History:
We will continue examining history with
Chapter one in your required text of readings. This chapter will introduce you to parasites
important because of the diseases they cause on a global scale. Some of them you have already met,
such as the species that causes malaria.
Please answer questions 1-5 after
reading chapter one.
Parasites, Tales of Humanity's Most Unwelcomed Guests, Rosemary Drisdelle, 2010, University of California Press----History Online Resources QL757 .D75 2010 ebook----available as a digital download in our library. I cannot distribute this resource, but you each can download your own copy from the library.
Are the stories you have been reading true?
According to most historians, they fit the evidence better than other causes for the events recounted. It is know for example that Jericho was a city in serious decline at the time of the proposed battle. It is also known when sleeping sickness became epidemic in parts of Africa. At this point in time being sent to serve for any reason in that area was considered a sentence of death for Europeans. We also know how many American soldiers died from malaria during the Vietnam conflict.
The author indicates when she
is speculating, as with Captain Cook’s change in behavior as he gets older. I would even be a bit
surprised to find that Captain Cook’s intestinal blockage was due to another
worm. Ascaris lumbricoides has been the species tied to humans, well,
since there were humans. It
could be due to tapeworms, but tapeworms (Platyhelminthes) are better tolerated
than nematodes as Ascaris. It is differences such as these among
major groups in virulence and other factors that we will explore as we go
through the major groups of parasites. In any case worms probably are a likely
cause of any blockage, since Captain Cook's problems were probably not due to some benign or
cancerous tumor. Given
the dangers of what passed for surgery in those days, Captain Cook would not
have survived the surgery for that large a blockage. His improvement probably attests to a
long period between re-infestations and the worms themselves just getting old.
Finally note that the author
already at the end accepts the premise that intelligence is correlated
negatively with worm presence. It was stated in assignment one that this fairly unsubstantiated hypothesis would
find its way into popular as well as medical research publications. I am not saying that the proponents of
this theory are wrong. They may
have discovered one of the major underlying factors responsible for differences
in intelligence scores across nations. It is just that the suggestion is based
to date on little evidence. Also other alternative hypotheses (such as developing brains of the poor often have to cope on substandard diets) have not been ruled out.
Evolutionary ecology:
Focus on communities
Today, modern conservation
efforts are interested in the effects of parasites on communities whether they
contain humans or not. For
example, conservation itself often involves confining select species to a small
area where they are protected. Are we erecting new cities of Jericho? As human communities become larger, we
as a species are coming into closer contact with wildlife and their parasites. You have already learned about the dangers
of getting too cozy with raccoons, or at least their latrines. A major area of research in
parasitology today then is the effect parasites have on wild communities. In the face of global warning,
scientists want to examine how changes in climatic and other factors change parasitic
loads, effects on hosts, and other species in the community.
Before we start this
discussion, we must consider how to estimate
parasite load. This is difficult to do in a way that allows us to compare populations with regard to parasite load. This is
because not all individuals are infected and the numbers of parasites that may
be found per individual vary greatly.
Frogs have been the
traditional animals to sacrifice in high school and college classrooms not only
for an examination of organs but parasites. They have also been a model species for
naturalists interested in parasite loads. So we have a lot of information on parasite distribution in frogs and
toads from all over the world. Below
is a table from a paper that attempts to determine parasitic load in some species
of frogs. The terms used in
this paper are terms you will see today in ecological papers on parasite loads
carried by various species.
Examine the table below and then
answer question 6 on your assignment sheet.
Table one. Prevalence and
intensities of Haematoloechus spp. in
infected frogs.
Prevalence =percentage of
infected frogs. Intensity
=number of lung flukes per infected frog. Mean intensity = arithmetic mean or mean of lung flukes in infected individuals.
Frog
species
|
Fluke
species
|
No.
examined
|
No.
infected
|
Prevalence
|
Mean
intensity
|
Highest
intensity
|
Rana clamitans
|
H. longiplexus
H. complexus
|
95
|
40
|
42
|
8.9
|
71
|
Rana catesbeiana
|
H. breviplexus
H. longiplexus
H. complexus
|
63
|
24
|
38
|
6.3
|
56
|
Rana utricularia
|
H. complexus
|
13
|
7
|
54
|
13.6
|
33
|
Adapted from C. Whitehouse. 2002. Pro.
Ind. Acad. of Sci. 1:67-76.
Ecologists have always found
fault with terms such as prevalence and intensity. Although used in almost all publications
on parasite loads in population, they feel that they often do not give a true
picture of the load carried by a population. In question 7, you will be dealing with an example that
shows how individuals can vary in the parasites they carry and skew means, etc.
Answer question 7.
Community effects
As with any community study,
several factors may affect the parasites effect on the host populations. For example, read this
report that deals with the effects of multiple parasite infections in frogs.
https://Coloradoan/today/2012/05/21/richer-parasite-diversity-leads-healthier-frogs-says-new-cu-study
Answer question 8.
The paper on this website remains one of the most readable that attempts to document what is known about the effects of parasites in general on communities. We obviously have a lot to learn, but the effects documented to date are both complex and dramatic.
Answer question 9
using examples from this paper
Evolution of parasitic relationships-- coming next week