Medical Parasitology Lecture 1-2 PDF

Medical Parasitology Dr. Khalis Ahmed Lecture..1-2 Introduction to Medical Parasitology Parasitology is an important component of clinical laboratory medicine. The results obtained through specimen examination for parasites, provide invaluable information regard...

Medical Parasitology Dr. Khalis Ahmed Lecture..1-2 Introduction to Medical Parasitology Parasitology is an important component of clinical laboratory medicine. The results obtained through specimen examination for parasites, provide invaluable information regarding the diagnosis and treatment of human disease. Tracking the epidemiology of such organisms as well as establishing prevention mechanisms may be accomplished with the assistance of this information. Although numerous advances in technology have been developed during recent years, the traditional technique of manually processing and examining the samples both macroscopically and microscopically still occurs in select clinical settings. It is critical that well-educated and highly trained individuals perform these procedures as well as read and interpret the results. Thus, the goal of this course learning is to provide such information for students preparing for a career in laboratory medicine, for learners in related disciplines, which include parasitology, and for clinical practitioners. HISTORICAL PERSPECTIVE The documentation of parasite existence by the ancient Persians, Egyptians, and Greeks dates back to prehistoric times. Just as the people of that era were primitive, relatively speaking, so too were parasites. Although underdeveloped areas still exist, humans have progressed through the years into an age of civilization. Parasites have evolved as well. A number of discoveries over the years has contributed to our current knowledge of parasitology. For example, as increased awareness that parasites were becoming a problem and the realization that they were responsible for invasion in the body (infection), invasion on the body (infestation), and disease, defined as a process with characteristic symptoms, emerged, determining an effective means of healing infected persons became a priority. As  Medical Parasitology Dr. Khalis Ahmed Lecture..1-2 more information was discovered regarding parasitic life cycles, especially the fact that transport carriers known as vectors were frequently responsible for transmission, parasite control and elimination also became important. Advances in other areas of medical and biologic science, coupled with the discovery of useful tools, such as microscopes, not only expanded our knowledge of parasites and their makeup, but also their relationships with hosts— that is, plants, animals, and humans known to harbor parasites. Today, parasitologists and clinicians have a wealth of parasite knowledge from which to draw. The escalation of disease caused by the presence of parasites (a concept known as parasitic) because of global travel tends to result in higher parasite recovery rates. The increased number and diversity of these organisms may allow practitioners to gain high levels of expertise in parasite identification and treatment. Enhanced preservation of specimens now allows parasites that otherwise might have been destroyed to remain viable. A number of advances in parasitology, particularly in the area of parasite laboratory diagnosis, promise to be exciting. Measures are also now in place that are designed to protect the practitioner when handling samples for parasite study. EPIDEMIOLOGY Even though treatment, prevention, and control measures are available, parasitic infections still occur and thus it is important to study and monitor their trends, a field known as epidemiology. Although they are distributed worldwide, most parasitic infections are found in underdeveloped tropical and subtropical countries such as Haiti, Guatemala, and Myanmar (Burma) and countries on the African continent. Increased population density, poor sanitation, marginal water sources, poor public health practices, and environmental changes affecting vector breeding areas account for the prevalence of parasites. The habits and customs of the people living in these regions are also contributing factors. The increased prevalence  Medical Parasitology Dr. Khalis Ahmed Lecture..1-2 of global travel likely accounts for parasitic infections being spread to areas other than where these infections originated. Individuals who travel to endemic areas are at risk of contracting parasitic infections. Refugees, immigrants, and foreign visitors may bring parasites with them when entering a non-endemic area. Representative additional human populations at risk of contracting a parasitic infection are listed in Box 1-1. Historically, a dramatic increase in parasite infection incidence occurred in the homosexual population but it is now also occurring more in the heterosexual population. More recently, parasitic infections have become more prevalent in underdeveloped countries, regardless of a person’s sexual orientation. The means whereby a parasite gains entry into an unsuspecting host, referred to as mode of transmission, vary by specific parasite species and those associated with the parasites covered in this text are summarized in Box 1-2. Consuming contaminated food or water and hand-to mouth transfer are common ways of transmitting select parasites. Others require an insect (arthropod) vector through which a parasite is passed on to an uninfected host, most often via a blood meal (bite). Still others will drill their way into the body via the skin through an unprotected bare foot or when an unsuspecting human is swimming in contaminated water. Sexual transmission, mouth-to-mouth contact through kissing, droplet contamination, and eye contact with infected swimming water also serve as routes for parasite transmission. BOX 1-1 Populations at Risk for Contracting Parasites Individuals in underdeveloped areas and countries Refugees Immigrants Visitors from foreign countries Individuals who are immunocompromised Individuals living in close quarters (e.g., prisons) Children who attend day care centers  Medical Parasitology Dr. Khalis Ahmed Lecture..1-2 BOX 1-2 Modes of Parasite Transmission Ingestion of contaminated food or drink (primarily water) Hand-to-mouth transfer Insect bite Entry via