parasitic protozoa

Malaria

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 and has sexual and asexual stage .

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).

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 re-invade liver cells initiating further Exo-erythrocytic schizogony.

Erythrocytic schizogony (blood phase) is completed in 48 hrs in P. vivax. 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 develops to form the microgametes, which actively motile and separate from the gametocyte.
The macrogametocyte by reduction division becomes a macrogamete.
Fertilization occurs by entry of a microgamete into the macrogamete 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.




















The life cycle of malaria

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






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.

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.

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.














Toxoplasmosis

Toxoplasmosis is an infection caused by the protozoan obligate intracellular
parasite Toxoplasma gondii.
The incidence of toxoplasmosis varies greatly by country and by age, but may affect up to one-third of the global human population. the majority of immunocompetent adults, pregnant women and children infected with Toxoplasma gondii .

The house cat and other members of the family Felidae serve as definitive hosts in which the sexual stages of the parasite develop. the life cycle of Toxoplasma gondii begins when a cat ingests toxoplasma-infected tissue from an intermediate host, usually a rodent. Tissue cysts within the muscle fibers or brain are digested in the cat’s digestive tract. the parasite then undergoes sexual development, multiplies in the intestine of the cat and is eventually shed in cat feces, mainly into litter boxes and garden soil. A human may become infected in one of the following ways:
1. By accidentally ingesting oocysts passed in cat feces through contaminated soil or handling of cat litter.
2. By ingesting tissue cysts within raw or undercooked meat (lamb, pork and
beef ), drinking unpasteurized milk, contaminated water, or unwashed fruits
or vegetables.
3. By direct transmission of tachyzoites from mother to fetus through the placenta (congenital infection) or, rarely, by blood transfusion or solid organ transplantation from a positive donor to a previously uninfected host.

Life Cycle

The asexual stages of T. gondii can cause disease in humans and most animals . There are two asexual forms. The first form, called the tachyzoite, can invade all types of cells and divides rapidly, leading to cell death . The second form, called the bradyzoite, divides slowly and forms cysts, most prominently in muscle and brain. Tachyzoite replication causes acute disease, while encysted bradyzoites are long-lived, with slow turnover, and are responsible for maintaining the latent infection. Reactivation of bradyzoites from cysts is responsible for most disease in immunosuppressed hosts.


(A) tachyzoite and (B) bradyzoite of T. gondii.

Rodents and birds ingested by cats keep the sexual cycle going in the wild. The sexual cycle takes place in the superficial epithelium of the small intestine of both wild and domestic members of the cat family , Oocysts, which are shed in feces of recently infected cats,. Human food animals, especially sheep, pigs and goats, may harbor cysts in muscle, which are infectious for people and other carnivores when ingested in raw or undercooked meat . Oocysts may be ingested, either directly by hand-to-mouth transmission or on raw vegetables.



















Life cycle of T. gondii

Pathology
T. gondii invades numerous organs, infecting a broad spectrum of cell types. Symptomatic, or clinical, toxoplasmosis may be classified as acute, subacute, chronic, or congenital. Acute toxoplasmosis in humans is characterized by parasitic invasion of the mesenteric lymph nodes and liver parenchyma. The most common symptom is painful, swollen, lymph glands in the inguinal, cervical, and regions, frequently accompanied by fever, headache, anemia, muscle pain, and sometimes pulmonary complications.
The tachyzoites proliferate in many tissues and tend to kill host cells rapidly. When cells from sites such as the retina or brain are involved, serious lesions often develop. Subacute toxoplasmosis is merely a prolongation of the acute stage.

Congenital toxoplasmosis is caused by infection with Toxoplasma gondii in a pregnant woman. Infants born to women who were infected before conception do not develop disease due to protection by maternal antibodies. fetuses exposed early are at much higher risk for severe symptoms or death and spontaneous abortion.

Treatment
Oral administration of pyrimethamine, usually accompanied by sulfadiazine, is the treatment of choice at this time.

Prevention
1. Protect children’s play areas from cat and dog feces. Cover sandboxes when not in use to avoid cat defecation.
2. Wash hands thoroughly after contact with soil that may be contaminated with animal feces.
3. Control flies and cockroaches as much as possible. they can spread contaminated soil or cat feces onto food.
4. Avoid rubbing eyes or face when preparing food, especially raw meat or poultry. After food preparation, wash hands thoroughly with soap and water and clean the counter.
5. Avoid ingesting raw or undercooked meat or poultry, raw eggs and unpasteurized milk. Fruits and vegetables should be peeled or thoroughly washed.