Disease epidemiology

Question 1

Four causes for emerging infectious diseases (Anderson et al)

Answer 1

1. Pathogen pollution - Citrus tristeza virus (CTV) into South America
2. Climate change - Grey leaf blight in maize (Cercospora zeae-maydis)
3. Change in agricultural practices - Panama disease in cultivated banana (Fusarium oxysporum)
4. Host-pathogen evolution - Tomato yellow leafcurl virus (TYLCV) jumping host to the common bean.

Question 2

Emerging infectious diseases

Answer 2

EIDS are caused by pathogens that:

(i) have increased in incidence, geographical or host range
(ii) have changed pathogenesis
(iii) have newly evolved
(iv) have been discovered or newly recognized

Question 3

Pathogen pollution

Answer 3

The introduction of alien plant pathogens via trade, travel etc

Question 4

Landscape structures can affect the risk of pathogen dispersal

Answer 4

• Topographical features effect the risk of wind dispersed pathogens e.g. plants on exposed hills or next to wind breaks are more vulnerable
• Barriers in the landscape such as windbreaks and walls can slow the spread of pathogens between fields
• ‘Corridors’ of pathogen habitat between two non contiguous areas can facilitate the spread of pathogens between these areas e.g. rivers, irrigation channels
• Linear structures such as rivers and roads can facilitate pathogen spread
• Landscape complexity/heterogeneity is related to pathogen risk

Question 5

The effect of alternative hosts on te focal species

Answer 5

• Can be positive or negative
• Depends on the properties of the focal host compared to the entire host population
• If the focal host is a poor reservoir, another more competent host can dramitically increase incidence e.g. barley yellow dwarf virus in grasses, also acting as a sink
• In the case of a heteroecious pathogen, eradicating the alternative host can decrease infection of the focal host e.g. stem rust of grains has its sexual phase in barberry

Question 6

Diversity can reduce the risk of disease

Answer 6

• Genetic diversity reduces pathogen risk e.g. a mixture of resistant and non resistant rice strains vs rice blast
• Pathogen genetic diversity increases the risk of a virulent strain
• Diverse landscapes generally have less pathogen pressure