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Western University
Biology 3332A
Ron Podesta

Biology 3332A Midterm Study Notes (Sections I, II, & III) Section I - Introduction to Parasitology Introduction - Parasites are very hard to study since they cannot be incubated in petri dishes and are very difficult to isolate - Since we cannot infect people with parasites and study them, we must use animal models, however we face the problem of parasites without animal models, aka human exclusive parasites - Furthermore, very little money is spent on 3 world, tropical diseases (majority of which are associated with some parasite) when compared to diseases in the 1 world st (cancer, heart conditions etc) - However, it is noteworthy that as the effects of global warming are becoming more severe every year, these tropical parasites are finding their way up north, to our part of the world - Experiments with frogs have shown that approximately 90% of frog species at certain elevations have gone extinct, due to Chytrid fungus (which infects frogs) increasing and killing more frog species - Furthermore, malaria and filarial parasites (and their vectors/intermediate hosts) have moved north from Africa into Italy and Spain, due to global warming - Parasites fall under the umbrella of symbiosis, since they are not free-living organisms. They have 3 basic relationships - Mutualistic - (+)(+) - a mutualistic relationship is when 2 organisms interact, and both organisms receive some sort of benefit to their fitness (i.e bacteria that live in our gut) - Commensalistic - (+)(/) - a commensalistic relationship is when 2 organism interact, and 1 organism’s fitness is increased without doing any harm to the other organism - Parasitic - (+)(-) - a parasitic relationship is when 2 organism interact, and 1 organism’s fitness is increased and the other organism’s fitness (or life) is harmed in a negative way - Hosts and their parasites can evolve together, one evolves mechanisms to deter the parasite, while the other develops mechanism to deter the host’s defenses - A general trend in parasitology is that over time, parasitic relationships can become commensalistic or even mutualistic. Parasites don’t really want to kill their host, for if they did, the hosts would all die, and bring the parasites down with them. - However this relationship is not always true, as we will see in an experiment done with 3 Schistosoma species: - S. mansoni - Oldest and least harmful to humans - S. hematobium - Younger and more harmful than S. mansoni to humans - S. japonicum - Youngest and most harmful of all schistosoma parasites - An experiment on Daphnia (crustacean, and an intermediate host that carries the larval form of the parasite) was conducted to determine if the general trend is true. - Researchers selected 2 geologically isolated ponds with Daphnia present. They then added one strain of schistosoma in one pond and another strain in the other pond. Biology 3332A Midterm Study Notes (Sections I, II, & III) - Once the hosts and parasites had been mixed, they expected to see an increase in pathology for the 2 different ponds. However, this was not the case, as both bonds showed a pathology of 20%. - This experiment showed that the trend of parasitism decreasing over age is not true for all parasites - In the past, parasites used to be identified and classified based on their morphological features - However new advances in genomics and cladistics have allowed for better parasite classification and taxonomy - Furthermore, in the past, the idea of one host, one parasite species was widely accepted, until Podesta did an experiment - He found oocysts from fecal samples of pikas infected with coccidian parasites - In these oocysts, he found some that contained 4 sporocysts (98% of oocysts), while other oocysts contained none, 1, 2, 3, 5, and 6 sporocysts. - This showed that there was natural variation within a single species population, which was not widely accepted. Why exactly, I’m not sure Host Specificity & Evolution - Experiments have shown that parasites are very “picky” when they infect hosts. Even similar organisms will not be infected by the same parasite - An experiment was conducted with the flatworm parasites H. diminuta and S. Mansoni to determine their host specificity. - The results showed that H. diminuta would establish itself in rats but not in mice, and S. mansoni would establish itself in mice but not in rats - Furthermore, there is also host specificity in intermediate hosts. Trematodes (flat worm parasites) will only infect one species of snail as an intermediate host. - This specificity can be a result of ecological factors or other factors such as small changes in protein structure which prevent monkey parasites from infecting human blood, even though we share a considerable amount of our genome with them. - So how is the host specificity relationship/data important? We can use parasites and archaeoparasitology to determine where humans crossed from Africa/Europe/Asia to the Americas - Researchers used host-specific (human) intestinal parasites (hookworms, whipworms and pinworms) found in archaeological sites in north and south America, which could not survive the cold temperatures of the Bering strait (when it froze over), suggesting that that was not the route taken by migrating people - The study showed that whipworms and hookworms were found in both old world tropical archaeological sites and new world tropical archaeological sites, and since these worms cannot survive in the cold (fecal/oral contamination, no intermediate host, thus cannot survive frigid wether), humans must have brought them over by either a sub-arctic route or a transpacific migration - In terms of evolution, is it believed that parasites with invertebrate intermediate hosts initially evolved as parasites of invertebrates, which were then transmitted to vertebrates. Biology 3332A Midterm Study Notes (Sections I, II, & III) Models of Parasite Populations - Some studies have suggested that parasites can control or affect host populations - These studies have focused on ectoparasites, such as ticks, fleas and lice and reduced bird clutch sizes (# of bird eggs/offspring) - Other examples include many domesticated animals. - Coccidia in chickens - if left untreated, can wipe out entire chicken populations - Lungworms in sheep - Trypanosoma brucei - cattle of sub-sahara Africa - Hookworms parasites and wild red grouse populations. Studies have shown that red grouse populations crash every 3 years. The crashes in their population is correlated with a high prevalence and intensity of the parasitic hookworms that affect them. - There is also some speculation as to parasites influencing human populations and population growth rates. Most human parasites are concentrated in developing countries, such as Africa, where populations continue to rapidly expand, despite the abundance of parasites. - But if we don’t see an increase in parasite mortality (especially in young and old individuals) can we conclude that these parasites are limiting population increases? Were not sure yet. II - Parasites and Host Behavior Exploitation of Natural Host Behavior - Many parasites have developed an excellent ability of exploiting the natural behavior of their final or intermediate hosts to ensure and/or increase their transmission. Most parasites exploit the feeding and/or foraging behaviors of their host to increase the chance of infection. Some examples of parasite host exploitation include: - Diphyllobothrium (parasite of fish-eating birds) - This parasite exploits predation to increase its transmission. It’s eggs are laid in water (from feces of bird) where they become free-swimming coracidium. The coracidium are then ingested by a tiny copepod which is eaten by fish, then these fish are predated on by birds or humans. - Entobdella - This parasite uses temporal differences to increase its transmission. It lays its eggs in sand close to its host. It uses its biological clock to determine when the eggs should hatch, depending on the species of parasite and the host species. Eggs that hatch are night (E. hippoglossus) are consumed by halibut (their appropriate feeding time) and eggs that hatch during the day (E. soleae) infect sole. - Dracunculus - Consumption of infected copepods release larvae into the intestine of humans, where they then travel to the connective tissue and become adults. The adults then use toxic substances to cause an ulcer on a limb (usually the leg) and whenever the host (human) gets water (goes swimming or something) onto the wound, the parasite expells eggs into the water. Example of parasite exploiting the human behavior of washing and/or Biology 3332A Midterm Study Notes (Sections I, II, & III) entering bodies of water. Drugs designed to rid this parasite are available to 3 rd world countries for free, however research has shown that in recent years where the parasite has not been eradicated, it has developed resistance to the drug (ivermectin) - Since parasites have developed to exploit host behavior, hosts have also evolved behavioral responses to avoiding infections and reducing parasite loads - Avoiding infections - Selective Foraging by Cattle - A study looked at cattle pastures and lungworm levels (remember that lungworms can be fatal to livestock). Random pastures showed very high levels of parasitic larvae, but pastures that had recently been grazed on by cattle showed 1/4 the levels of parasitic larvae when compared to the random sample. This shows that cattle grazed on “clean” pastures to decrease the rate of transmission - Nest Use by Great Tits - Nests are a breeding ground for ectoparasites like fleas and ticks. 