Virology

1. VIROLOGY BIO342
Derrick Banda MSc

2. How to get a hold of Derrick
Banda
• Office: Mulungushi University Main Campus Staff
Room
• E-mail: dbanda@mu.ac.zm
dbanda45@yahoo.com
• Phone Number:0974420585, 0955556060
•

3. SUBVIRAL PATHOGENS
(HDV,Prions and Viroids)

4. SUBVIRAL PATHOGENS
• Subviral agents are composed of three kinds: satellite
viruses, viroids, and prions. These transmissible agents
are classified as subviral agents as they are less than a
virus in some respects.
• The first subviral agent is the “satellite virus,” which is
morphologically indistinguishable from ordinary virus
particles, but it depends on another virus, a host or
helper virus, for propagation. Therefore, a satellite virus
is often called “a parasite of a parasite,” as it relies on
another parasite, a virus.
• The second subviral agViroids are comprised of “RNA
only,” ent are “viroids” found in plants. and are devoid
of any protein component. In contrast to the viroids, prions
are transmissible agents that are comprised of “protein

5. Hepatitis Delta Virus (HDV)
• In 1977, an Italian doctor named Mario Rizzetto
discovered a new nuclear antigen in the liver cells of
patients infected with Hepatitis B Virus (HBV).
• The antigen was thought to be a new protein encoded
by HBV, and it was labeled as the delta antigen.
Subsequent research on chimpanzees, however,
indicated that this antigen was derived from a new
virus, named the Hepatitis Delta Virus (HDV).
• The hepatitis delta virion is the smallest RNA
pathogen known to interact with a human host and to
cause substantial global morbidity and mortality.

6. HDV STRUCTURE
• HDV consists of a single stranded, negative sense,
circular RNA virus, with an envelope made up of
HBAg.
• HDV is a defective virus, whose genome is
surrounded by three HBV envelope proteins and
host lipids.

7. Hepatitis Delta Virus (HDV)
• HBV plays an essential role as a helper virus for HDV,
since its envelope proteins are stringently necessary
for HDV propagation. The production and transmission
of HDV is entirely dependent on HBV to provide
HBsAg. The hepatitis delta virions from the infected
hepatocytes can only occur if the cells are coinfected
with HBV.

8. HDV (Hepatitis Delta Virus)
Symptoms and Incubation
• HDV causes a unique infection that requires the
assistance of HBV viral particles to replicate and infect
hepatocytes. Its clinical course is varied and ranges
from acute, self-limited infection to acute, fulminant
liver failure.
• Although variable, the clinical course of HDV is
typically more severe than that of the other hepatitis
viruses. After an incubation period of 3-7 weeks,
nonspecific clinical symptoms, including fatigue,
lethargy, nausea, and anorexia, begin and last for
about 3-7 days.

9. VIROIDS
• Viroids are the small pathogenic RNA molecules known to
be the causative agents of several plant diseases'.
• In 1971, a plant pathologist named Theodor Otto Diener
first discovered the Viroids. He found an infectious RNA
particle smaller than a virus that causes diseases in plants
when he was working in an Agriculture Research Service,
and named this particle as viroid, meaning “virus-like.”
• Viroids appear to be transmitted mechanically from one
cell to another through cellular debris. Viroids are of much
interest because of their subviral nature and their
obscure mode of action.

10. VIROIDS
Biological properties of viroids
• The molecular weight of the known viroids is
about 80,000 to 125,000 daltons and their
nucleotide chain is only 246-371 nucleotides
long.
• The mechanisms of viroid replication is not fully
elucidated, but it is certain that viroids depend
upon the nuclear enzymes already present in
the host plants for their replication.

11. VIROIDS
• Viroids are even more simple than viruses. They are
small, circular, single-stranded molecules of infectious
RNA lacking even a protein coat.

12. VIROIDS
• The viroid diseases are of worldwide
occurrence and have caused serious economic
losses in the temperate and subtropical
climates. Millions of dollars are lost each year in
crop failures caused by viroids.
• They are the cause of a few plant diseases
such as potato spindle-tuber disease, cucumber
pale fruit, citrus exocortis disease, and cadang-
cadang (coconuts).

13. Viroid Diseases Symptoms
• In viroid diseases, characteristic stunting of the
entire plants is known to be common and there
are usually other symptoms such as smaller
upper leaves, and internodal shortening in
plants infected by viroids.
• Discoloration of the leaves such as mosaic or
mottling has scarcely observed in any viroid
infected original hosts.

