All the factors that influence the distribution of zoonotic infections impact on each other in a dynamic manner and thus it is almost impossible to predict the exact distribution of zoonotic infections into the future on a global long term scale due to the number of variables involved and the complex interactions between the various factors. There are, however, systems that can provide early warning for local and imminent outbreaks of infection.
Perhaps the greatest factor involved in the distribution of zoonotic infections is human behavior and human influence on the biosystems and ecosystems. Our response and ability to predict and mitigate for the factors listed (including our ability to adapt and develop new technology), as well as change behavior on a global scale, will play a major role in the future of humans, pathogens and the planet we inhabit.
The "disease triangle"
The disease triangle is a model that can be used to demonstrate the interactions between the various factors that can affect the distribution of zoonotic infection. (3)
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There are host factors , pathogen factors and environmental factors and they all interact dynamically with each other on the basis of exposure and sensitivity. (3)
Geo Climatic considerations
Climate and disease distribution
Climate is defined as the prevailing long term pattern (usually 30 years) of weather conditions of a region.
Weather is defined as the short-term state of the atmosphere in a region with regard to temperature, air pressure, rainfall, humidity, winds, cloudiness and storms.
August Hirsch (4) first noted the relationship between distribution of certain tropical disease (dengue, malaria, yellow fever, cholera, plague etc.) and seasonality in the second edition of his book Handbook of Geographical and Historical Pathology. Jacques May (5) also noted the effect of climate and geography on malaria, cholera and other tropical diseases in the 1950s. (6)
Temperature extremes - terrestrial and water, global warming
The effect of temperature on mosquito and other insect vectors
Insect vectors and snails flourish in environments that are warm. They can often not survive in freezing temperatures and therefore are found in the tropics. The infective organism is also often reliant on temperature for replication. For example the both Anopheles species of mosquitoes and plasmodium falciparum can be directly affected by temperature in the following ways:
At low temperatures (<18-20 C) P. falciparum can't complete its maintenance life cycle in the mosquito. At low temperatures sporogony is not completed and transmission can't occur.
The life span of the mosquito is affected by temperature - in desserts and areas of very high temperatures the life span of the mosquito is shortened. The portion of the life cycle that takes place in the mosquito (gametocyte ingestion to sporogony) takes 8 - 30 days depending on the ambient temperature. Therefore in order to transmit malaria the mosquito needs to survive more than a week. At very high temperatures the life span of the mosquito may be shortened.
At higher temperatures the life cycle of plasmodium is completed more quickly
It is believed that the mosquito bite more at higher temperatures.
According to Harrison's Principles of Internal Medicine transmission of malaria is directly proportional to the density of the vector, the square of the number of human bites per day per mosquito, and the tenth power of the probability of the mosquito surviving for one day. Change in global temperature could therefore have an effect on malaria distribution. Some areas may become deserts and act as geographic barriers to the spread of disease while other areas may experience increased rainfall and vegetation becoming areas where the Anopheles mosquito and plasmodium could thrive. However human factors and interventions also play a large role.
In 1996 already Bryan, Foley and Sutherst (7) published an article on climate change and malaria transmission in Australia. They stated that it had been hypothesized that Australia (where malaria was eradicated in 1981) may again become a hot spot for malaria because of excepted climate change and the fact that the vectors are still present (Anopheles farauti sensu strict). They mention that among the climate changes that could be expected are more frequent cyclones and floods, which will increase vector density and the risk of malaria. In fact, we are indeed seeing this type of climatic activity this is Australia today.
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(See excerpt below taken from http://www.grforum.org/pages_new.php/australia-floods/917/1/707/769/ about the recent Cyclone and floods in northeastern Australia)
"Flooded cities and villages, blocked roads, crocodiles - The Northeast ofÂ Australia is struggling with one of the worst flood catastrophes of the last decades.
After Cyclone Tasha has devastated the country, especially the northeast of the country is still affected by immense floods. Entire villages are isolated and need to be supplied by helicopter.
The floods haveÂ flushed many animals into the area. Snakes, crocodiles and mosquito s have become a dreadful blight and dangerous threat to the people." (8)
Another climatic changes mentioned by Bryan, Foley and Sutherst (7) is inundation of low-lying areas, due to raised sea levels. Within the Asia-Pacific Region, many such areas are malarious and refugees from them could provide a large reservoir of infection. Emergency relocation of refugees, particularly if aircraft are used, will increase the possibility of introducing exotic vectors into Australia.
However in their conclusion Bryan, Foley and Sutherst (7) felt that the malaria threat to Australia was over dramatized and that for many reasons malaria is unlikely to become as significant problem in Australia. They give the following reasons:
1. Australian mosquitoes may not be susceptible to imported parasites
2. Many people with P. falciparuim are diagnosed and treated early before the gametocytes can enter the vector
3. Mosquito to human density is low
4. A low proportion of blood meals from humans are taken and
5. Vector longevity is short. (7)
In general eggs hatch earlier, cycles process more rapidly and adults occur earlier in warm tropical environments for most insect vector borne agents e.g. Plasmodium, arboviruses, vector borne rickettsia, borrelia, typanosomia, leshmania.
