Life At Risk: Infectious Disease

Life At Risk: Infectious Disease Chapter 18

Virus • Noncellular infectious agent • Consists of protein wrapped around a nucleic-acid core • Cannot reproduce itself; can only be reproduced using a host cell

Viral Body Plans • Genetic material is DNA or RNA • Coat is protein Complex virus (bacteriophage) Helical virus Polyhedral virus

Enveloped Virus (HIV) lipid envelope; proteins span the envelope, line its inner surface, spike out above it viral coat (proteins)

Replication of anEnvelopedVirus Transcription of viral genes Translation DNA replication Proteins Assembly

Prions • Small proteins • Linked to human diseases • Kuru • Creutzfeldt-Jakob disease (CJD) • Animal diseases • Scrapie in sheep • Bovine spongiform encephalopathy (mad cow disease)

Bacteria • Were first living organisms • Prokaryotic

Bacterial Characteristics • No membrane-bound nucleus • Single chromosome • Cell wall in most species • Prokaryotic fission • Metabolic diversity

Bacterial Shapes coccus bacillus spirillum

Prokaryotic Body Plan capsule pilus bacterial flagellum DNA cell wall plasma membrane ribosomes in cytoplasm

Bacterial Genes • Bacteria have a single chromosome • Circular molecule of DNA • Many bacteria also have plasmids • Self-replicating circle of DNA that has a few genes • Can be passed from one cell to another

Prokaryotic Fission

Antibiotics • Can destroy or inhibit the growth of bacteria and some other microorganisms • Have no effect on viruses • Antibiotic-resistance is a growing problem

Parasitic Protozoa • Entamoeba histolytica - amoebic dysentery • Giardia lamblia - giardiasis

Parasitic Worms • Pinworms • Tapeworms • Hookworms • Ascaris roundworms

Malaria • Symptoms known for more than 2,000 years • Most prevalent in tropical and subtropical parts of Africa • Kills a million Africans each year • Caused by four species of Plasmodium • Transmitted by Anopheles mosquitoes

Plasmodium Life Cycle sporozoites sporozoites Gametes form in mosquito gut, combine to form zygotes merozoite Offspring enter blood, cause malarial symptoms Male and female gametocytes in blood

Disease Transmission • Direct contact with a pathogen • Indirect contact • Inhaling pathogens that have been spewed into the air • Contact with a vector

Disease Patterns • Sporadic disease (whooping cough) • Endemic disease (common cold) • Epidemic (cholera in Peru) • Pandemic (AIDS)

Virulence • Relative ability of a pathogen to cause serious disease • Low virulence - common cold • High virulence - rabies

AIDS • Combination of disorders that follows infection with HIV • Includes • Yeast (Candida) infections • Pneumocystis pneumonia • Karposi’s sarcoma

HIV Replication (1) • RNA retrovirus • A protein (gp120) at virus surface binds to host cells with CD4 and chemokine receptors • These receptors occur on helper T cells • Once bound, RNA and viral enzymes enter the host cell

HIV Replication (2) • Viral RNA is reverse-transcribed to DNA • HIV DNA is called provirus; it inserts into host DNA • The host cell makes copies of viral DNA and viral proteins that assemble to form new virus particles

T-Cell Decline • Release of new viral particles kills the host T cell • The body is constantly making new T cells, but cannot outpace the rate of destruction • As infection proceeds, T-cell numbers inevitably decline

Effect of T-Cell Decline • CD4 helper T cells play a vital role in immune function • They are required for both cell-mediated and antibody-mediated immunity • Infected individual becomes vulnerable to other infections, which eventually result in death

Transmission of HIV • HIV does not live long outside human body • Most often spread by exchange of bodily fluids with an infected person • Sexual intercourse and needle sharing are main modes of transmission

Human Papillomaviruses • HPV can cause bumplike warts on the genitals and anus • One strain, 16 HPV, does not cause symptoms • It can lead to cancers of cervix, vagina, vulva, penis, and anus • There is no cure

Life At Risk: Infectious Disease