Reservoir of malarial plasmodium

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Here is a complete, textbook-grounded answer:

Reservoir of Malarial Plasmodium

Primary Reservoir: Humans

Humans are the only natural reservoir of the four main human malarial plasmodia (P. falciparum, P. vivax, P. malariae, P. ovale). The infected human is the source from which the Anopheles mosquito (the vector) acquires the parasite.

"With the possible exception of chimpanzees in tropical Africa, which may carry the infection with P. malariae, no other animal reservoir of human plasmodia is known to exist."

Park's Textbook of Preventive and Social Medicine

"All forms are transmitted by the bite of female Anopheles mosquitoes and humans are the only natural reservoir."

Robbins & Kumar Basic Pathology

Who Qualifies as a Reservoir?

A human acts as a reservoir only when certain conditions are met. The person must:

Harbor gametocytes (the sexual forms) - both male and female - in the blood. Asexual forms alone do not transmit.
Have mature gametocytes - immature forms cannot undergo further development in the mosquito. Maturation takes 2-4 days after gametocytes first appear.
Have viable gametocytes - antimalarial drugs render gametocytes non-infective.
Have sufficient gametocyte density - at least ~12 gametocytes per cubic mm of blood are thought necessary to infect a mosquito.

Children are better reservoirs than adults because they are more likely to carry gametocytes.

Period of Communicability

Species	Gametocytes appear
P. vivax	4-5 days after asexual parasites appear
P. falciparum	10-12 days after asexual parasites appear

Gametocyte density is highest in early infection (can exceed 1,000/mm³), and they appear in waves in peripheral blood.

Special Case: P. knowlesi

P. knowlesi is a zoonosis - the reservoir here is macaques and monkeys in Southeast Asia, transmitted to humans by Anopheles leucosphyrus group mosquitoes. This is the exception to the human-only rule.

The Mosquito's Role

The Anopheles mosquito is the vector and definitive host (sexual cycle occurs here), not the reservoir. The mosquito ingests gametocytes from an infected human, and after 10-20 days (extrinsic incubation period), sporozoites develop in the salivary glands and can infect a new host.

Summary table:

Species	Reservoir
P. falciparum	Humans
P. vivax	Humans
P. ovale	Humans
P. malariae	Humans (possibly chimpanzees in Africa)
P. knowlesi	Macaques/monkeys (zoonosis)

Park's Textbook of Preventive and Social Medicine, Reservoir of Infection section
Robbins & Kumar Basic Pathology, Malaria section

Why is Anopheles mosquito not the reservoir?

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I now have enough textbook and conceptual material to give a thorough answer. Here it is:

Why is Anopheles Mosquito NOT the Reservoir?

This comes down to the precise epidemiological definition of a reservoir and the biological role the mosquito plays in the parasite's life cycle.

1. Definition of a Reservoir

A reservoir of infection is a living host (human, animal, or arthropod) in which the infectious agent:

Normally lives and multiplies
Depends on for survival
From which it can be transmitted to a susceptible host

The key idea is that the reservoir is the natural, long-term habitat of the pathogen - the place where the agent is maintained in nature between transmission events.

2. The Mosquito is a Vector, Not a Reservoir

The Anopheles mosquito fails to qualify as a reservoir for several reasons:

Criterion	Human	Anopheles mosquito
Long-term survival of parasite	Yes (years - P. malariae up to 40 years)	No (only days to weeks)
Asexual multiplication occurs	Yes (liver + RBCs)	No (only sexual cycle)
Parasite maintains population	Yes	No - entirely dependent on infecting a new human
Natural, sustained habitat	Yes	Transient phase only
Can remain asymptomatic and infective for long periods	Yes (carriers)	No - mosquito lifespan is short

The mosquito:

Acquires gametocytes from an infected human
Allows the sexual cycle (sporogony) to complete in ~10-20 days
Transmits sporozoites to the next human host

It is an essential transmitter, but the parasite does NOT persist or maintain itself in the mosquito population long-term.

3. The Mosquito is the Definitive Host - Yet Still Not the Reservoir

Here is where it gets conceptually interesting. As Sherris & Ryan's Medical Microbiology notes:

"People argue whether mosquito or man is the definitive host as the sexual union of gametes occurs in the mosquito."

Technically, in parasitology:

Definitive host = where sexual reproduction occurs → the Anopheles mosquito
Intermediate host = where asexual reproduction occurs → humans

So the mosquito is the definitive host (from a parasitology standpoint) but is still not the reservoir (from an epidemiological standpoint). These are two separate concepts from two different frameworks:

Framework	Mosquito's role	Human's role
Parasitology	Definitive host (sexual cycle)	Intermediate host (asexual cycle)
Epidemiology	Vector	Reservoir

4. Why Humans ARE the Reservoir

The human is the reservoir because:

The parasite survives long-term in humans (P. malariae: up to 40 years of low-level asymptomatic parasitemia)
Asexual multiplication (schizogony) sustains the parasite population in the body
Humans harbor gametocytes in the blood - the infective stage for mosquitoes
Even after clinical cure, relapses occur in P. vivax and P. ovale (hypnozoites in liver) - the parasite persists

In other words: if all Anopheles mosquitoes disappeared today, Plasmodium would still survive in millions of human bodies. If all humans were cleared of the infection, the parasite would die out - because the mosquito cannot maintain it alone.

Simple Analogy

Think of it this way:

The petrol station (human) stores and maintains fuel (parasite)
The delivery truck (mosquito) picks it up and drops it off at the next station
The truck is essential for distribution, but fuel is not stored in the truck

The mosquito is the vehicle of transmission, not the storehouse of infection.

Sherris & Ryan's Medical Microbiology, 8th Ed. - Definitive and Intermediate Hosts
Park's Textbook of Preventive and Social Medicine - Reservoir of Infection (Malaria)
Robbins & Kumar Basic Pathology - Malaria

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