Introduction to viruses, fungi protozoa, and helminths

Introduction to viruses

Viruses cause infections and disease as pathogenic bacteria, fungi or protozoa do

Are obligate intracellular parasites Require living host cells to multiply

Viruses consist of a(n)

DNA or RNA genome Protein coat (capsid made of capsomers) Envelope: not always present (derived from the plasma

membrane of the host cell) Surface proteins/glycoproteins (spikes) that bind to receptors

of host cells

Host range of viruses

Viruses can infect bacteria (bacteriophages) fungi protozoa plants animals

Certain viruses can infect one species or even a particular

cell type within a given host HIV and T cells

Some others can infect a wider range of hosts

Flu virus, WNV (West Nile virus)

Host range of viruses

Eight segments of ssRNA

Antigenic shift

Size of viruses

Examples of shape of viruses

Polyhedral Complex

Common cold virus - ssRNA

Rabies virus - ssRNA

Smallpox virus - dsDNA

ssRNA virus

Helical

Viral envelopes

Envelopes are acquired from the membrane of the host cell

Viruses can attach to host cells using their spikes Influenza virus undergoes changes in its spikes (antigenic drift)

resulting in new strains!!!!

Influenza virus causes infection of the lower respiratory tract

Influenza virus budding from a host cell

Infects epithelial cells lining the lungs

Death of the cells eliminates the lung’s first line of defense against infection

Flu patients become susceptible to

secondary bacterial infections such as Haemophilus influenzae

Viruses are released by coughing or sneezing

Elderly, young children, people with chronic diseases are at risk of bronchitis, pneumonia, heart failure

A scene from the flu pandemic of 1918-19

Killed millions of people (influenza A virus - strain H1N1)

Replication of animal viruses

Replication can be summarized into five basic steps Attachment to the host cell

Entry of the virus or its genome Synthesis of viral DNA or RNA

and viral proteins

Assembly of virus Nucleus or cytoplasm

Release

Replication of animal viruses

Release of viruses from infected cells Enveloped viruses

Budding

Nonenveloped viruses Exocytosis

Or lysis and death

of the cells

AIDS (Acquired immunodeficiency syndrome)

Opportunistic infections associated with low CD4 T cells and infection by HIV

HIV is a retrovirus Structure

Two major types

HIV-1 HIV-2

Estimated to have emerged in the human population around 1930

HIV attachment

Active HIV infection

Latent HIV infection

HIV infection of CD4 T cells (Helper T cells)

Opportunistic infections associated with AIDS

HIV transmission

HIV is found in blood, semen, vaginal secretions, saliva, or breast milk

Must be injected into the body or contact a lesion in the skin or mucous membranes

Infected body fluids transmit HIV via Sexual contact Intravenous drug use Blood-contaminated needles Blood transfusion Organ transplants Placenta and breast milk

The global distribution of HIV/AIDS

Certain behaviors increase risk of infection: anal intercourse, sexual promiscuity, and intravenous drug use

HIV

Diagnosis

Detection of antibodies against HIV

Seroconversion may take up to 3

months

Plasma viral load (PVL) Detects viral nucleic acid (RNA)

Chemotherapy

HAART (Highly Active Antiretroviral

Therapy)

Cocktail of several antiviral drugs

Reduces viral replication, but

infection remains

Vaccine development has been

problematic

Mutations, subtypes

Antibody-binding sites “hidden”

Latency and reservoirs of the virus

Preventing AIDS

Use of condoms, reciprocal monogamy, use of sterile needles Health care workers use Universal Precautions (CDC)

Fungi

The study of fungi is called mycology

Fungi

Cell type Eukaryotic

Metabolism Chemoheterotrophs

Oxygen Aerobic or facultative anaerobes; few anaerobes

Cellular arrangement Unicellular; filamentous

Reproduction Budding; fission; Sexual and asexual spores

Fungi are beneficial: decomposers of plant and animal matter, symbiotic fungi (plants), used by human to produce foods and drugs

Out of thousands of species, few hundreds are pathogenic to humans and animals

Molds (filamentous fungi)

The fungal body is called thallus and consists of hyphae A mass of hyphae is called mycelium

Colony characteristics Opportunistic fungus

Yeasts are non-filamentous fungi

Unicellular fungi

Reproduce by fission or budding

Saccharomyces cerevisiae (not of clinical significance)

Candida albicans is an

opportunistic pathogen

Attaches to human epithelial cells as a yeast

Usually it requires pseudohyphae to invade deeper tissues In this form is resistant

to phagocytosis Buds form a short chain of cells

called a pseudohypha

Fungal Dimorphism

Histoplasma capsulatum is a pathogenic dimorphic fungus

Examples of pathogenic dimorphic fungi Blastomyces dermatitidis Coccidioides immitis Paracoccidioides brasilensis

Clinical identification Yeasts Biochemical analysis Filamentous fungi Colony Characteristics Microscopic analysis of asexual spores

Mycosis are usually chronic infections

Superficial Surface of epidermal cells

Cutaneous Epidermis, hair, nails

Subcutaneous Occur beneath the skin

Systemic From lungs spread to other body tissues

Opportunistic

Debilitated/traumatized hosts, under treatment

with broad-spectrum antibiotics, immune system

suppressed by drugs, immune disorder (AIDS)

