PARASITOLOGY - CHAPTER   TWO  

BLOOD AND TISSUE PROTOZOA 

PART 3

OTHER PROTOZOA

Dr Abdul Ghaffar 
Professor Emeritus
University of South Carolina

 

• Part 1: Trypanosomiasis and Leishmaniasis

• Part 2: Malaria

• Part 3: Other blood and tissue protozoa

Etiology
Babesia microti is the most important member of the genus that infects man, although a few cases of infection by Babesia sp. have been detected.

Epidemiology
In the United States, infections are usually seen in the northeast and the upper mid-west (figure 21E) during the summer months (figure 21F)  when ticks are more likely to come in contact with humans. In 2012, there were 911 reported cases of babeosis (figure 21G). Patients had a median age of 62 (figure 21H) and two thirds were male, probably reflecting the fact that men are more likely to come in contact with Ixodid ticks.

Morphology
The trophozoite is very similar to the ring form of the Plasmodium species (figure 21A and B).

   Figure 21B  
Infection with Babesia. Giemsa-stained thin smears. Note  the tetrad (left side of the image), a dividing form pathognomonic for Babesia.  A 6 year old girl, status post splenectomy for hereditary spherocytosis, infection acquired in the US.
CDC

  Figure 21C
Thin blood film of B. microti ring forms with a typical Maltese Cross (four rings in cross formation).
© MicrobeLibrary and Lynne Garcia, LSG & Associates

 
Life cycle
The organism (sporozoite) is transmitted by a tick (Ixodes scapularis) and enters the red cell where it undergoes mitosis and the organisms (merozoite) are released to infect other red cells. Ticks acquire the organism during feeding on an infected individual. In the tick, the organism divides sexually in the gut and migrates into the salivary gland (figure 21D).

Babesiosis has also be spread by blood transfusion and from other to fetus.

Symptoms
Infections are often asymptomatic and in others there are flu-like symptoms:

• fever

• malaise

• chills sweats

• general aches and pains

However, the destruction of erythrocytes can lead to:

• hemolytic anemia

• jaundice

• hepatomegaly

These occur usually 1 to 2 weeks after infection.  Although usually not severe, babeosis can be life-threatening as a result of additional complications including thrombocytopenia, low blood pressure, disseminated intravascular coagulation (consumptive coagulopathy) leading to thromboses, and organ collapse. This can be fatal, especially in immunosuppressed patients, the elderly and those that have undergone splenectomy.

Diagnosis
Diagnosis is based on symptoms, patient history and detection of intraerythrocytic parasite in the blood (figure 21B,D) or transfer of blood in normal hamsters which can be heavily parasitized.

Treatment and Control
Drugs of choice are clindamycin combined with quinine or atovaquone combined with azithromycin.

The patient may recover spontaneously. One should avoid tick exposure and, if bitten, remove the tick from the skin immediately.

 

  Figure 21D

The Babesia microti life cycle involves two hosts, which includes a rodent, primarily the white-footed mouse, Peromyscus leucopus.  During a blood meal, a Babesia-infected tick introduces sporozoites into the mouse host .  Sporozoites enter erythrocytes and undergo asexual reproduction (budding) .  In the blood, some parasites differentiate into male and female gametes although these cannot be distinguished at the light microscope level .  The definitive host is a tick, in this case the deer tick, Ixodes dammini (I. scapularis).  Once ingested by an appropriate tick , gametes unite and undergo a sporogonic cycle resulting in sporozoites .  Transovarial transmission (also known as vertical, or hereditary, transmission) has been documented for “large” Babesia spp. but not for the “small” babesiae, such as B. microti .  Humans enter the cycle when bitten by infected ticks.  During a blood meal, a Babesia-infected tick introduces sporozoites into the human host .  Sporozoites enter erythrocytes and undergo asexual replication (budding) .  Multiplication of the blood stage parasites is responsible for the clinical manifestations of the disease.  Humans are, for all practical purposes, dead-end hosts and there is probably little, if any, subsequent transmission that occurs from ticks feeding on infected persons.  However, human to human transmission is well recognized to occur through blood transfusions .