14. Viroid Diseases
Potato spindle tuber viroid (PSTVd)
• Potato spindle tuber viroid (PSTVd) causes
serious disease in potato (Solanum tuberosum),
tomato (Lycopersicon esculentum), and
eggplant (Solanum melongena).
• Natural infections have also been reported on
pepino (Solanum muricatum), avocado (Persea
americana) and sweet potato (Ipomoea
batatas) plus a wide range of wild solanaceous
hosts.

15. Viroid Diseases
Potato spindle tuber viroid (PSTVd)
Symptoms
• There are both mild and severe strains. Symptoms may be
confused with those of nutrient imbalance, spray damage,
insect damage or other plant diseases such as true
viruses. Symptoms become more pronounced in warm
conditions and under high light intensity.

16. Viroid Diseases
Potato spindle tuber viroid (PSTVd)
On tomatoes
• In mature tomato plants, infection with severe
PSTVd strains causes purpling and yellowing of
the leaflets, shortening of leaflet internodes
resulting in a ‘bunchy top’ effect, leaflet down-
curling and twisting and general plant stunting.
• Spindly shoot growth can occur, flowers may
abort and fruit can be dark green, fail to ripen
normally and have thicker outer walls.

17. Viroid Diseases
Potato spindle tuber viroid (PSTVd)
Tomato Symptoms
• Mild strains may not cause any symptoms, while severe
strains cause plants to appear stunted and bunched due to
shortened internodes. Leaves are small with yellowing and
purpling, down curling and twisted. Flowers may abort.
Fruit size is reduced and ripen erratically.

18. Control measures of viroid diseases
• No chemotherapeutants are yet available for
controlling the viroid diseases. Therefore,
control measures have to correspond to the
breaking of the infection chains for each viroid.
• Control measures suggested include;
1. Prompt removal of all the infected plants
including root systems
2. Replanting with certified viroid-free plants and
3. Disinfection of the tools contaminated with
viroids.

19. PRIONS
• Prions are infectious protein particles responsible
for a group of transmissible neurodegenerative
diseases.
• Abnormal prions (PrPsc) are the pathologic
isoforms of prions.
• PrPsc has a high β-sheet content, is extremely
resistant to heat and proteases, and is insoluble in

20. One protein: Two structures
PrPsc
“BAD”
conformation
PrPc
“NORMAL”
conformation

21. PRIONS
• Prion Disease occur when the normal ‘cellular’
form of the prion protein (PrPc) is converted to
the abnormal form (PrPsc). PrPc and PrPsc differ
in conformation. The conversion is
‘autocatalytic’ - PrPsc facilitates the conversion
of more PrPc to PrPsc

22. PRIONS
• Prions causes transmissible spongiform
encephalopathies (TSEs) such as Creutzfeldt-
Jakob disease and kuru in humans, as well as
scrapie in sheep and goats, and bovine
spongiform encephalopathy (mad cow
disease) in cattle.

23. Prions and Creutzfeldt-Jakob disease
• Prions causes transmissible spongiform
encephalopathies (TSEs) such as Creutzfeldt-Jakob
disease (CJD).
• Creutzfeldt-Jakob disease (CJD) is a rare
neurodegenerative condition. It has severe effects on
the brain.

24. Prions Creutzfeldt-Jakob disease
The hallmark symptoms of CJD are a rapid
progression of dementia and myoclonus —
spasmodic, involuntary movement of muscle
groups.
• Other common symptoms of CJD include:
Changes in mood, personality, or behaviour
Memory loss
Impaired judgment

25. Prions and Bovine Spongiform
Encephalopathy
• Bovine spongiform encephalopathy (BSE),
commonly known as mad cow disease is a
progressive neurological disorder of cattle that
results from infection by a prion.
• Prion damages the central nervous system of
cattle causing the brain to degenerate until it
becomes “spongy,”.
• It attacks the central nervous system - the brain
and spinal cord - causing infected animals to
lose muscle control.

26. Prions and Bovine Spongiform
Encephalopathy
• Bovine spongiform encephalopathy (BSE), commonly
known as mad cow disease symptoms include
abnormal behavior, trouble walking, and weight loss.
• They become unsteady on their feet and also become
aggressive, nervous or frenzied - leading to the name
"mad cow disease".

27. Prions and Sheep scrapie
• Scrapie is a neurodegenerative disease,
caused by a prion, that affects sheep and
goats, but less is known about scrapie in
goats than in sheep.

