Introduction to Parasites PDF

M.A. Freeman Definitions A facultative parasite is an organism that may become parasitic, but does not require a host for completion of its life cycle. Examples of facultative parasitism occur among many species of fungi (e.g. Blastomyces dermatitidis, Blastomycosis) An opportunistic infection is an...

M.A. Freeman Definitions A facultative parasite is an organism that may become parasitic, but does not require a host for completion of its life cycle. Examples of facultative parasitism occur among many species of fungi (e.g. Blastomyces dermatitidis, Blastomycosis) An opportunistic infection is an infection caused by pathogens (bacteria, viruses, fungi, or protozoa) that take advantage of an opportunity not normally available, such as a host with a weakened immune system, an altered microbiota (such as a disrupted gut flora), or breached integumentary barriers PARASITISM: is when an organism (the parasite) for all or part of its life, derives at least some, but often all of it’s food from a living organism of another species (the host), usually living in or on the body or cells of the host, which is usually harmed to some extent by the association, ‘ONE WHO EATS AT ANOTHER’S TABLE’ A flea living on a dog: the flea feeds off the dog’s blood, which is detrimental to the dog. This is an examples of an ectoparasite A caligid copepod living on the skin of fish, again an ectoparasite, but these parasites spend part of their life cycle as free-living A cestode platyhelminth living in the intestine of fish and mammals, an example on an endoparasite EPIBIOSIS AND PHORESIS: ‘THE ACT OF BEING CARRIED’ In this case the two organisms or symbionts are nutritionally independent of each other and have a facultative association (nonobligatory). The larger, basibiont, carries the smaller epibiont around on its surface. The epibiont benefits from this transportation by gaining access to differing environments. Example: Stalked ciliates using copepod as transport to gain access to waterborne particles. Such ciliates may however inhabit other surfaces and are not restricted to the copepods. COMMENSALISM Latin com mensa meaning ‘sharing a table’ This occurs when the smaller symbiont, the commensal, feeds on the food available in or on the surface of the host, for whom it is unusable or unwanted, while the host neither benefits nor is harmed. There are both ectocommensals and endocommensals. Examples: flagellates feeding on bacteria on the surface of fish, and Entamoeba sp. also feeding on bacteria in the human intestine. Host-parasite balance PARASITISM = Ht – X X COMMENSALISM = Ht ± 0 (- X) Latin: com = together; mensa = table MUTUALISM = Ht + Y (symbiosis – long-term) Ht = HOST TABLE COMPLEX (FOOD AVAILABLE) VIRULENCE OR PATHOGENICITY Parasites of veterinary importance Helminths: parasitic worms, large and multicellular The term "helminth" includes a number of phyla, many of which are unrelated (phylogenetically) but they have superficial similarities: -they are vermiform or “worm-like” in form- Phylum Nematoda (roundworms) Phylum Platyhelminthes (flatworms) – Class Cestoda (tapeworms) – Class Trematoda (flukes: endoparasites) – Class Monogenea (skin flukes: ectoparasites) – Class Turbellaria (free-living flatworms, some parasitic forms) Phylum Acanthocephala (thorny-headed worms) Phylum Annelida (segmented worms) not parasitic (leeches) Parasite of veterinary importance Kingdom Protozoa (Protists) – – – Phylum Mastigophora (flagellates) Phylum Apicomplexa Phylum Ciliophora (ciliates) Kingdom Fungi Kingdom Animalia – – Trichodina Phylum Arthropoda (insects/arachnids, copepods) Phylum Cnidaria (Myxozoa) Lecture objectives Key concepts with parasites Mode of infection Types of hosts Life cycle Parasite fitness / survival – Food – Life cycle strategies Host-parasite balance Key concepts Parasitism: a non-mutual non-symbiotic relationship between species, where one species, the parasite, benefits at the expense of the other, the host How many species of parasite are there?? And how many full stop!!!? Key concepts Live in or on their host – Ectoparasite – Endoparasite Smaller than their host Parasites reduce host biological fitness, to some extent (pathogenicity/virulence) Typically do not kill their host Reproduce at a faster rate than their host Key concepts Parasites use the host for survival – Food – Water – Heat – Habitat – Transmission Mode of infection Oral – – – – – – Spore in environment Oocyst/eggs in environment Larvae in environment Larvae in IH, PH Cyst in other host Larvae in milk Skin penetration Injection Transplacental (vertical transmission) Hosts and life cycles Intermediate host (IH) – Required, serves as temporary but necessary host for continued development of essential life stages Paratenic host (PH) or transport hosts – A non-required transport ‘intermediate’ host in which no development of the parasite occurs Aberrant or abnormal host – Host not usually used by the parasite, development slow or incomplete Dead-end or incidental host – An ‘intermediate’ host that generally does not allow transmission to the definitive host, thereby preventing the parasite from completing