Figure 21E
Number of reported cases of babesiosis, by county of residence — 27 states, 2013
CDC

Figure 21F
Number of reported cases of babesiosis, by month of symptom onset — 2013
CDC

Figure 21G
Number of reported cases of babesiosis, by year
CDC

Figure 21H
 Number of reported cases of babesiosis, by age group — 2013
CDC

TOXOPLASMOSIS

Etiology
Toxoplasma gondii is the organism responsible for toxoplasmosis

Epidemiology
Toxoplasma has worldwide distribution and 20%-75% of the population is seropositive without any symptomatic episode. In the United States, 22.5% of the population is seropositive. However, the infection poses a serious threat in immunosuppressed individuals and pregnant females.

The most common routes for human infection are:

• Food, resulting from

  • consumption of contaminated, undercooked meat, especially pork, lamb and venison

  • ingestion after handling contaminated meat

  • using contaminated utensils
     

• Zoonotic transmission via

  • cat feces in a litter box

  • contact with something that has contacted cat feces

  • contaminated soil (especially sand covered play areas where a cat may have defecated

  • contaminated water

Toxoplasma may also be spread congenitally (from a mother with no symptoms)  and rarely via blood transfusions and organ transplants.

Morphology
The intracellular parasites (tachyzoite) are 3x6 microns, pear-shaped organisms that are enclosed in a parasite membrane to form a cyst measuring 10-100 microns in size. Cysts in cat feces (oocysts) are 10-13 microns in diameter (figure 22).

Life cycle
The natural life cycle of T. gondii occurs in cats and small rodents, although the parasite can grow in the organs (brain, eye, skeletal muscle, etc.) of any mammal or birds (Figure 22). Cats gets infected by ingestion of cysts in flesh. Decystation occurs in the small intestine, and the organisms penetrate the submucosal epithelial cells where they undergo several generations of mitosis, finally resulting in the development of micro- (male) and macro- (female) gametocytes. Fertilized macro-gametocytes develop into oocysts that are discharged into the gut lumen and excreted. Oocysts sporulate in the warm environment and are infectious to a variety of animals including rodents and man. Sporozoites released from the oocyst in the small intestine penetrate the intestinal mucosa and find their way into macrophages where they divide very rapidly (hence the name tachyzoites) (figure 23) and form a cyst which may occupy the whole cell. The infected cells ultimately burst and release the tachyzoites to enter other cells, including muscle and nerve cells, where they are protected from the host immune system and multiply slowly (bradyzoites). These cysts are infectious to carnivores (including man). Unless man is eaten by a cat, it is a dead-end host.

Symptoms
Although Toxoplasma infection is common, it rarely produces symptoms in normal individuals and when symptoms do occur, they are flu-like and sometimes associated with lymphadenopathy. Serious consequences are limited to pregnant women and immunodeficient hosts.

Congenital infections
These occur in about 1 to 5 per 1000 pregnancies of which 5 to 10% result in miscarriage and 8 to 10% result in serious brain and eye damage to the fetus. 10 to 13% of the babies will have visual handicaps. Although 58 to 70% of infected women will give birth to a normal offspring, a small proportion of babies will develop active retino-chorditis or mental retardation in childhood or young adulthood. Eye lesions are often not identified at birth but are found in 20 to 80% of infected patients by adulthood. In the United States fewer than 2% of patients develop eye lesions.

Immunocompromized patients
In immunocompromized individuals, infection results in generalized parasitemia involvement of brain, liver lung and other organs, and often death.

Immunology
Both humoral and cell mediated immune responses are stimulated in normal individuals. Cell-mediated immunity is protective and humoral response is of diagnostic value.

Diagnosis
Suspected toxoplasmosis can be confirmed by isolation of the organism from tonsil or lymph gland biopsy and by serologic testing.

Treatment
Acute infections benefit from pyrimethamine or sulphadiazine. Spiramycin is a successful alternative. Pregnant women are advised to avoid cat litter and to handle uncooked and undercooked meat carefully.

