Parasitology

Parasitology

OBJECTIVES

At the end of this section the student is expected to:

·         Discuss the various types of parasites and hosts.

·         Explain the relationship between a parasite and the host and their effects.

·         Discuss in detail the classification of medically important parasites. 

·         Explain the difference between the Cestodes, Nematodes, Trematodes and protozoa

INTRODUCTION
Man and other living things on earth live in an entangling relationship with each other. They don’t exist in an isolated fashion. They are interdependent; each forms a strand in the web of life. Medical parasitology is the science that deals with organisms living in the human body (the host) and the medical significance of this host-parasite relationship.

ASSOCIATION BETWEEN PARASITE AND HOST

A parasite is a living organism, which takes its nourishment and other needs from a host; the host is an organism which supports the parasite. The parasites included in medical parasitology are protozoa, helminthes, and some arthropods. The hosts vary depending on whether they harbor the various stages in parasitic development.

DIFFERENT KINDS OF PARASITES

·         Ectoparasite – a parasitic organism that lives on the outer surface of its host, e.g. lice, ticks, mites etc.

·         Endoparasites – parasites that live inside the body of their host, e.g. Entamoeba histolytica.

·         Obligate 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

·         Accidental parasite – when a parasite attacks an unnatural host and survives. E.g. Hymenolepis diminuta (rat tapeworm).

·         Erratic parasite -is one that wanders in to an organ in which it is not usually found. E.g. Entamoeba histolytica in the liver or lung of humans.

Most of the parasites which live in/on the body of the host do not cause disease (non-pathogenic parasites). However, understanding parasites which do not ordinarily produce disease in healthy (immunocompetent) individuals but do cause illness in individuals with impaired defense mechanism (opportunistic parasites) is becoming of paramount importance because of the increasing prevalence of HIV/AIDS in our country.

DIFFERENT KINDS OF HOSTS

·         Definitive host – a host that harbors a parasite in the adult stage or where the parasite undergoes a sexual method of reproduction.

·         Intermediate host - harbors the larval stages of the parasite or an asexual cycle of development takes place. In some cases, larval development is completed in two different intermediate hosts, referred to as first and second intermediate hosts.

·         Paratenic 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 parasites life cycle.

·         Reservoir host – a host that makes the parasite available for the 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.

There is a dynamic equilibrium which exists in the interaction of organisms. Any organism that spends a portion or all of its life cycle intimately associated with another organism of a different species is considered as Symbiont (symbiote) and this relationship is called symbiosis (symbiotic relationships). 

The following are the three common symbiotic relationships between two organisms:

·         Mutualism -an association in which both partners are metabolically dependent upon each other and one cannot live without the help of the other; however, none of the partners suffers any harm from the association. 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.

·         Commensalism - an association in which the commensal takes the benefit without causing injury to the host. E.g. Most of the normal floras of the humans’ body can be considered as commensals.

·         Parasitism - an association where one of the partners is harmed and the other lives at the expense of the other. E.g. Worms like Ascaris lumbricoides reside in the gastrointestinal tract of man, and feed on important items of intestinal food causing various illnesses.

Once we are clear about the different types of associations between hosts and parasites, we can see the effect the parasite brings to the host and the reactions which develop in the host’s body due to parasitic invasion.

EFFECT OF PARASITES ON THE HOST

The damage which pathogenic parasites produce in the tissues of the host may be described in the following two ways;

(a) Direct effects of the parasite on the host

·         Mechanical injury - may be inflicted by a parasite by means of pressure as it grows larger, e.g. Hydatid cyst causes blockage of ducts such as blood vessels producing infraction.

·         Deleterious effect of toxic substances- in Plasmodium falciparum production of toxic substances may cause rigors and other symptoms.

·         Deprivation of nutrients, fluids and metabolites -parasite may produce disease by competing with the host for nutrients.

(b) Indirect effects of the parasite on the host:

·         Immunological reaction: Tissue damage may be caused by immunological response of the host, e.g. nephritic syndrome following Plasmodium infections. Excessive proliferation of certain tissues due to invasion by some parasites can also cause tissue damage in man, e.g. fibrosis of liver after deposition of the ova of Schistosoma.

BASIC CONCEPTS IN MEDICAL PARASITOLOGY

In medical parasitology, each of the medically important parasites are discussed under the standard subheadings of morphology, geographical distribution, means of infection, life cycle, host/parasite relationship, pathology and clinical manifestations of infection, laboratory diagnosis, treatment and preventive/control measures of parasites. In the subsequent section some of these criteria are briefly presented.

Morphology - includes size, shape, color and position of different organelles in different parasites at various stages of their development. This is especially important in laboratory diagnosis which helps to identify the different stages of development and differentiate between pathogenic and commensal organisms. For example: Entamoeba histolytica and Entamoeba coli.

