Micro and Macro Parasites

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來自 The Pennsylvania State University 的課程

流行病：传染病动力学

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The Pennsylvania State University

流行病：传染病动力学

619 個評分

Not so long ago, it was almost guaranteed that you would die of an infectious disease. In fact, had you been born just 150 years ago, your chances of dying of an infectious disease before you've reached the tender age of 5 would have been extremely high. Since then, science has come a long way in understanding infectious diseases - what they are, how they spread, and how they can be prevented. But diseases like HIV/AIDS, Malaria, Tuberculosis, or the flu are still major killers worldwide, and novel emerging diseases are a constant threat to public health. In addition, the bugs are evolving. Antibiotics, our most potent weapon against bacterial infections, are losing their power because the bacteria are becoming resistant. In this course, we'll explore the major themes of infectious diseases dynamics. After we’ve covered the basics, we'll be looking at the dynamics of the flu, and why we're worried about flu pandemics. We'll be looking at the dynamics of childhood diseases such as measles and whooping cough, which were once considered almost eradicated, but are now making a comeback. We'll explore Malaria, and use it as a case study of the evolution of drug resistance. We'll even be looking at social networks - how diseases can spread from you to your friends to your friends' friends, and so on. And of course we’ll be talking about vaccination too. We’ll also be talking about how mobile phones, social media and crowdsourcing are revolutionizing disease surveillance, giving rise to a new field of digital epidemiology. And yes, we will be talking about Zombies - not human zombies, but zombie ants whose brains are hijacked by an infectious fungus. We're looking forward to having you join us for an exciting course!

從本節課中

Pathogens

In the first module, you will learn about the organisms that can infect us and make us sick: pathogens. We will first take a look at the various types of pathogens that exist, and then learn about what it means to be infectious, and what it means to cause disease. After that, we will cover the various methods by which pathogens can transmit from one person to the next, thereby causing an epidemic. You will learn about the most important number in all of epidemiology, R0, and why this number is so important. Finally, we will learn that microorganisms are not always harmful, but can be beneficial to us as well.

What is an Epidemic?6:00

Micro and Macro Parasites5:41

Infection and Disease6:27

Transmission Types4:30

與講師見面

Dr. Ottar N. Bjornstad
Professor of Entomology and Biology
Department of Biology
Dr. Rachel A. Smith
Associate Professor
Department of Communication Arts & Sciences and Human Development & Family Studies
Dr. Mary L. Poss
Professor of Biology
Department of Biology
Dr. David P. Hughes
Assistant Professor of Entomology and Biology
Department of Biology
Dr. Peter Hudson
Willaman Professor of Biology
Department of Biology
Dr. Matthew Ferrari
Assistant Professor of Biology
Department of Biology
Dr. Andrew Read
Alumni Professor in the Biological Sciences, and Professor of Entomology
Center for Infectious Disease Dynamics
Dr. Marcel Salathé
Assistant Professor of Biology
Department of Biology

Parasites ubiquitous.

The majority of living organisms are parasitic in one form or

another and they're the predominant mode of life on the planet.

Since almost all living organisms are parasites, even parasites have parasites.

This was nicely caught, in this little ditty.

Great fleas have little fleas upon their back

to bite 'em, and little fleas have lesser fleas,

and so ad infinitum.

>> There are two ways to think about the types of parasites we encounter.

The first is a taxonomic classification that divides all the pathogens and

parasites into taxonomic groups, depending on

the relationship with you other organisms.

These groups include viruses, bacteria, helminths, fleas, ticks, etcetera.

The categories are based on how related they are by common descent and not by how

they appear.

So, bats and birds fly, but they are in different

groups because one is a mammal and the other's a bird.

>> The second is to make a

functional division that reflects fundamental biological features.

And in this case, we simply divide them into two broad

groups known as the micro parasites, and the macro parasites respectively.

>> As the name implies, micro is small and macro is larger.

But more that

just being small versus large, the different

categories have very different ways of life.

These two groups have very different biological characteristics.

Which means their epidemiology is very different.

The microparasites replicate inside the host, whereas the macroparasites

have a special transmission state, outside of the hosts.

And with macro parasites, every parasite has entered the host body in you,

and there is no replication within the host.

As such, the impact of macro-parasites is really a function

of the number of parasites that are within the host.

With just a few, the host may be able to tolerate the infection.

But the more you have, the sicker you feel.

And if you have many parasites the likelihood is you're going to die.

One consequence of this, is that hosts often have a moderate

level of infection.

So the parasites reduce condition and tend

to cause more morbidity rather than mortality.

Now that word morbidity, is a wonderful old English word,

that means the unhealthy symptoms of lethargy associated with disease.

And it is this, that is characteristic of macroparasite infections.

This morbidity means the impact, the parasites have on the host,

tend to be sublethal.

So the parasites reduce the competitive ability of the host.

Reduce their ability to rear their offspring.

And even though the infection is not in their young, it

can result in the young not being healthy, or even dying.

The macroparasites tend to incite a complex immune response, and since the

moderate to lower burdens are tolerated, the infections can be persistent and

last for most of your life.

In contrast, microparasites give you a very short-term infection that leads

to a crisis from which you either die or recover from.

Indeed there is one interesting study on a parasitic tapeworm of

a rat, where the tapeworm has survived longer than its host.

And the researchers have taken the tapeworm and moved

it from one rat to another rat to another rat.

And successfully transplanted at least five times.

And the tapeworm has continued to live while of course the hosts have died.

>> A useful example of a microparasite infection would be the cold virus that

reproduces inside your body by turning your

cells into viral factories releasing more virus.

Since we mentioned viruses, let's begin with these.

These are very small, and many hundreds of times smaller than

the human cells they infect. They only develop within whole cells.

Many human diseases such as the common cold,

HIV, measles, and herpes are due to viruses.

But there are also parasites of lots of other types

of life such as bacteria, fungi, plants and non-human animals.

Even smaller than virus are prions, which are

proteins that are folded in an abnormal way which

allows them to become infectious.

Many prions are not parasitic, but some are.

And the name derives from these, proteinaceous infectious.

They can cause devastating diseases such as scrapie in sheep, mad cow

disease, and the debilitating diseases of

humans called CreutzfeldtâJakob disease, and Kuru.

The last one, Kuru, which translates as a laughing disease, in a

Papua New Guinea language, is spread as people eat the brains of others.

Best avoided.

Prions are definitely are not alive, but interestingly, they can ev, evolve.

And our genome carries a signature of

co-evolution as we evolve defenses against them.

Next in our list are bacteria.

Bacteria are typically much larger than virus.

Although there is an overlap, with the largest

virus being larger than the smallest bacteria.

Many bacteria are highly beneficial to humans, and others live

on us in a commensal fashion, neither harming nor helping.

Fungi are also microorganisms because they can occur in a single cell stage.

Although when you see or eat a mushroom you would

be forgiven for not thinking it was a micro anything.

We humans daily inhale millions of fungal

spores and our immune system is constantly battling them.

For AIDS patients whose immune system breaks down, they're often killed

by fungal disease that would be benign in a healthy adult.

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