Parasitology lecture- characteristic features of a vector organism

1. Characteristic features of a
vector organism
Semester IV Zoology Major
Module 1- Introduction to parasitology and Vector Biology
Dr. Puja Ray
Dept. of Life Sciences
Presidency University, Kolkata

2. Vectors
•from Latin word vectus. An organism that transmits
pathogen.
•A live organism that serves to communicate disease.
For example, mosquitoes and other arthropods.
•A host that carries a pathogen without injury to itself
and spreads the pathogen to susceptible organisms.
•asymptomatic carriers of pathogens

3. • Vector-borne infections, diseases caused by pathogens transmitted by
insects and ticks, have long impacted human affairs.
• Alexander the Great, conqueror of many nations, was vanquished by
the bite of a tiny mosquito bearing malaria parasites in the marshes of
what is now called Iraq.
• The Black Death, decimator of Europe, killer of tens of millions
worldwide is the work of a tiny flea vectoring the bacilli that cause
bubonic plague from rats to people. Vector-borne infections remain
influential to this day, filling the hospitals of sub-Saharan Africa with
malaria victims, suppressing the economies of nations and
interrupting industrial operations where it remains endemic.
• Some less common agents cause blindness and horrible
disfigurement. Together they form a fearsome array of potential
threats to the health and livelihood of those who visit, work, or live in
the tropics where they exert their greatest impact.

4. Types of vectors
Vectors may be mechanical or biological:
Mechanical vector
• Microbes do not multiply within
mechanical vectors - only physically
transport microbes from host to host.
• An example of a mechanical vector
is a housefly, which lands on cow
dung and then lands on food, which
is then eaten. The bacteria travel
from the dung to the food without
ever actually entering the body of
the fly.
Biological vector
• microbes must propagate within a
biological vector before it can
transmit the microbes.
• It has the pathogens within its body,
and delivers them to new hosts in an
active manner, usually a bite.
• Mosquitoes, ticks, fleas and lice are
examples of biological vectors and are
often responsible for serious blood-
borne diseases, such as malaria.

5. Vector Borne Diseases
• A disease that is transmitted to humans or
other animals by an insect such as a
mosquito or another arthropod is called a
vector-borne disease.
• Nearly half of the world's population is
infected by vector-borne diseases,
resulting in high morbidity and mortality
• There are many types of vector borne
diseases: Denque Fever, Japanese
Encephalitis, Yellow Fever, West Nile
Virus and Lyme Disease.
• A common strategy used to control vector
borne infectious diseases is to interrupt
the life cycle of a pathogen by killing the
vector.

6. Denque Fever
• Dengue is primarily a disease of the
tropics, and the viruses that cause it are
maintained in a cycle that involves
humans and Aedes aegypti, a domestic,
day-biting mosquito that prefers to feed
on humans.
• Infection with dengue viruses produces
a spectrum of clinical illness ranging
from a nonspecific viral syndrome to
severe and fatal hemorrhagic disease.
Important risk factors for DHF include
the strain and serotype of the infecting
virus, as well as the age, immune status,
and genetic predisposition of the patient
Dengue and dengue hemorrhagic
Image: The stylets (needle-like structures) and proboscis (elongated mouth) of an Aedes aegypti
feeding. Dengue viruses are transmitted during the feeding process.

7. Encephalitis
• Aseptic meningitis or encephalitis. Many
cases have only fever with headache
• Can progress to focal paralysis, intractable
seizures, coma and death
• Varies with occurrence and intensity of
epidemic transmission; usually 150-3,000
cases/year
• Infrequent but unpredictable epidemics
• No human vaccines available
• Treatment not always effective
• Knowledge of geographic distribution
incomplete
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• Japanese encephalitis (JE) virus: flavivirus antigenically related to St. Louis
encephalitis virus
• Leading cause of viral encephalitis in Asia with 30-50,000 cases reported annually
• Mosquito-borne Culex tritaeniorhynchus group

8. Yellow Fever
• Yellow fever occurs only in Africa and
South America. In South America
sporadic infections occur almost
exclusively in forestry and agricultural
workers
• The virus damages many body tissues,
but especially the liver.
• The Aedes aegypti mosquito carries
the yellow fever virus from one person
to another.
• Yellow fever is found in jungle areas,
especially in South America. The
disease can now be prevented by a
vaccine.

9. •Lyme disease is caused by the bacterium, Borrelia
burgdorferi. These bacteria are transmitted to humans
by the bite of infected deer ticks and caused more than
23,000 infections in the United States in 2002
•Lyme disease was named in 1977 when arthritis was
observed in a cluster of children in and around Lyme,
Conn
Lyme disease

10. West Nile Virus
• West Nile virus (WNV) has
emerged in recent years in
temperate regions of Europe
and North America, presenting
a threat to public and animal
health. The most serious
manifestation of WNV
infection is fatal encephalitis
(inflammation of the brain) in
humans and horses, as well as
mortality in certain domestic
and wild birds. WNV has also
been a significant cause of
human illness in the United
States in 2002 and 2003.

