What does malaria cause?

by

QUESTION:

What does malaria cause?

ANSWER:

Malaria is a disease. It is caused by tiny single-celled parasites called Plasmodium, which are transmitted through the bite of infected Anopheles mosquitoes. There are many symptoms that occur as a result of infection with malaria, namely fever, chills, headaches and nausea, among others. Malaria can be a very serious disease, especially if not treated promptly, and so when spending time in malaria-infection areas (such as many areas of low to moderate elevation in the tropics and sub-tropics) precautions should be taken to avoid mosquito bites and thus infection. If a person finds themselves suffering from some of the above symptoms after being in a malaria transmission region, it is crucially important they get diagnosed straight away; if they do indeed have malaria, then they can be given treatment to facilitate a quick and safe recovery.

When was malaria first found in humans?

by

QUESTION:

When was the first case of malaria in humans discovered? What did the name malaria originate from?

ANSWER:

Malaria has been known to humans for thousands of years; its earliest record is from around 2700 BCE in an ancient Chinese medical text. The ancient Greeks, Egyptians and Indians also recorded cases of malaria and described its symptoms. However, the parasite that causes malaria was first observed in a suffering patient in 1880; in 1897 mosquitoes were discovered to be the agents transmitting the parasite, finally allowing doctors to understand the true nature of the disease. The word “malaria” comes from “mala aria”, Italian for “bad air”, hinting at the long-held association between malaria and foul marshy regions with bad smelling air, which dates back as far as the ancient Romans. Indeed, the mosquitoes that spread malaria breed in stagnant water, so the Romans weren’t too far off!

Who discovered the disease malaria?

by

QUESTION:

Who discovered the malaria disease and when?

ANSWER:

Malaria has been known for thousands of years; there are written accounts of the disease, including a description of its symptoms, in ancient Chinese medical texts from 2700 BCE. The ancient Romans and Greeks also knew about malaria.

However, the causes of the disease were only discovered in the 19th century. In 1880, a French surgeon called Charles Louis Alphonse Laveran stationed in Algeria was the first person to see the parasites that cause malaria in the blood of an infected person. Then, a few years later, in 1897/1898, a British army doctor called Ronald Ross discovered that mosquitoes transmitted the parasites between patients. This allowed for further clarification of the life cycle of malaria, and thus opened up the path for modern malaria research and control.

How do mosquitoes cause malaria?

by

QUESTION:

How can the mosquito cause malaria?

ANSWER:

Actually, mosquitoes don’t cause malaria themselves, they just transmit it from one person to another! The disease is actually caused by a tiny parasite, of the genus Plasmodium, several species of which infect humans. Part of the parasite’s life cycle takes place in the human host, and the other part in the mosquito; it needs to go through both to survive. Mosquitoes become infected when they feed on the blood of a human host that is infected; the parasites are then transmitted to a new, uninfected human hosts in the mosquitoes saliva, when they again take a blood meal.

Malaria in Southern United States

by

QUESTION:

Why is there not a prevalence of malaria in the southern United States when we are bitten almost daily by the “little beasts?”

ANSWER:

Malaria once was relatively common in the southern United States. Transmission used to be possible due to the favorable climatic conditions for the development both of the mosquito as well as the malaria parasite. Huge advances in the control and treatment of malaria were made directly as a result of increased interest in the disease after the US occupation of Cuba and the building of the Panama Canal in the early years of the 20th century. This vastly reduced the number of cases of the disease, but the final, concerted effort to eradicate malaria came in the 1940s.

This was due to a federal public health program called the National Malaria Eradication Program (NMEP), and as a result of its actions, malaria transmission was halted throughout the United States by 1951. The program was launched in 1947, coordinated by the newly formed Communicable Disease Center (now the Center for Disease Control and Prevention, or CDC) and mostly involved reducing the number of mosquitoes in and around people’s homes. This was done through the wide-spread spraying of DDT—during the years of NMEP, it has been estimated that more than 6.5 million homes were sprayed with the insecticide. Alongside spraying, mosquito breeding habitats were also removed, through wetland drainage, and human monitoring and treatment efforts were stepped up. By 1949, malaria was no longer considered a disease of public health importance, and it was declared eradicated from the United States in 1951.

Having said that, the species of mosquito that transmit malaria still exist in the USA, and particularly in the southern states, which means that there is always a risk of small, localized outbreaks of the disease, particularly during hot and wet seasons.

Climate change may also increase the zones where malaria is at risk of being able to develop within the United States. For this reason, the CDC continually monitors the small number of cases reported each year in the USA (there were about 1500 cases in 2007—all but four of these cases, however, were the result of travelers to malarial areas outside of the USA bringing the disease back with them) to ensure that they are prepared and well-informed should an outbreak arise.

Does malaria spread from a person to another?

by

QUESTION:

Can malaria spread from one person to another?

ANSWER:

Usually, no. In most cases, the malaria parasite has to first pass from a human host into a mosquito as the mosquito takes a blood meal, and then from the mosquito into another human via the mosquito’s saliva. This severely limits the amount of person-to-person transmission that exists. In fact, the only mechanisms for direct transmission between humans are when malaria parasites are passed between a mother and her unborn child via the placenta (congenital transmission) and through unscreened blood transfusions.

Congenital malaria is the more common type of human-to-human transmission; across various surveys of newborns in West Africa, between 8-24% were found to be infected with malaria parasites.