Geographical distribution - Even though revolutionary advances in transportation has made geographical isolation no longer a protection against many of the parasitic diseases, many of them are still found in abundance in the tropics. Distribution of parasites depends upon:

a.   The presence and food habits of a suitable host:

·         Host specificity, for example, Ancylostoma duodenale requires man as a host where Ancylostoma caninum requires a dog.

·         Food habits, e.g. consumption of raw or undercooked meat or vegetables predisposes to Taeniasis

b.   Easy escape of the parasite from the host- the different developmental stages of a parasite which are released from the body along with faeces and urine are widely distributed in many parts of the world as compared to those parasites which require a vector or direct body fluid contact for transmission.

c.   Environmental conditions favoring survival outside the body of the host, i.e. temperature, the presence of water, humidity etc.

d.   The presence of an appropriate vector or intermediate host – parasites that do not require an intermediate host (vector) for transmission are more widely distributed than those that do require vectors.

Once we are clear about the geographical distribution and conditions favoring survival in relation to different parasites, effective preventive and control measures can more easily be devised and implemented.

Life cycle of parasites: The route followed by a parasite from the time of entry to the host to exit, including the extracorporeal (outside the host) life. It can either be simple, when only one host is involved, or complex, involving one or more intermediate hosts. A parasite’s life cycle consists of two common phases one phase involves the route a parasite follows inside the body. This information provides an understanding of the symptomatology and pathology of the parasite. In addition the method of diagnosis and selection of appropriate medication may also be determined. The other phase, the route a parasite follows outside of the body, provides crucial information pertinent to epidemiology, prevention, and control. 

Host parasite relationship - infection is the result of entry and development within the body of any injurious organism regardless of its size. Once the infecting organism is introduced into the body of the host, it reacts in different ways and this could result in:

a.       Carrier state - a perfect host parasite relationship where tissue destruction by a parasite is balanced with the host’s tissue repair. At this point the parasite and the host live harmoniously, i.e. they are at equilibrium. 

b.      Disease state - this is due to an imperfect host parasite relationship where the parasite dominates the upper hand. It can result either from lower resistance of the host or a higher pathogenecity of the parasite.

c.       Parasite destruction – occurs when the host takes the upper hand.

Laboratory diagnosis – depending on the nature of the parasitic infections, the following specimens are selected for laboratory diagnosis:

a) Blood – in those parasitic infections where the parasite itself in any stage of its development circulates in the blood stream, examination of blood film forms one of the main procedures for specific diagnosis. For example, in malaria the parasites are found inside the red blood cells. In Bancroftian and Malayan filariasis, microfilariae are found in the blood plasma.

b) Stool – examination of the stool forms an important part in the diagnosis of intestinal parasitic infections and also for those helminthic parasites that localize in the biliary tract and discharge their eggs into the intestine.  In protozoan infections, either trophozoites or cystic forms may be detected; the former during the active phase and the latter during the chronic phase. Example, Amoebiasis, Giardiasis, etc.  In the case of helmithic infections, the adult worms, their eggs, or larvae are found in the stool.

c) Urine – when the parasite localizes in the urinary tract, examination of the urine will be of help in establishing the parasitological diagnosis. For example in urinary Schistosomiasis, eggs of Schistosoma haematobium are found in the urine. In cases of chyluria caused by Wuchereria bancrofti, microfilariae are found in the urine.

d) Sputum – examination of the sputum is useful in the following:

·         In cases where the habitat of the parasite is in the respiratory tract, as in Paragonimiasis, the eggs of Paragonimus westermani are found.

·         In amoebic abscess of lung or in the case of amoebic liver abscess bursting into the lungs, the trophozoites of E. histolytica are detected in the sputum.

e) Biopsy material - varies with different parasitic infections. For example spleen punctures in cases of kala-azar, muscle biopsy in cases of Cysticercosis, Trichinelliasis, and Chagas’ disease, Skin snip for Onchocerciasis.

f) Urethral or vaginal discharge – for Trichomonas vaginalis

Indirect evidences – changes indicative of intestinal parasitic infections are:

        i.            Cytological changes in the blood – eosiniphilia often gives an indication of tissue invasion by helminthes, a reduction in white blood cell count is an indication of kala-azar, and anemia is a feature of hookworm infestation and malaria.

      ii.            Serological tests – are carried out only in laboratories where special antigens are available.

Treatment – many parasitic infections can be cured by specific chemotherapy. The greatest advances have been made in the treatment of protozoal diseases.  For the treatment of intestinal helminthiasis, drugs are given orally for direct action on the helminthes. To obtain maximum parasiticidal effect, it is desirable that the drugs administered should not be absorbed and the drugs should also have minimum toxic effect on the host.

Prevention and control - measures may be taken against every parasite infectiving humans. Preventive measures designed to break the transmission cycle are crucial to successful parasitic eradication. Such measures include: Reduction of the source of infection- the parasite is attacked within the host, thereby preventing the dissemination of the infecting agent. Therefore, a prompt diagnosis and treatment of parasitic diseases is an important component in the prevention of dissemination.

·         Sanitary control of drinking water and food.