11. Vector organisms
• Vectors typically become infected by a disease agent while feeding on infected
vertebrates (e.g., birds, rodents, other larger animals, or humans), and then pass
on the microbe to a susceptible person or other animal. In almost all cases, an
infectious microbe must infect and multiply inside the arthropod before the
arthropod is able to transmit the disease through its salivary glands. The most
common vector-borne diseases are carried by mosquitoes and ticks.
• Disease Transmission: Diseases may be transmitted by being bitten by an animal
or insect carrying the disease.
• Weather condition: outbreaks of human disease in the tropics generally are
more common during the wet season than in the dry season. Disease vectors,
like mosquitoes and flies, are more abundant during the wet season.

12. Characteristic features of vector organisms: Ticks
Ticks possess certain characteristics that make them highly efficient vectors of
disease:
1. Adaptability - Man's increasing encroachment on the wild habitat has
allowed ticks to adapt to feeding on domestic stock, pets and humans. New
hosts, having not been previously exposed to such pathogens, have no
immunity and thus succumb to disease. Ticks can also adapt to climate change
by entering a state of torpor.
2. Firm attachment - Ticks are hard to dislodge, lying close to the skin
amongst hair, feathers or scales. This makes them difficult for the host to groom
out and allows the tick plenty of time to feed in relative security.
3. High agent dispersal - By selecting mobile hosts, such as large mammals
or birds, ticks can be transported to new regions either locally or across oceans.
This helps spread disease far and wide.
4. High reproductive potential - Dermacentor and Ixodes species lay
approximately 1-10,000 eggs, while Hylomma species lay 10,000 or more.

13. Characteristic features of vector organisms: Ticks
5. Slow feeding - This allows ample time for ingestion, and transmission of
large quantities of infective agents. Argasids and certain male Ixodids will take
multiple feeds, increasing the opportunity for transmission to occur.
6. Starvation resistance - Argasids can endure long periods without food by
slowing their metabolic rate and entering a state of torpor, which can last a year
or more until a host is found.
7. Wide host range - Throughout its life cycle, Ixodes ricinus will feed on small
to large mammals, birds and reptiles, spreading disease throughout a wide
variety of fauna.
8. Versatile saliva - Tick saliva is produced during the biting and feeding
process. It has anaesthetic, anticoagulant, anti-inflammatory,
immunosuppressive, and (in the case of Ixodids) adhesive properties. The
introduction of saliva allows the transmission of disease agents. The
importance of saliva in disease transmission is considerable.

14. Characteristic features of vector organisms: Mosquitoes
• The mosquito has been described as the most dangerous animal in
the world and the mosquito-borne disease with the greatest
detrimental impact is undoubtedly malaria.
• There are about 3,500 mosquito species and those that transmit
malaria all belong to a sub-set called the Anopheles.
• Approximately 40 Anopheles species are able to transmit malaria
well enough to cause significant human illness and death.

15. Characteristic features of vector organisms: Mosquitoes
• To be effective at transmitting malaria between people, a mosquito
species needs to have a number of characteristics including:
1. Abundance - the species needs to exist in numbers high enough to
ensure individuals encounter an infectious human to pick up the
malaria parasite
2. Longevity - individual mosquitoes need to survive long enough after
feeding on infected blood to allow the parasite time to develop and
travel to the mosquito’s salivary glands ready to infect the next
person bitten
3. Capacity – each mosquito needs to be able to carry enough malaria
parasites in the salivary glands to ensure the parasite is transmitted to
the next human
4. Contact with humans – the species needs to prefer to feed on
humans rather than other animals, and be able to survive and breed in
places close to homes, and be able to find people (usually by entering
their houses)

16. Characteristic features of vector organisms: Mosquitoes
• To be effective at transmitting malaria between people, a mosquito
species needs to have a number of characteristics including:
1. Abundance - the species needs to exist in numbers high enough to
ensure individuals encounter an infectious human to pick up the
malaria parasite
2. Longevity - individual mosquitoes need to survive long enough after
feeding on infected blood to allow the parasite time to develop and
travel to the mosquito’s salivary glands ready to infect the next
person bitten
3. Capacity – each mosquito needs to be able to carry enough malaria
parasites in the salivary glands to ensure the parasite is transmitted to
the next human
4. Contact with humans – the species needs to prefer to feed on
humans rather than other animals, and be able to survive and breed in
places close to homes, and be able to find people (usually by entering
their houses)

17. Any queries?