Medical Parasitology

&Pathology By Dr: Yara

Shamikh

Medical Parasitology•Parasitology: is the science that deals with parasites, which infect man temporarily or permanently.

•Parasitism: indicates that one species (parasite) depends upon another species (host) to derive benefits such as food, shelter and maintenance of species. This association may show harmful effects to the host.

The association (relation) between 2 organisms

• Parasitism: One organism (parasite) gets

benefit and lives on the expense of another

organism (host) that usually loses or suffers.• Mutualism: The two organisms get benefit from

each other.• Symbiosis: Mutual benefit but the two organisms

can’t live independently.• Commensalism: One organism gets benefit while

the other (host) is not affected.

Description of Parasites• A- Ectoparasite: Live on the outside of the host

(infestation).• B- Endoparasite: Live within the body of the host

(infection).

They are either obligate or facultative parasites• Obligate Parasite: Can’t live independent from the host

i.e. permanent residence. ex: Trichomonas• Facultative Parasite: Can live free far from the host or as

parasite. ex.: Fasciola

They are also classified into: Permanent Parasite, Temporary Parasite,

Accidental Parasite, Coprozoic Parasite and also Pseudo parasite,

while the Zoonotic parasites are those which are normally found in

wild /domestic animals and may harm man.

Types of hosts• Final or Definitive Host: Harbors the

sexually mature parasite (adult worm or sexual stage of protozoa).

• Intermediate Host: Harbors the immature or asexual stage.

• Reservoir Host: Animal that harbors the same species of parasites as man and so is a potential source (store) of infection to man with this parasite.

• Vector: Usually an arthropod that carries the parasite to its host.

Modes of Transmission of Parasitic DiseasesContact- Food & drink- Arthropods- Contact with

larvae- Blood borne- Transplacental

1- Contact:• Direct contact with infested persons e.g. scabies, lice.• Indirect contact with infected person’s towels, bed

sheets e.g. scabies.• Sexual contact e.g. Trichomonas vaginalis.

2- Food and drink: Is the most common.• Eating or drinking contaminated water or food

e.g. helminthes ova, protozoan cysts.• Eating raw or under cooked or under salted fishes

e.g. Heterophyes heterophyes.• Eating raw or undercooked meat

e.g. Taenia saginata (in cattle meat), T. solium and Trichenella spiralis (in pig or pork meat).

3- By arthropods Mechanically by non-blood sucking insects e.g. housefly transmit

protozoal cysts or helminthes ova. Biologically by bite of blood sucking insects e.g. malaria,

filariasis, trypanosomiasis. Ingestion of insect containing the infective stage: e.g. infected

flea (in Hymenolepis nana) and other insects (in H. diminuta) or infected Cyclops in water (in Dracunculus medinensis).

4- By contact with motile larval stages in the external environment:– Cercaria of schistosoma in water (swimming, washing or

irrigation).– Larvae of ancylostoma in soil (penetrate the skin of persons

walking bare-footed in muddy soil).

5- Blood transfusion and contaminated syringes

e.g. malaria.

6- Transplacentally e.g. Toxoplasma, Trypanosomiasis.

Effects of parasitic infection on the host

1- Anemia due to: Sucking of blood & blood loss e.g. Ancylostoma. • Destruction and feeding on Red cells hemoglobin e.g. malaria.• Depression of bone marrow e.g. Leishmania donovani.• Vit B12 deficiency e.g. Diphyllobothrium latum.

2- Loss of weight due to: Sucking digested food e.g. Taenia and Ascaris.• Preventing fat absorption e.g. Giardia lamblia.

3- Mechanical effects:• Intestinal obstruction in heavy ascariasis and in taeniasis.• Intestinal perforation (ascariasis, taeniasis).• Pressure atrophy in Hydatid (liver and lung).

4- Poisoning or allergic reaction due to toxic substances produced by the parasite in blood.

5- Localized irritation e.g. gastrointestinal disturbances with colic, dyspepsia, diarrhea, dysentery.

6- Facilitating secondary bacterial infection.

Anemia- Loss of weight- Mechanical effects- Poisoning or allergic reaction- Localized irritation- Facilitating secondary bacterial infection

Methods of control of parasites

1- Control of hosts:• Man • Vector• Intermediate hosts• Reservoir host

2- Personal hygienic measures

3- General hygienic measures

Parasitology is classified into three main groups

Protozology(proto = primitive)

Study of protozoaHelminthology

(Helminth = Worm)Study of helminth

Entomology

Study of arthropods

Helminthes = Worms• I. Phylum Platyhelminths (means flat worms)

– i. class Trematoda (trematodes or flukes)– i i. class Cestoda (cestodes or tapeworms)

• II. Phylum Aschelminths – class Nematoda (nematodes or roundworms)

Important Pathogenic Trematodes:

1. Fasciola sp.: include F. hepatica, F. gigentica

2. Heterophyes heterophyes

3. Schistosoma sp.: include S. haematobium, S. mansoni,

S. Japonicum

General Features• All consist of flat, fleshy, leaf-shaped, un segmented body

except Schistosoma sp. which are cylindrical. • They don’t have a respiratory system nor a blood vascular

system.• The body is equipped with 2 muscular suckers for

attachment; (oral and a ventral), except the genus Heterophyes, which has a third “genital” sucker.

• All are hermaphroditic (i.e. male and female reproductive organs exist in a single body), except schistosomes that have 2 separate sexes.

