Lecture 26: Vector Borne Infections and Climate

Medically Important Vector Groups

List of vector groups (genus level) and the diseases they transmit:
- Mosquitoes: Transmit malaria, dengue, zika, chikungunya, west nile virus and rift valley fever.
- Black flies: Transmit onchocerciasis (river blindness), a macro parasite.
- Tsetse flies: Transmit trypanosomiasis (sleeping sickness).
- Sandflies: Transmit leishmaniasis.

Vector Borne Dynamics

Anthopanotic: Human-arthropod-human cycle (e.g., malaria).
Zoonotic: Animal-arthropod-human cycle (e.g., leishmaniasis with dogs as the main reservoir).
West Nile Virus: Mosquitoes-birds (maintenance), humans/horses (dead end hosts).
- Dead end hosts: Hosts that can be infected, but the pathogen cannot develop and be taken away by another vector.

Vector Capacity

Vector capacity (C): Average number of potentially infected bites delivered by all vectors feeding upon a single host in one day.
R0 calculation: $R_0 = C \times D$ (where D = duration of host infectiousness).
Vectorial capacity is directly related to $R_0$ .
Vectorial capacity is particularly sensitive to vector biting rate per day, which is represented in the equation as being proportional to A squared ( $A^2$ ), because you need a bite to infect and a bite to take the infection back away again.

Elements That Make Up Vectorial Capacity:

*   Vector biting rate per day ( $A^2$ ).
*   Proportion of blood meals taken on the host (host choice).
*   Daily survival rate (vector life expectancy).
*   Latent period of the agent in the vector (extrinsic incubation period, n).

Vector Control Methods

Human bait traps (e.g., insecticide-treated bed nets).
- Act as a barrier and kill vectors via insecticide.
Non-human bait traps (attract vectors to something harmful).
Urban breeding site source reduction.
Rural drainage of breeding sites.

Examples

Tsetse Flies:
- Older methods: Expensive, reactive (identifying and treating infected individuals).
- Newer methods: Tiny targets that attract and kill tsetse flies (reduced tsetse populations by 90% at a small cost in Uganda).
Insecticide Treated Bed Nets (ITNs):
- Potential to provide herd immunity if coverage is sufficient.
- Protect against multiple vector species and pathogens.

Other Effects on Vectors:

*   Increased vector mortality (desirable).
*   Exitorepellency (undesirable; vectors avoid bed nets).
*   Increased zoophagy (vectors feed on other animals).

Modern Developments in Vector Control

Genetically Modified Mosquitoes:
- Modified males mate with wild females; offspring larvae don't survive, interrupting reproduction.
- Sterile males mate with wild females; larvae treated with tetracycline can develop, increasing the population of genetically modified males.
Wolbachia:
- Endosymbiont bacteria in many insects (but not normally mosquitoes).
- Infected mosquitoes are less able to transmit viruses (prevents viral replication).
- Interferes with reproduction, affecting population size.
- 20 years old study in Australia showed Wolbachia infected Aedes aegypti blocked the transmission of dengue.

Summary of Control Methods

Control can mean different things, depending on how it is defined.
Vaccination, change in behavior (host and vector), and other methods can be used.

Vector Dynamics and Climate

Learning Objectives

Consider which biological mechanisms are affected by climate.
Whether climate models adequately explain current disease incidence and distributions.

Climate Change

Temperature Anomaly: Increase over from baseline.
Antarctica: Decrease in mass.
Climate change brings extreme weather events (more floods, etc.).

Infectious Disease Impact and Extreme Weather Events

Many instances affected by extreme weather events are vector borne infection.
Examples: El Nino and La Nina.

Sensitivity of Vector Borne Diseases to Climate Change

Vertebrate host, vector stage, and environmental stage.
Host is robust due to thermoregulation.
Environmental stage is directly impacted by climate.
Both parasite and vector are sensitive to changes in climate.

Vector Capacity

Population size of vectors and hosts, biting rates, preference, survival, and extrinsic incubation period.
Extrinsic incubation period of parasite within the vector is highlighted as a factor that can change with environment.

Example: Anopheles gambiae and Malaria

Gonotrophic cycle of 48 hours (won't feed again for 48 hours).
Average life expectancy of 14 days.
Extrinsic incubation period of 12 days.
Only has a 2 day window between 12 days and 14 days to transmit the pathogen.
Extrinsic incubation period must be less than life expectancy for transmission to be possible.

