Parasitology

Parasitology is the study of all Parasites, their association with hosts, and their causes of infectious diseases. So, whole Parasitology covers mainly Parasite, Host and its association with them.

Parasite

Simply parasitism is living in association with the host.  A living organism that receives nourishment and shelter from another organism where it lives is called a parasite. A parasite does not necessarily always cause disease. The parasite derives all benefits from the association and the host may either not be harmed or may suffer the consequences of this association, a parasite disease.  

Host 

An organism which provides nourishment and shelter to another organism is known as the host. The host does not always get harmed. A parasite that is entirely dependent on another organism, referred to as its host, for all or part of its life cycle and for metabolic requirements. The host always does not get harmed, as it depends on their association with parasites like parasitic, mutualistic, and commensalism. 

Terms for the association of Parasites:

Symbiosis

An association in which both are so dependent upon each other that one can’t live without the help of the other. None of the partners suffers any harm from the association. The living together of two or more organisms. One classic example is the relationship between certain species of flagellated protozoa living in the gut of termites. The protozoa, which depend entirely on a carbohydrate diet, acquire their nutrients from termites. In return, they are capable of synthesizing and secreting cellulases; the cellulose digesting enzymes, which are utilized by termites in their digestion. 

Commensalisms

An association in which the parasite only is deriving benefit without causing injury to its host. A relationship between members of different species living in proximity (the same cultural environment) in which one organism benefits from the association but the other is not affected. An association in which the commensal takes the benefit without causing injury to the host. E.g. Most of the normal floras of the human body can be considered as commensals. 

Parasitism

An association in which the parasite derives benefit and the host gets nothing in return but always suffers some injury, however slight the injury may be. The host, at the same time, offers some resistance to the injury done by the parasite and there may be some adaptation (tolerance) between the parasite and the host. A parasite has lost its power of independent life. An association where one of the partners is harmed and the other lives at the expense of the other. For E.g. Worms like Ascaris lumbricoides reside in the gastrointestinal tract of man and feed on important items of intestinal food causing various illnesses.

Classes of parasites

Parasites are mainly classified into the following classes along with some other types of Parasites.

Ecto-parasite (ectozoa): A parasitic organism that lives on the outer surface of its host, e.g. lice, ticks, mites, etc.

 Endo-parasite (entozoa): A parasite that lives inside the body of their host, e.g. Entamoeba histolytica,  hookworms, etc.

Temporary parasite: The parasite visits its host for a short period e.g leeches visit their host only for a short period of time

Permanent parasite:  A parasitic life throughout the whole period of its life, e.g Ascaris lumbricoides, Entamoeba histolytica, etc.

Obligatory parasite: This parasite is completely dependent on the host during a   segment or all of its life cycle, e.g. Plasmodium spp. 

Facultative parasite: An organism that exhibits both parasitic and non-parasitic modes of living and hence does not absolutely depend on the parasitic way of life, but is capable of adapting to it if placed on a host, e.g. Naegleria fowleri.

Occasional or accidental parasite:  When a parasite attacks an unnatural host and survives, e.g. Hymenolepis diminuta (rat tapeworm). 

Wandering or aberrant parasite:   One that wanders into an organ in which it is not usually found, e.g. Entamoeba histolytica in the liver or lung of humans.

Opportunistic parasites: The parasites which do not ordinarily produce disease in healthy (immune-competent) individuals but do cause illness in individuals with impaired defense mechanisms (opportunistic parasites). It is becoming of paramount importance because of the increasing prevalence of HIV/AIDS in the world, e.g cylosphora,

Classes of host

Definitive host

Parasites either harbors the adult stage of the parasite or where the parasite utilizes the sexual method of reproduction. In the majority of human parasitic infections, man is the definitive host. For example, Plasmodium species causing malariae has distinct developmental cycles in the female Anopheles mosquito and in the human host. The mosquito serves as the definitive host and the human host is intermediate.

Intermediate host

Parasites either harbors the larval or asexual stages of the parasite. In some cases, larval developments are completed in two different intermediate hosts which are been referred to as first and second intermediate hosts respectively. For example, in W. brancrofti causing filariasis), the intermediate host is mosquito (Culex, Aedes, Anopheles) and the definitive host is the man.

Paratenic host (a carrier or transport host)

A host that serves as a temporary refuge and vehicle for reaching an obligatory host, usually the definitive host, i.e. it is not necessary for the completion of the parasite’s life cycle.

Reservoir host

A host that makes the parasite available for transmission to another host and is usually not affected by the infection.

Natural host: A host that is naturally infected with certain species of parasite.

Accidental host: A host that is under normal circumstances not infected with the parasite.