28. Prions and Sheep scrapie
• Scrapie has an incubation period of more
than two years. As a result, the disease
symptoms are only seen in adult animals.
• As the result of nerve cell damage,
affected animals usually show behavioral
changes, tremor (especially of the head
and neck), pruritus, and locomotor
incoordination, which progresses to
recumbency and death.

29. Prions and Sheep scrapie
• Scrapie is the ancient form of TSEs. It is
known since 1732 and has occurred in
sheep and goats.

30. Transmission pathway for Bovine
Spongiform Encephalopathy
• Parts of sheep infected with scrapie are turned
into agricultural feed. Feed is eaten by cow.
Cow is turned into hamburger. Man eats
hamburger.
• Prion enters the system orally, and from the
digestive system is distributed throughout the
body, into the lymphoid areas and crossing the
blood-brain barrier into the brain.
• The disease may lie dormant for years before
activating and resulting in neurodegeneration

31. Transmission pathway for Bovine
Spongiform Encephalopathy

32. Viral diseases in agricultural crop
• Viral diseases in crop plants constitute a
major threat to crop production and
obstacle to food security in the developing
world.
• Subsistence crops, including cassava,
sweet potato, potato, banana, papaya,
common bean, rice and maize are often
infected with RNA and/or DNA viruses that
cannot be controlled with pesticides.

33. Viral diseases in agricultural crop
• Healthy planting materials and virus-
resistant cultivars are essential for high
yields of good quality.
• However, resistance genes are not
available for all viral diseases of crop
plants. Therefore, virus resistance
engineered in plants using modern
biotechnology methods is an important
addition to the crop production toolbox.

34. Viral diseases in agricultural crop
• Africa suffers from major recurring plant
virus pandemics and epidemics in
important crops.
• Well-known viral diseases caused by DNA
viruses include cassava mosaic disease
(CMD) caused by begomoviruses, banana
bunchy top disease caused by a
babuvirus and maize streak disease
caused by maize streak virus (MSV).

35. Viral diseases in agricultural crop
• Maize streak virus is transmitted to maize by small
insects called leafhoppers. The disease is therefore a
result of a complex interplay between the plant, the
virus and insect.
Maize streak virus symptoms in a commercial maize field in Klerksdorp,
NorthWest South Africa, showing chlorotic streaking and deformed cob

36. Viral diseases resistance
transgenetics
• Considerable efforts have been invested to
develop virus resistance to many different
viruses affecting crops relevant to agriculture
and food production in developing countries.
• The main target crops have been maize, rice,
cassava, potato, tomato, peanuts, sugarcane,
rice and soybean.
• Therefore, development and transfer of
resistance to crops by biotechnological means
offers an attractive alternative solution.

37. Viral diseases in agricultural
crop
Transgenic resistance to maize viruses
• The first all-African genetically modified crop plant with
resistance to the severe maize streak virus (MSV),
which seriously reduces the continent’s maize yield,
was developed by scientists from the University of
Cape Town and PANNAR PTY Ltd, a South African
seed company.

38. Viral diseases in agricultural
crop
Transgenic resistance to maize viruses
• They created a MSV-resistant maize variety by genetic
engineering, using an approach known as pathogen-
derived resistance.
• This means that a gene from the viral pathogen is
used to protect the plant from that pathogen. They
mutated a viral gene that under normal circumstances
produces a protein that is essential for the virus to
replicate itself and inserted it into the maize plant’s
genome, creating genetically modified maize. When
the virus infects one of these transgenic maize plants,
it displays a significant delay in symptom
development, a decrease in symptom severity and

39. Viral diseases resistance
transgenetics
Transgenic resistance to cassava viruses
• Cassava is currently the third most important source of
calories in the tropics, after rice and corn.
• Cassava mosaic virus is the most important single
factor limiting cassava production.
Cassava plant affected by an 'unusual' form of cassava mosaic disease (CMD)

40. Viral diseases resistance
transgenetics
Transgenic resistance to cassava viruses
• They have developed transgenic cassava plants with
increased African Cassava mosaic virus (ACMV)
resistance using improved antisense RNA technology
by targeting the viral mRNAs of Rep (AC1), TrAP
(AC2) and REn (AC3).
• Viral DNA replication assays in detached leaves
demonstrated that replication of two ACMV isolates
was strongly reduced or inhibited in most transgenic
lines.
• After ACMV infection of plants using biolistic
inoculation, several lines remained symptomless at
lower infection pressure (100 ng viral DNA/plant).

41. Tropical crops engineered for virus
resistance

42. Questions?

43. END OF LECTURE!
THANK YOU.