Calculations of average duration a vector might be infectious

$\text{Probability of survival of a single day} = 0.9$
$\text{Extrinsic incubation period} = 12$

In this example, this vector would be infectious for 2.7 days of its lifespan.
In hotter climates, extrinsic incubation periods could reduce the pathogen that's being affected by the climate.
Warmer weather, faster development means that it actually reaches the end of its incubation after only 6 days instead of 12.
Then the duration that the vector is infectious then becomes 5 days.
If increase in temperature increases survival of the vector from 0.90 to 0.95 per day, but we are back to a 12 day extrinsic incubation period, you can see they have now 10.5 days being infectious.

Dengue as an Example

Viral, spread by mosquitoes (primarily Aedes aegypti).
Disease state is flu like, but can develop into dengue hemorrhagic fever (potentially lethal).
Possible to be asymptomatic, and therefore not diseased, but infected with dengue.
Found around the world in tropical and subtropical regions; mostly in urban and semi urban areas.
No specific treatment for dengue or severe dengue, and so prevention control means, or requires effective vector control.
Some studies estimate 390 million cases a year, but then only a proportion of them are manifesting clinically because of the proportion which remain asymptomatic and don't know that they are infected and they're not diseased.
Geographic range puts ≈ half of the world's population at risk for dengue.
Reason I'm using dengue as an example is because it correlates with an extreme weather event so El Nino and La Nina.

Southern Oscillation Index and Dengue Epidemics

Evidence that an infectious disease can very much correlate with climactic activity.
Climate can play an important role in vector borne disease dynamics, both by its effect on the vector and its effect on the parasite or pathogen.
El Nino offers us evidence of that with dengue, so one very clear example there.

Blue Tongue Virus

Double stranded RNA virus found in livestock.
Often subclinical, but chronic infection.
Cattle are the reservoir host.
Causes severe disease in other species, notably sheep and deer.
Vector borne: Midge species (Culicoides imicola).
Traditionally a subtropical, tropical disease. The original or traditional or classic midge species is Cudacoides amicula.

Blue Tongue Invasion into Europe

Different strains present.
In 02/2006, first outbreak. Affected 2,000 herds in five countries.
A year later, more countries, leading to then quite large numbers being dying, but mostly being cold (twenty five thousand).
Range of the vector has increased geographically.
Outbreak in this part of Europe is outside of the range of the vector that spreads the infection.
Infection has not been spread by the traditional vector, but a different species of vector has become competent at spreading the infection.
Infection was not possible in the original range of the vector. But then the vector's range changed, moved north, and outbreaks were then possible by the original vector.
Another vector (cudicoides obsolescence and cudicoides cudicaris) has become competent at spreading the infection.

What has happened due to the climate?

Temperature is the main one.
If you have warmer, hotter periods in the autumn and summer, this increases transmission potential.
There's a particular danger in warmer winters and warmer nights, because this would have been where viruses would have failed to persist in the past.
Increased temperature means the extrinsic incubation period may have shrunk, and the lifespan may have increased maybe on of those or both.

Rainfall

Governs size and persistence of breeding sites for these vectors.
They require wet breeding sites, so semi aquatic and precipitation.
They need moisture to have a suitable habitat to live and survive.
The wet organic matter is good for them, but flooding wouldn't be good for them.

What has happened due to the climate's impact on vector spread

Northern range of Cudicoides amicula in Southern Europe correlated with annual mean year round warmth of 12 to 20.
So warmer average temperatures increasing the range of the vector species.
Some of the new outbreaks were possible becauseThe minimum temperatures have increased.

Potential changes at other levels

Vector Species

Transoverial transmission, the pathogen remaining in the vector species and being transmitted to its offspring.
Overwinter.
You may have adult midges that previously wouldn't have survived the winter, now survive in the winter because that minimum temperature is increased.

The Host

Transplacental transmission within the host.

Summary

Vector borne disease have complex dynamics and patterns, and this is important, particularly around control.
Climate plays an important role part in vector borne disease dynamics.
There are multiple stages at which it can impact: vector survival, vector reproduction, then extrinsic incubation period, and so on.
Climate change has changed the characteristics of infectious disease presence and transmission.
There's a potential for that kind of tipping point of vector survival and extrinsic incubation periods, kind of parasite survival, to impact upon the transmission of vector borne infection further.
GM mosquitoes can be fertile; they block reproduction because they're competing with healthy males and stopping females being successfully reproduced.