3 experimental studies were carried out with very similar results. Birds preferred the nests without parasite (new or old nest boxes, no preference) and would not breed if their nests were parasite infected - Clean nest box vs old parasite free nest - birds had no preference - Parasite free vs parasite infested - strong preference for parasite free - Parasite infested vs no nest box - birds did not use nests - Mate Choice - sex is very risky since it can pass ectoparasites and micro- parasites, but is a necessary behavior. Some of the benefits of parasite- mediated mate choice include reduced exposure to directly transmitted parasites, a reduced chance of passing parasite to offspring and selecting for parasite resistance genes. Many species undergo this process, and it involves secondary sexual traits associated with health. Whenever parasites reduce these secondary traits (such as display rates in guppies, and tail length in swallows) females will not mate with these individual males, and will prefer healthier males. - Reducing parasite loads - Self Preening in Chickens - Chickens were debeaked and after 30days researchers counted the numbers of lice in their feathers. Debeaked chickens had significantly higher levels of lice than the control group, which had beaks and could effectively remove parasites and their vectors. - Reciprocal Grooming - Unpaired Macaroni penguins harbor 2-3 times the number of ticks than paired penguins. Males impalas (which do not perform reciprocal grooming) had 6 times the number of ticks than their female counterparts (which practice reciprocal grooming) - Nest Fumigation - Starlings (nesting birds) weave fresh green material into their nests. Their choice of plants is interesting however, since they choose plants like fleabane and wild carrot, which retard the growth of parasitic louse eggs. Experiments removed these plants and found increased levels of larvae, so these plants are clearly superior fumigants. Biology 3332A Midterm Study Notes (Sections I, II, & III) The Mixed Phenotype Idea - The mixed phenotype idea states that when infections cause changes in host behavior, a parasite’s genes find phenotypic expression in the behavior of the host - In other words the behavior of the parasitized host is therefore a mixed phenotype, influenced by the host and the parasite genome. - So basically if the hosts behavior changes in any way, the parasite’s behavior also changes, so we get a mixed phenotype - meaning that both parasite and host genes are at play. - Parasites are also capable of altering host behavior - Dicrocoelium - More commonly known as the liver flukes of sheep, alters the behavior/habitat selection of its 3 host to increase its infectivity. After the cercariae are secreted as slim balls by snails on blades of grass, they are consumed by ants. Most of the cercariae encyst in the haemocoel of the ant and mature into metacercariae but one moves to the sub-esophageal ganglion (a cluster of nerve cells underneath the esophagus)(ASG). There, the fluke takes control of the ant's actions by manipulating these nerves and the ant’s circadian rhythm. During the day the infected ant is drawn away from other members of the colony and upward to the top of a blade of grass (unlike regular ants which forage during the night). Once there, it clamps its mandibles onto the top of the blade and stays there until dusk. Day after day, the ant goes back to the top of a blade of grass until a sheep comes along and eats the blade, ingesting the ant along with it. It is noteworthy that the metacercariae that enters the ASG can no longer infect the final host, so we must ask is this an altruistic act by the metacercariae? - Toxoplasma - More commonly known as the cat parasite, alters the anti- predator behavior of its 2nd host. Rats (the 2 ndhost) either ingest the oocysts from the soil or from consuming an infected beetle or worm. Once in the rat is infected with the parasite it becomes less cautious of novel stimuli, is more active (which allows it to be more easily seen) and is more likely to be caught in traps. Further experiments showed that uninfected rats avoided areas sprayed with cat urine, but infected rats readily approached the areas with cat urine, significantly increasing their susceptibility to predation by the cat, the parasites final host - So what are the mechanism that change host behavior. There are indirect manipulations and direct manipulations - Indirect Manipulation: - Diplostomum (fish eating birds) - diplostomum invades the eyes and the brain of infected freshwater fish, and this is related to many behavioral changes. Infected fish cannot feed properly and make a higher proportion of failed attacks on prey, expose themselves to predation by foraging at the water’s surface and they spend less time hiding from predators. Furthermore, infected fish suffer from parasitic cataract infections and when the parasite enters the brain in minnows, they form less compact shoals which swim closer to the surface, exposing them to predation. - Direct Manipulation: Biology 3332A Midterm Study Notes (Sections I, II, & III) - Anisakis nematodes in Cod - these parasites encyst in the musculature of their fish intermediate host which will be eaten by marine mammals. Once they encyst in the musculature, they produce waste products from their normal physiological processes (alcohols and ketones) which have an anesthetic affect on fish muscles, impairing their ability to swim, making them