·         Proper waste disposal – through establishing safe sewage systems, use of screened latrines, and treatment of night soil.

·         The use of insecticides and other chemicals used to control the vector population. 

·         Protective clothing that would prevent vectors from resting in the surface of the body and inoculate pathogens during their blood meal. . Good personal hygiene.

·         Avoidance of unprotected sexual practices.

CLASSIFICATION OF MEDICAL PARASITOLOGY

Parasites of medical importance come under the kingdom called protista and animalia. Protista includes the microscopic single-celled eukaryotes known as protozoa. In contrast, helminthes are macroscopic, multicellular worms possessing well-differentiated tissues and complex organs belonging to the kingdom animalia. Medical Parasitology is generally classified into: 

·         Medical Protozoology - Deals with the study of medically important protozoa.  

·         Medical Helminthology - Deals with the study of helminthes (worms) that affect man.

·         Medical Entomology - Deals with the study of arthropods which cause or transmit disease to man.

Describing animal parasites follow certain rules of zoological nomenclature and each phylum may be further subdivided as follows:

GENERAL CHARACTERISTICS OF MEDICALLY IMPORTANT PARASITES

Medically important protozoa, helminthes, and arthropods, which are identified as causes and propagators of disease have the following general features. These features also differ among parasites in a specific category. 

(1) PROTOZOA

Protozoan parasites consist of a single "cell-like unit" which is morphologically and functionally complete and can perform all functions of life. They are made up of a mass of protoplasm differentiated into cytoplasm and nucleoplasm. The cytoplasm consists of an outer layer of hyaline ectoplasm and an inner voluminous granular endoplasm. The ectoplasm functions in protection, locomotion, and ingestion of food, excretion, and respiration. In the cytoplasm there are different vacuoles responsible for storage of food, digestion and excretion of waste products. The nucleus also functions in reproduction and maintaining life.

The protozoal parasite possesses the property of being transformed from an active (trophozoite) to an inactive stage, losing its power of motility and enclosing itself within a tough wall. The protoplasmic body thus formed is known as a cyst. At this stage the parasite loses its power to grow and multiply. The cyst is the resistant stage of the parasite and is also infective to the human host.

Reproduction – the methods of reproduction or multiplication among the parasitic protozoa are of the following types:

1. Asexual multiplication:

(a) Simple binary fission – in this process, after division of all the structures, the individual parasite divides either longitudinally or transversely into two more or less equal parts.

(b) Multiple fission or schizogonies – in these process more than two individuals are produced, e.g. asexual reproduction in Plasmodia.

 2. Sexual reproduction:

(a) Conjugation – in this process, a temporary union of two individuals occurs during which time interchange of nuclear material takes place. Later on, the two individuals separate.

(b) Syngamy – in this process, sexually differentiated cells, called gametes, unite permanently and a complete fusion of the nuclear material takes place. The resulting product is then known as a zygote.

Protozoa are divided into four types classified based on their organs of locomotion. These classifications are: amoebas, ciliates, flagellates, and sporozoans.

Table 1: Classification of the pathogenic protozoa

PROTOZOA	ORGAN OF LOCOMOTION
1. Rhizopoda (Amoeba)	Pseudopodia
2. Mastigophora (Flagellates)	Flagella
3. Sporozoa	None, exhibit a slight        Amoeboid movement
4. Ciliates	Cilia

IMPORTANT HUMAN PATHOGENS

Entamoeba histolytica

Trypanosomes Leishmania

Trichomonas Giardia

Plasmodium spp.

Balantidium coli

(2) HELIMINTHS:

The heliminthic parasites are multicellular, bilaterally symmetrical animals having three germ layers. The helminthes of importance to human beings are divided into three main groups with the peculiarities of the different categories described in table 2.

Table 2: Differentiating features of helminthes

	CESTODE	TREMATODE	NEMATODE
Shape	Tape like, segmented	Leaf like, Unsegmented	Elongated, Cylindrical
Sexes	No separate             (monoecious)	Not separate (monoecious) Except blood flukes which are dioecious	Separate. (diecious)
"Head" End	Suckers: with hooks	Suckers: no hooks	No suckers, and hooks
Alimentary canal	Absent	Present   but incomplete	Present and           complete
Body cavity	Absent	Absent	Present

ENTAMOEBA HISTOLYTICA

It is a causative agent of Amoebaisis. It is a primitive unicellular amoeba with a relatively simple life cycle which can be divided into two stages:

• Trophozoite – actively motile feeding stage. 

• Cyst – quiescent, resistant, infective stage. 

Their reproduction is through binary fission, e.g. splitting of the trophozoite or through the development of numerous trophozoites within the mature multinucleated cyst. Motility is accomplished by extension of pseudopodia (“false foot”) 

Morphological features:

(a) Trophozoites

Viable trophozoites vary in size from about 10-60 μm in diameter. Motility is rapid, progressive, and unidirectional, through pseudopods. The nucleus is characterized by evenly arranged chromatin on the nuclear membrane and the presence of a small, compact, centrally located karyosome. The cytoplasm is usually described as finely granular with few ingested bacteria or debris in vacuoles. In the case of dysentery, however, RBCs may be visible in the cytoplasm, and this feature is diagnostic for E. histolytica.

