MMI 133 Session 6: Parasitic Infections

MMI 133 Session 6: Parasitic Infections

Learning Objectives

• Differentiate between protozoa and metazoa.
• Describe how different parasitic organisms cause disease in humans.
• Identify organisms causing specific diseases and their pathogenic characteristics (e.g., Plasmodium falciparum).
• Identify some commonly occurring ecto-parasitic infections.
• Describe the epidemiology and transmission of parasites.

Parasite Classification

Parasites are broadly classified into three main categories:

• A. Protozoa: One-celled eukaryotes.
• B. Metazoa: Multicellular animals (commonly referred to as Helminths, or worms).
• C. Ectoparasites: Free-living "bugs" that live on the surface of the host.

Protozoa

General Characteristics of Protozoa

• Some protozoa have a "cyst" form, which features a tough protective layer outside the cytoplasmic membrane. This form provides protection from inhospitable environments.
• The growing form in many species is called a "trophozoite." This form lacks a cyst wall and only has a cytoplasmic membrane.
• The cyst form may also be termed an "oocyst" in certain types of protozoa, such as Cryptosporidium.

Key Protozoans and Associated Diseases

1. Entamoeba histolytica

   • Agent of: Amoebic dysentery.
   • Primary Food Source: Red Blood Cells (RBCs), leading to ulcer formation and bloody diarrhea.
   • Forms: Both trophozoite and cyst forms exist.
   • Resistance: Cysts exhibit a high degree of resistance to chlorine.
   • Treatment: Can be treated with a nitroimidazole, such as metronidazole (Flagyl).

2. Giardia lamblia

   • Type: Flagellate.
   • Causative Agent of: Giardiasis, commonly known as "beaver fever."
   • Symptoms: Intestinal upsets, flatulence, nausea, abdominal cramps, and diarrhea.
   • Forms: Cyst and trophozoite.
   • Resistance: Cysts are hardy and show high resistance to chlorine.
   • Treatment: Can be treated with nitroimidazoles.

3. Trichomonas vaginalis ("Trich")

   • Type: Flagellate, motile.
   • Forms: No cyst stage. Infection is always person-to-person.
   • Transmission: Primarily sexual.
   • Location: Found in the vagina and male urethra.
   • Symptoms: Intense itching and inflammation of tissues.
   • Treatment: Can be treated with nitroimidazoles.

4. Acanthamoeba sp.

   • Habitat: Grows in water; cyst form exists in dust.
   • Significance: Particularly important for contact lens wearers.
   • Outcome: Infection can lead to eye loss.
   • Forms: Both cyst and trophozoite stages occur in humans.

5. Cryptosporidium parvum

   • Hosts: Transmitted from animals like cows, rats, dogs, and cats to people.
   • Symptoms: Causes chronic diarrhea, which is severe in immunocompromised individuals but can also be serious in immunocompetent persons.
   • Diagnosis: Diagnosis requires the use of a Ziehl-Neelsen (ZN) stain.
   • Resistance: Oocysts have high resistance to chlorine.
   • Treatment: No consistently effective treatment available.

6. Toxoplasma gondii

   • Life Cycle: Primary life cycle occurs in cats.
   • Source: Cat feces contain oocysts.
   • Transmission: Oocysts are ingested by humans or other animals, or tissue cysts containing the protozoan can form in other animals and be acquired by humans from undercooked meat.
   • Risk: Seriously dangerous for the fetus during pregnancy, especially in the first trimester.

7. Trypanosoma species

   • Type: Blood parasites.
   • Transmission: Mostly through insect vectors.
   • Two Main Types:
     • T. brucei gambiense/rhodesiense: Causes African Sleeping Sickness (found in Africa; transmitted by the tsetse fly vector).
     • T. cruzi: Causes Chaga's Disease (found in South America; transmitted by the reduviid or kissing bug vector). Chaga's disease is very common in South America, affecting around $10$% of patients with cardiac mortality.

8. Leishmania sp.

   • Species Diversity: Approximately $20$ different Leishmania species cause disease.
   • Transmission: All species are transmitted by the sand fly vector.
   • Syndromes: Causes visceral, cutaneous, and mucocutaneous syndromes.
   • Location in Host: Live and develop within macrophages (immune cells).

