Lecture 8 : New Infectious Diseases Overview

1. Lecture 8: New Infectious DiseasesOverview • AIDS / HIV • NEW INFECTIOUS DISEASES - FACTORS • EXAMPLES • Machupo • Marburg • Lassa Fever • Ebola • Rift Valley Fever • Sin Nombre • Other • CONCLUSION

2. 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.

3. 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.

5. 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.

6. 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.

8. 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.

9. 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.

10. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.