(b) Cyst

The size of Cysts ranges from 10-20 μm. The immature cyst has inclusions namely; glycogen mass and chromatoidal bars. As the cyst matures, the glycogen completely disappears; the chromotiodials may also be absent in the mature cyst. 

Life cycle

Intestinal infections occur through the ingestion of a mature quadrinucleate infective cyst, contaminated food or drink and also by hand to mouth contact. It is then passed unaltered through the stomach, as the cyst wall is resistant to gastric juice.

In terminal ileum (with alkaline pH), excystation takes place. Trophozoites being actively motile invade the tissues and ultimately lodge in the submucous layer of the large bowel. Here they grow and multiply by binary fission.  Trophozoites are responsible for producing lesions in amoebiasis.
Invasion of blood vessels leads to secondary extra intestinal lesions. Gradually the effect of the parasite on the host is toned down together with concomitant increase in host tolerance, making it difficult for the parasite to continue its life cycle in the trophozoite phase. A certain number of trophozoites come from tissues into lumen of bowel and are first transformed into pre-cyst forms. Pre-cysts secret a cyst wall and become a uninucleate cyst. Eventually, mature quadrinucleate cysts form.  These are the infective forms. Both mature and immature cysts may be passed in faeces. Immature cysts can mature in external environments and become infective.

Figure-1: Life cycle of Entamoeba histolytica

Pathogenesis

Trophozoites divide and produce extensive local necrosis in the large intestine. Invasion into the deeper mucosa with extension into the peritoneal cavity may occur. This can lead to secondary involvement of other organs, primarily the liver but also the lungs, brain, and heart. Extraintestinal amebiasis is associated with trophozoites.  Amoebas multiply rapidly in an anaerobic environment, because the trophozites are killed by ambient oxygen concentration.

Epidemiology

E. histolytica has a worldwide distribution. Although it is found in cold areas, the incidence is highest in tropical and subtropical regions that have poor sanitation and contaminated water. About 90% of infections are asymptomatic, and the remaining produces a spectrum of clinical syndrome. Patients infected with E. hisolytica pass non­infectious trophozoites and infectious cysts in their stools. Therefore, the main source of water and food contamination is the symptomatic carrier who passes cysts. Symptomatic amoebiasis is usually sporadic. The epidemic form is a result of direct person-to-person faecal-oral spread under conditions of poor personal hygiene.  

Clinical features

The outcome of infection may result in a carrier state, intestinal amebiasis, or exteraintestinal amebiasis. Diarrhoea, flatulence, and cramping are complaints of symptomatic patients. More severe disease is characterised by the passing of numerous bloody stools in a day. Systemic signs of infection (fever, leukocytosis, rigors) are present in patients with extraintestinal amebiasis. The liver is primarily involved, because trophozoites in the blood are removed from the blood by the portal veins. The right lobe is most commonly involved, thus pain over the liver with hepatomegaly and elevation of the diaphragm is observed. 

Immunity

E.histolytica elicits both the humeral and cellular immune responses, but it is not yet clearly defined whether it modulates the initial infection or prevents reinfection. 

Laboratory diagnosis

Intestinal amoebiasis:

·         Examination of a fresh dysenteric faecal specimen or rectal scraping for trophozoite stage. (Motile amoebae containing red cells are diagnostic of amoebic dysentery).

·         Examination of formed or semiformed faeces for cyst stage. (Cysts indicate infection with either a pathogenic E. histolytica or non-pathogenic E. dispar.)

Figure-2: E. histolytica trophozoite (A) E. histolytica Cyst (B)

Extraintestinal amoebiasis:

·         Diagnosed by the use of scanning procedures for liver and other organs. 

·         Specific serologic tests, together with microscopic examination of the abscess material, can confirm the diagnosis. 

Treatment

Acute, fulminating amebiasis is treated with metrondiazole followed by iodoquinol, and asymptomatic carriage can be eradicated with iodoquinol, diloxanide furoate, or paromomycin. The cysticidal agents are commonly recommended for asymptomatic carriers who handle food for public use. Metronidazole, chloroquine, and diloxanide furoate can be used for the treatment of extra intestinal amoebiasis.

Prevention

·                     Introduction of adequate sanitation measures and education about the routes of transmission.

·                     Avoid eating raw vegetables grown by sewerage irrigation and night soil.

GIARDIA LAMBLIA 

Important features – the life cycle consists of two stages, the trophozoite and cyst. The trophozoite is 9-12 μm long and 5-15 μm wide anteriorly. It is bilaterally symmetrical, pear-shaped with two nuclei (large central karyosome), four pairs of flagella, two axonemes, and a suction disc with which it attaches to the intestinal wall. The oval cyst is 8-12μm long and7-10μm wide, thick-walled with four nucleus and several internal fibera? Each cyst gives rise to two trophozoites during excystation in the intestinal tract. Transmission is by ingestion of the infective cyst.