drilling through the skin Unprotected sexual relations Mouth-to-mouth contact Droplet contamination Eye contact with infected swimming water PARASITE-HOST RELATIONSHIPS The study of parasite-host relationships is over 100 years old. The main focus of this research has been threefold: (1) recognition of these relationships; (2) search for patterns of the relationships; and (3) development of methodologies to study these patterns. Table 1-1 lists the terms associated with parasite-host relationships, along with their definitions. There are several types of parasites that may be members of a parasite-host relationship. An organism may be an obligatory parasite or a facultative parasite. It may be an endoparasite or an ectoparasite. In the same manner, a number of different hosts may be part of this parasite host relationship. These include accidental or incidental hosts, definitive hosts, intermediate hosts, reservoir hosts, transport hosts, and carriers. When a parasite infects a host, symbiosis results. The primary function of the host is to carry on the parasite’s life cycle. This newly formed relationship may develop into commensalism, mutualism, or parasitism. Some of these associations exist as commensal under certain circumstances and pathogenic under others. Parasites have an amazing capability to adapt to their host surroundings. In addition to a number of morphologic adaptations, parasites are capable of protecting themselves from the host’s immune system. Parasites alter their antigenic makeup so that the host will not recognize the modified parasites as foreign, and thus the initiation of an immune response does not occur.  Medical Parasitology Dr. Khalis Ahmed Lecture..1-2 TABLE 1-1 Terms Associated with Parasite-Host Relationships Parameter Definition or Description Type of Parasite Obligatory parasite Parasite that cannot survive outside of a host Facultative parasite Parasite that is capable of existing independently of a host Endoparasite Parasite that is established inside of a host Ectoparasite Parasite that is established in or on the exterior surface of a host Type of Host Accidental or incidental host Host other than the normal one that is harboring a parasite Definitive host Host in which the adult sexual phase of parasite development occurs Intermediate host Host in which the larval asexual phase of parasite development occurs Reservoir host Host harboring parasites that are parasitic for humans and from which humans may become infected Transport host Host responsible for transferring a parasite from one location to another Carrier Parasite-harboring host that is not exhibiting any clinical symptoms but can infect others Parasite-Host Relationship Terms Symbiosis Living together; the association of two living organisms, each of a different species Commensalism Association of two different species of organisms that is beneficial to one and neutral to the other Mutualism Association of two different species of organisms that is beneficial to both Parasitism Association of two different species of organisms that is beneficial to one at the other’s expense Commensal Relating to commensalism; the association between two different organisms in which one benefits and has a neutral effect on the other Pathogenic Parasite that has demonstrated the ability to cause disease PARASITIC LIFE CYCLES Although parasitic life cycles range from simple to complex, they all have three common components—a mode of transmission, a morphologic form that invades humans, known as the infective stage, and one (or more) forms that can be detected via laboratory retrieval methods, known as the diagnostic stage. Some parasites require only a definitive host, whereas others also require one or more intermediate hosts.  Medical Parasitology Dr. Khalis Ahmed Lecture..1-2 A parasitic life cycle consists of two common phases (Fig. 1-1). One phase involves the route a parasite follows when in or on the human body. This information provides an understanding of the symptomatology and pathology of the parasite. Insights about the best the method of diagnosis and selection of appropriate antiparasitic medication may also be determined. The other phase, the route a parasite follows independently of the human body, provides crucial information pertinent to epidemiology, prevention, and control.  Medical Parasitology Dr. Khalis Ahmed Lecture..1-2 DISEASE PROCESSES AND SYMPTOMS A parasitic disease may affect the entire body or any of its parts. The major body areas associated with such processes include the following: (1) the gastrointestinal (GI) and urogenital (UG) tracts; (2) blood and tissue; (3) liver, lung, and other major organs; and (4) miscellaneous locations, such as cerebrospinal fluid (CSF), eye, skin, and extremities. A wide variety of representative symptoms, summarized in Box 1-3, may occur when a parasite infects a human host. Some persons remain asymptomatic, whereas other parasites produce severe symptoms and may result in death. The most commonly observed symptoms include diarrhea, fever, chills, abdominal pain, and abdominal cramping. Other symptoms, such as elephantiasis (an enlargement of areas such as the breast, leg, and scrotum caused by a parasite’s presence), anemia, vitamin deficiency, bowel obstruction, edema, enlargement of major organs, skin lesions, and blindness, may develop. TREATMENT There are several options for treating parasitic infections. Examples of such measures are listed in Box 1-4. There are a variety of antiparasitic medications available. Many of these drugs are toxic to the host and care should be exercised  Medical Parasitology Dr. Khalis Ahmed Lecture..1-2 when selecting the proper course of treatment. Therapies such as a change in diet, vitamin supplements, fluid replacement, blood transfusion, and bed rest may be indicated solely or in addition to chemotherapy. Treatment for nonpathogenic parasitic infections is usually not indicated. PREVENTION AND CONTROL Prevention and control measures may be taken against every parasite infective to humans. Preventive measures designed to break the transmission cycle are crucial for successful parasite eradication. Examples of such measures are listed in Box 1-5 and include the following: education programs, use of insecticides and other chemicals, protective clothing, protective netting, proper water treatment, good personal hygiene, proper sanitation practices, proper handling and preparation of food, and avoidance of unprotected sexual relations. The vast capital expenditures required to accomplish these measures are not available in many endemic countries in the world. The problem of eradicating parasites is an ongoing process and is a key goal of international health groups such as the World Health Organization (WHO) and Doctors Without Borders (Médecins Sans Frontières [MSF]).  