more susceptible to predation by water mammals. Directly secret chemicals that alter host behavior - Polymorphus in Gammarids (amphipods) - Is an intestinal parasite of ducks with a gammarus as an intermediate host. The parasite affects the gammarus, and infected gammarids can be discerned by a large orange dot on their abdomen. Uninfected gammarids avoid light and dive when disturbed. However experiments have shown that infected gammarids do not dive and do not avoid light; they skim around the surface and cling to the surface of plants, and act like sitting ducks. Another experiment conducted showed that similar behavior can be induced in non infected gammarids by injecting serotonin near the ganglion, where the parasite in the infected gammarids resides. Thus the parasite is directly affecting the host behavior by releasing specific neurotransmitters which mimic the gammarid’s innate mating practices. Ecological Affects of Parasites - Some have suggested that parasitic mechanism of manipulation and host behavioral changes have a significant affect on the ecology of ecosystems - To test this, Lafferty & Morris did an experiment that examined the behavior of parasitized and clean killifish. The parasite of interest to the killifish was Euhaplorchis califormiensis, a trematode parasite. - They first noted that infected fish showed many conspicuous behaviors such as flashing, jerking and surfacing, all of which would expose them to predation - They then created 2 enclosures, one open to allow predation by birds and the other closed, to prevent predation by birds. In each enclosure they placed the same number of healthy and infected fish, and waited 20 days - They then examined the proportion of infected to non infected fish in each type of enclosure - Their main discovery was that in the open enclosure, infected fish were 40 times more likely to be consumed by birds. This shows that parasites have a huge affect on the ecology of ecosystems. (if 40% of fish were eaten, imagine the alterations to the food web and the alterations in biomass for each species involved - huge complications) III - Biochemistry of Parasites Biochemistry of Parasites - In the past, it was assumed that the small intestine was anaerobic, which had a huge affect the research on parasites done at the time. Biology 3332A Midterm Study Notes (Sections I, II, & III) - Podesta however conducted an experiment in the 70s called perfused intestinal loop studies (PIL) on rats which showed that there was indeed O2 in the lumen of the small intestine. - Of course this pissed off many people, something Podesta clearly does very often - Now that we’ve established that the intestine is indeed aerobic, we now look at parasites and how they produce energy in the environment which they live in (note that most parasites live in the lumen and suck up all our nutrients) - In human tissue, glucose is oxidized to CO2 and H2O, producing 38 ATP molecules per glucose molecule where in parasites, glucose is oxidized to succinate, which produces 4 ATP molecules per glucose. Clearly this is very inefficient, so why do parasites do this? - They do this due to the chemical composition of the intestine. The intestine secretes high quantities of bicarbonate ions to neutralize all the stomach acid that passes through the intestine along with the bolus. This increase in bicarbonate ions produces significant levels of CO2, about 600mmHg compared to the 50mmHg of O2. - So parasites use this high concentration of CO2 in their environment and in their tissues to their advantage. Depending on the chemical environment, they have 2 pathways to produce energy: - Aerobic Conditions (O2 present) - this pathway is active at a more basic pH. When O2 is present, parasites do oxidize glucose to pyruvate and undergo a functional Krebs cycle, also known as aerobic respiration (PK-> Pyruvate-> TCA Cycle) - Anaerobic Conditions (CO2 present) - this pathway is active a more acidic pH. In the absence of O2, carbon from bicarbonate ions (which have diffused into parasitic tissue) undergo carbon fixation to produce a 4 carbon compound (3-> 4 carbons). After oxidation by oxaloacetate, succinate is produced, which can then produce 4 ATP molecules. - So now that we’ve established the 2 pathways in which parasites oxidize glucose (aerobic and anaerobic), what about mitochondria? - Parasites do not have mitochondria, which means they technically are anaerobic organisms. However, parasites (E. histolytica in this case) have an organelle called a Biology 3332A Midterm Study Notes (Sections I, II, & III) hydrogenosome, which are now considered to be degenerate mitochondria. These hydrogenosomes contain the same enzymes found in mitochondria. - Furthermore, some parasites (trypanosoma) have an organelle, which is believe to have evolved from a peroxisome, called a glycosome. This organelle contains the enzymes for glycolysis and enzymes normally found in chloroplasts. What the importance of the chloroplast enzymes are, I’m not sure, but a s
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