Figure 3: Life cycle of Giardia lamblia.

Pathogenesis

Infection with G. lamblia is initiated by ingestion of cysts. Gastric acid stimulates excystation, with the release of trophozoites in duodenum and jejunum. The trophozoites can attach to the intestinal villi by the ventral sucking discs without penetration of the mucosa lining, but they only feed on the mucous secretions. In symptomatic patients, however, mucosa-lining irritation may cause increased mucous secretion and dehydration. Metastatic spread of disease beyond the GIT is very rare. 

Epidemiology

Giardia lamblia has a worldwide distribution, particularly common in the tropics and subtropics. It is acquired through the consumption of inadequately treated contaminated water, ingestion of contaminated uncooked vegetables or fruits, or person-to-person spread by the faecal-oral route. The cyst stage is resistant to chlorine in concentrations used in most water treatment facilities. Infection exists in 50% of symptomatic carriage, and reserves the infection in endemic form.

Clinical features

Clinical disease: Giardiasis Symptomatic giardiasis ranges from mild diarrhea to severe malabsorption syndrome. Usually, the onset of the disease is sudden and consists of foul smelling, watery diarrhea, abdominal cramps, flatulence, and streatorrhoea. Blood & pus are rarely present in stool specimens, a feature consistent with the absence of tissue destruction. 

Immunity

The humoral immune response and the cellular immune mechanism are involved in giardiasis. Giardia – specific IgA is particularly important in both defense against and clearance of parasite.

Laboratory diagnosis

Examination of diarrhoeal stool- trophozoite or cyst, or both may be recovered in wet preparation. In examinations of formed stool (e.g. in asymptomatic carriers) only cysts are seen. Giardia species may occur in “showers”, i.e. many organisms may be present in the stool on a given day and few or none may be detected the next day.  Therefore one stool specimen per day for 3 days is important.

Figure 4: Giardia lamblia tphozoite (A), cyst (B)

If microscopic examination of the stool is negative in a patient in whom giardiasis is highly suspected duodenal aspiration, string test (entero-test), or biopsy of the upper small intestine can be examined. In addition to conventional microscopy, several immunologic tests can be implemented for the detection of parasitic antigens.

Treatment

For asymptomatic carriers and diseased patients the drug of choice is quinacrine hydrochloride or metronidazole.

PLASMODIUM SPP.

There are four species normally infecting humans, namely, Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, and Plasmodium malariae.

Life cycle

The life cycle of malaria is passed in two hosts (alternation of hosts) and has sexual and asexual stage (alternation of generations). Vertebrate host; man (intermediate host) where the asexual cycle takes place. The parasite multiplies by schizogony and there is formation of male and female gametocytes (gametogony). Invertebrate host; mosquito (definitive host) where the sexual cycle takes place. Union of male and female gametes ends in the formation of sporozoites (sporogony).

The life cycle passes in four stages:

Three in man:              Pre - erythrocytic schizogony 

Erythrocytic schizogony

Exo- erythrocytic schizogony

One in mosquito:         Sporogony

Introduction into humans: when an infective female Anopheles mosquito bites man, it inoculates saliva containing sporozoites (infective stage).

Pre- Erythrocytic schizogony: sporozoites reach the blood stream and within 30 minutes enter the parenchymal cells of the liver, initiating a cycle of schizogony. Multiplication occurs in tissue schizonts, to form thousands of tiny merozoites. Merozoites are then liberated on rupture of schizonts about 7th – 9th day of the bites and enter into the blood stream. These merozoites either invade the RBC’s or other parenchymal liver cells. In case of P. falciparum and possibly

P. malariae, all merozoites invade RBC’s without re-invading liver cells. However, for P. vivax and P. ovale, some merozoites invade RBC’s and some re-invade liver cells initiating further Exo-erythrocytic schizogony, which is responsible for relapses. Some of the merozoites remain dormant (hypnozoites) becoming active later on.

Erythrocytic schizogony (blood phase) is completed in 48 hrs in P. vivax, P. ovale, and P. falciparum, and 72 hrs in P. malariae. The merozoites reinvade fresh RBC’s repeating the schizogonic cycles

Erythrocytic merozoites do not reinvade the liver cells. So malaria transmitted by blood transfusion reproduces only erythrocytic cycle

Gametogony

Some merozoites that invade RBC’s develop into sexual stages (male and female gametocytes). These undergo no further development until taken by the mosquito.

Sporogony (extrinsic cycle in mosquito)

When a female Anopheles mosquito vector bites an infected person, it sucks blood containing the different stages of malaria parasite. All stages other than gametocytes are digested in the stomach.

