There is no evidence that mosquitoes feeding on an infected person can subsequently infect another person by means of transmission of infected blood. Rather, the parasite must pass through several developmental stages in the mosquito before it is infective to humans. This takes 10-14 days. The infective stage for the mosquito, as we said before, is the gametocyte. Once inside the intestine of the mosquito, sexual reproduction of the parasite occurs, and the parasites continue to multiply in outgrowths of the mosquito's stomach called oocysts. It is in these oocysts that sporozoites are formed. When the mosquito next takes a blood meal, these parasites mix with the saliva, and are injected into the bite.

How do malarial parasites cause disease?

The manifestations of malaria may be attributed to three main factors:

1. the species of parasite involved

2. the inflammatory response by infected humans to the presence of the parasite, resulting in headache, fever, chills, etc., and

3. anaemia, arising from the destruction of an enormous number of red blood cells.

The most important species causing human malaria is P. falciparum, the same one that is implicated in the resurgence of infections in Jamaica. In a nutshell, P. falciparum infects a higher proportion of red blood cells; has a higher rate of reproduction; and is responsible for more clinical complications than any of the other species causing human malaria.

The most common complication is cerebral malaria. This occurs in about 10 per cent of untreated falciparum malaria cases, and accounts for up to 80 per cent of such deaths. Cerebral malaria, which presents itself as progressive headache and increasingly high fever, has been blamed on the blocking of the very fine blood vessels (capillaries) in the brain by blood clots, or by adherence of infected red blood cells to the walls of capillaries causing parts of the brain to be starved of oxygen. Another suggestion involves the release of nitrous oxide from capillary cells, which would have the effect of disrupting signaling in the brain.

Blackwater fever is another grave condition associated with falciparum malaria. This is an acute, massive lysis (destruction) of red blood cells marked by high levels of free haemoglobin, and breakdown products of both haemoglobin and malaria parasites in the blood and urine. The presence of these compounds in the urine causes the fluid to appear dark, hence the name of the condition. It is possible that blackwater fever is an autoimmune phenomenon driven by antibodies that attack not only infected cells but uninfected ones as well. Hence the massive levels of destruction. Excessively high fever, vomiting and jaundice ensue. Free haemoglobin and red blood cell membranes damage the kidneys, and renal failure is the usual cause of death.

What is the nature of the relationship between malaria and sickle cell anaemia?

In persons who have sickle cell anaemia, the amino acid glutamic acid in haemoglobin is replaced with valine. This interferes with conformation of the haemoglobin molecule, and adversely affects its oxygen-carrying capacity. Further, sickled red cells tend to cluster together, and cannot easily move through the blood vessels. The cluster causes a blockage and stops the movement of normal oxygen-carrying blood. This blockage is what causes the painful and damaging complications of sickle cell disease.

In 1954, Anthony Allison (among others) hypothesised that sickle cell trait offered protection against malaria. He suggested that those with the trait did not succumb to malaria as often as those without it; but, when they did, their disease was less severe. It is now known that, when invaded by the malarial parasite, normally stable red cells of someone with the sickle cell trait can sickle in a low oxygen environment, such as occurs in the veins. The sickling process destroys the malaria parasite within, and prevents it from spreading through the body. This apparent ability of a genetic condition to protect carriers is particularly important in infants. Thus, in regions repeatedly devastated by malaria, people who carry the sickle cell trait will have a greater chance for survival than other individuals.

The selective pressure of malaria in Africa has led to the retention of this otherwise undesirable gene in the population. Interestingly, in the U.S.A. one out of 10 African Americans still has sickle cell trait, while 1/400 has the disease even though there is no longer a threat of malaria.

How soon will a person feel sick after being bitten by an infected mosquito?

For most people, symptoms begin 10 days to 14 weeks after infection, although a person may feel ill as early as 7 days, or as late as 1 year. Two kinds of malaria, P. vivax and P. ovale, can relapse. In these infections, some parasites can remain dormant in the liver for several months up to about 4 years after a person is bitten by an infected mosquito. When these parasites come out of hibernation and begin invading red blood cells, the person will again become sick.