Classification of parasites

Protozoa: Single-cell for all function	Helminths: Multicellular cells for the specialized function
1. Amoeba: move by pseudopodia	1. Nematodes: cylindrical body, unsegmented
2. Flagellates: move by flagella	2. Trematodes: leaf-like body, unsegmented
3. Ciliates: move by cilia	3. Cestodes: tape-like body, segmented
4. Apicomplexan: sporozoa

Example of protozoa

• Amoeba: Entamoeba histolytica, Entamoeba coli, Entamoeba gingivalis, Acanthamoeba species , Naegleria species etc.
• Flagellates: Giardia lamblia, Trichomonas vaginalis, Trypanosoma species, Leishmaniaspecies etc.
• Ciliates:  Balantidium coli
• Coccidia: Plasmodium species, Toxoplasma gondii, Isospora belli Cryptosporidium parvum, Cyclospora cayetanensis etc.
• Microsporidia:  Encephalitozoon species,  Enterocytozoon species etc.

Helminthes

The helminthic parasites are multicellular, bilaterally symmetrical animals having three germ layers (triploblastic metazoa).

Cestodes: Taenia species (Tapeworm), Echinococcus granulosus (Dog worm), etc.

Nematodes: Ascaris lumbricoides (large roundworm), Enterobius vermicularis (threadworm), Trichuris trichiura (whipworm), Strongyloides stercoralis, Ancylostoma duodenale (hookworm), Necator americanus (hookworm) Wuchereria bancrofti etc.

Trematodes (Flukes): Schistosoma species, Paragonimus species Fasciolopsis buski, Fasciola hepatica etc.

References

1. K.D. Chatterjee  (2011). Parasitology Protozoology And Helminthology,  Chatterjee Publications.

2. Apurba Sankar Sastry and bhakta s (2014). Essentials of medical parasitology,   Jaypee Brothers Medical Publishers (P) Ltd.

2. Entamoeba histolytica

Introduction

 Entamoeba histolytica is one of six parasitic amebae of the genus Entamoeba that are known to infect humans. Entamoeba coli,Entamoeba gingivalis,Entamoeba moshkovskii, and Entamoeba hartmanni are associated with non-patholognic but Entamoeba polecki and Entamoeba histolytica are pathogenic.  E. histolytica are pathogenic amoeba that is widely known for causing intestinal and extraintestinal infections in human beings. E. histolytica falls under the phylum Protozoa, the class Sarcodina, and the order Lobosa. E. histolytica causes dysentery by invading the mucosa and submucosa layers of the large intestine in human beings.

Geographical distribution

E. histolytica is found in all populations throughout the world but is more prevalent in the tropics than in cooler climates. Found more in unsanitary communities of temperate and subartic areas. Occurs worldwide, but is more common in areas or countries with poor sanitation, particularly in the tropics.

History

In 1828, James Annesley wrote in Prevalent Diseases of India, hepatic disease seems to be induced by the disorder of the bowels, more particularly when this disorder is of a sub-acute or  chronic kind’, recognizing for the first time a link between dysentery and liver abscess. Approximately 30 years later, in 1855, Lambl described amebae in the stool of a child who had diarrhea. Fedor Losch described amebae in the stool of a young farmer with dysentery from his first evaluation in November 1873 until his death in April 1874.

Epidemiology

E. histolytica infections occur worldwide but are more prevalent in the tropics. It has been estimated that approximately 480 million people or 12% of the world’s population are infected and that annual mortality is 40,000 to 110,000 persons. E. histolytica are commonly found in tropical and subtropical countries. The life cycle of E. histolytica begins and ends inside one single host, i.e., an individual human being. Among the protozoans, Entamoeba histolytica infects 500 million individuals per year, causing disease in 50 million and ultimately resulting in 100,000 deaths. The global mortality due to E. histolytica infections was reported to be 50–75 thousand deaths per year.  The clinical manifestations vary from asymptomatic colonization to amebic dysentery and invasive extraintestinal infections. Liver abscess is the most common extra intestinal presentation. It is widely acknowledged that Entamoeba infections are of public health importance but the geographical distribution and regional burden are yet to be determined.

E. histolytica is the most common intestinal protozoan parasite in Nepal ranging from less than 3% to high as 28.8%. Humans are the major reservoir of infection with E. histolytica, although natural infections in macaque monkeys and pigs have been reported.

Risk group

Worldwide, with a higher incidence of amoebiasis in developing countries due lack of sanitary conditions and poor water supplement systems. High-risk groups include male homosexuals, travelers, and recent immigrants.

Mode of transmission

Ingestion of food and drink contaminated with E. histolytica cysts from human feces and direct fecal-oral contact are the most common means of infection.

