Parasitology •Parasites include things that live internal (endoparasites) or external (ectoparasites) to the infected (or infested) host •Broadly speaking, all pathogens may be classified as parasites •Narrowly speaking, Parasitology is the science that studies the relatively large parasites including parasitic Protists, Worms, and, to a lesser degree, pathogenic Fungi and Arthropod ectoparasites •Today we’ll consider Protists,
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Parasitology• Parasites include things that live internal
(endoparasites) or external (ectoparasites) to the infected (or infested) host
• Broadly speaking, all pathogens may be classified as parasites
• Narrowly speaking, Parasitology is the science that studies the relatively large parasites including parasitic Protists, Worms, and, to a lesser degree, pathogenic Fungi and Arthropod ectoparasites
• Today we’ll consider Protists, Helminths (worms), Fungi, and the Arthropod Vectors of infectious disease
Vectors and Hosts
• A Vector is a living organism that carries a disease-causing organism to new Hosts
• This distinction can be arbitrary, however, depending on which organism’s infection we are most concerned with
• Thus, the Anopheles mosquito is a vector for the parasitic disease malaria because we care more about the health of the human host than that of the mosquito
• However, in addition, the malaria parasite has a much greater impact on the health of the human host than it does on the health of the mosquito vector
Biological vs. Mechanical Vector• The real confusion between host and vector comes
from the concept of Biological Vector
• Within (or upon) both a host and a Biological Vector the parasite undergoes some aspect of its life cycle
• Thus, in one sense, all hosts that can pass a parasite on, particularly to another species of host, is also a Biological Vector
• However, if the parasite does not undergo some aspect of its life cycle as it is transported by one organisms to a second, then the first organism is described as a Mechanical Vector
• E.g., flies can be mechanical vectos of feces-born pathogens such as Salmonella
Types of Hosts
• Definitive Host: The host in which the parasite goes through its sexual cycle (I.e., fertilization & meiosis)
• E.g., mosquitoes serve as definitive hosts of the malaria parasite (Plasmodium spp.)
• E.g., snakes (~30 species) serve as definitive hosts of Sarcocystis singaporensis, a disease of mammals
Types of Hosts
• Intermediate Host: Host in which the parasite replicates but does not go through its sexual cycle
• E.g., mammals, including humans, serve as intermediate hosts of the malaria parasite (Plasmodium spp.)
• E.g., mammals (~30 species, including humans) serve as intermediate hosts of Sarcocystis singaporensis
S. singaporensis has been proposed as biological control of rodents pests—apparently it can selectively kill them!
Types of Hosts• Reservior Hosts: The reservoir host is the population
in which a parasite resides when it isn’t affecting a population that we care more about
• E.g., the rabies virus normally is passed back and forth among wild mammals (these serve as the reservoir hosts—the populations that we don’t much care about)
• Occasionally, however, rabies can infect pets, and thereby us (the population we do care about)
• To prevent the latter (i) we vaccinate pets, (ii) avoid handling wild animals, and (iii) otherwise attempt to reduced the incidence of rabies in reservoir pops.
• Mycelia (tangled masses of hyphae, typically that has grown into a substrate)
• Thallus, the word used to describe the “body” of a fungus (same word as used to describe the body of an algae
• Fungi live by growing mycelia into (or onto) substrate (e.g., a dead plant), secreting exoenzymes from these cells, and then absorbing digested nutrient into the mycelia