Yersinia pestis is the bacterium that causes plague. It is usually transmitted to humans by the bites of infected fleas that live on rodents. The rat flea Xenopsylla cheopsis is the most common and efficient vector. Certain environmental conditions like warm temperatures between 20-25 degrees Celsius and 60% humidity provide favorable conditions for plague transmission. Human activities and movement can also influence plague spread. Monitoring flea indices on rodents is important for evaluating plague risk and the effectiveness of control programs.

1. EPIDEMOLOGICAL
DETERMINANTS OF
PLAGUE
Dr Nikhil Bansal
J.N.M.C.,Wardha

2. Epidemiological Determinants

3. AGENT FACTORS
Agent
Reservoir of infection
Source of
infection

4. AGENT
Yerisinia pestis
Family :- Enterobacteriaceae
Genus :- Pasteurella
Discovered by :-
YERSIN and
KITASTO.

5. Morphology :-
• Y. pestis is a short, plump,
ovoid, gram-ve bacillus,
1.5μm×0.7µm in size.
• In smears stained with
giemsa or methylene blue,
it shows bipolar staining.
• It is non-motile, non-
sporing and non-acid fast.
• It is surrounded by slime
layer.

6. BIOCHEMICAL REACTIONS:-
• Glucose , maltose & mannitol are fermented
with the production of acid but not gas.
• Indole not produced.
• It is urease negative.
• It is oxidase positive.
• It is citrate negative.

7. RESISTANCE:-
• Plague bacillus is easily destroyed by exposure
to heat, sunlight, drying and chemical
disinfectants.
• It is destroyed by heat at 55 degrees or by 0.5
percent phenol in 15 minutes.
• They are lysed by specific antiplague
bacteriophages.

8. VIRULENCE FACTORS:-
• A heat labile protein envelope antigen seen in
virulent strains only.
• Two antigens designated as V & W are always
produced together.
• Virulent strains produce bacteriocin,coagulase
and fibrinolysin.
• Plague toxins endotoxins and murine toxins.
• Virulence is associated with purine synthesis .

9. RESERVOIR OF INFECTION
• Wild rodents like field mice, gerbils, skunks,
other small animals are natural reservoirs of
plague
• In India, wild rodent, Tatera indica has been
incriminated as main reservoir
• Generally disease is maintained and spread by
resistant species of wild rodents.

10. Source of infection:-
• Infected rodents
• Infected fleas
• Case of a pneumonic plague

11. HOST - FACTORS
• Age and sex
• Human activities
• Movement of the people.
• Immunity.

12. ENVIRONMENTAL FACTORS
Season:-
• Out breaks are usually seasonal in nature
• In northern India plague season start
from september until may
• Plague season depends primarily on field
rodent factors

13. Temperature and humidity:-
A mean temperature of 20-25 degrees and
relative humidity of 60% is considered to be
favourable for spread of plague.
Rainfall :-
Heavy rainfall is unfavourable for spread of
plague.

14. Urban and Rural areas:-
Plague have unable to gain a foothold in
many towns of India perhaps due to unto-
ward ecological conditions and lack of
efficient flea vectors.
Human dwellings :-
Rats are abundantly found in places of
poor housing conditions and frequent
dwellings.

15. VECTORS OF PLAGUE
Commonest and most efficient vector of
plague is Rat flea, Xenopsylla cheopsis

16. But other flies may also transmit the infection.
Examples:-
Xenopsylla astia
Xenopsylla brasilensis
Pulex irritants
Ceratophyllus fasciatus
Pulex irritants Xenopsylla brasilensis Ceratophyllus fasciatus

17. BLOCKED FLEA
• A flea may ingest upto 0.5 cu.mm of blood which
may contain as many as 5000 plague bacilli.
• Plague bacilli multiply enormously in gut of rat
flea and may block the proventriculus so that no
food can pass through it
• Such a flea is called blocked flea
• A blocked flea may eventually faces starvation
and dies as it is unable to obtain blood meal.

18. • A blocked flea is an efficient transmitter of
plague
• A partially blocked flea is more dangerous then a
completely blocked flea because it can live
longer
• Infected fleas may live upto an year and certain
species in burrows for as long as 4 years.
BLOCKED FLEA:

19. FLEA INDICES
• They are useful measurements for density of fleas.
• They are useful in evaluating the effectiveness of
spraying programme.
• Following flea indices are widely used:-
Total flea index.
Cheopsis index.
Specific percentage of fleas.
Burrow index.

20. Total flea index:-
It is average number of fleas of all species
per rat.

21. Cheopsis index:-
It is the average number of x.cheopsis per
rat. It is a specific flea index.

22. Specific percentage of fleas:-
It is percentage of different species of
fleas that are found in rats.

23. Burrow index : -
It is the average number of free living
fleas per species per rodent burrow.

24. Thanks…