IV. Sexually Transmitted Diseases (STDs)

STD Facts

• 20% of people in the US have had an STD.

• 50% of sexually active individuals will contract an STD by age 25.

• There are 19 million new cases in the U.S. each year.

• More than 65 million people are living with an incurable STD.

• The U.S. has the highest STD infection rate of any developed country.

• Women suffer more from serious complications than men.

• Viral STDs are as common as bacterial ones. Viral STDs are often incurable.

Bacterial STDs

• Gonorrhea

  • Caused by Neisseria gonorrhoeae.

  • A gram-negative diplococcus, non-motile.

  • Transmitted via sexual contact or to a newborn at birth.

  • Affects only humans, infecting the penis, vagina, anus, mouth, or eye.

  • Symptomless infections are common (10% of infected males, 50% of infected females).

  • Symptoms include painful urination and discharge.

  • Causes 700,000 new cases in the U.S. every year.

  • Condoms are effective in preventing transmission.

  • Antibiotic treatments are effective, but antibiotic resistance is a problem.

  • Infection rates and serious complications are higher in women, including Pelvic Inflammatory Disease (affecting > 1 million women annually).

  • Infection of newborns can lead to blindness.

• Chlamydia

  • Caused by Chlamydia trachomatis.

  • A small gram-negative bacterium.

  • An obligate intracellular pathogen, meaning it lacks a peptidoglycan-based cell wall.

  • Humans are the only reservoir.

  • Symptoms and signs are similar to Gonorrhea.

  • Newborns can also become infected.

  • Causes 3-4 million cases each year in the USA.

  • Condoms can prevent transmission.

  • Treated effectively with antibiotics, such as Azithromycin.

  • It is the #1 cause of infertility worldwide. Antimicrobial drug resistance is an important problem.

Viral STDs (as common as bacterial but often incurable)

• Genital Herpes (herpes simplex virus HSV-1 and HSV-2)

  • Very common and often asymptomatic. An estimated 45 million people in the U.S. have had a genital HSV infection.

  • Causes painful lesions or blisters on the genitalia.

  • HSV-1 typically causes oral cold sores, while HSV-2 is the most common cause of genital infections; however, both can cause either type of infection.

  • They are enveloped, icosahedral DNA viruses.

  • Infection of newborns and infants can be fatal. Cesarean sections are common if the mother has an active infection to reduce the chance of transmission during delivery.

  • Herpes viruses can become latent. Reactivation results in new lesion outbreaks.

  • Antiviral drugs may help manage symptoms but do not cure the infection.

  • Condoms provide some, but not complete, protection as lesions may be outside the protected area.

• Human Papillomavirus (HPV)

  • There are over 40 different strains.

  • May be the most widespread STD; 75-80% of sexually active individuals acquire HPV at some point. It is the most common STD in the U.S..

  • Causes genital warts.

  • Some high-risk strains infect the cervix and can lead to cancer (genitals, mouth, throat). The Pap smear test is used for screening.

  • The Gardasil vaccine was approved by the FDA in 2006. It protects against 4 strains (6, 11, 16, 18) responsible for 70% of cervical cancers and 90% of genital warts. It is not effective against other HPV strains.

  • The Advisory Committee on Immunization Practices (ACIP) recommends the vaccine for children ages 11-12. No serious side effects are listed.

  • It is a non-enveloped DNA virus.

• Zika Virus

  • Spread mostly by the bite of an infected Aedes species mosquito (Ae. aegypti and Ae. albopictus), which bite during the day and night.

  • Can also be passed from an infected person to a sexual partner.

  • Can be passed from a pregnant woman to her fetus.

  • Infection during pregnancy can cause certain birth defects, notably microcephaly. There is a very high correlation between Zika infection during pregnancy and microcephaly in the newborn.

  • There is currently no vaccine or medicine for Zika.

  • Local mosquito-borne Zika virus transmission has been reported in the continental United States.

  • Typically causes a mild flu-like infection.

  • It is an enveloped, single-stranded RNA virus.

• Human Immunodeficiency Virus (HIV) / Acquired Immune Deficiency Syndrome (AIDS)

  • AIDS is the clinical condition resulting from advanced HIV infection.

  • AIDS is considered the most significant new infectious disease to emerge during the 20th century.

  • Over 40 million people are currently infected worldwide; over 20 million have died. New infections occur every 6 seconds, and deaths every 5 minutes.

  • It is a zoonosis originating from African chimpanzees. Probable spread to humans occurred during butchering chimpanzees for meat in the 1920s in the Democratic Republic of Congo.

  • AIDS in humans was described in 1981, but probably existed since the 1950s.

  • The HIV-1 virus was isolated in 1983 by Dr. Luc Montagne (who won a 2008 Nobel Prize).

  • HIV is transmitted in humans by body fluid exchange. This includes during sexual activity, via contaminated blood transfusions, through contaminated needles used by intravenous drug users, and via perinatal transmission. AIDS can be prevented.

  • HIV is a retrovirus with an RNA genome. It is an enveloped virus.

  • The enzyme reverse transcriptase is packaged inside the viral particle. Reverse transcriptase converts the viral RNA into cDNA (a DNA copy). This effectively reverses the usual direction of transcription (DNA to RNA). (This corresponds to option D in a study question).

  • HIV primarily targets CD4 T-cells (also called TH cells or Helper T-lymphocytes), which recognize antigens presented on MHC II. HIV-1 infection kills these cells.

  • AIDS develops during the late stages of HIV-1 infection when CD4 T-cell populations become very low, severely compromising the immune system.

  • Death typically occurs due to opportunistic infections and cancers that invade following the collapse of the immune system. Examples include Encephalitis, Meningitis, Retinitis, Pneumocystis pneumonia, Tuberculosis, various Tumors (lungs, skin, GI), Esophagitis, and Chronic diarrhea.

  • As noted earlier, HIV/AIDS and TB co-infections are very problematic globally. 1/3 of HIV-infected individuals worldwide are co-infected with M. tuberculosis. TB is a common cause of death for those suffering from AIDS.

  • Treatment involves drug therapy targeting viral enzymes and functions. Drug mechanisms include inhibition of viral entry, integrase, maturation, reverse transcriptase (nucleoside and non-nucleoside types), and protease.

  • Development of an effective vaccine is difficult due to the virus's high mutation rate and the lack of a good animal model.

  • Prevention is the best approach to AIDS management.

  • Highly Active Anti-Retroviral Therapy (HAART) is the standard treatment. It's a combination or "cocktail" of drugs, typically two reverse transcriptase inhibitors plus one protease inhibitor, potentially also including a fusion or integrase inhibitor. HAART is highly effective and reduces the chance of drug resistance. However, it has high cost and serious side effects. It is not a cure; the virus is not eliminated from the body.

  • As of 2018, a person in the US diagnosed with HIV has no shorter life expectancy than someone without HIV.

STD Transmission During Pregnancy and Birth

• Some STDs can be passed from mother to infant during delivery, often due to skin-to-skin contact as the infant passes through an infected birth canal. Examples include Chlamydia, Gonorrhea, Genital herpes, and Cytomegalovirus (CMV).

• A few STDs can infect the fetus before birth during pregnancy by migrating into the placental fluid. Examples include Syphilis, HIV, and CMV.

• A cesarean delivery may be recommended to reduce the chance of transmission for certain STDs/STIs during delivery.