its development Hosts and life cycles Host spectrum varies by parasite and taxonomic group Direct life cycle – Only one host required to complete the life cycle – Stages in this host and the environment – Host specific or broad host spectrum (nonspecific) – Definitive host (DH) – sexual reproduction takes place – Parasites that (sexually) reproduce without an intermediate host have direct life cycles Direct life cycle; host specific DH Sexual repro Direct life cycle; broad host spectrum Hosts and life cycles Indirect life cycle – Two or more hosts required to complete the life cycle – Host specific or broad host spectrum (nonspecific) – Final host (FH): sexual reproduction (DH) – Intermediate host (IH) – Involves stages in the hosts and the environment Indirect life cycle: e.g. Dirofilaria immitis Dog heartworm development Direct life cycle; Paratenic Host Toxocara canis: as paratenic hosts, a number of vertebrates and some invertebrates can become infected. Mouse host is not required No development Direct life cycle; aberrant host (human) Toxocara canis Indirect life cycle with Paratenic Host Nematode lungworms No development just transportation PH IH = some important development Host, life cycle, mode of infection Sexual – Genetic diversity – Longer life cycle – Must have at least 2 parasites (M & F) Asexual – Lack of genetic diversity – Short life cycle – One is enough! Direct life cycle; sexual reproduction in DH DH Sexual repro Indirect life cycle; a/sexual reproduction in IH Sibley et al. 2009. Genetic diversity of Toxoplasma gondii in animals and humans. DOI: 10.1098/rstb.2009.0087 Lecture objectives Key concepts with parasites Mode of infection Types of hosts Life cycle Parasite fitness / survival – Food – Life cycle strategies Host-parasite balance Parasite fitness - survival Benefits from the host Survival: heat, food and water, habitat Food: blood, mucosal lining, etc. Transmission or maturation/ reproduction Arrested development / hypobiotic stage – Hypobiosis is the term most often used for arrested development that has a seasonal basis (nematodes). Can also be caused by host immune responses and an overcrowding effect, high numbers of adults inhibit larvae Parasite fitness - survival Habitat and food: where in the host? Predilection site: preferred site in or on the host Food source, able to avoid immune system (for at least some time period) Feeding and location related to pathology Aberrant site: site in or on a host which is not a normal location Site(s) of infection/infestation Normal and aberrant migration of larvae Oestrus ovis (the sheep botfly) Parasite fitness – life cycle strategies A complex life cycle involving more than one host is a common feature of parasitic animals. This has many advantages but also creates some potential problems – how to get from one host to the other – ‘Transmission’ Some parasites have life cycle stages that are motile and actively seek out hosts, others use hosts (vectors) that deliver them, and many use hosts that are linked by a food chain and are transmitted to the next host when their current one is eaten. This is known as trophic transmission. Many trophically transmitted parasites have tricks up their sleeves to enhance the chance that their hosts are eaten; but only once they have developed to a stage capable of infecting the next host. For example, when infected, their intermediate/paratenic host may lose its predator avoidance behaviour and even behave in a way that attracts predators. https://blogs.biomedcentral.com/bugbitten/2018/03/09/parasite-induced-behavioural-changes-sterility-linked/ Parasite fitness – life cycle strategies https://blogs.biomedcentral.com/bugbitten/2018/08/17/mind-control-from-beyond-thehost-tapeworm-increases-risky-behaviour-even-in-uninfected-group-members/ Infected fish behave in an unusual or atypical manner Due to heavy infections but also altered behavioral traits Even showing risky behavior like swimming close to the water surface Parasite fitness – life cycle strategies Leucochloridium paradoxum – ‘Zombie Snails’ A parasitic flatworm (trematode) Gastropod is the intermediate host The snail consumes bird droppings with eggs shed from adult flukes in the bird Larvae develop and migrate to the snail’s eye tentacle, forming a pulsating sac This attracts the definitive bird host, that consumes the unsuspecting zombie snail http://dailyparasite.blogspot.com/2016/12/leucochloridi um-paradoxum-revisited.html https://www.youtube.com/watch?v=J5x_cUUY6e4 http://thescienceexplorer.com/nature/natures-zombies-mind-altering-infections-snails-humans Parasite fitness – life cycle strategies Toxoplasma gondii, despite being capable of infecting almost any warm-blooded animal, it can only reproduce sexually in cats Infected rodents appear to lose their fear of cats, a phenomenon dubbed “ Fatal Feline Attraction” Researchers estimate that as much as 30 percent of the people on earth are carrying T. gondii tachyzoites around in our brains. Does this affect human behavior? Parasite – host balance The parasite The host & host response Environment Parasite x host x environment Parasite – host balance The host response Previous