There are approximately 3,500 species of mosquitoes grouped into 41 genera. Human malaria is transmitted only by females of the genus Anopheles. Of approximately 430 Anopheles species, only 30 to 40 transmit malaria in nature. An. gambiae and An. funestus are the primary vectors in Africa.
The malaria parasite, Plasmodium falciparum accounts for the most severe cases of malaria and for over 90% of infections in most areas of tropical Africa where malaria is endemic. Three other malaria parasites, P. vivax, malariae and ovale contribute significantly to the pool in Sub-Sahara Africa.
In general, these parasites only affect people and certain Anopheles mosquito species pass the disease from one infected person to another.
This is very important.
The disease requires both the Anopheles mosquito and people, Homo sapiens, to complete its life cycle and to propagate. No other mosquito genus or warm blooded host will suffice. One part of the disease life cycle can only occur in the Anopheles mosquito and the other, only in a human being.
Part 1 The Malaria Life Cycle in the Mosquito
The successful disease cycle requires that the mosquito obtain a blood meal containing the disease from an infected person. During the bite, the infected blood is "siphoned" into the mosquito's stomach.
|Fully engorged, the mosquito uses the blood protein for egg development.|
The mosquito's stomach provides the required environment for the disease to multiply. After about 14 days, increased numbers of the organism now in an infective stage make their way through the mosquito's body to its salivary glands and "wait" to be transmitted into the mosquito's next victim. The mosquito's salivary glands produce saliva with an anticoagulant that is injected to prevent the blood from clotting when the mosquito begins feeding. The parasites are injected with the saliva.
The mosquito does not appear to be adversely affected by the parasite in its body.
Several points of interest -
- Only female mosquitoes bite. She requires human blood as a source of protein to develop eggs. If mosquitoes can be prevented from obtaining a blood meal, they will not be able to reproduce and the disease will not be acquired or transmitted. Bed nets and indoor residual sprays are intended to prevent mosquitoes from obtaining a blood meal and transmitting the disease. Integrated vector management with larviciding could eliminate the majority of adult mosquitoes before they emerge to bite and transmit malaria.
- The female mosquito will continue to take blood meals and produce eggs under conducive environmental conditions until her body parts wear out and she is no longer able to fulfill her function - reproduction.
- If mosquitoes are controlled early in their life cycle (with larviciding) or the adults are controlled in a timely fashion (with adulticiding), the reproductive life span of the female adult mosquito would be inconsequential.
- The malaria infected mosquito may feed multiple times but is not able to transmit the disease until it has completed its development cycle inside the mosquito which takes about 14 (10 to 18) days.
- If infected mosquitoes can be killed within 14 days, the disease will not be transmitted by those mosquito. If mosquitoes are controlled early in their life cycle (with larviciding) or the adults are controlled in a timely fashion (with adulticiding), the incubation period of the disease within the adult mosquito would be inconsequential.
- Indirect estimates of daily survivorship could indicate that less than 10% of female Anopheles gambiae would survive longer than the 14-day incubation period. Control measures that rely on insecticides (for example, indoor residual sprays) could impact on malaria transmission more through their effect on adult longevity than through their effect on the population of adult mosquitoes. If mosquitoes are controlled early in their life cycle (with larviciding) or the adults are controlled in a timely fashion (with adulticiding), the survivorship would be inconsequential.
- I was discussing the malaria situation with an associate, Mr. Okeke, in Nigeria where the incidence of the disease may be the highest in Sub-Sahara Africa. He stated we all have it, that is to say most adults are probably carriers. In other words most of the human population is a reservoir from which the mosquitoes can easily obtain the disease.
- If mosquitoes can be prevented from reaching infected people or infected people are cured, the disease would not be acquired. If mosquitoes are controlled early in their life cycle (with larviciding) or adults are controlled in a timely fashion (with adulticiding), people being the reservoir would be inconsequential.
- Anopheles gambiae and funestus prefer to feed on humans (strongly anthropophilic) and therefore are more likely to transmit the malaria parasites from one person to another. These two species are considered two of the most efficient malaria vectors in the world. Those that prefer to feed on other animals such as cattle are classified as zoophilic. Most Anopheles mosquitoes are not exclusively anthropophilic or zoophilic. This means most species although they may form a smaller proportion of the population are capable transmitting malaria and should be considered as potential targets in a control program. If mosquitoes are controlled early in their life cycle (with larviciding) or adults are controlled in a timely fashion (with adulticiding), regardless of whether the mosquitoes are anthropophilic or generalized feeders would be inconsequential.
- Insecticide resistance is becoming a major challenge. Resistance has been noted to the pyrethroids used in the treatment of nets and to DDT used in indoor residual sprays. Larviciding and adulticiding as an added component to integrated vector management have not been used operationally for many years and the newer materials such as Bti not at all. Introduction of these practices and alternation of larvicides could negate or impede the development of resistance to the pyrethroids and DDT.