9. Plasmodium species

   • Type: "Apicomplexa" parasites.
   • Causative Agent of: Malaria. Currently, there is no vaccine available.
   • Mortality: Responsible for approximately $2-4$ million deaths per year, predominantly among children.
   • Main Types (transmitted by Anopheles mosquito):
     • P. falciparum (most dangerous type)
     • P. malariae
     • P. ovale
     • P. vivax
     • (New species: P. knowlesii, also known as monkey malaria)
   • P. falciparum Characteristics: It infects red blood cells (RBCs) of all ages, leading to a high level of parasitized RBCs.
   • Life Cycle (Human Infection): The Anopheles mosquito carries sporozoites in its saliva. When the mosquito bites, these sporozoites enter the bloodstream and then quickly enter liver cells within approximately $30$ minutes. The liver then becomes the primary focus of infection initially.
   • Symptoms of Falciparum Malaria:
     • Fever and chills: Caused by the release of toxic breakdown products when the infected RBCs rupture and release developing parasites into the bloodstream.
     • Sequestration: Parasites cause infected red blood cells to become sticky. These sticky collections occlude capillaries, leading to organ death. This process is mediated by a virulence factor called PfEMP1 (a protein).
   • Severe Consequences of P. falciparum Malaria:
     • Cerebral malaria: Characterized by clumping and occlusion of blood vessels in the brain, as well as vasculitis.
     • Anemia: Caused by the lysis (destruction) of RBCs.
     • Kidney failure: Results from high levels of hemoglobin breakdown products accumulating in the kidneys.

Metazoa (Worms)

Metazoa are multicellular animals, often referred to as helminths or worms. They are classified into three major groups:

1. Nematodes: Roundworms.
2. Trematodes: Flukes.
3. Cestodes: Tapeworms.

A. Nematodes (Roundworms)

• Characteristics: Roundworms with a complete digestive system, including a mouth, intestine, and anus.
• Categories Based on Infective Stage:
  • a) Eggs are the infective stage.
  • b) Larvae are the infective stage.

a) Eggs as Infectious Stage

1. Enterobius vermicularis (Pinworm)

   • Life Cycle: Spends its entire life cycle in humans.
   • Location: Lives in the large intestine. Female worms migrate to the anus to lay eggs on the perianal skin.
   • Egg Adhesion: Produces a glue (an irritant) to stick the eggs to the skin.
   • Severity: Usually not serious.
   • Size: Small worms, females are about $1$ cm long.
   • Transmission: Eggs are the infectious stage and are transmitted by direct contact with eggs or ingestion of eggs (e.g., stirred up in dust). Highly infectious, especially in settings like daycares.

2. Ascaris lumbricoides

   • Characteristics: Large roundworms, about $30$ cm long.
   • Hosts/Habitat: Live in the small intestine; eggs found in soil are very hardy due to a thick protein coat.
   • Infection: By ingestion of eggs.
   • Sexes: Adult worms have distinct sexes (dioecious).
   • Diagnosis: Infections are diagnosed by examination of feces to look for eggs.
   • Life Cycle: Ascaris eggs hatch in the intestine. The larvae then migrate to the lungs, up the trachea, and are swallowed down into the nose or mouth, eventually returning to the small intestine (common in children).

b) Larvae as Infectious Stage

1. Hookworms

   • Infection: Occurs when larvae penetrate the intact skin of bare feet.
   • Migration: Larvae enter the lymphatic and circulatory systems, travel to the lungs, are coughed up, and swallowed down to the small intestine, where they attach by hooks on their scolex (head).
   • Pathology: They suck blood, leading to anemia.
   • Excretion: Eggs are excreted in feces.
   • Geography: Primarily found in warm climates.

2. Anisakis (Fish Nematodes)

   • Infection: These fish nematodes accidentally infect humans in their larval form, typically through the consumption of raw or undercooked fish (e.g., sushi).
   • Prevalence: A rare type of infection, but increasing.
   • Prevention:
     • Heat fish to $60^{ ext{o}}C$ for $10$ minutes.
     • Blast-freeze to $-35^{ ext{o}}C$ for $15$ hours.
     • Freeze at $-23^{ ext{o}}C$ for $7$ days.
     • Cleaning of fish (evisceration) as soon as possible after catch.

3. Trichinella spiralis

   • Infection: Ingestion of encysted larvae found in undercooked pork, boar, bear, or walrus meat.
   • Location in Host: Larvae encyst in striated muscle fibers and the heart.
   • Symptoms: Muscle weakness is a dominant symptom.
   • Treatment: Difficult to treat.

4. Blood Nematodes (Filaria)

   • Main Filarial Infections:
     • Lymphatic filariasis (elephantiasis): Adult worms reside in lymph tissue.
     • Onchocerciasis (river blindness): Adult worms live in subcutaneous tissue.
   • Microfilariae: The offspring of adult worms, microfilariae, circulate through the bloodstream or migrate through subcutaneous tissue.
   • Transmission: By mosquitoes.
   • Prevalence: High prevalence in Africa, Asia, and tropical Latin America.
   • Loa Loa: While not explicitly detailed as a blood nematode, an image depicts Loa Loa in the eye, indicating another type of filarial infection.

B. Trematodes (Flukes)

• Characteristics: Leaf-shaped worms.

