The document discusses host-parasite relationships, defining hosts as organisms that support parasites, which are dependent on the host for survival. It distinguishes between infections and diseases, explains types of pathogens, and describes factors affecting pathogenicity and virulence. Key concepts include the definitions of primary and opportunistic pathogens, as well as methods for measuring virulence such as lethal and infectious doses.

1. Host parasite relationships
-Nidhi Jodhwani

2. • Host: A larger organism that supports the
survival and growth of a smaller organism is
called the host.
• Parasites : those organisms that live on or
within a host organism and are metabolically
dependent on the host.(any organism that
causes disease is a parasite)

3. Host: human
Parasite: mosquito
Ecto parasite

4. Host: Human
Parasite: Plasmodium spp.
Endo-parasite

5. Infection: When a parasite is growing and
multiplying within or on a host, the host is
said to have an infection .
Infection and disease are same?
• No. When host’s normal body is not capable
of doing its normal function due to presence
of parasite or its products, host is said to be
suffering from disease.
• Pathogen: Any organism or agent that
produces such a disease is also known as a
pathogen
• Pathogenicity: Its ability to cause disease is
called pathogenicity .

6. Types of pathogen
Primary pathogen
• A primary pathogen is any
organism that causes
disease in a healthy host by
direct interaction.
Opportunistic pathogen
• An microorganism that is
normally a commensal or does
not harm its host but can
cause disease when there is
favourable condition is
created e.g. Weakened host
immune system.

7. latency
Intermittent latency
Initial infection
No fixed time
for reactivation
(weeks or years)
reactivation
Quiescent latency
Initial infection
years
for reactivation
reactivation
Present in host but do not cause any symptoms.

8. outcome of most host-parasite
relationships
the number of microorganisms infecting
the host
the degree of pathogenicity (or virulence)
of the organism
the host’s defenses or degree of
resistance

9. Pathogenicity vs. virulence
pathogenicity
• Qualitative term
Yes/ No
virulence
• Quantitative term
How much??
• Highly virulent(more
pathogenic)
• Less virulent (less
pathogenic)

10. Virulence factors
• Various physical and chemical characteristics
contribute to pathogenicity are called
virulence factors
Infectivity
• ability of the
organism to
establish
infection
Invasiveness
• ability of the
organism to
spread to
adjacent or
other
tissues.
Toxigenicity
• the
pathogen’s
ability to
produce
toxins—
chemical
substances
that damage
the host and
produce
disease

11. How we can measure virulence?
• lethal dose 50 (LD 50 )
number of pathogens that can kill 50% of an
experimental group of hosts within a specified
period
• infectious dose 50 (ID 50 )
number of pathogens that can infect 50% of
an experimental group of hosts within a specified
period

12. LD50 inversely proportional to virulence.

16. 1.1 Host parasite relationships

17. Reference
Joanne M. Willey , Linda M. Sherwood ,
and Christopher J.
Woolverton . 2011. McGraw-Hill Higher
Education, New York, NY.