Parasitology PDF

# Parasitology ## Definition Parasitology is the area of biology concerned with the phenomenon of dependence of one living organism on another. Medical Parasitology is concerned primarily with parasites that affect humans and their medical significance, as well as their importance in human communit...

# Parasitology ## Definition Parasitology is the area of biology concerned with the phenomenon of dependence of one living organism on another. Medical Parasitology is concerned primarily with parasites that affect humans and their medical significance, as well as their importance in human communities. ## Tropical Medicine Tropical Medicine is a branch of medicine that deals with tropical diseases and other special medical problems of tropical regions. A tropical disease is an illness that is indigenous to or endemic in a tropical area but may also occur in sporadic or epidemic proportions in areas that are not tropical. Many tropical diseases are parasitic diseases. ## Biological Relationships Organisms may develop unique relationships due to their habitual and long associations with one another. These relationships are very important to their survival. **Symbiosis** is the living together of unlike organisms. It may also involve protection or other advantages to one or both organisms. **Different forms of symbiosis may be distinguished on the basis of whether or not the association is detrimental to one of the two organisms.** ### Commensalism Commensalism is a symbiotic relationship in which two species live together and one species benefits from the relationship without harming or benefiting the other. For example, *Entamoeba coli* in the intestinal lumen are supplied with nourishment and are protected from harm, while it does not cause any damage to the tissues of its host. ### Mutualism Mutualism is a symbiosis in which two organisms mutually benefit from each other, such as termites and the flagellates in their digestive system, which synthesize cellulase to aid in the breakdown of ingested wood. ### Parasitism Parasitism is a symbiotic relationship where one organism, the **parasite**, lives in or on another, depending on the latter, for its survival and usually at the expense of the host. One example of a parasite is *Entamoeba histolytica*, which derives nutrition from the human host and causes amebic dysentery. ## Parasites Parasites are often described according to their habitat or mode of development. A parasite living inside the body of a host is known as an **endoparasite**, whereas a parasite living outside the body of a host is an **ectoparasite**. The presence of an endoparasite in a host is called an **infection**, while the presence of an ectoparasite on a host is called an **infestation**. A parasite is considered **erratic** when it is found in an organ that is not its usual habitat. Most parasites are **obligate parasites** in that they need a host at some stage of their life cycle to complete their development and to propagate their species. Obligate parasites, such as tapeworms, depend entirely upon their host for existence. A **facultative parasite** may exist in a free-living state or may become parasitic when the need arises. A parasite, which establishes itself in a host where it does not ordinarily live, is called an **accidental or incidental parasite**. A **permanent parasite** remains on or in the body of the host for its entire life, while a **temporary parasite** lives on the host only for a short period of time. A **spurious parasite** is a free-living organism that passes through the digestive tract without infecting the host. ## Hosts Hosts can be classified into various types based on their role in the life cycle of the parasite. A **definitive or final host** is one in which the parasite attains sexual maturity. In taeniasis, for example, humans are considered the definitive host. An **intermediate host** harbors the asexual or larval stage of the parasite. Pigs or cattle serve as intermediate hosts of *Taenia spp.*, while snails are hosts of *Schistosoma spp*. If there is more than one intermediate host, these can be classified as first and second intermediate hosts. A **paratenic host** is one in which the parasite does not develop further to later stages. However, the parasite remains alive and is able to infect another susceptible host. For example, *Paragonimus metacercaria* in raw wild boar meat can pass through the intestinal wall of humans and complete its development. In this case, the wild boar serves as a paratenic host transferring the infective stage to humans. Paratenic hosts are important because they widen the parasite distribution and bridge the ecological gap between the definitive and intermediate hosts. There are also other animals that harbor the parasite other than definitive, intermediate, and paratenic hosts. These are known as **reservoir hosts**. They allow the parasite's life cycle to continue and become additional sources of human infection. Pigs are reservoirs of *Balantidium coli*, field rats of *Paragonimus westermani*, and cats of *Brugia malayi*. Humans may be the most important host in the spread of the disease or an incidental host of parasites prevalent in other animals. ## Vectors Vectors are responsible for transmitting the parasite from one host to another. A **biologic vector** transmits the parasite only after the latter has completed its development within the host. A biologic vector is therefore an essential part of the parasite's life cycle. When an *Aedes* mosquito sucks blood from a patient with filariasis, the parasite undergoes several stages of development from first stage larva to third stage larva before the latter (infective stage) is transmitted to another susceptible host. A **mechanical phoretic vector**, on the other hand, only transports the parasite. Flies and cockroaches that feed on fecal material may carry