Temperature affects vertebrate less and they are usually more adaptable. Some vertebrate hosts like the vampire bat (rabies) though can't survive at freezing temperatures.
Temperature and water
The temperature of the ocean water can also affect the distribution of some zoonotic infections. A good example of a rise in ocean water temperature affecting a zoonotic pathogen is Cholera.
In 1991 Vibrio Cholera occurred in South America and spread from Peru to Mexico. There are two prevailing theories for why this occurred:
A ship from the orient contaminated the water in the Peruvian harbor. Local shellfish were thus contaminated and an epidemic focus was established.
A more likely theory is that due to global warming and associated El Niño conditions sea water temperatures increased. This then resulted in expansion of phytoplankton followed by a zooplankton copepod boom. These carry V. Cholerae on their surfaces and in their gut. This resulted in wide spread dissemination of cholera along the coastal region of South America, (9)
Vibrioses are usually halophilic (require salt water) with the exception of Vibrio Cholerae and Vibrio Mimicus. They usually reside in tidal rivers and bays. They proliferate in warmer months when the water temperature in >20Â°C. The vibrioses live in close relationship to plankton which it can survive in non culturable form.
The effects of Global warming on rainfall patterns
It is difficult to predict the exact effect global warming will have on rainfall patterns in the future.
Climate models project that the global average temperature will rise about 1Â°C by the middle of the century, if we continue with business as usual and emit greenhouse gases as we have been.
We are however not certain how this will affect rainfall patterns. Some scientists believe that the topics will experience more rainfall and that there will be more extreme weather conditions worldwide. ScienceDaily (Aug. 29, 2007) (10) states that:
"NASA scientists have detected the first signs that tropical rainfall is on the rise with the longest and most complete data record available." "Using a 27-year-long global record of rainfall assembled by the international scientific community from satellite and ground-based instruments, the scientists found that the rainiest years in the tropics between 1979 and 2005 were mainly since 2001. The rainiest year was 2005, followed by 2004, 1998, 2003 and 2002, respectively. (10)
The effect of rainfall on Zoonotic infections
Increased rainfall in the tropics could significantly affect the distribution and frequency of outbreaks of zoonotic and epizoonotic infections. However other areas may experience drought and this may limit or lead to a decrease of zoonotic infections in these areas as the vectors/ hosts may not be able to survive in such warm and dry areas. Deserts such as the Sahara desert in Africa also act as barriers to the spread of infections.
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There are a number of studies that show a link between certain zoonotic infections and rainfall.
An example of a zoonotic infection that has been shown to have a direct link to rainfall patterns is discussed below.
Rift Valley Fever
Rift Valley fever is an example of a zoonotic infection that can be linked to climate and rainfall in particular. It is caused by Rift Valley Fever Virus, a member of the genus Phlebovirus (family Bunyaviridae) and occurs primarily in livestock and can be transmitted to humans. The vector is the Aedes aegypti mosquito.
Rift Valley Fever Distribution Map
Countries with endemic disease and substantial outbreaks of RVF:
Gambia, Senegal, Mauritania, Namibia, South Africa, Mozambique, Zimbabwe, Zambia, Kenya, Sudan, Egypt, Madagascar, Saudi Arabia, Yemen
Countries known to have some cases, periodic isolation of virus, or serologic evidence of RVF:
Botswana, Angola, Democratic Republic of the Congo, Congo, Gabon, Cameroon, Nigeria, Central African Republic, Chad, Niger, Burkina Faso, Mali, Guinea, Tanzania, Malawi, Uganda, Ethiopia, Somalia
Map taken from the Centre for Disease Control website (11)
The relationship of Rift Valley Fever to climatic factors was documented by Anyamba, A; Linthicum, K.J & Tucker, C.J (12) in 2001. In the article by Anyamba et al. (12)they showed that all known outbreaks of Rift Valley Fever followed abnormally high periods of rainfall. They stated that periods of above normal rainfall in East Africa are associated with the warm phase on ENSO (El Nino Southern Oscillation) phenomenon. The increased rainfall floods dambos (grassland depressions) which are the mosquito breeding grounds. Historical data also showed that more than two thirds of Rift Valley Fever outbreaks occurred during warm ENSO periods. In their conclusion they state that:
"Rift Valley fever is a good example of a disease that is well coupled with climatic anomalies.
The ability to forecast these ENSO events almost a year in advance means that we can in principle anticipate with some level of confidence the areas that are likely to be impacted.
This provides a valuable lead-time to take measures to reduce negative societal impacts of
ENSO on health and economic well-being."
Today Satellites can be used to measure and monitor the greening of vegetation in response to increased levels of rainfall by Remote Sensing Satellite Imagery. This may allow experts to forecast when there is a high risk for RVF (Rift Valley Fever) outbreaks. This information can be used as early warning systems and are essential to enable effective and timely control measures to be implemented.
Rainfall and food supply
Rainfall can also be linked to an increase or decrease of food supply for the reservoirs of three and four-factor complex zoonotic pathogens. This in turn can influence the influence the reservoir population density, which may increase the risk of an outbreak. An example is discussed below. (6)