cavazzoni, Elena1; Finemore, Taran3; Watts, Matthew R3; Stelzer-Braid, Sacha2; Rawlinson, William2

10. https://pubmed.ncbi.nlm.nih.gov/19648047/ - Outbreak investigation of needle sharing-induced malaria, Ahvaz, Iran by Seyed Mohammad Alavi 1, Leila Alavi, Farhad Jaafari

11. https://pmc.ncbi.nlm.nih.gov/articles/PMC4921569/ - Congenital Malaria by RK Thapar *, A Saxena +, A Devgan #

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