Figure 23    

  Figure 23A  
Toxoplasma gondii in the bronchoalveolar lavage (BAL) material from an HIV infected patient. Numerous trophozoites (tachyzoites) can be seen, which are typically crescent shaped with a prominent, centrally placed nucleus. Most of the tachyzoites are free, some are still associated with bronchopulmonary cells. 
CDC

   Figure 23B 
Toxoplasma gondii in tissue from a cat.
CDC  

Figure 23C
Toxoplasma gondii in mouse ascitic fluid. Smear
CDC

PNEUMOCYSTIS PNEUMONIA

Pneumocystis jiroveci (formerly known as Pneumocystis carinii)

Pneumocystis jiroveci was formerly thought to be a protozoan but is now known to be a fungus. It is included here because pneumocystis pneumonia is often described as an opportunistic parasitic disease. 

Pneumocystis pneumonia is an infection of immunosuppressed individuals and is particularly seen in AIDS patients. In the United States, about 10% of AIDS patients and about 1% of solid organ transplant recipients are infected.

The organism is pleomorphic, exhibiting, at various stages of its life cycle: 1-2 micron sporozoites, 4-5 micron trophozoites and 6-8 micron cysts. It spreads from person to person in cough droplets. Infection in immunosuppressed individuals results in interstitial pneumonia characterized by thickened alveolar septum infiltrated with lymphocytes and plasma cells. Pneumonia is associated with fever, tachypnea, hypoxia, cyanosis and asphyxia. Diagnosis is based on isolation of organisms from affected lungs.

Trimethoprim-sulphamethoxazole is the treatment of choice (figure 24). The mortality rate for P. jiroveci infections is 5 to 40% when treated and near 100% when untreated.

     Figure 24 B and C
Pneumocystis jiroveci cysts
B. 3 cysts in bronchoalveolar material, Giemsa stain; the rounded cysts (size 4-7 µm) contain 6-8 intracystic bodies, whose nuclei are stained by Giemsa; the walls of the cysts are not stained; note the presence of several smaller, isolated trophozoites.
C. cysts in lung tissue, silver stain; the walls of the cysts are stained black; the intracystic bodies are not visible with this stain; baby who died with pneumonia in California. CDC

This is a generalized life cycle proposed by John J. Ruffolo, Ph.D. (Cushion, MT, 1988) for the various species of Pneumocystis.  These fungi are found in the lungs of mammals where they reside without causing overt infection until the host's immune system becomes debilitated.  Then, an oftentimes lethal pneumonia can result.  Asexual phase: trophic forms replicate by mitosis to .  Sexual phase: haploid trophic forms conjugate and produce a zygote or sporocyte (early cyst) .  The zygote undergoes meiosis and subsequent mitosis to produce eight haploid nuclei (late phase cyst) .  Spores exhibit different shapes (such as, spherical and elongated forms).   It is postulated that elongation of the spores precedes release from the spore case.  It is believed that the release occurs through a rent in the cell wall.  After release, the empty spore case usually collapses, but retains some residual cytoplasm .  A trophic stage, where the organisms probably multiply by binary fission is also recognized to exist.  The organism causes disease in immunosuppressed individuals.  

FACULTATIVE PARASITIC PROTOZOA

These are free-living amebae that occasionally cause serious human disease. They are of particular significance in immunocompromised hosts.

Naegleria fowleri

This organism causes a rare disease. It is a flagellate that may inhabit warm waters (spas, warm springs, heated swimming pools, etc.) and gain access via the nasal passage to the brain and cause primary amebic meningoencephalitis which is almost always fatal (figure 25). Only three people out of 132 have survived primary amebic meningoencephalitis in the last 50 years.

Naegleria fowleri is sometimes call "the brain eating ameba". Although Naegleria can be found in contaminated tap water, human infection does not result from drinking the water.

