Mechanisms of Survival of Viruses in Nature

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Mechanisms of Survival of Viruses in Nature 

• Transmissibility related to virion release from
  the patient and quantity
• Seasonality affects the survival of viruses
• Community size determines if acute or
  persistence infections are perpetuated
• Effects of immunity types
• Antigenic drift, shift and reassortment 

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Mechanisms of Survival of Viruses in
Nature Persistent Infections

•  Herpesviridae,
• Adenoviridae,
• Papovaviridae,
• Hepadnaviridae,
• Arenaviridae, Togaviridae,
• Flaviviridae,
• Retroviridae.

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Mechanisms of Survival of Viruses in
Nature Persistent Infections

• Arenaviruses,
• Herpesviruses,
• Retroviruses
• and some Togaviruses

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Know Your Enemy
MSV

• Pauline Ruiru sows less and less maize each
  year.

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Know Your Enemy
MSV

• Although this cereal (maize) is the staple food
  in her part of Kenya, she has had to gradually
  give up trying to grow it. The reason?
• The damage inflicted by a devastating disease
  known as maize streak.

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Know Your Enemy
MSV

• Why have farmers in eastern Africa chosen not to
  adopt maize varieties allegedly resistant to the
  destructive maize streak virus (MSV) ?
• Are these varieties really resistant or do they
  merely have undesirable characters associated
  with resistance?  

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Know Your Enemy
MSV

• In order to answer these questions, much more
  needs to be known about the virus,
• about the insects that carry it, about maize ...
• and also about Pauline Ruiru, who can no longer
  obtain meaningful yields on her two acre
  subsistence farm.

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MSV

• Maize streak monogeminivirus .
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• Total genome size 2.69 kb.
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• Transmitted by a vector.
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MSV - Transmitted by a vector

• Transmitted in a persistent manner.
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• Virus does not multiply in the vector.
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• Not transmitted congenitally to the progeny of
  the vector.
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MSV - Transmitted by a vector

• Not transmitted by mechanical inoculation.
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• Not transmitted by contact between plants.
• Not transmitted by seed not transmitted by
  pollen.
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Know Your Enemy
MSV

• In southern and central Africa, MSV is
  transmitted chiefly by Cicadulina mbila

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Latent Period
MSV

• MSV undergoes a latent period in the vector
  before transmission can occur.
• The length of this period is temperature
  dependent.
• At 30øC, the minimum latent period is 6 - 12
  hours, the medium period being 16 - 20 hours.
• At 16øC, the minimum latent period is 85 hours.
• The latent period represents the time required
  for the virus to pass from the insect's gut lumen
  to the salivary glands, where it becomes
  available for inoculation.

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Epidemiology
MSV

• Disease avoidance can be practiced by adjusting
  planting dates to avoid migrating leafhoppers
  landing on young plants.
• The vector can be controlled by applying systemic
  insecticides to the planting furrow during maize
  planting.

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?????
MSV

• However, the development and use of
  streak-resistant cultivars is probably the most
  effective and economically viable means of
  preventing streak epidemics.
• Naturally occurring resistance to MSV has been
  found in maize on Reunion Island (where MSV has
  long been endemic). The resistance appeared to be
  simply inherited and was rapidly fixed in
  breeding, being easily transferred to other maize
  lines.

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?????
MSV

• The incorporated resistance resulted in lowered
  disease incidence and reduced disease severity.
• More recently, it has been shown that resistance
  to MSV in maize is quantitatively inherited, with
  relatively small numbers of genes involved.
• Thus, it is envisaged that simple recurrent
  selection or modified back-cross breeding methods
  could be used to breed for MSV resistance in
  Africa.

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West Nile
West Nile is transmitted by mosquitoes, in
particular a species called the culex.

• Family Flaviviridae
• Genus Flavivirus Japanese Encephalitis Antigenic
  Complex
• Flaviviruses share a common size (40-60nm),
  symmetry (enveloped, icosahedral nucleocapsid),
  nucleic acid (positive-sense, single stranded RNA
  approximately 10,000-11,000 bases), and
  appearance in the electron microscope

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  ?????? ???? ????? ?? ??? ????? ?????? (culex).
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  ???????? ???????? ????? ????.
• ???? 2000 ????? ??????? ?? ????? ???? ??? ???????
  ?????? ????? ?????? ????? ????? ??????? ?????.

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  ???? ???? ?????? ?? ?????? ???.
• ??????? ??????? ???? ???? ???? ????? ???? ?????
  ??????.
• ?? ?????? ????? ????? ??????, ???????? ???????
  ???????. ?? ????? ????? ?? (?????) ????? ???????
  ??????.

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• ??? ??? ????? ??? ?????? ????? ?????? ?? ????
  ???? ?????? ?????? ????? ???? (????? ???? ???)
  ????.
• ????? ?? ????? ?????? ??? ???? ?????? ????? ?????
  ?? ????? ???? ???? ?????? ?????? ???? ???.

