What is reservoir and source and what's the difference between them

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Here is a thorough breakdown, drawn from Park's Textbook of Preventive and Social Medicine (a leading epidemiology reference):

Reservoir vs. Source of Infection

Definitions

Reservoir

"Any person, animal, arthropod, plant, soil or substance (or combination of these) in which an infectious agent lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host."

In short: the reservoir is the natural habitat where the pathogen metabolizes and replicates over the long term.

Source of Infection

"The person, animal, object or substance from which an infectious agent passes or is disseminated to the host."

In short: the source is the immediate point of departure - where the host actually picks up the infection.

The Key Difference

The two terms are not always synonymous, even though they are often used interchangeably. The "source" refers to the immediate origin of transmission, and it may or may not be part of the reservoir.

Feature	Reservoir	Source
Role	Long-term survival & multiplication of pathogen	Immediate origin of transmission to the host
Duration	Persists indefinitely	The point of actual transmission
Can be the same?	Yes, sometimes	Yes, sometimes
Examples	Human, animal, soil, water	Feces, urine, contaminated food/water, soil, aerosols

Illustrative Examples

Hookworm infection
- Reservoir: Man (the pathogen lives in the human intestine)
- Source: Soil contaminated with infective larvae (where humans pick it up)
- Here, reservoir ≠ source
Tetanus
- Reservoir: Soil (Clostridium tetani spores persist there)
- Source: Soil (directly contaminates wounds)
- Here, reservoir = source
Typhoid fever
- Reservoir: A case or carrier (a person harboring Salmonella typhi)
- Source: Feces or urine of the patient, or contaminated food, milk, or water
- Here, the reservoir is the person, but the source is the excreta or contaminated vehicle
Cholera (Vibrio cholerae)
- Uses the concept of a homologous reservoir - man is the principal reservoir (same species is the victim)
Salmonella (from animals/birds)
- Uses the concept of a heterologous reservoir - infection derived from a species other than man

Types of Reservoirs

Reservoirs fall into three categories:

Human reservoir - The most important. A person can be a case (symptomatic) or a carrier (asymptomatic but infectious)
Animal reservoir - Infections passed from animals to humans are called zoonoses (e.g., rabies, brucellosis, plague)
Reservoir in non-living things - Soil (tetanus, histoplasmosis), water (Legionella), etc.

Chain of Infection (Visual)

Chain of Infection diagram showing Source/Reservoir, Modes of Transmission, and Susceptible Host as three overlapping circles

The three essential links in disease transmission are: Reservoir/Source → Mode of Transmission → Susceptible Host. Breaking any one link interrupts transmission and controls the disease.

Summary: Every reservoir can potentially become a source, but not every source is the reservoir. The reservoir is where the pathogen lives and persists; the source is from where it actually reaches the new host.

Park's Textbook of Preventive and Social Medicine, p. 107

What is amplifying and maintaience reservoir of Japenese Encephalitis

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Here is a complete answer drawn directly from Park's Textbook of Preventive and Social Medicine and Goldman-Cecil Medicine:

Japanese Encephalitis - Amplifying and Maintenance Reservoirs

The Two Basic Transmission Cycles

JE virus infects several extra-human hosts. The two primary cycles established in nature are:

(a) Pig → Mosquito → Pig (b) Ardeid bird → Mosquito → Ardeid bird

Man is only an incidental "dead-end" host - man-to-man transmission does not occur.

Amplifying Host - PIGS

Pigs are the major amplifying hosts of JE virus.

In some areas, up to 100% of pigs may be sero-positive for JE virus
Infected pigs do not show any overt signs of illness
However, they develop a high-level viremia - the virus circulates freely in their blood
When Culex mosquitoes feed on these viremic pigs, they pick up large amounts of virus and become highly infectious
Pigs therefore amplify the viral population exponentially - they multiply the amount of virus available for mosquitoes to pick up and pass on to humans

This is why JE outbreaks are closely tied to pig-rearing areas near rice fields, where the Culex vector also breeds.

Maintenance (Reservoir) Host - ARDEID BIRDS (Herons and Egrets)

Ardeid birds (family Ardeidae - herons, egrets, bitterns) are the maintenance/natural reservoir hosts of JE virus.