The microgametocyte undergoes ex-flagellation. The nucleus divides by reduction division into 6-8 pieces, which migrate to the periphery. At the same, time 6-8 thin filaments of cytoplasm are thrust out, in each passes a piece of chromatin. These filaments, the microgametes, are actively motile and separate from the gametocyte.

The macrogametocyte by reduction division becomes a macrogamete. Fertilization occurs by entry of a micro gamete into the macro gamete forming a zygote.

The zygote changes into a worm like form, the ookinete, which penetrates the wall of the stomach to develop into a spherical oocyst between the epithelium and basement membrane. The oocystes increase in size. Thousands of sporozoites develop inside the oocysts. Oocysts rupture and sporozoites are liberated in the body cavity and migrate everywhere particularly to the salivary glands. Now the mosquito is infective

The sporogonous cycle in the mosquito takes 8-12 days depending on temperature

Figure 5: Life cycle of Plasmodium species

Plasmodium falciparum

Plasmodium falciparum demonstrates no selectivity in host erythrocytes, i.e. it invades young and old RBCs cells.  The infected red blood cells also do not enlarge and become distorted.

·         Multiple sporozoites can infect a single erythrocyte, and show multiple infections of cells with small ring forms.

·         The trophozoite is often seen in the host cells at the very edge or periphery of cell membrane at accole position.

·         Occasionally, reddish granules known as Maurer’s dots are observed 

·         Mature (large) trophozoite stages and schizonts are rarely seen in blood films, because their forms are sequestered in deep capillaries, liver and spleen.

·         Peripheral blood smears characteristically contain only young ring forms and occasionally crescent shaped gametocytes.

Epidemiology

P. falciparum occurs almost exclusively in tropical and subtropical regions. Weather (rainfall, temperature & humidity) is the most obvious cause of seasonality in malaria transmission. To date, abnormal weather conditions are also important causes of significant and widespread epidemics. Moreover, drug-resistant infection of P. falciparum is the commonest challenge in many parts of the world. In Ethiopia, even though all the four species of plasmodium infecting man have been recorded, P.falciparum is the one that most causes the epidemic disease and followed by vivax and malariae. P.ovale is rare. Infection rates in Ethiopia are 60%, 40%, 1%, and <1 for="" i="">P. falciparum, P. vivax, P. malariae,

and P. ovale, respectively.

Clinical features

Of all the four Plasmodia, P. falciparum has the shortest incubation period, which ranges from 7 to 10 days. After the early flu-like symptoms, P.falciparum rapidly produces daily (quotidian) chills and fever as well as severe nausea, vomiting and diarrhea. The periodicity of the attacks then becomes tertian (36 to 48 hours), and fulminating disease develops. Involvement of the brain (cerebral malaria) is most often seen in P.falciparum infection. Capillary plugging from an adhesion of infected red blood cells with each other and endothelial linings of capillaries causes hypoxic injury to the brain that can result in coma and death.  Kidney damage is also associated with P.falciparum malaria, resulting in an illness called “black water” fever. Intravascular hemolysis with rapid destruction of red blood cells produces a marked hemoglobinuria and can result in acute renal failure, tubular necrosis, nephrotic syndrome, and death.  Liver involvement is characterized by abdominal pain, vomiting of bile, hepatosplenomegally, severe diarrhea, and rapid dehydration.

Figure 6: Ring form of P. falciparum, with multiple infection of an erythrocyte

Figure 7: mature gametocyte of P. falciparum

Treatment

Because chloroquine – resistant stains of P. falciparum are present in many parts of the world, infection of P. falciparum may be treated with other agents including mefloquine, quinine, guanidine, pyrimethamine – sulfadoxine, and doxycycline.  If the laboratory reports a mixed infection involving P. falciparum and P. vivax, the treatment must eradicate not only P. falciparum from the erythrocytes but also the liver stages of P. vivax to avoid relapses provided that the person no longer lives in a malaria endemic area.

Plasmodium vivax

P. vivax is selective in that it invades only young immature erythrocytes. Infections of P. vivax have the following characteristics:

·         Infected red blood cells are usually enlarged and contain numerous pink granules or schuffner’s dots.

·         The trophozoite is ring-shaped but amoeboid in appearance.

·         More mature trophozoites and erythrocytic schizonts containing up to 24 merozoites are present.

·         The gametocytes are round

Epidemiology

P. vivax is the most prevalent of the human plasmodia with the widest geographic distribution, including the tropics, subtropics, and temperate regions. However, it is the second most prevalent in Ethiopia following P. falciparum

Clinical features

After an incubation period (usually 10 to 17 days), the patient experiences vague flu-like symptoms, such as headache, muscle pains, photophobia, anorexia, nausea and vomiting. As the infection progresses, increased numbers of rupturing erythrocytes liberate merozoites as well as toxic cellular debris and hemoglobin in to circulation. In combination, these substances produce the typical pattern chills, fever and malarial rigors. These paroxysms usually reappear periodically (generally every 48 hours) as the cycle of infection, replication, and cell lyses progresses. The paroxysms may remain relatively mild or may progress to severe attacks, with hours of sweating, chills, shaking persistently, high temperatures (103⁰F to 106⁰F) and exhaustion. Since P. vivax infects only the reticulocytes, the parasitemia is usually limited to around 2 to 5% of the available RBCs. 