At the onset of the disease, most persons have fever, sweats, chills, headaches, muscles aches, nausea and vomiting. The surest way for you and your local clinic to know whether you have malaria is to have a diagnostic test where a drop of blood is examined under the microscope for the presence of malaria parasites. Anyone who has recently travelled and is ill with a fever or flu-like illness up to 1 year after returning to Jamaica should also seek the advice of a doctor.

How does one protect oneself from contracting malaria?

Preventive measures for malaria exist at three levels: national, community, and personal. At a national level, there needs to be sufficient political commitment for improved health services, adequate surveillance and control programmes (where infections are known to be imported, or endemic), a strong lobby directed at sufficient international public funding, adequate access by the public to health care services for early diagnosis and prompt treatment, and adequate training and career opportunities for local malaria scientists and health professionals.

Community-based measures largely involve the provision of an adequate sanitary environment that is unfavourable for mosquito breeding (such as removal of water- filled containers, repairing of leaky standpipes, regular mosquito spraying (especially after the rainy seasons). Facilities that convey information on how to protect oneself from infection, how to recognise infections, and what to do if you think you may have an infection are also useful at a community level.

On a personal level, one would be well advised not to go outdoors between dusk and dawn - anopheline mosquitoes commonly bite at night. A difficult call in Jamaica! If someone needs to go out, the wearing of long sleeved shirts and long trousers would help, although mosquitoes can feed through tight clothing! The application of insect repellent to exposed skin is also useful. Use of fly sprays in the home is an expensive and only temporary means of protecting the family, compared with the mosquito coil which is a more efficient and economic approach. Insecticide-impregnated bed nets have proved to be very successful in mosquito control programmes in Africa, but are unlikely to catch on in Jamaica. Otherwise, in Jamaica, the home owner could also consider mosquito screens over doors and windows.

What is the most effective treatment for malaria?

Although malaria caused by P. falciparum has become increasingly resistant to front-line medications, combination drugs containing artemisinin - currently the most effective treatment for malaria - show nearly 100 per cent effectiveness after a short three-day regimen. Yet, at a price of US$2.40 per adult course through the World Health Organisation, the drugs are beyond the reach of millions of the world's poorest people. Moreover, artemisinin, which is derived from the wormwood tree, is in short supply, and producing it is labour-intensive and relatively expensive.

Is there a vaccine for malaria?

There is currently no malaria vaccine approved for human use. The malaria parasite is a complex organism with a complicated life cycle. The surface of the parasites is constantly changing, and developing a vaccine against these is very difficult. In addition, scientists do not yet totally understand the complex immunological responses that protect humans against malaria.

However, many scientists all over the world are working on developing an effective vaccine. Because other methods of fighting malaria, including drugs, insecticides, and bed nets, have not succeeded in eliminating the disease, the search for a vaccine is considered to be one of the most important research projects in public health.

Why has malaria not been eradicated?

We did not anticipate: the development of insecticide-resistance of Anopheles; drug-resistant Plasmodium; significant role of host and parasite genetics; the difficulty of elucidating the pathogenesis; the variety of animal reservoirs (e.g. P. knowlesi); the logistical problems associated with control in wilderness areas, and dealing with traditionalist peoples; lack of financial and technical resources; the disruptive effect of wars and political upheavals on control efforts; the non-involvement of Africa in early eradication try and the problems surrounding the development of a malaria vaccine.

Malaria will be with us for a long time to come; probably as long as there are people.

- Additional reading: 'Malaria' http://www.cdc.gov/malaria

Ralph Robinson is Professor of Parasitology at the University of the West Indies, Mona.

Page Produced by the Mona Institute of Applied Sciences

To comment on this article contact MIAS at 970-2021 or email: mias@uwimona.edu.jm