Influence of parasites and hosts The damage that pathogenic parasites produce in host tissuecan be explained in the following two ways, namely: a.Direct effects of parasites on the hostmechanical injury, can be caused by pressure from parasites due to larger growth, for example: hydatid cysts causing blockage of the ducts. The damaging effect of toxic substances onPlasmodium falciparum, producing toxic substances that can cause rigors and other symptoms. uptake of nutrients, fluids, and metabolites by parasites can produce disease through competition with the host for nutrients. b.Indirect effects of parasites on the host Immunological reactions, tissue damage can be caused by the responsehost immunology, for example: nephritis syndrome after infection Plasmodium. Excessive proliferation of certain tissues due to invasion by some parasites can also cause tissue damage in humans, for example liver fibrosis after deposition of ova from Schistosoma • Parasite transmission Parasite transmission depends on the source or reservoirinfection, and how it is transmitted.a. Source of infection 1) Human Humans are the biggest source or intermediary of infectionparasitic (for example taeniasis, amoebiasis, etc.). A condition in which an infection is transmitted from one person to another is called anthroponysis. 3) Animals In many parasitic diseases, animals act as a source of infection. A condition where an infection is transmitted from animals to humans is called zoonosis (for example, hydatidiasis). b. Mode of Transmission Transmission of parasites from one host to another, caused by a particular form of parasite is known asinfection stage. The infectious stages of various parasites are transmitted from one host to another in the following ways: 1) Oral route. Consuming food, water, vegetables or places contaminated by parasite infection stages. This method of transmission for some parasites is known as the fecal-oral route (for example Giardia intestinalis and Entamoeba histolytica cysts, Ascaris lumbricoides eggs, and Trichuris trichura. a) Consuming raw or undercooked meat. Infection can be transmitted orally if raw or undercooked meat containing infective parasites is consumed (for example: pork containing cysticercus cellulose, larval stage Taenia solium). b) Consuming undercooked or raw fish and crab. Infection can also be transmitted by consuming raw or undercooked fish and crabs that contain infective stages of parasites (for example: crabs containing infective stages of parasites, crabs or freshwater shrimp containing metacercariaeParagonimus westermani, fish contain metacercariaeClonorchis sinensis, and others). c) Consuming raw or uncooked water. Infection can be transmitted through raw food or uncooked water which hides the infective form of the parasite (for example: breast bean water, etc. containing metacercariae on theFasciolopsis buskiAnd Fasciola hepatica). 2) Penetration of skin and mucous membranes. Infection is transmitted by: a) Skin penetration by filarial larvae (filariformy larvae) in hookworms,Strongyloides stercoraliswho come into contact with soil contaminated with feces. b) Skin puncture by cercariae onSchistosomajaponicum, S. Mansoni, AndS.haematobiumwho come into contact with infected water. The part of the skin that is penetrated is the thin part of the skin, for example: in the finger area, perianal skin and perineal skin. 3) Inoculation of arthropod vectorsInfection can also be transmitted by inoculation into the bloodthrough mosquitoes, such as malaria and filariasis. 4) Sexual contact Trichomoniais can be transmitted through sexual contact.Entamoebiasis can be transmitted through oral anal sexual contact, such as among homosexuals. • Medical parasitology In the concept of medical parasitology, each important parasite is discussed regarding morphology, geographical distribution, mode of infection, life cycle, host/parasite relationship, pathology and clinical manifestations of infection, laboratory diagnosis, treatment and parasite prevention/control measures. Below are some of these criteria. a. Morphology Morphology includes size, shape, color and position of organelleswhich differ in parasites at various stages of their development. This is important in laboratory diagnosis which helps to identify the different stages of development and differentiate between pathogens and commensal organisms. Example:Entamoeba histolyticaAndEntamoeba coli. b. Geographic distribution Some of the parasites are found in tropical areas. The distribution of parasites dependson: 1) Host specificity, for example:Ancylostoma duodenalerequires humans as hosts, temporarilyAncylostoma caninumrequires a dog as a host. 2) Eating habits, for example consumption of raw or undercooked meat or vegetables predisposes to Taeniasis. 3) Ease of parasite escape from the host, parasites that are released from the body along with feces and urine are distributed more quickly than parasites that require a vector or direct contact with body fluids for transmission. 4) Environmental conditions that support survival outside the host body, namely temperature, presence of water, humidity, and so on. 5) In the presence of a suitable host, parasites that do not require an intermediate host (vector) for transmission are more widely distributed than parasites that require a vector.