Parasitology 101 for Environmental Health

Key Concepts  Parasitism – Literally “to feed at the table of another” (from the Greek parasitos)  Parasite -- A species that is metabolically dependent on another species  Life cycles --The key to eradication of parasites in an environment

Key Definitions  Endoparasite --Resides within the host, e.g., Angiostrongylus  Ectoparasite --Resides outside the host, i.e., on the exterior, e.g., Larva migrans  Aberrant parasite - The right host but the wrong tissue, e.g., S. haematobium in the eyes  Incidental parasite - Wrong host, wrong everything.

More Terms  Facultative parasite - A parasite that can exist in a parasitic relationship as easily as free-living  Obligatory parasite—Parasitism is necessary— “obligatory”—for the parasite to survive  Pseudoparasite --Literally a false parasite (pseudo)  Hyperparasite --A parasite that parasitizes another parasite

Types of Hosts  Definitive host --Host on/in which parasite reaches sexual maturity  Intermediate host --Host on/in which parasite develops sexually but to full maturation  Paratenic (transport) host --Host that moves parasite from one host or site to another

More Hosts  Biologic (Vector) --Living (usually invertebrate) host (e.g., mosquitoes)  Mechanical (Vehicle) – Non-living host (e.g., water)  Reservoir host --A host that maintains parasite in environment (e.g., mammals and Giardia lambliae)

Basic Varieties of Life Cycles  Direct --Literally direct transmission, e.g., fecal-oral, i.e., Enterobius vermicularis (pinworm)  Indirect --Several hosts that maintain sexual development (e.g., Angiostrongylus)

Basic Types of Transmission  Horizontal – Indirect in which one host eats another host  Vertical -- Direct in which mother may pass parasite to foetus  NB:Host specificity: If pathogen can jump from non-humans to humans, then pathogen is generally considered zoönotic.

Characteristics of Parasitism of Domestic “Animals”  Parasitism is extremely common in domestic animals  Parasitic disease is often relatively rare, however, and usually is a consequence of management.

Characteristics  Parasitiasis = parasitism (infection), but in the absence of the disease  Parasitosis = parasitism (infection) with the presence of the disease

Characteristics  Parasitic disease is usually related to the numbers of parasites present or relative susceptibility of the host.  Parasites damage hosts by a variety of pathophysiologic mechanisms; the spectrum of damage ranges from no effect to death.  Parasitized animals eventually develop some degree of acquired immunity to most parasitic infections or at least to associated disease.

Characteristics  The ultimate goal of parasite control is disruption of transmission (i.e., prevention of infection/reinfection).  Disruption of parasitic transmission is rarely done by treatment with parasiticides alone.  Development of effective host immunity, changes in clinical management, and/or modifications of the environment are usually necessary to achieve effective control.

Characteristics  The ultimate goal of a parasite’s existence is perpetuation of the species. Many parasites can alter their biologic functions to enhance the odds for survival and reproduction.  Parasite populations are plastic and dynamic; they have the capacity to respond to a variety of conditions and stimuli (selection pressures) by alterations of their genetic composition.

Parasites May Injure Their Hosts in a Variety of Ways

Injury Risk  They may suck blood (mosquitoes, hookworms), lymph (midges), or exudates (lungworms).  They may feed on solid tissues, either directly (giant kidney worms, liver t flukes) or after first liquefying them (chiggers, amoebas).  They may compete with the host for food it has ingested, either by ingesting intestinal contents (ascarids) or absorbing them through body wall (tapeworms).

Injury  They may cause mechanical obstructions of the intestine (Ascaris spp.), bile ducts (Ascaris spp.), blood vessels (Dirophilaria immitis), lymph channels (Wuncheria bancrofti), bronchi (Strongyloides stercoralis), or other body channels.  They may cause pressure atrophy (Hydatid cysts from Fasciola hepatica).

Injury  They may destroy host cells by growing in them (Plasmodium spp.).  They may produce various toxic substances such as hemolysins, histolysins, and anticoagulants, exudate from biting arthropods.

Injury  They may cause allergic reactions.  They may cause various host reactions such as inflammation, hypertrophy, hyperplasia. and nodule formation.  They may stimulate the development of cancer (Schistosoma haematobiuim)

Injury  They may carry diseases and parasites, including malaria (mosquitoes), swine influenza (lungworms),salmon poisoning (flukes), and heartworms (mosquitoes).  They may reduce their host’s resistance to other diseases and parasites, thereby setting the host from a premature death from something other than a parasitic disease.