All four main species of human malaria (P. falciparum, P. vivax, P. ovale and P. malariae) have been implicated in congenital transmission. Infection with malaria during pregnancy not only puts the mother at greater risk of severe disease episodes (probably through reduced immunocompetence during pregnancy) but may also negatively impact the baby; although in endemic areas it is rare for babies to be symptomatic for malaria when acquired congenitally, even if they have parasitaemia, they have been shown to have a higher mortality rate than non-infected newborns. In non-endemic areas, babies with congenital malaria often display symptoms, which usually manifest themselves between 2 and 8 weeks after birth. Both quinine and artemisinin-based therapies have been successfully used to treat congenital malaria.

Malaria infection as a result of blood transfusion was first identified in 1911 and is one of the most common illness transmitted via transfusion, although the risk of being infected, particularly in non-endemic countries, is very low.

As it is difficult to screen blood directly for malaria infection, a number of standards have been put in place by blood-collection services to reduce the risk of obtaining blood containing malaria parasites. For example, in many places, you will not be allowed to donate whole blood if you have visited an endemic malarial region in the last three months, nor should you donate if you have previously had malaria unless you have been symptom-free for at least three years.

Due to the longevity of Plasmodium malariae in the blood, you are unlikely to be able to donate blood if you have ever been confirmed as positive for P. malariae. Serological screening of blood for malaria antibodies has recently been shown to be a sensitive method for testing for malaria in blood, although it is expensive and therefore not cost-effective for screening every sample, especially in non-endemic countries. However, it can be effective and efficient to avoid wastage when employed together with a travel-based questionnaire to ascertain donors who are high-risk for malaria.

It is worth mentioning that transmission of malaria via plasma only is very uncommon, and so frequent travellers or residents in malarial areas, who may be denied the right to donate whole blood, should ask about the possibility of donating plasma instead.

Mosquitoes with Malaria

by

QUESTION:

Do mosquitoes with malaria get sick like people do?

ANSWER:

That’s a great question! As far as I know, the jury is still out in terms of what overall effect being infected with malaria has on mosquitoes, and to a large extent, it appears to vary due to lots of factors, such as the compatibility between the mosquito and the particular type of malaria parasite, the intensity of malarial infection, and even environmental conditions, to name but a few. However, what is clear is that mosquitoes don’t seem to get ‘sick’ like we do, with fevers, chills and the like—their immune system is just too different from ours! Instead, with mosquitoes, it’s more a question of whether malaria decreases their lifespan, reduced their reproductive success, or other such effects.

A meta-analysis (Ferguson & Read, 2002, ‘Why is the effect of malaria parasites on mosquito survival still unresolved?’, in Trends in Parasitology) of studies that looked on the effect of malaria on mosquito survival demonstrated that 41% of studies reported a detrimental effect of malaria on mosquitoes, whereas 59% reported no effect (none reported a positive effect, which is interesting).

Also worth noting is that the length of the study seemed to have an effect on whether it would report a detrimental effect; this might be due to early stages of infection with malaria not having a negative impact on the host, while the malaria parasite is developing; then, once it is mature, it may be that the parasite’s virulence towards its host increases. Similarly, at this stage in its life cycle, the parasite might induce changes in the mosquito’s behaviour, encouraging it to feed more, which can result in higher mortality to the mosquito (think of swatting away annoying, biting mosquitoes, whereas you’re less likely to go for ones that leave you alone!).

More recently, a study by EJ Dawes and colleagues at Imperial College, London (2009, ‘Anopheles mortality is both age- and Plasmodium-density dependent: Implications for malaria transmission, in Malaria Journal) found evidence for the age of the mosquito and the intensity of malaria parasitism influencing mortality of the insects. Similarly, another research group found that infected female mosquites had significantly lower fecundity than non-infected mosquitoes (Gray & Bradley, 2006, ‘Malarial infection in Aedes aegypti: Effects on feeding, fecundity and metabolic rate’, in Parasitology). These examples begin to swing the balance of the debate in favour of malaria having a negative impact on mosquitoes, at least at certain points in the timeline of an infection (i.e. near the beginning, and then after parasite maturation).

You might ask why mosquitoes that are commonly infected with malaria don’t become resistant to infection, given that they seem to suffer ill-effects when they are parasitised—well, some recent research may have provided the answer to that one. Researchers looking at population growth rates in malaria-susceptible mosquitoes versus those resistant to infection noticed significantly slower growth rates in the resistant population, suggesting that in terms of reproduction, resistance at a population level might come at a cost to growth (Voordouw et al., 2009, ‘Rodent malaria-resistant strains of the mosquito, Anopheles gambiae, have slower population growth than -susceptible strains’, in BMC Evolutionary Biology). However, there may be ways in which this effect can be by-passed, in order to control malaria transmission at the level of the mosquito; one group of researchers recently published the results of a study, where they showed that when resistant and susceptible male mosquitoes rae released into a population of females (bearing in mind only the females feed on blood, and so males are not implicated in transmission), the females tend to mate first with the earlier-hatching resistant males, and moreover, lay a greater number of eggs with these males! So it may be that research on the effect of malaria on mosquitoes can be used in the future to decrease malaria transmission, which would be a hugely positive step for public health in many parts of the world.

It would be great if any malaria researchers out there would comment on the above answer, especially if there are more up-to-date examples of research on the effect of malaria on mosquitoes!