PDA

View Full Version : Epidemiology?



King Neptune
06-11-2014, 02:28 AM
If blankets used by people with smallpox in a smallpox hospital were given to people who were susceptible to smallpox, then how long before the disease had run through the local population of a few thousand?

This is in regard to infection of American Indians with smallpox in the 18th century. I have some idea of how it might go, but I don't know much about smallpox.

Cath
06-11-2014, 02:45 AM
Difficult question to answer because it's going to depend on how the people involved live and interact and how frequently. Smallpox is airborne, which means it can be spread with face to face contact, e.g. at at market stall. It can be spread by bedding, but that's most likely the trigger, once you have one or two cases in the wild, the transmission is most likely to be airborne.

It is most commonly transmitted in the first week after symptoms appear, but the incubation period is about 12 days - which means there will be a short delay in onward transmission. What that means is that an infected person can travel however far they travel in twelve days and start a new cluster of infection in another community. If your population is spread out or migratory you need to factor that in.

Basically, work out how your population interacts and map out how the disease will spread from there if one or two people get infected from the blankets.

There are different types of smallpox; some more survivable than others. This Wikipedia article (http://en.wikipedia.org/wiki/Smallpox) is helpful in outlining the differences.

King Neptune
06-11-2014, 02:57 AM
Difficult question to answer because it's going to depend on how the people involved live and interact and how frequently. Smallpox is airborne, which means it can be spread with face to face contact, e.g. at at market stall. It can be spread by bedding, but that's most likely the trigger, once you have one or two cases in the wild, the transmission is most likely to be airborne.

It is most commonly transmitted in the first week after symptoms appear, but the incubation period is about 12 days - which means there will be a short delay in onward transmission. What that means is that an infected person can travel however far they travel in twelve days and start a new cluster of infection in another community. If your population is spread out or migratory you need to factor that in.

Basically, work out how your population interacts and map out how the disease will spread from there if one or two people get infected from the blankets.

There are different types of smallpox; some more survivable than others. This Wikipedia article (http://en.wikipedia.org/wiki/Smallpox) is helpful in outlining the differences.

I thank you. Population distribution is the unknown now, but I can't imagine that there would be many cases active in the immediate area twelve months later.

buirechain
06-11-2014, 08:18 PM
The other number you need for your math is the basic reproduction number R0 (R nought), which is the number of people each patient will infect in a completely susceptible population. For small pox that number is 5-7.

But that doesn't mean it will take 33 generations of infection to get from patient zero to everyone in a community of 2000 people having it (and thus about 500 days). The definition I said is for a completely susceptible population, but as more and more people have had the disease, each person will transmit the infection to fewer and fewer people. The epidemic will die out long before every last person has been infected. And that assumes that the community makes no attempt to limit the spread by isolating sick individuals (some communities have various taboos to prevent the spread of disease -- figure that out for the community that you're writing about). And the R0 number assumes a community with a lot of contact, but that's not the case where, as Cath suggested, the community is spread out.

If you're just asking how long before the epidemic finishes itself off, consider some of the numbers from the wiki page history of smallpox: http://en.wikipedia.org/wiki/History_of_smallpox Most of the epidemics listed lasted one or a few years--and that's for very large communities, whole countries or larger where they had time to spread. So in your example it could stop spreading after a few months when a fraction of the population has been infected.

But, there's yet another factor to consider. How resistant is your population to small pox? Have they experienced it before, or have they been isolated (in the 18th century they probably aren't completely unexposed to Small pox). But if they are more susceptible, than the R0 is an underestimate of how quickly the disease would spread.

Maybe you want to consider this http://en.wikipedia.org/wiki/Mandan#Smallpox_epidemic_of_1837.E2.80.9338 that I just randomly found. It sounds a lot like the size and scope of the epidemic that you're interested in; I'd do some more research if I were you to find out the details of how quickly it spread and anything else of interest. There are probably other incidents that you could look up. It's a community of 1600 decimated by smallpox between 1837 and 1838. I'm sure there are other examples you can find, and anything of that sort is going to be more accurate than just running the numbers.

My one reaction though to the idea is that myths and rumors of the use of small pox blankets are for more widespread than their actual use. There is one incident for which we have any evidence for their use as a genocidal tool and another which it was discussed, but we don't know if it was carried out. Both occurred during Pontiac's Rebellion 1763-6. In the former case, we don't know if the attempt to use blankets actually spread the disease, because there was already an epidemic. That isn't to say that it never happened without reaching the historical record, and certainly plenty of nasty things were done, but most of the times when anyone is accused of having distributed smallpox blankets--including anytime outside of warfare--are unsubstantiated. For instance, I learned growing up that Smallpox blankets were handed out during the trail of tears, but later found out that there is no evidence for such.