1. Schistosoma
   • Infection: Humans are infected by cercariae (larval forms) that penetrate intact skin in freshwater ponds.
   • Immune Evasion: Adult worms hide from the host immune system by acquiring a coat of proteins from the host.
   • Location: Worms reside in the venous plexus of the bladder or intestine, depending on the species.
   • Pathology: The eggs, not the adult worms, cause inflammation.
   • Three Types of Schistosoma in Human Infections:
     • S. haematobium: Causes urinary schistosomiasis (bilharzia). Results in inflammation of the bladder wall, with chronic sequelae including cancer. Prevalent in Africa and the Middle East.
     • S. japonicum: An intestinal parasite found in the Far East.
     • S. mansoni: An intestinal parasite found in Africa, South America, and the Caribbean.
     • Global Impact: Over $250$ million people are infected worldwide.
   • "Swimmer's Itch"
     • Cause: Caused by a duck schistosome.
     • Pathology: These cercariae do not enter the human circulation; the infection is self-limiting. It causes skin itching due to the immune response.

C. Cestodes (Tapeworms)

• Characteristics:
  • Intestinal parasites that are dependent on the host for nutrients; they lack a digestive system.
  • Possess a head, or "scolex," which has suckers or hooks for attachment to the gastric mucosa.
  • The body consists of segments called proglottids, each of which contains both male and female reproductive organs.
• Acquisition: Acquired by eating inadequately cooked contaminated beef, pork, or fish.
• Two Types of Disease Caused by Tapeworms:
  • Intestinal infection: Typically presents as a mild clinical picture, caused by pork, beef, fish, or rodent tapeworms.
  • Deep tissue infection: More serious infections, such as those caused by the pork tapeworm (cysticercosis) or Echinococcosis (dog tapeworm).

1. Taenia saginata (Beef Tapeworm)

   • Life Cycle: Requires both humans and cattle.
   • Infection: Infectious tissue larvae (cysticerci) are ingested by humans in infested meat, and they mature into adult tapeworms in the intestine.
   • Adult Size: Adult worms may be up to $10$ m long.
   • Transmission: Eggs passed in feces may be taken up by cattle, completing the cycle.
   • Symptoms: Usually mild, though sometimes playfully associated with weight loss.

2. Taenia solium (Pork Tapeworm)

   • Life Cycle: Same life cycle as the beef tapeworm, but the intermediate host is the pig.
   • Severity: More dangerous than the beef tapeworm because cerebral cysts can cause cysticercosis; person-to-person transmission is also possible.
   • Two Modes of Infection:
     • a) Eggs: Ingestion of eggs results in the hatching of larvae in the intestine, which then escape and penetrate body tissues, including the brain. They encyst in the brain and can cause symptoms such as fainting and seizures.
     • b) Larvae: Ingestion of larvae results in an adult worm developing in the intestine (intestinal infection).

3. Echinococcus granulosus (Dog Tapeworm)

   • Characteristics: A small tapeworm.
   • Hosts: Dogs, coyotes, and dingos are definitive hosts; humans are dead-end hosts.
   • Pathology in Humans: The larva develops into a hydatid cyst containing hundreds of worm parts, but it cannot form a complete adult tapeworm in humans.

Ectoparasites

• Location: Live on the skin surface and do not typically enter deep tissues.
• Complications: Often lead to secondary bacterial infections due to scratching.

1. Sarcoptes scabei (Scabies)

   • Infection Sites: Commonly found in finger webs, elbows, armpits, breasts, groin, and buttocks; rarely on the head.
   • Mechanism: Mites burrow into the skin, feed, mate, and lay eggs within the burrow during their $30$-day lifespan.
   • Symptoms: Host defenses cause inflammation around the burrows, leading to intense itching.
   • Complications: Secondary infections with skin bacteria are common.
   • Specificity: Other Sarcoptes mites, such as those causing dog mange, do not thrive on humans and do not establish persistent infections.

2. Bed Bug Bites

   • (Image provided, implying common knowledge of their existence and bites)

3. Pubic Lice ("Crabs", Phthirus pubis) and Head Lice (Pediculus humanus)

   • Characteristics: Surface dwellers that penetrate the skin with their mouthparts to suck blood.
   • Reproduction: The female glues eggs (nits) to hair shafts.
   • Hatching: Eggs hatch in $5-10$ days.
   • Complications: Secondary infections can occur when lice feces are crushed into wounds on the skin.

Summary of Key Points

• Parasites are generally classified into two main types: protozoa and metazoa (also known as helminths).
• Certain protozoa are significant causes of diarrhea, even in regions like Canada.
• Other protozoa are transmitted by vectors and infect internal organs, the Central Nervous System (CNS), and other deep tissues (e.g., Trypanosoma and Plasmodium species causing malaria).
• Helminths (worms) include nematodes, trematodes, and cestodes, and usually do not cause diarrhea.
• Filaria are a specific type of nematode that causes diseases like elephantiasis and river blindness.
• Ectoparasites include organisms such as lice, scabies mites, and bed bugs, which primarily infest the skin surface.