enteric organisms and transfer these to food, which could be ingested by humans. ## Exposure and Infection Majority of parasites are pathogens which are harmful and which frequently cause mechanical injury to their hosts. A **carrier** harbors a particular pathogen without manifesting any signs and symptoms. **Exposure** is the process of inoculating an infective agent, while **infection** connotes the establishment of the infective agent in the host. The **incubation period** is the period between infection and evidence of symptoms. It is sometimes referred to as the **clinical incubation period**. The **pre-patent period**, also known as the **biologic incubation period**, is the period between infection or acquisition of the parasite and evidence or demonstration of infection. **Autoinfection** results when an infected individual becomes his own direct source of infection. In enterobiasis, infection may occur through hand-to-mouth transmission. Infective eggs may end up in the hands by scratching the perianal areas where the gravid females lay their eggs. Alternatively, parasites may multiply internally, such as *Capillaria philippinensis*. **Superinfection or hyperinfection** happens when the already infected individual is further infected with the same species leading to massive infection with the parasite. An alteration in the normal life cycle of *Strongyloides* results in a large increase in worm burden, which may lead to severe debilitation or even death due to an increase in the proportion of rhabditiform larvae that transform into filariform larvae while in the gut. ## Sources of Infection There are various sources of parasitic infections. The most common sources are contaminated soil and water. Lack of sanitary toilets and the use of night soil or human excreta as fertilizer allow the eggs to come in contact with the soil and favor the development of *Ascaris lumbricoides*, *Trichuris trichiura*, *Strongyloides stercoralis*, and hookworm. Water may be contaminated with cysts of amebae or flagellates, as well as cercariae of *Schistosoma*. Another possible source of infection is food, which may contain the infective stage of the parasite, as exemplified by a number of trematode and cestode infections. Consumption of undercooked or raw freshwater fish can result in several intestinal and liver fluke infections. Raw crabs are considered a delicacy in areas where paragonimiasis is endemic, while raw *Bullastra* snails are associated with *Artyfechinostomum malayanum* infection. Arthropods can also transmit infection. Mosquitoes are vectors of malaria and filarial parasites. *Triatoma* bugs are carriers of *Trypanosoma cruzi* causing Chagas disease. Sand flies (e.g., *Phlebotomus spp.*) are the natural vectors of all types of *Leishmania*. Other animals, whether wild or domesticated, may also harbor parasites. Cats are direct sources of *Toxoplasma* infection, while rats may be infected with *Hymenolepis nana*. Other sources of infection include another person, his beddings and clothing, as well as the immediate environment he has contaminated, or even one's self. Asymptomatic carriers of *Entamoeba histolytica* working as food handlers in food establishments may be important sources of infection. Autoinfection, where the infected person himself is the source of infection, is seen in the life cycles of *Capillaria philippinensis*, *Enterobius vermicularis*, *Hymenolepis nana*, and *Strongyloides stercoralis*. ## Modes of Transmission Since the most common source of parasitic infection is contaminated food and water, the most likely portal of entry is the mouth. Majority of infections with cestodes, trematodes, and intestinal protozoans are foodborne: *Taenia solium*, *Taenia saginata*, and *Diphyllobothrium latum* from eating food harboring the infective larval stages; *Entamoeba histolytica* and *Giardia lamblia* from drinking water contaminated with cysts; and *Clonorchis*, *Opistorchis*, and *Haplorchis* through ingesting raw or improperly cooked freshwater fish containing infective larvae. Skin penetration is another route of transmission. Hookworms and *Strongyloides* enter via exposure of skin to soil, while *Schistosoma* species enter skin via water. Arthropods also serve as vectors and transmit parasites through their bites. Examples are agents of malaria, filariasis, leishmaniasis, trypanosomiasis, and babesiosis. Another way of acquiring infection is through congenital transmission. *Toxoplasma gondii* trophozoites can cross the placental barrier during pregnancy. In transmammary infection with *Ancylostoma* and *Strongyloides*, the parasites may be transmitted through mother's milk. Other ways of acquiring the infection include inhalation of airborne eggs of *Enterobius* and sexual intercourse as in the case of *Trichomonas vaginalis*. ## Nomenclature Animal parasites are classified according to the International Code of Zoological Nomenclature. Each phylum is divided into classes, which are further subdivided into orders, families, genera, and species. At times, the further divisions of suborder, superfamily, and subspecies are employed. Scientific names are latinized; family names are formed by adding -idae to the stem of the genus type; generic names consist of a single word written in initial capital letter; the specific name always begins with a small letter. The names of the genera and species are italicized or underlined when written. ## Life Cycle Through adaptation to their hosts and the external environment, parasites have developed life cycles, which may be simple or complicated. Most parasitic organisms attain sexual maturity in their definitive hosts. Some spend their entire lives within the host with one generation after another, while others are exposed to the external environment before being taken up by an appropriate host. The larval stage of the parasite may pass through different