Question

Which one of the following life cycle stages of the malarial parasite is responsible for the relapse of malarial symptoms?
(a) Merozoites
(b) Sporozoites
(c) Hypnozoites
(d) Gametocyte

Answer 1

Hint: In the life cycles of certain parasitic protozoa this stage of the parasite is dormant forms that belong to the Phylum Apicomplexa (Sporozoa). Plasmodium ovale and P. vivax cause malarial infections in humans, they are best known for their probable association with latency and relapse.

Complete answer:
2 hosts are involved in the life cycle of the malaria parasite. A female Anopheles mosquito which is malaria-infected inoculates sporozoites into the human host during a blood meal. Sporozoites infect liver cells and mature into schizonts, which rupture and release merozoites that infect red blood cells and periodically replicate every 48 h. A new sexual or asexual cycle of development can be initiated by the small amoeboid sporozoan trophozoites (as of a malaria parasite) which are produced by schizogony. At a later time, this hypnozoite can reactivate and undergo schizogony resulting in a relapse which is again reactivation of the infection via hypnozoites. If individuals are treated with an effective regimen of primaquine, the only licensed drug capable of eliminating hypnozoites from the liver, the relapses can be excluded.

So, the correct answer is, ‘Hypnozoites’.

Additional information:
1) The term "hypnozoite" is derived from the Greek words “hypnos” (sleep) and “zoon” (animal) .
2) In the life cycles of certain parasitic protozoa, hypnozoites are in dormant forms that belong to the Phylum Apicomplexa (Sporozoa) . In human malarial infections, they are best known for their probable association with latency and relapse caused by Plasmodium ovale and P. vivax.
3) Only three types of relapsing malaria have been examined to date and only one species of Plasmodium causing non-relapsing malaria has been investigated for hypnozoites.
4) Although little is experimentally known about the life span of liver cells in cases of infection with P. vivax, the longevity of relapsing malaria cases suggests that the estimate of 1 year is less than precise. Hence, in a daughter cell, it is conceivable that the hypnozoite could survive cell upon hepatocyte division.

Note: Besides being genetically determined by the specific strain, a relapse of vivax malaria could also be induced by the bites of uninfected vectors. The Finnish domestic vector, Anopheles mosquito, has a strong seasonality. Through winter the females hibernate as adults and lay eggs in the spring. In summer, the development of the larvae takes place in permanent water, which freezes during the winter. The same pattern can be found in British material. When domestic relapses of P. vivax cases are analyzed, they show an adaption to the phenology of local vectors. Plasmodium vivax relapses of malaria can be prevented only by 8-aminoquinoline antimalarials. If rapidly eliminated antimalarials are given for treatment, tropical P. vivax relapses at three-week intervals. The infections caused by P. vivax in temperate regions and parts of the subtropics are characterized either by a long incubation or a long-latency period between illness and relapse - in both cases approximating 8-10 months.