Medical Parasitology Dr. Khalis Ahmed Lecture..1-2 SPECIMEN PROCESSING AND LABORATORY DIAGNOSIS Proper specimen selection and processing are crucial to parasite recovery. There are a variety of acceptable specimen types that may be examined for parasites. Stool is the most commonly submitted sample for such studies. Typical stool analysis consists of performing macroscopic and microscopic techniques on a portion of unpreserved sample when available. A process to remove fecal debris, which often resembles parasitic forms, is performed on a portion of sample after a preservative is added to it. Microscopic analysis of the resultant processed sample follows. This traditional parasite recovery method, often referred to as an O&P, in which “O” stands for ova (eggs) and “P” stands for parasites, is still widely used today. Other specimens, including blood, tissue biopsies, CSF, sputum, urine, and genital material, may also be examined for the presence of parasites. In some cases, the sample is basically processed the same as for stool. Other specimens, such as blood, are traditionally processed differently. For example, a Giemsa stain followed by microscopic examination is the procedure of choice for blood samples submitted for parasite study. A number of other traditional and new parasite recovery techniques are available. Cellophane tape preparation, a methodology for recovery of pinworm eggs, and the Enterotest (string test) for recovery of several parasites are among the traditional tests. Representative newer methodologies are listed in Box 1- 6. Details regarding these various specimen processing techniques are found in Chapter 2, “Specimen Collection and Processing.” It is important to note that Chapter 2 was designed to provide representative examples of laboratory methodologies that may be used to recover parasites. In some cases, Chapter 2 contains laboratory methodologies that are not covered in the corresponding individual parasite laboratory diagnosis sections. Similarly, the laboratory diagnosis section of select individual parasites mentions additional possible laboratory  Medical Parasitology Dr. Khalis Ahmed Lecture..1-2 techniques that are not specifically identified as being associated with these parasites or are not mentioned at all in Chapter 2. Thus, examination and study of the methods covered in Chapter 2 and those identified in the individual parasite laboratory diagnosis sections are required to understand and appreciate fully the extent of laboratory techniques available. Careful and thorough microscopic examination of samples for parasites is essential to ensure that accurate patient results are obtained and ultimately reported. Suspicious forms that visually resemble parasites in terms of size and morphology are commonly encountered and are often referred to as artifacts and/or confusers. For example, the Entamoeba histolytica cyst (described in detail in Chapter 3), a single-celled eukaryotic animal known as a protozoa, typically measures 12 to 18 microns (µm), a measurement defined as one millionth of a meter (10−6 m). Similarly, polymorphonuclear leukocytes average 15 µm in size. In addition, the nuclear structures, although very different on further inspection, may often initially appear almost identical. Plant cells, as another example, resemble the Ascaris lumbricoides egg (see Chapter 8 for details), a member of the subkingdom Metazoa, which includes multicellular organisms such as parasitic worms. Not only do they share structural similarities, but both may measure in the diameter range of 30 to 50 µm. There are numerous artifacts and confusers (also often referred to as pseudoparasites) that may be present in samples submitted for parasite study. Brief detailed descriptions of a select group of commonly encountered artifacts and confusers are discussed in Chapter 12.  Medical Parasitology Dr. Khalis Ahmed Lecture..1-2 PARASITE NOMENCLATURE AND CLASSIFICATION The scientific names of parasites are written in italics and consist of two components, genus (pl., genera) and species. An example of a parasite name is Giardia intestinalis, in which Giardia is the genus name and intestinalis is the species name. When a parasite name first appears in a document, the entire parasite name is written out. Subsequent references to a parasite are often abbreviated by recording only the first letter of the genera name followed by a period, followed by the entire species name. Thus, the abbreviation of our example parasite Giardia intestinalis is G. intestinalis. Variations of scientific genus names are used to identify diseases and conditions associated with their presence. The suffix- iasis is often used to denote such diseases or conditions. For example, giardiasis refers to the disease or condition associated with Giardia intestinalis. In some cases, a variation of a scientific genus name may be used to refer to a genus of parasites. Here is an example of this use of a genus name. Chapter 5 of this text discusses two genera of parasites, Leishmania and Trypanosoma. In general, reference to infections with these two genera is often written as leishmanial infections and trypanosomal infections. Along with specific parasite name variations, variations of parasite category names are common. An example of this terminology is the amebas. When appropriate, reference to the amebas may be written in several ways, such as amebic or ameboid. There are several different parasite classification systems, ranging from very basic to complex. The system used in this text delineates three major groups of clinically significant parasites: 1. Single-celled parasites—Protozoa (Fig. 1-2) 2. Multicellular worms— Metazoa helminths (Fig. 1-3) 3. Arthropods (insects and their allies)—Animalia (Fig. 1-4) Medical Parasitology Dr. Khalis Ahmed Lecture..1-2