stages in an intermediate host before it reaches a final host. As the life cycle becomes more complicated, the lesser the chances are for the individual parasite to survive. The perpetuation of a species of parasite depends upon its ability to ensure transmission from one host to the next. The parasite must, therefore, adapt to protect itself from the host's defenses and the external environment, and it must overcome the attrition in the species by producing numerous progeny. ## Epidemiologic Measures Epidemiology is the study of patterns, distribution, and occurrence of disease. **Incidence** is the number of new cases of infection appearing in a population in a given period of time. **Prevalence** is the number (usually expressed as percentage) of individuals in a population estimated to be infected with a particular parasite species at a given time. **Cumulative prevalence** is the percentage of individuals in a population infected with at least one parasite. **Intensity of infection** refers to burden of infection that is related to the number of worms per infected person. This may be measured directly or indirectly and is also referred to as the worm burden. In the case of soil-transmitted helminths, it can be measured directly by counting expelled worms during treatment, or indirectly by counting helminth eggs excreted in feces. The latter is expressed as the number of eggs per gram (epg). **Clinical consequences** of infections or diseases that affect an individual's well-being refer to morbidity. ## Treatment **Deworming** is the use of anthelminthic drugs in an individual or a public health program. **Cure rate** refers to the number (usually expressed as a percentage) of previously positive subjects found to be egg negative on examination of a stool or urine sample using a standard procedure at a set time after deworming. **Egg reduction rate (ERR)** is the percentage fall in egg counts after deworming based on examination of a stool or urine sample using a standard procedure at a set time after the treatment. **Selective treatment** involves individual-level deworming with selection for treatment based on a diagnosis of infection or an assessment of the intensity of infection, or based on presumptive grounds. This strategy can be used in whole populations, or in defined risk groups. **Targeted treatment** is group-level deworming where the (risk) group to be treated (without prior diagnosis) may be defined by age, sex, or other social characteristics irrespective of infection status. **Universal treatment** is population-level deworming in which the community is treated irrespective of age, sex, infection status, or other social characteristics. **Preventive chemotherapy** is the regular, systematic, large-scale intervention involving the administration of one or more drugs to selected population groups with the aim of reducing morbidity and transmission of selected helminth infections. **Coverage** refers to the proportion of the target population reached by an intervention. It could be the percentage of school-age children treated during a treatment day. **Efficacy** is the effect of a drug against an infective agent in ideal experimental conditions and isolated from any context. **Effectiveness** is a measure of the effect of a drug against an infective agent in a particular host, living in a particular environment with specific ecological, immunological, and epidemiological determinants**. Effectiveness is usually measured by means of qualitative and quantitative diagnostic tests that detect eggs or larvae in feces or urine after an optimal time interval, which is variable for each parasite. **Cure rate** and **egg reduction rate** are indicators that are commonly used to measure the reduction in prevalence and reduction in intensity of infection, respectively. **Drug resistance** is a genetically transmitted loss of susceptibility to a drug in a parasite population that was previously sensitive to the appropriate therapeutic dose. ## Prevention and Control **Morbidity control** is the avoidance of illness caused by infections. It may be achieved by periodically deworming individuals or groups, known to be at risk of morbidity. **Information-education-communication (IEC)** is a health education strategy that aims to encourage people to adapt and maintain healthy life practices. **Environmental management** is the planning, organization, performance, and monitoring of activities for the modification and/or manipulation of environmental factors or their interaction with human beings with a view to preventing or minimizing vector or intermediate host propagation and reducing contact between humans and the infective agent. **Environmental sanitation** involves interventions to reduce environmental health risks including the safe disposal and hygienic management of human and animal excreta, refuse, and waste water. It also involves the control of vectors, intermediate hosts, and reservoirs of disease. It also covers the provision of safe drinking water and food safety; housing that is adequate in terms of location, quality of shelter, and indoor living conditions; facilities for personal and domestic hygiene; as well as safe and healthy working conditions. **Sanitation** is the provision of access to adequate facilities for the safe disposal of human excreta, usually combined with access to safe drinking water. ## Eradication versus Elimination **Disease eradication** is defined as a permanent reduction to zero of the worldwide incidence of infection caused by a specific agent, as a result of deliberate efforts. Once this is achieved, continued measures are no longer needed. On the other hand, **disease elimination** is a reduction to zero of the incidence of a specified disease in a defined geographic area as a result of deliberate efforts. Continued intervention or surveillance measures are still required.