Epidemiology
In the United States, infections are rare with only 34 cases between 2004 and 2013. These resulted from

• Contaminated recreational water (3o cases)

• Nasal irrigation with contaminated tap water (3 cases)

• Contaminated tap water on a backyard slide (1 case)

Symptoms
One to seven days after nasal exposure, the patient suffers:

• Sever headache

• Nausea/vomiting

• Fever

As the meningoencephalitis develops, patients then experience:

• Stiff neck

• Seizures

• Hallucinations

• Coma

• They usually die 1 to 12 days after experiencing symptoms

Treatment
There is an investigational drug, miltefosine, that may show promise. In 2013, two children survived an infection. One started treatment 36 hours after onset of treatment. She was treated with therapeutic hypothermia and miltefosine. She made a complete recovery. Another child did not receive hypothermia and was treated later after the onset of symptoms. He did receive miltefosine. He suffered permanent brain damage.

  Figure 25B  
Naegleria fowleri trophozoite in spinal fluid. Trichrome stain. Note the typically large karyosome and the monopodial locomotion. Image contributed by Texas SHD.
CDC

   Figure 25D   
Histopathology of Naegleria infection of brain.
CDC 

Free-living amebae belonging to the genera Acanthamoeba, Balamuthia, and Naegleria are important causes of disease in humans and animals.  Naegleria fowleri produces an acute, and usually lethal, central nervous system (CNS) disease called primary amebic meingoencephalitis (PAM).  N. fowleri has three stages, cysts , trophozoites , and flagellated forms , in its life cycle.  The trophozoites replicate by promitosis (nuclear membrane remains intact) .  Naegleria fowleri is found in fresh water, soil, thermal discharges of power plants, heated swimming pools, hydrotherapy and medicinal pools, aquariums, and sewage.  Trophozoites can turn into temporary flagellated forms which usually revert back to the trophozoite stage.  Trophozoites infect humans or animals by entering the olfactory neuroepithelium and reaching the brain.  N. fowleri trophozoites are found in cerebrospinal fluid (CSF) and tissue, while flagellated forms are found in CSF.
Acanthamoeba spp. and Balamuthia mandrillaris are opportunistic free-living amebae capable of causing granulomatous amebic encephalitis (GAE) in individuals with compromised immune systems.  Acanthamoeba spp. have been found in soil; fresh, brackish, and sea water; sewage; swimming pools; contact lens equipment; medicinal pools; dental treatment units; dialysis machines; heating, ventilating, and air conditioning systems; mammalian cell cultures; vegetables; human nostrils and throats; and human and animal brain, skin, and lung tissues.  B. mandrillaris however, has not been isolated from the environment but has been isolated from autopsy specimens of infected humans and animals.  Unlike N. fowleri, Acanthamoeba and Balamuthia have only two stages, cysts and trophozoites , in their life cycle.  No flagellated stage exists as part of the life cycle.  The trophozoites replicate by mitosis (nuclear membrane does not remain intact) .  The trophozoites are the infective forms and are believed to gain entry into the body through the lower respiratory tract, ulcerated or broken skin and invade the central nervous system by hematogenous dissemination .  Acanthamoeba spp. and Balamuthia mandrillaris cysts and trophozoites are found in tissue.  

Acanthameba

Several species of free-living Acanthameba are pathogenic to man. They normally reside in soil and can infect children who swallow dirt while playing on the ground. In normal individuals, the infection may cause mild disease (pharyngitis) or remain asymptomatic, but in immunodeficient individuals, the organism may penetrate the esophageal mucosa and reach the brain where it causes Granulomatous Amebic Encephalitis (figure 26).

Granulomatous Amebic Encephalitis
This is a rare infection that can affect the brain and disseminate to the rest of the body. It can affect healthy people but is normally associated with immunocompromized individuals (organ transplants, lymphocyte disorders) and patients with diabetes, cancer, liver cirrhosis, lupus and people who have used antibiotics and steroids excessively.

Most cases are fatal. The use of miltefosine is recommended by CDC.

Acanthameba Keratitis
Most cases of this disease in the United States occur in contact lens users (1 to 33 cases per million). It results from improper storage and cleaning of lenses in tap water.

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