Morphology

1. Trophozoite

Outside the cyst, the nuclei within the quadrinucleate ameba begin to separate from the surrounding cytoplasm and undergo division to form eight uninucleate metacystic trophozoites. The resulting trophozoites are always smaller (8μm) than the trophozoites seen in the bowel of an infected human. The metacystic trophozoites continue to feed and grow, finally achieving the size normally associated with the trophozoite.

• Precyst

In the precyst stage, the trophozoite becomes approximately the same size as the cyst. The precystic form is uninucleate, and the enlarged nucleus contains a karyosome that is more or less escentric.

• Cyst

A cyst wall develops around the precystic form, and the single nucleus divides to form the mature quadrinucleate stage. Again because of shrinkage caused by the dehydration reagents, cysts maybe 1 to 1.5 μm smaller than organisms seen on wet preparations. They are usually spherical and contain four nuclei. The nuclear membrane is uniformly lined with peripheral chromatin. The karyosome is small and usually centrally located within the nucleus.

• Metacyst

During the process of excystation, the encysted ameba containing four nuclei becomes very active, separating from the cyst wall. The quadrinucleate ameba escapes from the cyst wall through a tiny pore, and the nuclei clump together.

Reproduction: various modes of reproduction seen in E. histolytica are

Encystatition:- This is the process of transformation of trophzoite into cysts that occurs in the lumen of an infected individual.

Excystation:- This is the process of transformation of cysts into trophzoite and occurs only in the alimentary canal of the susceptible host. During excystation a quadrinucleate cyst gives rise to eight amoebulae each one of which is capable of developing into a trophozoite.

Multiplication:- Multiplication occurs by simple binary fission first of the nucleus and then of cytoplasm.

Life-cycle of Entamoeba histolytica

The active (trophozoite) stage exists only in the host and in fresh loose feces, cysts survive outside the host in water, in soils, and on foods, especially under moist conditions on the latter. The cysts are readily killed by heat and by freezing temperatures and survive for only a few months outside of the host. When quadrinucleate cysts are swallowed they cause excysting by releasing the trophozoite stage due to lysing the cyst cell wall by the action of trypsin in the digestive tract. A single trophozoiteof E. histolytica with four nuclei are liberated. The trophozoite divides by binary fission giving rise to eight daughter trophozoites. The daughter trophozoites are actively motile and are unique among the intestinal amebae parasitizing humans because it is able to invade tissue. The amoeba can actually ‘bore’ into the intestinal wall, causing lesions and intestinal symptoms, and it may reach the blood stream. From there, it can reach different vital organs of the human body, usually the liver, but sometimes the lungs, brain, spleen, etc. When the infection disseminates to extraintestinal sites, it is found most frequently in the right lobe of the liver. Some trophozoites transformed to cyst by the process of encystation, and some cysts along with trophozoites excreted in the feaces which are susceptible to the human host. These remain viable for 2 months if taken up by another host, its life cycle continues in another host.

Pathogenesis

Two mechanisms of pathogenesis have evolved:

•       A. Secretion of soluble toxins

•       B. Cellular contact

A number of E. histolytica adherence receptors have been identified, but the galactose-specific lectin receptor has been the most thoroughly studied and is thought to be responsible for mediating attachment to colonic mucus and colonic epithelial cells. Human colonic epithelial cells and mucus contain large numbers of galactose or N-acetylgalactosamine residues. E. histolytica also enhances mucus secretion, which correlates with its pathogenicity. More virulent strains stimulated mucus secretion and depleted goblet cells of mucin, thereby making epithelial surfaces more vulnerable to invasion. Penetration of the mucous blanket was thought to be due primarily to mechanical ameboid movement. The pore-forming ameba pore protein is transferred from the trophozoite to the target cell, causing a disruption of the transmembrane gradient and contributing to the cell’s death by the colloid-osmosis lysis mechanism. They are involved in amebic invasiveness at intercellular junctions, thereby allowing amoebae to invade host tissues. Cysteine proteinase can degrade cellular attachment and matrix proteins such as collagen, laminin, and fibronectin. There is a direct correlation between the amount of proteinase activity and the pathogenicity of E. histolytica. Furthermore the extracellular products of  E. histolytica cleaves the ground substances and also cleave the complement components making them paralyze to neutralize this parasite.

Clinical manifestation

Asymptomatic infection

 Upto 90% of E. histolytica infections, the symptoms are absent or very mild. These patients have normal rectosigmoidoscopic findings, without a history of blood in stool samples.  Cysts and trophozoites lacking ingested RBCs may be visible on microscopy. Interestingly, most individuals infected with E. histolytica, but not E. dispar, develop serum antibody responses to the parasite even in the absence of invasive disease.