exposure Type of host response (inflammatory, immune, etc.) Nutritional status (health of host) Age of the host (immune status) Parasite – host balance Parasite x host x environment Environmental factors that – ↑ parasites – ↑ hypobiosis (synchronization of the nematode life cycle to changing host and environmental conditions) – ↓ immunity Management / treatment Geographical location Temperature / weather Parasite – host balance Physical presence, activity or production of toxic products Size, numbers and behavior of the parasites How and how much the parasite feeds -iasis = presence of parasite (sub-clinical) -osis = disease caused by parasite Example: Coccidiasis vs Coccidiosis Host-parasite balance PARASITISM = Ht – X X COMMENSALISM = Ht ± 0 (- X) Latin: com = together; mensa = table MUTUALISM = Ht + Y (symbiosis – long-term) Ht = HOST TABLE COMPLEX (FOOD AVAILABLE) VIRULENCE OR PATHOGENICITY Can parasites benefit the host? Immune system (boost?) Allergy relief? Reproduction? Julius Lukeš Scientist swallows tapeworm to prove intestinal parasites can be our friends Healthy skin Do we really understand their role? Remember the number of species and individuals!!! Many taxa have co-evolved with their parasites Can parasites benefit the host? Ectoparasites: on the outside! Ectoparasitic mite: Demodex (Arachnida) Diagnosed microscopically from a deep skin scrape of infected animal (present in low numbers in healthy dogs) Not usually contagious: suppressed immune system Ectoparasitic mite: Demodex Interesting case in Iceland a few years ago of blepharitis Post treatment with tea tree oil Ectoparasitic mite: Demodex First documented case of Demodex in humans in Iceland ! Particularly prevalent in the old, and very under diagnosed in humans Ectoparasites as vectors “Any organism (vertebrate or invertebrate) that functions as a carrier of an infectious agent between organisms of a different species.” Many biting ectoparasites like fleas and ticks do carry serious pathogens in their salivary glands Therefore ectoparasites have a very high veterinary importance Ehrlichia is a genus of rickettsial bacteria that is transmitted to vertebrates by ticks Anaplasma phagocytophilum is a bacterial infection of the white blood cells (neutrophils) that’s transmitted by the deer tick Heart worm: parasitic roundworm that is spread from host to host through the bites of mosquitoes Class: Monogenea – ectoparasitic flatworms ▪ Part of the Phylum Platyhelminthes ▪ Ectoparasites: especially on the skin, fins and gills of fish, also in exotics like frogs and reptiles and hippos! (in the eye) ▪ Poorly developed suckers, attachment is via hooks (classification) ▪ Ancestrally related to Turbellaria (free-living flatworms) and closest to modern cestodes ▪ Direct life cycle leads to significant problems in cage culture of fish Class: Monogenea – ectoparasitic flatworms Gyrodactylus: viviparous monogeneans and some species are extremely pathogenic Class: Monogenea – ectoparasitic flatworms Udonella spp. have lost their attachment hooks during evolution Udonella fugu Ectoparasites: on the outside A parasite that lives on or in the skin but not within the body. Fleas and lice are ectoparasites. Arachnids: ticks and mites Insects: lice, fleas (wingless flies), blow flies The copepod is clearly an ectoparasite Is Udonella an ectoparasite? Lives on the copepod, but feeds on the fish (damage to fish not copepod!) Really it should be classed as an epibiont or an ectocommensal EPIBIOSIS AND PHORESIS: ‘THE ACT OF BEING CARRIED’ In this case the two organisms or symbionts are nutritionally independent of each other and have a facultative association (nonobligatory). The larger, basibiont, carries the smaller epibiont around on its surface. The epibiont benefits from this transportation by gaining access to differing environments. Example: Stalked ciliates using copepod as transport to gain access to waterborne particles. Such ciliates may however inhabit other surfaces and are not restricted to the copepods. COMMENSALISM Latin com mensa meaning ‘sharing a table’ This occurs when the smaller symbiont, the commensal, feeds on the food available in or on the surface of the host, for whom it is unusable or unwanted, while the host neither benefits nor is harmed. There are both ectocommensals and endocommensals. Examples: flagellates feeding on bacteria on the surface of fish, and Entamoeba sp. also feeding on bacteria in the human intestine. Class: Monogenea – ectoparasitic flatworms In the rather complex case of Udonella on caligid copepods Udonella have an obligate relationship with the copepod, but they feed directly from the fish, or from fish mucus disturbed by the copepod. But they can’t move back to live on the fish as they have lost their hooks, hence they must depend on the copepod to gain access to food Therefore they can’t be epibionts, as not a facultative relationship It is not clear whether the copeods are negatively impacted, they don’t seem to be and can swim as normal with many udonellids ‘hanging on’ Therefore they are best described as ectocommensals (of copepods) They are the ultimate ‘hitchhikers’ (but still really fish parasites)