Parasites 1

Definition of Parasite

A parasitic disease is defined as an infectious disease that is caused by or transmitted by a parasite. According to the Oxford English Dictionary, a parasite is described as:

Types of Parasites

Obligate Parasites

• Obligate parasites are completely dependent on their host to complete their life cycle.

Facultative Parasites

• Facultative parasites are not completely dependent on a host for their life cycle and can be free-living.

Groups of Parasitic Organisms

Endoparasites

• Definition: Endoparasites are defined as organisms that live inside the host, including all parasitic worms (helminths).

• Sometimes, the definition of a "parasitic disease" may be restricted to those caused by endoparasites.

Example 1: Protists (Microparasites)

• Protists tend to rely on another organism, often known as a carrier or vector, for transmission or for maturation in their life cycle.

  • Hosts: Man, Cattle, Horses.

  • Carrier: Tsetse fly (Glossina).

  • Disease: African trypanosomiasis, also known as Sleeping Sickness or Nagana.

  • Pathogen: Trypanosoma brucei (T.brucei).

  • Primary host where sexual reproduction occurs is the Tsetse fly and the secondary hosts are humans and animals.

Example 2: Helminths (Macroparasites)

• Helminths are parasitic worms that inhabit spaces within the host's body, not restricted to intestinal locales. They can be classified into three types:

  • Tapeworms (Cestodes): For example, the Pork Tapeworm, where the young form (cysticercus) resides in the brain, eyes, muscles, and skin, while the adult lives in the gut.

  • Flukes (Trematodes): For example, Schistosoma, which resides in blood vessels.

  • Roundworms (Nematodes): For example, Heartworm, which resides in the pulmonary artery.

Ectoparasites

• Definition: Ectoparasites are generally macroparasites that live on the outside of the host, including on the skin or its outgrowths.

  • Examples include: Lice, Fleas, Ticks, Leeches, some Mites, Biting Flies, and Mistletoe.

Additional Concepts

• The terminology in parasitology can involve more complex interactions such as:

  • Hyperparasitism: One parasite living on another.

  • Kleptoparasitism: Stealing food from another organism.

  • Social Parasitism: Exploiting the social behavior of other species.

  • Intraspecific Social Parasitism: Taking advantage of individuals within the same species.

  • Brood Parasite: Laying eggs in the nests of other species.

Parasitology Insights

• In medical terminology, parasites can be broadly categorized to include protozoa (motile protists) and multicellular endo- or ecto-parasites.

• The diverse study fields include virologists studying viruses (all considered parasites), microbiologists focusing on bacteria and fungi (some of which are parasitic), and parasitologists who explore the remaining organisms.

Important Considerations Regarding Parasites

Characteristics of Protozoa and Helminths

• Both protozoa and helminths are eukaryotic organisms, which complicates treatment for infections.

• Parasites exist at the host's expense and evolve alongside the host and its immune system.

• Parasitic infections are characterized by complex life cycles and prolonged duration within or on the host due to their ability to evade the immune response.

Co-evolution of Parasites and Hosts

• The close relationship between parasites and hosts often leads to the loss or gain of metabolic pathways, resulting in the parasite becoming reliant on the host.

• This dependency can present vulnerabilities that may be exploited for treatment or prevention.

Diseases Caused by Protozoa and Helminths

Table of Common Diseases and Associated Organisms

General Characteristics of Parasitic Infections

• Most parasites generally lead to infections that, if not cleared by the host soon after infection, can become long-term.

• Chronic infections are typically not lethal to the host in the short term or in most cases.

• The length of time associated with parasitic infections can lead to high morbidity rates.

• Compared to bacteria, viruses, and fungi, parasites have necessitated a significant evolutionary interplay between host and pathogen due to their prolonged lifecycle.

Mechanisms of Pathogenesis

1. Direct damage caused by the parasite itself.

2. Damage induced by the host's immune system.

3. Death of the parasite.

4. Immune effects.

5. Pathogenic degeneration, such as calcification.

6. Carrying other agents, including viruses (e.g., African Swine Fever Virus) and bacteria, as well as other parasites (e.g., mosquitoes transmitting malaria).

Immune Responses to Parasitic Infections

• Most parasites are susceptible to both innate and adaptive immunity. However, they have developed various strategies to evade these immune responses.

• Microendoparasites and macroendoparasites elicit different immune responses. This leads to a skew towards a specific subset of CD4+ T helper cells.

• Naive T helper cells develop into:

  • TH1 cells: Respond to intracellular infective agents, driven by macrophage responses (M1 cells) and specifically target micro-endoparasites.

  • TH2 cells: Respond to extracellular infective agents, driven by eosinophil and basophil responses (M2 macrophages) and target macro-endoparasites.

  • TH17, Treg, Tfh: Other subtypes that alter inflammatory responses.

Protist Infections

• Microparasites usually are intracellular and small, rendering them susceptible to phagocytosis and destruction by infected cells.

• Early infections trigger the activation of macrophages and NK cells.

• The secretion of cytokines such as IL-12 leads to the induction of IFN-γ from NK cells and T cells:

  1. IFN-γ serves to activate macrophages and control infections.

  2. IL-12 and IFN-γ foster the development of parasite-specific TH1 cells.

Pattern Recognition Receptors (PRRs)

• PRRs include TLRs, C-type lectin receptors, and NOD-like receptors that recognize and respond to specific pathogen-associated molecular patterns.

• Activation of these pathways leads to the production of inflammatory cytokines.

Immune Responses to Macroparasites: Helminths

• Macroendoparasite infections present unique challenges due to their size and longevity.

  • Example: The Pork Tapeworm has a young form (cysticercus) that resides in various host tissues, with adults surviving in the gut. The prevalence in endemic areas can range from 5% to 20%.

  • Other helminths include Flukes (Schistosoma) and Roundworms (Heartworm).

Immune Response Divergence

• The immune responses to protists and helminths diverge significantly, mediated primarily by distinct CD4+ T helper cell responses:

  • Protist Response: TH1-mediated responses involve inflammation encouraging macrophage activation and oxidative bursts, driven by IL-12 and IFN-γ leading to IgG2 and IgG3 antibody responses.

  • Helminth Response: TH2-mediated responses lead to a shift towards eosinophil and basophil activation, with IL-4 stimulating the production of IgE and IgG1 (or IgG4 in humans), which cascades into inflammation suppression and recruitment of eosinophils.

Summary of Immune Responses and Parasite Interactions

Additional Notes on IgE and Immune Cells

• IgE:

  • Is the least abundant isotype, constituting about 0.05% of total Ig concentration.

  • Plays major roles in immunity to helminths and allergies, with high-affinity IgE receptors (FcεRI) found on eosinophils and mast cells.

  • Involved in degranulation processes that lead to the release of mediators such as histamine and eosinophil chemotactic factors.

• Eosinophils: Comprising roughly 1% of all white blood cells, are involved in the response to parasitic infections, particularly helminths, releasing cytotoxic substances.

• Basophils: Making up about 0.1% of WBCs, activate to release a variety of mediators and cytokines essential for IgE production and inflammation.

Co-evolution of Parasites and the Immune System

• Parasites influence the maturation and control of the host immune system, demonstrating a co-evolutionary relationship.

• Macroparasites like helminths play a role in allergy development due to their mechanisms in immune modulation.

Key Takeaways

1. Parasites are major causes of pathology, including high morbidity and chronic infections.

2. Distinct immune responses arise from interactions with protozoa and helminths.

3. Parasites have evolved complex strategies to survive within the host, necessitating co-evolution with the host's immune response.