Symptomatic infections

Symptoms commonly attributed to E. histolytica colitis or dysentery are abdominal pain or tenderness and diarrhea (watery, bloody, or mucous). Diarrhea can occur with up to 10 (or even more) bowel movements per day, and fever occurs in one-third of the patients. Patients are often reluctant to eat, and one-fifth develop weight loss. Although people can be asymptomatically colonized with E. histolytica, they should be treated. Colonic findings in amebiasis have varied from thickening of the mucosa to flask-shaped ulceration (mostly in the cecum or appendix or near the ascending colon, but rarely in the sigmoidorectal area). The development of fulminant colitis, ameboma, cutaneous amebiasis, and rectovaginal fistulas can occur as complications of intestinal amebiasis.

Extraintestinal Amebiasis

Liver abscess is the most common manifestation of extraintestinal amebiasis. Amebic liver abscess (ALA) is associated with fever and abdominal pain in most patients. Right upper abdominal pain or tenderness occurs in the acute phase, while weight loss, fever, and more diffuse abdominal pain occur in the subacute phase. ALA occurs more commonly in adults than in children. E. histolytica has been identified microscopically in the stool samples of only a minority of patients.

Biochemically, many patients also have elevated peripheral white blood cell counts and alkaline phosphate levels. Unusual sites or complications of extraintestinal amebiasis include direct extension from the liver to the pleura and/or pericardium, brain abscess, and genitourinary amebiasis. A common outcome of this invasion of tissues is a liver abscess, which can be fatal if untreated.

Laboratory Diagnosis

Diagnosis depends primarily on demonstration of haematophagus trophozoite of E. histolytica in stool samples, aspirates from intestinal and other organs, biopsy materials (pinch biopsy at protoscopy or sigmoidoscopy and surgical biopsy from elsewhere) and in mucus from rectal ulcers. Serology is the method of choice for diagnosis of amoebic liver disease.

Microscopy

Diagnosis of E. histolytica has historically relied on microscopic examination of protozoan morphology.Two techniques are currently employed that is wet mount for the demonstration of   trophozoites in freshly collected stool specimen and iodine preparation for the demonstration of cystic form. Current microscopy- and histology-based identification frameworks, however, are unable to differentiate among protozoa with similar morphological features.

Culture

Boeck and Drbohlav first cultivated E. histolytica in a diphasic egg slant medium. Today, the National Institutes of Health modification of Locke-egg medium has been used in some research laboratories. However, Robinson medium and TYSGM-9 of Diamond  are more often used for axenic cultivation of E. histolytica. After being used successful axenic cultivation by Diamond, TYI-S-33  is one of the most widely used axenic media.

Molecular technique

Molecular biology-based diagnosis (PCR) seems to be a modern research tool that may become the technique of choice in the future studies, because establishment of these protozoa in culture is not a routine process and is less sensitive than microscopy in detection.

Antibody and Antigen Detection

Serological tests are more helpful for the identification of E. histolytica infection  that have been used so far involve IHA, counter immune electrophoresis (CIE) , amoebic gel diffusion test , complement fixation (CF) , indirect fluorescence assay(IFA), latex agglutination, and ELISA.

Antigen-based ELISA have several significant advantages over other methods currently used for diagnosis of amebiasis. It also helps to differentiate the different species of amebiasis.

Treatment

      Metronidazole is the mainstay of therapy for invasive amebiasis.  Tinidazole has been approved by the US Food and Drug Administration (FDA) for intestinal or extraintestinal amebiasis. Other nitroimidazoles with longer half-lives (ie, secnidazole and ornidazole) are can be used.

 Prevention and Control

     Public education on public and personal hygiene, sanitary disposal of human feces, and food handling. Provision of potable water devoid of fecal contamination. Chemoprophylaxis focused mainly on the treatment of chronic cyst passers. Education of high-risk groups on sexual and other habitual practices that promote fecal-oral transmission. Thorough washing of fruits and vegetables using effective disinfectants before consumption.

References

·       Life cycle of Entamoeba histolytica (Available online: http://en.wikipedia.org/ wiki/File:Entamoeba_histolytica_life_cycle-en.svg). 

• https://www.jaypeedigital.com/book/9789351523291/chapter/ch3
• S. Khadka, S. Sapkota, S. Adhikari et al., (2021). “Intestinal parasitoses among Chepang and Musahar community people of Makwanpur and Nawalparasi Districts of Nepal,” Acta Parasitologica, vol. 66, no. 1, pp. 146–154,.
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• K.D. Chatterjee (2011). Parasitology (Protology and Helminthology) 13th edition., Thomson Press (India) Ltd., New Delhi.
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• C. K Jayaram Paniker’s (2018). Textbook of medical parasitology, Jaypee Brothers Medical Publishers (P)., Ltd, New Delhi.
• Parija, Subhash Chandra (2008). Textbook of medical Parasitology, Protozoology & Helminthology. (Text an Colour atlas). Third edition (reprint). New Delhi, All India Publishers & Distributors.

Giardia-lamblia