Figure 8: Plasmodium vivax ring form and trophozoites

Treatment

Chloroquine is the drug of choice for the suppression and therapeutic treatment of P. vivax, followed by premaquine for radical cure and elimination of gamatocytes. 

Plasmodium malariae

In contrast with P. vivax and P. ovale, P. malariae can infect only mature erythrocytes with relatively rigid cell membranes. As a result, the parasite’s growth must conform to the size and shape of red blood cell. 

This requirement produces no red cell enlargement or distortion, but it results in distinctive shapes of the parasite seen in the host cell, “band and bar forms” as well as very compact dark staining forms. The schizont of P. malariae is usually composed of eight merozoites appearing in a rosette.

Epidemiology

P. malariae infection occurs primarily in the same sub-tropical and temperate regions as infections with the other plasmodia but is less prevalent. 

Clinical features

The incubation period for P. malariae is the longest of the plasmodia, usually 18 to 40 days, but possibly several months to years. The early symptoms are flu-like with fever patterns of 72 hours (quartan or malarial) in periodicity. 

Treatment

Treatment is similar to that for P.vivax and P.ovale.

Plasmodium ovale

P. ovale is similar to P. vivax in many respects, including its selectivity for young, pliable erythrocytes. As a consequence the classical characteristics include: 

·         The host cell becomes enlarged and distorted, usually in an oval form.

·         Schiffner’s dots appear as pale pink granules.

·         The infected cell border is commonly fimbriated or ragged 

·         Mature schizonts contain about 10 merozoites. 

Epidemiology

P. ovale is distributed primarily in tropical Africa. It is also found in Asia and South America.

Clinical features

The incubation period for P. ovale is 16-18 days but can be longer. Clinically, ovale malaria resembles vivax malaria with attacks recurring every 48-50 hours. There are however, fewer relapses with P. ovale. Less than 2% of RBCs usually become infected.

Treatment

The treatment regimen, including the use of primaquine to prevent relapse from latent liver stages is similar to that used for P. vivax infection.

Laboratory diagnosis

Microscopic examination of thick and thin films of blood is the method of choice for confirming the clinical diagnosis of malaria and identifying the specific species responsible for disease.

Malaria parasites in thick and thin blood films are best stained at pH 7.1 – 7.2 using a Romanowsky stain (contains azure dyes and eosin).

The thick film is a concentration method that may be used to detect the presence of organisms. The thin film is most useful for establishing species identification.

Serologic procedures are available but they are used primarily for epidemiological surveys or for screening blood donors.

Immunity

There is evidence that antibodies can confer hormonal immunity against malaria infection.

Prevention

·         Chemoprophylaxis and prompt diagnosis and treatment. 

·         Control of mosquito breeding

·         Protection of insect bite by screening, netting and protective clothing 

·         Use of insect repellents.

Figure 9: Romanowsky stained thin malaria films and their different stages.

ASCARIS LUMBRICOIDES

These are common roundworms infecting more than 700 million people worldwide.

Morphology:

Male adult worm measures 15-20 cm in length. The posterior end is curved ventrally. The female worm measures 20-40 cm in length. Its posterior end is straight.

Infective stage and modes of infection:

The egg containing larva when ingested with contaminated raw vegetables causes ascariasis.

Life cycle:

Ingested eggs hatch in the duodenum. The larvae penetrate the intestinal wall and circulate in the blood. From the heart they migrate to the lungs, ascend to the trachea, descend to the esophagus and finally reach the small intestine to become adult. The female pass immature eggs which pass to the soil and mature in 2 weeks.

Figure 10: Life cycle of Ascaris lubriocoides

Pathogenecity and clinical features

Adult worms in the intestine cause abdominal pain and may cause intestinal obstruction especially in children. Larvae in the lungs may cause inflammation of the lungs (Loeffler’s syndrome) – pneumonia-like symptoms.

Diagnosis

1.      Examination of stool for eggs by direct saline smear method. The egg is ovoidal, 75x60 microns, covered by albuminous mamillatins.

2.      Demonstration of adult worms

Figure11: Egg of Ascaris lumbricoides

Treatment

Mebendazole, Albendazole and Piperazine 

TAENIA SOLIUM (PORK TAPEWORM)

The adult worms of T. solium reside or inhabit the upper jejunum. Infection has worldwide distribution.

