Friday, July 19, 2013

How the Malaria Parasite Hides Itself from the Mosquito’s Defenses

Malaria remains a potent killer to the human inhabitants of the tropical regions in the so-called “undeveloped world.”  This disease results in approximately one million deaths a year in Africa, alone.  An insect vector, the female Anopheles mosquito that requires blood to mature its eggs, is responsible for its transmission to humans.  There are a host of human maladies caused by such diverse organisms as viruses, protozoans and even worms that infect humans and that are carried by mosquitoes and other insects.

The causative agent for human malaria is a parasite referred to as Plasmodium falciparum.  The life cycle of P. falciparum is quite complex and intriguing.  When it is first inadvertently ingested by the mosquito from an infected host, the parasite undergoes a progression of transformations.  The specialized sexual precursor cells – male and female - are rapidly activated to form complete and functional sex cells – gametes.  Male and female gamete pairs subsequently fuse to from the incipient new organism called the zygote.  Within 18 – 24 hours, this zygote develops into a motile organism called the ookinete that is infectious to the mosquito.  This ookinete rapidly enters the insect’s midgut and forms an oocyst.  Within the oocyst more than 10,000 sporozoites are created within ten days.  Once this oocyst ruptures, these sporozoites navigate to the mosquito’s salivary glands where they are transferred to the next human host upon the mosquito’s bite.

Given this information, the questions that comes to mind are the following –
  • What are the mosquito’s natural defenses against this parasite?
  • How does the P. falciparum successfully elude these defenses?

The investigations of Dr. Alvaro Molina-Cruz and colleagues at the Laboratory of Malaria and Vector Research at the National Institute of Allergy and Infectious Diseases at the National Institute of Health in Rockville Maryland helped elucidate the mechanism by which the parasite escapes the immune defenses of its host.

It has been well established that insects have a first line of immune defense, as do vertebrates, referred to as the innate immune system (IIR).  IIR utilizes both humoral (chemical) and cellular defense mechanisms; it is quite sophisticated.  In spite of this ability to fend off foreign invaders, P. falciparum manages to survive and propagate in this hostile environment.

Molina-Cruz and the team of collaborators were able to identify a gene product from the parasite that enables the invading organism to infect its host mosquito without activating the innate immune system described earlier.  The gene responsible for producing this product was identified as Pfs47.  In fact when this gene was disrupted from its normal functionality, the parasite’s survival within the host was greatly reduced.

This finding is significant, for it establishes the fact that the Pfs47 gene contained within P. falciparum is essential for the efficient transmission of the parasite responsible for malaria from the vector to its human host.

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