King Neptune
06-11-2014, 11:13 PM
The other number you need for your math is the basic reproduction number R0 (R nought), which is the number of people each patient will infect in a completely susceptible population. For small pox that number is 5-7.

But that doesn't mean it will take 33 generations of infection to get from patient zero to everyone in a community of 2000 people having it (and thus about 500 days). The definition I said is for a completely susceptible population, but as more and more people have had the disease, each person will transmit the infection to fewer and fewer people. The epidemic will die out long before every last person has been infected. And that assumes that the community makes no attempt to limit the spread by isolating sick individuals (some communities have various taboos to prevent the spread of disease -- figure that out for the community that you're writing about). And the R0 number assumes a community with a lot of contact, but that's not the case where, as Cath suggested, the community is spread out.

If you're just asking how long before the epidemic finishes itself off, consider some of the numbers from the wiki page history of smallpox: http://en.wikipedia.org/wiki/History_of_smallpox Most of the epidemics listed lasted one or a few years--and that's for very large communities, whole countries or larger where they had time to spread. So in your example it could stop spreading after a few months when a fraction of the population has been infected.

But, there's yet another factor to consider. How resistant is your population to small pox? Have they experienced it before, or have they been isolated (in the 18th century they probably aren't completely unexposed to Small pox). But if they are more susceptible, than the R0 is an underestimate of how quickly the disease would spread.

Maybe you want to consider this http://en.wikipedia.org/wiki/Mandan#Smallpox_epidemic_of_1837.E2.80.9338 that I just randomly found. It sounds a lot like the size and scope of the epidemic that you're interested in; I'd do some more research if I were you to find out the details of how quickly it spread and anything else of interest. There are probably other incidents that you could look up. It's a community of 1600 decimated by smallpox between 1837 and 1838. I'm sure there are other examples you can find, and anything of that sort is going to be more accurate than just running the numbers.

Thanks, I reread what I was working from, and realized that my concept of the timing was wrong. When I realized the correct timing, everything made sense.


My one reaction though to the idea is that myths and rumors of the use of small pox blankets are for more widespread than their actual use. There is one incident for which we have any evidence for their use as a genocidal tool and another which it was discussed, but we don't know if it was carried out. Both occurred during Pontiac's Rebellion 1763-6. In the former case, we don't know if the attempt to use blankets actually spread the disease, because there was already an epidemic. That isn't to say that it never happened without reaching the historical record, and certainly plenty of nasty things were done, but most of the times when anyone is accused of having distributed smallpox blankets--including anytime outside of warfare--are unsubstantiated. For instance, I learned growing up that Smallpox blankets were handed out during the trail of tears, but later found out that there is no evidence for such.

Actually, I was looking into the 1763 job, and ended up realizing that the blankets were distributed before General Amherst authorized it, but it appears that the blankets had no effect. After having read the details it became clear that most of what I had known before was inaccurate. There wasn't an epidemic among the Indians, but there were scattered cases; that was the situation both before and after the blankets were given.

That was the only completely substantiated case that I know of, and some of the documentation appears to be by the man who did the job, Captain Trent whose journals have been published. If he had known anything about contagious diseases, then he would have been dangerous.
http://www.jstor.org/discover/10.2307/27774278?uid=3739696&uid=2&uid=4&uid=3739256&sid=21103841656611

(http://www.jstor.org/discover/10.2307/27774278?uid=3739696&uid=2&uid=4&uid=3739256&sid=21103841656611)
If you don't have access to JSTOR, then this page has most of the information, but some things are in the wrong order.
http://www.nativeweb.org/pages/legal/amherst/lord_jeff.html

buirechain
06-12-2014, 06:07 PM
That was the only completely substantiated case that I know of, and some of the documentation appears to be by the man who did the job, Captain Trent whose journals have been published. If he had known anything about contagious diseases, then he would have been dangerous.

If he had known anything about contagious diseases, he probably would have backed away from his plan. Germ warfare has always been a dicey proposition and it's incredibly hard to attack your enemy with while keeping your own people safe. Smallpox works in this situation a little better than in others because the British had immune systems that were a bit better prepared, but the difference was nowhere near as big as in 1492. And the British were still susceptible to small pox.

Of course, the other problem is that back in the 18th century in order to spread small pox (or anything else) you have to have at least one existing patient to spread it from, and so the impact isn't going to be as big because the enemy could just as easily catch the disease naturally. There was a reason that George Washington forced his troops to be variolated (a pre-vaccine method of inducing immunity by giving scratching small pox into the skin--it was not without risk), and it had nothing to do with germ warfare.