Morphology:

Adult worm measures about 3 meters in length. The globular scolex has rostellum with 2 rows of hooklets. There are <1000 30="" about="" br="" eggs.="" gravid="" liberates="" proglottid="" proglottids.="">

The pork tapeworm, Taenia solium, is the most harmful tapeworm in humans. Taenia solium infection is acquired either from human feces that contains Taenia solium eggs or from uncooked pork which contains larval cysts. If larvae are ingested, they mature into adults in the small intestine. This infection type is called taeniasis and is often asymptomatic. If eggs are ingested, the resulting disease is cysticercosis. It gets its name from larval Taenia solium called cysticercus. Both diseases are common in Africa, Asia, South America and Southern Europe. Taeniasis is rare in Muslim countries since people there do not consume pork.

Taeniasis

Taenia solium, as its Latin name suggests, uses pigs as intermediate hosts for its larval stage. A pig gets infected with cysticercosis. If a human eats pork without cooking it, the dormant larva excysts in the bowel. The larva matures into an adult tapeworm which absorbs nutrients from the passing food.

The flat body of an adult Taenia solium consists mostly of segments, proglottids. Pork tapeworm is attached to the intestinal wall with its head, the scolex. Its head has four suckers and two rows of hooks. It has a neck that produces the segments which grow bigger as they move towards the rectum. They absorb nutrients from the surrounding food. Each segment produces eggs that remain inside it until the segment is passed out in the feces. The segment is less than 1 cm long and 2 cm wide and contains up to 50000 eggs. Taenia solium grows up to 7 meters. A full grown pork tapeworm consists of 1000 segments and sheds six gravid proglottids per day. The segments are detached from the tail. Out in the nature they can be accidentally eaten by pigs or humans.

Taeniasis diagnosis is made by an endoscopic examination or by finding segments (or eggs) from the feces. Taeniasis is usually treated with niclosamide or endoscopic removal.

Cysticercosis

Microscopic tapeworm eggs are ingested by a human or pig due to poor hygiene. Tiny larvae called oncospheres hatch in the small intestine. They penetrate the intestinal wall and enter the bloodstream. They travel to muscles or other tissue such as the liver or the brain. Lastly, oncospheres transform into cysticerci and encyst. The smallest cysticerci are 0.5–1.5 cm long whereas the biggest forms are 20 cm long. About 60 % of patients with cysticercosis have cysticerci in the central nervous system which is called neurocysticercosis. Cysticerci molt into adults only in the intestine. Immune system does not recognize the cysts. They can live in the tissue for many years without causing any symptoms. Eventually they get old and their shell structures start to leak causing an inflammatory response. Common symptoms include: muscle spasms, dizziness, headaches and seizures. Major cysticerci infections can lead to a sudden death.

As the cysticerci die, the infected areas, lesions, shrink. The swelling goes down and symptoms start to go away. The area of the organ where they sited will be covered with fibrosis. Vital functions of the organ may be lost. If oncospheres travel to the eyes the developed cysticerci can float in the eye and cause disturbed or blurry vision. Infection in the eyes can also cause swelling or detachment of the retina.

The definitive host, human, can get infected with the same tapeworm over and over again. This autoinfection can occur in two ways. In some rare cases the mature segments dissolve too early releasing the eggs. It can happen, if the large intestine is not working properly. This retro-peristalsis reverses the direction of the stool and the gravid proglottids are carried back to the stomach. The larvae hatch and cause cysticercosis. Another way to autoinfect oneself with cysticerci is to scratch the anus and then put fingers into the mouth. This too requires that some microscopic eggs have been released from the segments before exiting the body. Normally the segments stay intact in the colon.

Cysticercosis diagnosis is possible from Magnetic Resonance Imaging scans or X-rays. The cysts resemble tumours so the diagnosis is not foolproof. Cysticercosis is generally treated with albendazole in combination with anti-inflammatory drugs. Drug treatment is not necessary, if the cysticerci are already dead. Surgical removal is possible, if the location of the cyst is known. All cases of cysticercosis are not treated. The decision of whether or not to treat neurocysticercosis is based upon symptoms and the number of cysticerci found in the brain. If only one is found, treatment is often not given.

Life cycle

Embryonated eggs passed with stool are ingested by pig and the embryo is released. It penetrates the intestinal wall and is carried by vascular channels to all parts of the body. After a period of 2-3 months of development the encysted larval stage called cysticerci or bladder worm occurs in the striated muscles of the tongue, neck, trunk brain, eye, and the nervous system. The cysticercus survives for 5 years. Humans become infected by eating pork containing larvae, cysticercus cellulosae. When improperly cooked cysticercus infected meat is eaten by man, the scolex remains undigested and attaches itself to the intestinal wall and chain of proglottids begin to grow to adult worm.

Figure 12: Life cycle of Taenia solium

Treatment

Niclosamide: 2 gm PO stat

Prevention:

·         Treatment of infected persons.

·         Thorough cooking of pork and proper processing

·         Proper disposal of human excreta (good hygiene/sanitation).

Reference:

1.         Dawit Assafa, Ephrem Kibru, S. Nagesh,, Solomon, Gebreselassie, Fetene Deribe, Jemal Ali (2006). Medical Parasitology, Ethiopia Public Health Training Initiative and USAID.