2. Epidemiology: some basic concepts and definitions
relationships in the rodent malaria parasite. Plasmodium in naıve and immunized mice, (4) virulence and infection length were positively correlated in immunized but not naıve mice, work was supported by the Leverhulme Trust, the Uni-. The words pathogenic, virulent and transmissible get tossed around a lot Infectivity, pathogenicity, virulence and transmissibility all have their own Here's an RSS question in relation to the transition to this new (cool) blogging site: .. You might try the Trust for America's health (google them) and ask. Of the surrogate markers of infection severity, or virulence, SPVL is the . a phylogeny of infections and estimate the relationship between virus genetic .. by a Wellcome Trust Institutional Strategic Support Award to LSTM.
Are they being exposed but not falling ill? And if that's the case, what is it about their infection that does not lead to disease? Is their body just developing anitbodies faster than the bodies of those who do fall ill? Or is it strictly a matter of viral load, as mentioned by "Name"? Log in to post comments By Dan not verified on 13 Jun permalink Jeez.
I go away for a few hours of meetings lucky me and return to find all these questions. I guess I didn't do such a good job of explaining. So let me try to clarify the good questions in reverse order because that way I don't have to scroll up first: There are a lot of ways a viral particle could not cause disease or even infection.
It might never find a competent cell to infect. The person might have adequate "pre-infection" defenses like copious mucus with dummy receptors in it, good clearance mechanisms, or who knows what. Some of those not sick really have inapparent infection.
Pathogenicity, virulence, transmissibility and all that | ScienceBlogs
Infection really refers to cells. It means the virus has successfully found a competent cell to hijack and make copies of itself. If those copies don't make it to other cells, that's the end of it. To be transmissible, the virus also has to get out of you and into someone else. Some viruses make you sick in ways to facilitate this, for example, making you cough or sneeze or have diarrhea. But it is not the same as infection, which is the ability to replicate in an individual cell.
The distinction you made in your comment does capture this idea. You are correct that there are different degrees of pathogenicity the ability to make someone sick, not just infect them.
Virulence refers to the severity of sickness, not just the fact of sickness. Thus a pathogenic virus can be of varying degrees of virulence, although we don't usually speak of mild diseas as "mild virulence. So you are sort of correct in saying pathogenicity is "on-off" and virulence is a matter of degree. I say "sort of" because what counts as being "sick" can be a gray area. The host-agent-envoironment idea is important. We often talk as I just did of a virulent virus, but in reality virulence isn't a property of the virus but in the combination of the virus, a particular host or host species and the environment both find themselves in.
- Virulence evolution in response to vaccination: The case of malaria
Certain viruses or more commonly bacteria are bad news in some environments and not others e. AIDS patients, for example, are prone to virulent infections from organisms that are harmless to others. It isn't the organism that is virulent but the combination of the organism and the host.
Unfortunately if I use an extended entry format only the above the fold stuff will be shown in your RSS reader.ID50 and LD50
For shorter posts I'll put the whole thing above the fold and you'll see it, but if there is an extended entry Read more. Not much I can do about it, I'm afraid. Hi, by the way. Log in to post comments By revere on 13 Jun permalink Revere, Do you have any whiz-wheel economists at the school or know of any who have done "bird flu" expected battled damage assessments in papers? I have seen a lot of speculative stuff, and I have seen the stuff that DHS has, but its only for a two million mortality rate.
Also remember that projection model that they used a month or two ago showing how it would spread? If you know the site that has both or either pls advise Log in to post comments By M.
Randolph Kruger not verified on 13 Jun permalink One more. He is asking what epidemiologists call an epidemic. This is purely a judgment call. Technically it is just the occurrence in a region or locality of cases beyond what you normally expect. This means it is relative to a time, a place and a population all of which, technically, should be precisely specified but often aren't.
Two cases of a cold aren't an epidemic but two cases of rabies in the same place might be one case is never an epidemic, however. When something is called an epidemic might differ between two health officials and it is a plastic word that is loaded with other meanings so is liable to be manipulated in either direction.