Any fungal infection is called mycosis

Examples

Superficial Black piedra - Piedra hortae - hair shafts

Cutaneous Tineas: ring worm and athlete’s foot

Pityriasis caused by Malassezia furfur*

Subcutaneous Sporotrix schenckii

Systemic

Blastomyces dermatitidis

Coccidioides immitis

Paracoccidioides brasiliensis

Histoplasma capsulatum

Cryptococcus neoformans (meningitis)

Opportunistic Mucor, Aspergillus, Candida*

Piedra hortae - hair shafts

Tineas: ring worm and athlete’s

foot

Protozoa are a large and diverse group

Protozoa inhabit water and soil

Relatively few cause human disease but with a significant impact: malaria is the fourth leading cause of death in the world

Protozoa

Cell type Eukaryotic

Cellular arrangement Unicellular

Metabolism Chemoeterotrophs

Oxygen Aerobic; anaerobic growth (intestinal protozoa)

Asexual reproduction Fission; budding; schizogony

Sexual reproduction Can produce gametocytes

Characteristic features Motility; cyst

Classification is currently based on rRNA sequences

Protozoa – Amoebozoa (amoebas)

Move by pseudopods

Acanthamoeba is a free-living amoeba found in water, including tap water Can infect the cornea and cause blindness

Protozoa – Amoebozoa (amoebas)

Pathogenic amoeba

Found in the human intestine, causes amebic dysentery

Transmitted through

ingestion of cysts excreted in the feces of infected person

Presence of ingested red blood cells is diagnostic of Entamoeba

Protozoa - Archaezoa

Giardia lamblia also called G. intestinalis or G. duodenalis

Found in the small intestine of humans and other mammals Trophozoite is the feeding and growing stage Cyst is the survival stage before being ingested by the next host

Diagnosis of giardiasis is based on the identification of cysts in feces

Protozoa – Apicomplexa Plasmodium species – The life cycle of P. vivax

Laboratory diagnosis: blood smear examined for the presence of Plasmodium

Plasmodium

Ring-stage - trophozoites within red blood cells

Protozoa – Euglenozoa

Hemoflagellates (blood parasites)

Transmitted by the bites of blood-feeding insects

Found in the circulatory system of the bitten host

Trypanosoma

Trypanosoma brucei gambiense (Sleeping sickness – Africa) Transmitted by the tsetse fly

Trypanosoma cruzei (Chagas’ disease - cardiovascular system) Transmitted by the kissing bug

Nucleus of Trypanosoma

Protozoa - Archaezoa Trichomonas vaginalis

Human parasite

Found in the vagina and in the male urinary tract

Transmitted by sexual intercourse

Multiple flagella (flagellate) No cyst stage – must be transferred from host to host quickly

before desiccation occurs

Parasitic helminths

Parasitic helminths

Cell type Multicellular eukaryotic animals

Nutrition Absorbs nutrient from their hosts

Cellular arrangement Tissues and organs Reduced digestive system or absent Reduced nervous system Reduced locomotion or absent

Reproductive system Complex

Trematodes (flukes) and Cestodes (tapeworms) Nematodes (round worms)

Parasitic helminths – Life cycle

The life cycle can involve Multiple intermediate hosts needed for the development of

the larval stages Definitive host for the adult parasite

Reproductive systems Separate male and female reproductive systems Hermaphroditic

Male and female reproductive systems in one animal

Egg larva(e) adult

Trematodes - flukes

Flukes are given names according to the tissue of the definitive host in which the adults lives

Lung fluke, liver fluke, blood fluke

Cause progressive damage to tissues and organs

The blood fluke Schistosoma will be discussed in a later lecture

Lung fluke Paragonimus westermani

Laboratory diagnosis: sputum and feces examined for fluke eggs

Cestodes (tape worms)

Cestodes are intestinal parasites

Absorb food through their cuticle

Diagnosis is based on the presence of mature proglottids and eggs in the feces

Cestodes (tape worms)

Common cestoda beef tapeworm, Taenia saginata pork tapeworm, Taenia solium

Cattle and pork are intermediate hosts

The larvae migrate to their muscles in which they encyst (cysticercus) Cysticerci can be seen macroscopically in meat (measly

meat) Human infection

Caused by ingestion of undercooked infected beef and pork (measly meat)

Cysticercosis in humans occurs when they ingest T. solium eggs cysticerci can form in muscle and brain

Nematodes (round worms)

Eggs infective for humans: Enterobius vermicularis (large intestine) and Ascaris lumbricoides (small intestine)

E. vermicularis: The female migrates to the anus to deposit her eggs on the perianal skin at night (Diagnosis: Graham sticky-tape method)

Nematodes (round worms)

Ascaris lumbricoides Infects over one billion people worldwide Small intestine: feeds on partially digested foods Eggs excreted with feces can survive for long periods

Nematodes (round worms)

Hookworms: Necator americanus and Ancylostoma duodenale Larvae infective for humans by penetrating the host’s skin Live in the small intestine of humans Feed on blood and tissue rather than on partially digested

foods, leading to anemia

Ancylostoma hookworm attached to intestinal mucosa