- And the last point is for my wife who reacts to mosquito bites with swelling, itching and reddening at the location of the bite. This is due to an allergic reaction to the saliva that mosquitoes inject to prevent coagulation of the blood. (our little bite reactions are inconsequential)
Like all mosquitoes, the Anopheles mosquito goes through four stages in its life cycle: egg, larva, pupa and adult . The first three are aquatic (i.e. in the water) for 5 to 14 days and the adult stage is considered terrestrial.
|Diagram - Anopheles eggs with floats are laid singly.|
|Photograph - Eggs form star shaped clusters on the water|
|Diagram - Larva at rest parallel to water surface.|
|Photograph - Anopheles larva.|
|Diagram - Non-feeding pupal stage.|
|Diagram - Showing adult feeding posture|
|Photograph - Typical feeding posture; hind legs raised.|
Examples of Malarial Mosquito Larval Habitats
|Roadside drainage ditch.|
|Garbage pit and abandoned containers, plastic bottles.|
|Flood plain adjacent to the Baro River, Gambella.|
|Intermittent stream bed.|
Part 2 The Malaria Life Cycle in People
- The parasites go to the liver and multiply within 30 minutes of entry into the human body, move to the blood stream and attack the red blood cells, multiplying again and rupturing the blood cells. Severe malaria can progress extremely rapidly and cause death within hours or days. Young children and pregnant women are especially vulnerable requiring immediate attention which often is not the case.
- Symptoms appear seven days or more (usually 10-15 days) after the infective mosquito bite. The first symptoms - fever, headache, chills and vomiting - may be mild and difficult to recognize as malaria. If not treated within 24 hours, P. falciparum malaria can progress to severe illness often leading to death. Children in endemic areas with severe disease frequently develop one or more of the following syndromatic presentations: severe anaemia, respiratory distress in relation to metabolic acidosis, or cerebral malaria. In adults multi-morgan involvement is also frequent.
- Approximately half of the world's population is at risk of malaria. Most malaria cases and deaths occur in Sub-Sahara Africa. Specific risk groups include:
- Young children in stable transmission areas who have not yet developed protective immunity. Young children contribute to the bulk of malaria deaths worldwide.
- Non-immune pregnant women are at risk as malaria causes high rates of miscarriage (up to 60% in P. falciparum infection) and maternal death rates of 10-50%.
- Semi-immune pregnant women in areas of high transmission, malaria can result in miscarriage and low birth weight, especially during the first and second pregnancies. An estimated 200,000 infants die annually as a result of malaria infection during pregancy.
- Semi-immune HIV-infected pregnant women in stable transmission areas are at increased risk of malaria during all pregnancies. Women with malaria infection of the placenta also have a higher risk of passing HIV infection to their newborns.
- People with HIV/AIDS are at increased risk of malaria disease when infected.
- Immigrants from endemic areas and their children living in non-endemic areas and returning to their home countries to visit friends and relatives are at risk because of waning or absent immunity.
- Early diagnosis and treatment of malaria reduces the disease and prevents deaths. It also contributes to reducing malaria transmission.
- The best available treatment, particularly for P. falciparum malaria, is artemisinin-based combination therapy (ACT).
- Growing resistance to antimalarial medicines has spread very rapidly, undermining malarial control efforts. When treated with an artemisinin-based monotherapy, patients may discontinue treatment early following the rapid clearance of malaria symptoms. This results in partial treatment and patients still have persistent parasites in their blood. Without a second drug given as part of the combination, these resistent parasites can survive and be passed onto another mosquito and then another person.
- If resistance to artemisinins develops and spreads to other large geographical areas, as has happened before with chloroquine and sulfacoxine-pyrimethamine, public health consequences could be dire.
- Black market medications are a major concern with respect to resistance because they tend to contain insufficient doses to be ineffective.
- People in malaria endemic areas still rely on over the counter medications such as aspirin brought by friends and relatives from abroad to address the disease within the home.
As stated in a previous blog entry, there has been a tremendous outpouring of effort, money and good will to address malaria from developed countries over the past 5 years. According to one article, global funding for malaria increased from $0.733 billion in 2006 to $1.94 billion in 2009. Approximately 289 million insecticide treated bed nets will have been delivered to Sub-Saharan Africa, enough to cover 76% of the 765 million people at risk of malaria by the end of 2010. New strategies of prophylactic medications and treatment of malaria cases have increased. Indoor residual spraying has had limited acceptance but appears to be on the increase. And .... there has been lots of research addressing biology and control strategies ... some of it "pie in the sky" but it all adds to the knowledge base.
We now have an opportunity, to look back and review the fruits of our labour knowing what we do about the the biology of the carrier and the epidemiology of the disease. The 2010 World Malaria Report asserts that the number of deaths due to malaria is estimated to have decreased from 985,000 in 2000 to 781,000 in 2009. The largest absolute decreases in deaths were observed in Africa. Are we feeling warm and fuzzy?
The above numbers translate to a 21% reduction in mortality after all that effort. I feel fairly secure in stating that when the mortality numbers are tallied for 2010, the numbers will still reflect a significant death toll. The limited success is a reflection of the complexity of the problem and our inability to come to terms with it.
It should be obvious that the current approaches, ITN's, IRS and medication strategies are certainly not going to eradicate the disease by 2015 or any other deadline. These approaches are saving lives but only a small proportion of those affected. To continue on as we are, could be described as nothing less than "man's inhumanity to man." We are responsible for a child dying every 45 seconds. Why are we allowing this to continue in developing countries? We certainly would not in our own!
Global eradication of malaria is political rhetoric with public appeal. We may eliminate deaths from malaria but not in the short term. We can certainly and significantly and cost effectively improve on the results of the current effort if we apply the FULL arsenal of tools. That would include responsible larviciding and adulticiding as part of Integrated Vector Management to create "vector free zones" at the community level.
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