Virulence evolution in response to vaccination: The case of malaria
Epidemics refer to human populations, strictly speaking it comes from Greek roots, meaning "upon the people". The same thing in animals is an epizootic. The technical word for this in birds is an epornithic, but in all the things I've read bout H5N1 in birds I've never seen this word used.
A pandemic crosses many international borders so that it involves large portions of the global population although not necessarily all of it.
Log in to post comments By revere on 13 Jun permalink Randy: I'm sure someone has done it and maybe a reader who follows this can tell us who and where. Log in to post comments By revere on 13 Jun permalink Thanks again, Revere - that did it. His most recent post explores implications of one possible scenario while pointing out that no one knows which scenario will actually come to pass, if any.
Log in to post comments By name not verified on 13 Jun permalink Thank you very much, Revere, for this very helpful post. These distinctions are something I've been wondering about for a while, and it doesn't help that these terms are often used incorrectly or interchangeably. I learn something valuable every day that I read your blog. I think its starting to weight in on people outside of the blog as to the enormity of what could happen.
Thi s request was made as the wife of a VP at a brokerage firm joined my just the facts email list. She read what was being posted and then started printing it off for her husband to read when he got home.
I got the call today for the "economics" question. I only deal with actualities when they happen. On the other hand predictives I am really big into. My ex-commander once told me that if you prepare for the worst case you were just that If something happens you didnt think of, you were underprepared.
If something happens that is bigger than the worst case projected, you are just screwed.
Revere, I say again I wish you would prepare. You are going to be needed to balance things if it comes. Log in to post comments By M.
Randolph Kruger not verified on 13 Jun permalink Dr Gleeson's "moderate" scenario, by the way, is based on a pandemic far less severe thanas are the scenarios being used by many planners. Worst case is downright paralyzing. Log in to post comments By Name not verified on 13 Jun permalink Reveres, Is it my computer settings or are the letters and lines squished together on this blog?
It doesn't appear the same near the top with the wording under "profiles" or "recent posts" I enjoy reading and learning from reading your posts. On this site, I will have to make sure I am very rested and very awake to unscramble the lines. I do not want to misinterpret what I am reading. Log in to post comments By Floridagirl not verified on 13 Jun permalink Floridagirl: What browser are you using?
Older versions of Internet Explorere IE seem to be having a problem with this and other sites, too. You'll get used to it fast and like it a lot more. Yo can import allyour IE bookmarks quickly and be ready to go in a flash.
If you are not using IE, let me know what you are using and I'll try to solve your problem with some help from the hivemind and Sb tech support.
Log in to post comments By revere on 13 Jun permalink I totally agree with the comment about the 'planning scenarios' that planners are using. In my interview last week with Dr. By Healthbizz not verified on 13 Jun permalink Reveres, If you don't mind my asking Why do you choose not to prepare?
Is it because you believe the likelihood is so remote that you do not? I am basing this question on MRK alluding to this. Possibly it was covered earlier in a post I missed? This document may be redistributed and reused, subject to certain conditions. This article has been cited by other articles in PMC.
Abstract One theory of why some pathogens are virulent i. Here we describe our studies in malaria that test and support this idea. We go on to show that host immunity can exacerbate selection for virulence and therefore that vaccines that reduce pathogen replication may select for more virulent pathogens, eroding the benefits of vaccination and putting the unvaccinated at greater risk. We suggest that in disease contexts where wild-type parasites can be transmitted through vaccinated hosts, evolutionary outcomes need to be considered.
Virulence, Evolution, Malaria, Parasite, Trade-off hypothesis 1. An evolutionary hypothesis for pathogen virulence Why are pathogens virulent?
Some evolutionary biologists believe that the answer to this question will make it possible to design vaccines and other control measures that, in the event of eradication being impossible, drive the pathogen towards lower virulence [1,2]. An alternative but not mutually exclusive  idea is that the level of pathogen virulence observed in nature is a well-adapted outcome of both positive and negative selective forces acting on virulence. Under this hypothesis, it is reckoned that to balance the fitness cost to the pathogen of host death, there must also be a virulence-related advantage to the pathogen's fitness.
Of all the explanations for virulence, the trade-off hypothesis has received most attention and a large body of theory has been derived from it.