They sustain the virus in nature long-term, even between epidemic seasons
They also develop viremia but do not die from infection
They serve as the enzootic cycle hosts - keeping the virus circulating quietly in the environment year-round
Migratory birds in this family may also spread the virus to new geographic areas

Why the Distinction Matters

Feature	Amplifying Host (Pig)	Maintenance Host (Ardeid Bird)
Role	Boosts viral numbers explosively	Keeps virus alive in nature long-term
Viremia level	Very high - infects many mosquitoes	Sustained but moderate
Disease in host	None (asymptomatic)	None (asymptomatic)
Epidemiological importance	Drives human outbreaks	Maintains enzootic cycle between seasons
Term used	Amplifier	Reservoir / maintenance host

Transmission Cycle Diagram

JE transmission cycle showing amplification cycle with pigs/birds and Culex mosquitoes, dead-end hosts (humans/horses), and possible overwintering maintenance mechanisms

Generalized transmission cycle of mosquito-borne flaviviruses. Humans and horses are dead-end hosts. Pigs play the amplifying role for JE specifically. Possible overwintering maintenance mechanisms include hibernating vectors, prolonged viremia in bats/birds, alternative vectors, and vertical (transovarial) transmission in mosquitoes.

Vectors

The primary mosquito vectors are:

Culex tritaeniorhynchus - the most important vector
Culex vishnui and Culex gelidus - also implicated
These mosquitoes breed in irrigated rice fields, shallow ditches, and pools - explaining the rural, agricultural epidemiology of JE

After feeding on a viremic host, the mosquito requires a 9-12 day extrinsic incubation period before it can transmit the virus.

Quick memory aid: Pigs Amplify (A for Animal that's a pig) - Birds Maintain (M for Maintenance - birds keep it going). The vector is Culex (C for Culex tritaeniorhynchus). Man is a dead-end host.

Park's Textbook of Preventive and Social Medicine, p. 331-332
Goldman-Cecil Medicine, p. 3664
Jawetz, Melnick & Adelberg's Medical Microbiology, 28th Ed., p. 572

How does pig amplify the virus

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Here is the complete explanation of how pigs amplify JE virus:

How Pigs Amplify Japanese Encephalitis Virus

Amplification is a step-by-step process. Think of it as the virus using the pig as a multiplier before it reaches humans.

Step-by-Step Mechanism

Step 1 - Mosquito bites an infected pig A Culex mosquito feeds on a pig that already has JEV in its bloodstream. The mosquito ingests the virus along with the blood meal.

Step 2 - Extrinsic incubation in the mosquito The virus replicates inside the mosquito for 9-12 days (the extrinsic incubation period). After this, the mosquito's salivary glands are loaded with virus and it becomes capable of transmission.

Step 3 - Why the pig is special: HIGH-LEVEL VIREMIA This is the core of amplification. When JEV infects a pig:

The virus replicates to extremely high titers in the pig's bloodstream
This is called a high-level viremia - the concentration of virus in pig blood is far higher than in most other hosts (including birds and certainly humans)
The pig shows no illness - it remains healthy, active, and continues to be bitten by many mosquitoes
Because pigs are large animals that attract many mosquitoes, and their blood contains enormous amounts of virus, every mosquito that bites them gets a high infectious dose

Step 4 - Infected mosquito bites another pig (or human) The now-infectious mosquito passes the virus on. If it bites another pig, that pig also develops high viremia and the cycle repeats - exponentially increasing the number of infected mosquitoes in the environment. Eventually some infected mosquitoes bite humans, causing disease.

Why Humans Can't Amplify

Humans (and horses) are dead-end hosts because:

They develop only low-level viremia - not enough virus in the blood for a mosquito to pick up and transmit
So the cycle stops at the human - humans cannot pass it on to mosquitoes

This is the fundamental contrast:

Host	Viremia level	Can infect mosquito?	Role
Pig	Very high	Yes - effectively	Amplifier
Ardeid bird	Moderate-high	Yes - effectively	Maintenance reservoir
Human	Very low	No	Dead-end host
Horse	Very low	No	Dead-end host

Additional Features That Make Pigs Efficient Amplifiers

Asymptomatic infection - pigs don't get sick, so they remain out in the open and continue to attract mosquitoes (sick animals hide or die)
High population density - pigs are raised in large numbers near rice fields, right next to the Culex mosquito breeding sites
Rapid reproduction - piglets are born susceptible, constantly providing new non-immune animals for the virus to infect and amplify through
Up to 100% seroprevalence in endemic areas - showing how efficiently the virus circulates through pig populations
Cattle and buffaloes in the same settings act as "mosquito attractants" - they draw more mosquitoes to the area, increasing the chance of pigs being bitten

Simple Analogy

Think of the pig as a viral factory. The mosquito delivers a small order (a few virus particles), the pig's body mass-produces them (high viremia), and the next mosquito that comes along picks up a huge consignment (large infectious dose) and delivers it to the next host - possibly a human.

Park's Textbook of Preventive and Social Medicine, p. 331-332

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