Lecture 8: New Infectious Diseases Overview

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Lecture 8 New Infectious DiseasesOverview

• AIDS / HIV
• NEW INFECTIOUS DISEASES - FACTORS
• EXAMPLES
• Machupo
• Marburg
• Lassa Fever
• Ebola
• Rift Valley Fever
• Sin Nombre
• Other
• CONCLUSION

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AIDS /HIV

• The discovery of AIDS / HIV in the early 1980s
  shattered the illusion than major infectious
  diseases were a thing of the past in developed
  countries.
• HIV attacks the immune system. HIV is capable of
  splicing DNA into the DNA of the host cells, so
  instead of controlling HIV the immune system
  helps reproduce it.
• About 40m people have been infected by HIV
  worldwide. The worst affected region is
  sub-Saharan Africa where it is transmitted
  heterosexually.
• Infection in developed countries is still mainly
  amongst homosexual men and intravenous drug users.

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New Infectious Diseases

• AIDS / HIV is by no means the only new infectious
  disease. There have been about 30 new diseases
  discovered in the past 20 years.
• Most originate in tropical countries, and most
  have so far been confined to these regions.
• The fear in developed countries is that one of
  these new diseases may be transmitted to a
  susceptible population in the developed world.

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Factors

• New infectious diseases may be regarded as a
  by-product of the knock-on effects of population
  growth.
• The increased demand for food has resulted in new
  areas being brought under cultivation, disrupting
  existing ecosystems
• Monocultivation has reduced biodiversity, forcing
  viruses to seek new hosts
• Rapid urbanisation results in high population
  densities and insanitary conditions
• Global warming is extending the habitats of
  mosquitoes and other vectors
• Competition for resources contributes to wars
  which disrupt public health systems and trigger
  mass population movements.

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Examples - Machupo

• Machupo (Bolivian Haemorrhagic Fever) originated
  in 1961 in the headwaters of the Amazon where the
  traditional cattle ranching / export economy was
  replaced by self-reliant peasant agriculture
  after a social revolution.
• Jungle was cleared to grow corn and vegetables,
  disrupting the habitat of Colomys callosis (a
  field mouse).
• The mouse population swelled given the
  availability of corn. They invaded the villages.
• A virus carried by the mice was passed by the
  mice in their urine.
• The virus in humans caused 50 people mortality.
• The disease was eventually contained by catching
  the mice.

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Examples - Marburg

• The Marburg virus causes a horrific disease.
• It was first observed amongst monkeys in 1961,
  but humans were infected in 1967 by monkeys
  imported into Germany from Uganda to be used to
  produce vaccines.
• The reservoir (i.e. the source which infected the
  monkeys) has never been discovered.

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Examples Lassa Fever

• Identified in Nigeria in 1969, but it is believed
  to have been around for a few decades before (but
  confused with malaria or yellow fever).
• It resembles malaria in the early stages, but
  this is followed by haemorrhaging.
• Now endemic in parts of west Africa.
• The reservoir has been identified as Mastomys
  natalensis (a brown rat) which thrives because
  humans killed off the larger black rat.
• It is fatal in about 10 per cent of cases.

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Examples - Ebola

• Occurred almost simultaneously in two locations a
  few hundred miles apart in Zaire (Congo) and
  Sudan.
• Ebola is a horrific disease with 90 per cent
  mortality.
• Transmission is by direct contact, but it is very
  contagious.
• The reservoir of the virus has never been
  discovered.
• Infected monkeys from the Phillipines almost
  caused an outbreak in Reston VA, near Washington
  in 1989.

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Examples Rift Valley Fever

• Originally confined to sheep and cattle in
  Africa, it jumped species and affected 200,000
  people in Egypt in 1977.
• The virus is transmitted by a mosquito.
• The 1977 outbreak is believed to have been
  triggered by the construction of the Aswan dam
  which created favourable conditions for the
  mosquitoes to breed.
• There was a similar outbreak in Mauritania after
  the Senegal river was dammed.

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Examples Sin Nombre

• Indians on the Navajo reservation in the south
  west USA were afflicted by a strange illness
  causing 75 per cent mortality.
• Caused by a virus carried by Peromyscus
  maniculatus (deer mouse).
• Two moist winters (caused by El Nino) resulted in
  an abundant crop of Penon nuts and a 10-fold
  increase in the mouse population.
• The mice invaded homes and transmitted the virus
  in their urine.

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Examples Lyme Disease

• Unknown before 1962, but now the most common
  vector disease in the USA.
• Caused by a bacteria transmitted by bites from a
  tick that lives on deer.
• Lyme disease has increased due to former farm
  land reverting back to scrub which favours the
  growth of the deer population.
• Especially common in the suburbs where people
  come in contact with the deer population.

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Examples Other

• Dengue Haemorrhagic Fever. Originally confined to
  South East Asia, but now a growing threat in
  other areas including the southern USA due to the
  diffusion of the vectors (Aedes aegypti or Aedes
  albopictus).
• Latin American Haemorrhagic Fevers (Sabia,
  Guaranito, Junin). Similar causes to Junin i.e.
  expansion of agriculture into new areas.

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Conclusion

• Developed countries may be protected from the new
  vector borne diseases because the vectors may not
  be present in developed countries.
• Diseases transmitted by person to person contact
  can hopefully be controlled.
• The main risk to developed countries is the
  possibility of a new air-borne disease. Hence the
  concerns about SARS (2003) and Asian bird flu
  (2004), both of which originated in Asia.