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Team:HZAU-China/Safety/the Safety of Flea
From 2013.igem.org
--Further works
Fleas are main vectors for the spread of plague, endemic typhus and other diseases. In our project, we had carefully considered the further works if we would use fleas. How will fleas influence the environment? Which flea species should we choose? Is it worth taking the risks of plague and other diseases to fight against rabies with fleas?
Plague and people
When flea-borne diseases are mentioned, the first one people think is plague. Also, plague is under strict observation as the most dangerous disease group in China. At the beginning of 20th century, Pavlovsky proposed a concept: when a disease in natural world does not depend on the existence or activity of human, it iscalled natural foci disease. Only when human occasionally involved in the disease cycle in nature, there may be an infection. Plague is the natural foci disease. The plague pathogen Yersiniapestisis obligatory parasitism[1] that cannot survive and transmit distantly along without hosts.(Fleas do well in jumping but are bad at moving，So fleas also transmit by their hosts and that’s why there will be a lot of mice before the pandemic). So the range of activity ofhosts and vectors is the natural geographical barriers of plague.
The pictures below are the map of plague focustypes in China and the Chinese lisarithmic map. Comparing two pictures, we will find that most of natural epidemic foci are far away from densely populated areas. Moreover, the distribution of thestray dogs is just reverse. So if we focus on deratizationand closely monitor the natural epidemic foci, there is remote possibility that the clean fleas we released will be infected by Yersiniapestis.(Yersiniapestis cannot vertically transmit in fleas, which means that the fleas will clean the Yersiniapestisin case of no external input source by reproduction. In fact, many researchers catch wild fleas from where never been polluted by plague and breed them for clean fleas.)
Map of plague focus types in China
Chinese lisarithmic map
The hosts, fleas and people
Here is a diagram to show the way of plague transmission. The following tips is the supplementary to the picture.
1. Hosts
In China, all identified main hosts in natural foci are rodents. Carnivores are infected by preying the infected rodents. Studies have shown that in the domesticated animals, dogs are notof high sensitivity to plague[1].
Flea mouthparts can only pierce the epidermis. And it iscalled bacteremia. Dogs are not sensitive enough to get bacteremia.
2. Hosts directly infect humans
The circumstances that hosts directly infect humans mainly occur in hunting or skinning wildlife, especially Marmotahimalayana[2]. As long as you don’t directly contact with wild animals in the natural epidemic foci, there will be no risk to be directly infected the plague.
3. Fleas
Yersiniapestis cannot vertically transmit in fleas[3]. To fight against plague, we should pay attention to the main hosts. History shows that deratizationcan successfully eliminate the natural epidemic foci in China[4].
Not all species of flea are vectors of plague. There are only a dozen kinds of fleas thatare main vectors in China[5]. We found that the cat fleas(Ctenocephalidesfelis) don’thave a bad record in plague history, so we choose it as our candidate. Of cause, it should be tested by kinds of experiments before use.
Only considering the main vector is not complete. There may not be only one species of flea in an area. So, we should take other flea species into consideration. The fleason the main transmission pathway of plague must occupy a certain position in the communityand then they can play the role as the main vector. It’s called the mass effect[6]. So, if we input the cat fleas in an area, they may replace the dominant position of the main vector and lower the risk of plague spread.
Growth and reproduction of fleas are significantly affected by the environment. The dust is the most important environmental factor for the larval growth. Covered with a solid ground, a house of good sanitary conditions is not suitable for the survival of larval fleas. So, we nearly never see fleas in house unless you have pets wandering in cities. If you have pets, keeping your pets clean can prevent fleas into your room.
Current situation
Fleas can spread many diseases, but the most significant diseases are two: plague and typhus. We got the patients and deaths data of these diseases during 2008-2012 from Chinese Center for Disease Control and Preventionand charted as follow.
As we can see, plague and typhus have been in very good control in China. But the death of rabies stays at a high level. So rabies prevention and control are extremely important missions in China. This is the reason why we try more ways to solve this issue.
Reference
[1][2][3][4][5][6]Dongzheng Yu. Plague,Epizootiology[M]. Science Press, 2009:31，31,35,62-65,36-37,55-56(In Chinese)