• Their life cycles involve a sexual and an asexual reproduction parts.

• All require one or more intermediate host(s) for completion of their life cycles.

• The 1st intermediate host of all flukes is snail, where asexual reproduction occurs.

• The adult worm develops in the secondary intermediate host (except the schistosomes which do require only one intermediate host).

• The eggs of flukes are equipped with a lid at the top called operculum, through which the larval worm comes out to find its appropriate snail host.

• The characteristic eggs are the diagnostic stages of trematodes’ diseases.

• The mode of transmission is by ingestion of cyst-contaminated food, except in Schistosoma, where cercaria penetrate the skin directly.

A. Hermaphroditic Trematodesthe most important ones

1. Fasciola sp.

2. Heterophyes heterophyes

1. Fasciola sp.

F. hepatica, F. gigentica

• Length: 30 - 75 mm• Common name: Sheep liver fluke {giant liver fluke}• Disease: Fascioliasis “liver rot”• Geographical distribution: Europe, Middle East, Asia. In

areas where sheep, cattle {pigs} are raised.• Definitive host: sheep, cattle, pigs and man.• Primary intermediadte host: Snail Lymnoea truncatula for

F. hepatica and Lymnoea caillaudi for F. gigantica.• Secondry intermediate host: Leaves of fresh-water plants

• Mode of infection: Ingestion of raw water-cress containing metacercaria

• Infective stage: Metacercaria.• Location of adult: Bile duct• Symptoms: Hepatomegaly, fever, vomiting, diarrhea,

eosinophilia, necrotic foci of liver (liver rot)

F. hepatica, adult worm measuring 2-5 cm by 8-13 mm, are flat, oval in shape with a cephalic cone containing the oral sucker. It can live in biliary ducts for up to 10 years.

Fasciola hepatica, living adult in bile duct of sheep.

F. gigantica adult worm

Fasciola sp.

Laboratory diagnosis

• Fasciola egg demonstration of operculated egg in stool; or in duodenal or biliary drainage

The eggs are ellipsoidal with small distinct operculum.  The operculum can be opened.

Treated with Triclabendazole with Bithionol

2.Heterophyes heterophyes

• Length: 1-1.7 mm

• Common name: Intestinal fluke

• Disease: Heterophiasis

• Geographical distribution: Africa, Middle East, Asia

• Definitive host: Dog, cat, man

• Primary intermediate host: Snail Pirenella conica

• Secondry intermediate host: Fresh water fish (Boury, Bolti)

• Mode of transmission: Ingestion of raw fish containing

encysted metacercaria

• Infective stage: Encysted metacercaria.

• Location of adult: Small intestine

• Symptoms: Abdominal pain, non-bloody diarrhea

Heterophyes heterophyes

Adult fluke of Heterophyes heterophyes.

Laboratory diagnosis

Operculated egg in stool containing metacercaria

Treated with Praziquantal

B. Non-Hermaphroditic Trematodes• These include the Schistosoma

sp. also known as {blood flukes} or {digenetic blood trematodes}.

• They exist as separate sexes but live attached to each other through a groove in the male, the {gynecophoric canal} or the {schist}

• They do not require a secondary intermediate host.

• No metacercaria are formed.• All species share a common

life cycle.

Schistosoma

All cause Bilharzia; also known as bilharziasis, schistosomiasis snail fever; or Katayama’s fever in acute schistosomiasis

All transmitted by direct skin penetration of cercariae during swimming

S. haematobium S. mansoni S. japonicum

Geog. distribution

Africa, Middle East especially Egypt

South America, Caribbean& Egypt

Far East (Japan, China, Philippines)

Location Veins of urinary bladder

Veins of large intestine, colon

Veins of small intestine, liver

Snail Bulinus truncatus Biomphalaria alexandrina

Oncomelania sp

Definitive host

Man Man Man, dogs, cats , rodents & pigs

Clinical features

The 1st sign is itching and pruritic rash (cercarial dermatitis) at penetration site.

Acute intestinal schistosomiasis (mostly by S. mansoni and S. japonicum) is characterized by fever (Katayama’s fever, that occurs after weeks of initial infection), headache, hepatosplenomegaly and eosinophilia.

Acute urinary schistosomiasis (by S. haematobium) is accompanied by fibrosis of bladder with hematuria.

Manifestations of chronic schistosomiasis include formation of bladder and urinary stones, bleeding oesophagal varices, hepato and splenomegaly, diarrhea (hematuria may progress to cancer).

egg in intestinal

wall

egg embolization

in hepatic venules

Egyptian with splenomegaly

Portal hypertension and ascitesdue to infection with S. mansoni

Laboratory diagnosis

• If no eggs appear, demonstration in rectal biopsy (for all species), or in biopsy of bladder (for S. haematobium).

• X-rays of urinary tract or intestine.• Serologic tests, and antibody detection by ID injection of

cercarial antigens (useful in recent infections)

• Microscopic examination of characteristic eggs in stool (for all species), or in urine (for S. haematobium)

S. haematobium eggs,prominent terminal spine

at the posterior end. 

S. mansoni eggs,prominent lateral

spine near the posterior end.

S. japonicum eggs,typically oval or subspherical, and has avestigial spine

Treatment

• Praziquantel is the drug of choice for infections by all Schistosoma.

• Oxamniquine is effective in treating infections by S. mansoni in some areas in which praziquantel is less effective