Week 4 ~ Diseases Jumping Species

Diseases that can jump from one species to another is a pretty freaky topic to think about, especially when the disease is transmitted from an animal to a human (this is known as zoonoses). According to an article by Robert Roy Britt (2011), there are more than 3,000 zoonoses we can catch directly through touch and more than 4,000 from bites. It's weird to think that these tiny, "invisible" agents can do so much damage to living things. "The cross-species infection can originate on farms or markets, where conditions foster mixing of pathogens, giving them opportunities to swap genes and gear up to kill previously foreign hosts (i.e. you)" (Britt, 2011). It's a bit scary but also very fascinating how bacteria and viruses deadly to one species can evolve to infect a completely different species. When these types of diseases start causing outbreaks, things can get pretty difficult and hectic (especially when there is no vaccine for the disease).

One major example of this type of disease is the group of influenza A viruses, which are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). The CDC's website (2012) says there are 17 different hemagglutinin subtypes and 10 different neuraminidase subtypes. Interestingly, all of these subtypes but one have been found among birds; H17N10 has only been found in bats. I also found it intriguing that most influenza viruses either cause asymptomatic or mild infection in birds, but when they transfer over to humans they can be much harsher.

I was also reading on the CDC website about pigs and influenza because I was curious how pigs, birds, and humans can all be connected when it comes to certain influenza viruses. Pigs are an interesting species because they can be infected with human and avian influenza viruses as well as their own (swine influenza viruses), and it's possible for them to be infected with two different species' influenza viruses at the same time. In this sort of situation, the creation of a new virus is possible by the mixing of genes among the two existing viruses. 

This could possibly lead to a change in the influenza A viruses known as antigenic shift: "if a pig were infected with a human influenza virus and an avian influenza virus at the same time, the viruses could mix (reassort) and produce a new virus that had most of the genes from the human virus, but a hemagglutinin and/or neuraminidase from the avian virus. The resulting new virus would likely be able to infect humans and spread from person to preson, but it would have surface proteins (hemagglutinin and/or neuraminidase) not previously seen in influenza viruses that infect humans" (CDC, 2012).

As seen from the above example, antigenic shift occurs when a new influenza A subtype infects humans. Due to the new surface proteins not previously seen in human flu viruses, people would have little to no immune protection against this new virus. This foreign virus could have the potential to cause illness and also be easily transmitted from person to person. If this were the case, then an influenza pandemic could easily occur. Fortunately, influenza infections transmitted directly from animals to humans is unusual, but of course it is still possible. Being aware of the possibilities and knowledgeable about these viruses is essential, even if the chances are low.

Life cycle of the parasitic microbe T. gondii

While doing some reading on zoonoses, I came across a few articles about a parasitic microbe that is commonly found in cats and can be transmitted to humans (Toxoplasma gondii). I have heard that cat litter is unsafe for pregnant women but never knew exactly why, so I was curious to learn more about this parasitic microbe. According to Ker Than's article "Study: Cat Parasite Affects Human Culture" (2006), infection by T. gondii could cause some people to become more prone to some forms of neuroticism, which could lead to differences among cultures if enough people were to become infected. This article also stated a finding by a U.S. Geological Survey scientist, Kevin Lafferty, that people living in countries with higher rates of T. gondii infection scored higher on average for neuroticism. I found this information about T. gondii very interesting because psychology is fascinating to me, especially mental disorders and possible causes.

T. gondii tissue cyst in mouse brain

This parasite is excreted by cats in their feces and causes toxoplasmosis. Pregnant women who become infected can end up transmitting the disease to the fetus, and in some cases severe brain damage or even death can occur to the baby. Another article I found also discusses the possibility of T. gondii affecting the human brain: it "may be changing connections between our neurons, altering how we act and feel" (Mcauliffe, 2012). In the early 1990s, a Czech scientist, Jaroslav Flegr, began to suspect that a single-celled parasite had invaded his brain and was subtly manipulating his personality. Flegr discovered in 1990 that he had T. gondii, and from then on he started researching more and more about the parasite. He learned that it can only sexually reproduce inside cats, so once the parasite is inside another animal or human host it starts working to return back to a cat. Flegr believes T. gondii may be able to manipulate behavior in other species in order for it to make its way back to the body of a cat so it can reproduce.

Reading about this parasitic microbe got me pretty interested in the possible affects it can have on the human brain. I'm glad it can't reproduce in species other than cats because who knows what it could do to our population if the neuroticism theory is true. Learning about new disease-causing organisms really gets me thinking about how many there are out there that we haven't even discovered yet. There could be some that function in ways we can't really even imagine right now, and maybe others could be beneficial to our health in some significant way rather than threatening. There is so much to learn and discover about diseases and their factors. I'm excited to continue growing my knowledge with this subject and encountering new and fascinating information.


References
1. Britt, R. R. (2011). 10 deadly diseases that hopped across species. Live Science. Retrieved from http://www.livescience.com/12951-10-infectious-diseases-ebola-plague-influenza.html 
2. CDC. (2012). Transmission of influenza viruses from animals to people. Retrieved from http://www.cdc.gov/flu/about/viruses/transmission.htm   
3. Mcauliffe, K. (2012). How your cat is making you crazy. The Atlantic. Retrieved from http://www.theatlantic.com/magazine/archive/2012/03/how-your-cat-is-making-you-crazy/308873/
4. Than, K. (2006). Study: cat parasite affects human culture. Live Science. Retrieved from http://www.livescience.com/933-study-cat-parasite-affects-human-culture.html
5. Photo (top): http://www.colourbox.com/preview/4326388-26830-swine-flu.jpg
6. Photo (middle): http://www.intechopen.com/source/html/44112/media/image6.jpeg
7. Photo: (bottom): http://upload.wikimedia.org/wikipedia/commons/3/3c/Toxoplasma_gondii_tissue_cyst
_in_mouse_brain.jpg