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• ?? ??? ????? ??????? ?????? ?? ???? ????? ???
  ???? ??? ?????? ???? ????????

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• ????? ????? - ???? ????? ?????? ??????.
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• ?????, ??????, ????? ?????? ????? - ?????.
• ?????? ??? - ?????? ??? ???????.

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???????? ???? ?????? ?????? (????? ???? ??????)

• 1 . ????? ????? ?????? ???? ????? ??????? ?????
  ????? ?? ???? ????.
• 2 . ????? ?????? ??????? ????? ??? ???? ?? ??????
  ????? ???? ??????? ???????? ?????? ?????? ??
  ????? ??????? ?????? ?? ??? ????? ?????? ??????.

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???????? ???? ?????? ?????? (????? ???? ??????)

• 3 . ????? ?????? ???? ???? ??????? ?????? ?????
  (?????? ?? ????), ???? ???? ??????? ?????? ??
  ????. ?? ?????? ???????? ?????? ?? ??? ?????
  ?????? ?????? .

• 4 . ????? ?????? ??????? ??????? ???????? ??????
  ?? ????. ??????? ??? ??????? ?? ??? ???? ???????.

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Borna Virus
August 31, 1998

• New research from Germany indicates some cases of
  serious depression may be caused by a virus.
• "We think that there is ... a lot of evidence
  that Borna virus has clinical significance for
  this type of disease," said Dr. Liv Bode of the
  Robert Koch Institute in Berlin.

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Borna Virus
August 31, 1998

• In the United States, at least 17 million people
  have some form of clinical depression -- not just
  a passing case of the blues, but a disabling and
  often long-term disease.
• Scientists are still unraveling the causes of the
  disease genetics, stress and possibly a virus.
• The virus was first identified in the late 1800s
  among horses near the town of Borna, Germany. The
  horses stopped eating, walked in circles and got
  sick. Some even killed themselves.

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Borna Virus
August 31, 1998

• Autopsies led scientists to the virus in the
  region of the horses' brains that controls
  emotions. Researchers in Berlin have found a
  similar strain in humans.
• "I think it is supporting our hypothesis that
  this virus, this particular agent, has really
  something to do with this type of disorder," Bode
  said.

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Borna Virus
Classification

• Borna disease virus (BDV) is a neurotropic
• negative
• single-stranded
• enveloped
• RNA virus
• that persistently infects various domestic animal
  species

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Borna Virus

• Infection causes disturbances in behavior and
  cognitive functions, but can also lead to a fatal
  neurologic disease.
• Human infections seemed likely, since serum
  antibodies were detected in neuropsychiatric
  patients.
• The markers seem to coincide with acute episodes
  of mood disorders, thus pointing to a new human
  virus infection possibly threatening mental
  health.

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Pathology

• It is believed that transmission between animals
  occurs through saliva or nasal secretions by
  direct contact or by exposure to contaminated
  food or water. 
• Borna is predominately, but not exclusively,
  neurotropic. 

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Pathology

• It appears to enter the CNS by intraaxonal
  migration through the olfactory nerve or nerve
  endings in the oropharyngeal and intestinal
  regions. 
• From there, it spreads to the peripheral nerves. 
  
• In addition to the CNS, T-cells play in important
  role in the pathology of BDV. 

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Epidemiology

• Evidence suggests an association with BDV and
  psychiatric disorders including depression and
  schizophrenia.
• Epidemiological studies showed an increased
  incidence of seropositive by 600 in young (ages
  17-30) psychiatric patient compared to young
  surgical patients (controls).

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Epidemiology

• These finding correspond with the fact that
  psychiatric disorders frequently initially
  manifest in young people.
• This strongly suggests that BDV plays a role in
  the pathogenesis of psychiatric disorders.

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BORNAVIRUS Not Linked to Psychosis

• A recent study in Japan suggests that despite
  recent research suggesting a link between Borna
  disease virus infection and some psychiatric
  disorders, the virus may not be responsible after
  all.
• The results of the study appear in the February
  2001 issue of the Journal of Clinical
  Microbiology.

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BORNAVIRUS Not Linked to Psychosis?

• Despite their results, the researchers still
  believe that some psychiatric disorders may be
  associated with BDV infection and suggest that
  further studies with a larger number of subjects
  may be necessary.

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Note the icosahedral symmetry which is clearly
visible in this image.
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  ?????.

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The Human Papilloma Virus

• ????? ?-HPV ???? ?????? ?-Papovaviridae.
• ????? ?-HPV ???? ??? ?? ????? ?????, ?????
  ?-8000 ??????. ???????? (????? ???), ??????
  ?????? ?????.
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??????? HIV-1
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• Ultraviolet, and other, radiation from the sun is
  sufficient to sterilize most pathogens within the
  space of about 30-60 seconds.
• This is the primary reason most infectious
  microorganisms die in the outdoor air.