Sexually transmitted diseases (STD) are caused by a variety of microorganisms. These agents cause genital tract infections that are often overlooked due to the absence of specific symptoms. The silent nature of these infections can prevent early diagnosis and delay possible treatment. Lack of symptoms will also facilitate disease transmission from to person to person or to the fetus during pregnancy. The availability of effective vaccines may effectively reduce the risk of contracting an STD and enhance existing prevention programs.
Bacterial STDs like chlamydia and gonorrhea can generally be treated with antibiotics, but are characterized by lack of symptoms in up to 80% of patients, especially women. These asymptomatic infections, when left untreated, can lead to severe complications including cervicitis and pelvic inflammatory disease, resulting in ectopic pregnancies and infertility. Undetected chlamydia and gonorrhoea can also be problematic in pregnant women because infection can be passed to the fetus causing eye and lung conditions after birth.
Many viral STDs are also asymptomatic or characterized by very mild clinical signs. One example is HIV infection. In some individuals, the first symptoms after contracting this virus often resemble those of a common cold or a mild flu, while others will not develop any apparent illness. After initial infection, the immune system will start working to develop antibodies to HIV. During this "window" period, lasting approximately three months, standard serological testing will show negative results but patients will still be highly infectious and can unknowingly spread the virus to others.
Infection with the hepatitis B virus (HBV) can be asymptomatic in up to 50% of all patients. Clinical signs appear several months following initial infection and can be misinterpreted for those of a flu-like illness. Hepatitis B can turn into a chronic disease, especially in children, causing extensive liver damage if untreated.
Genital herpes simplex virus (HSV) and human papilloma virus (HPV) are also contracted sexually. Infection with HSV is asymptomatic in up to 80% of patients. Although neurological complications are rare, HSV is extremely contagious, it can enhance transmission of HIV and cannot be treated. HPV also causes no apparent symptoms and can only be diagnosed by PAP smear screening and DNA testing in women. Patients with persistent infections (e.g. HIV-positive women or other underlying conditions) are at high risk of developing cervical or other types of cancer.
The use of vaccines to prevent STD is undoubtedly controversial. Many feel that their use may diminish the value of other prevention programs such as sexual education and condom use, ultimately encouraging irresponsible behavior. Nevertheless, while the use of male condoms is highly effective in preventing the spread of certain infections, it may not be useful against others. Transmission of viruses including HSV and HPV occurs by direct skin contact, and condoms will not necessarily ensure protection. The recent licensing of Gardasil and Cervarix, vaccines against HPV for use in young individuals, has been the center of heated debates but their use could have an impact to control virus transmission.
HBV vaccines are available in several parts of the world (e.g. Recombivax HB, Engerix-B). Immunization is based on the administration of three vaccine doses, and is highly recommended in newborn children or in individuals younger than 19 year of age. Due to the success of these current vaccines, ongoing research is focusing on decreasing the number of doses needed for protection.
HPV and HBV are currently the only two STD agents that can be prevented by vaccination. Vaccines to protect from chlamydia and gonorrhea have not been a priority because of the availability of treatment, but the emergence of antibiotic resistance especially for Neisseria gonhorrhoeae has shifted research efforts.
Development of vaccines against HIV has been a particularly challenging task. The virus destroys the human body by directly damaging the immune system. An ideal vaccine candidate should somehow work as a “switch” and induce immune cells to start fighting against HIV. In addition, the virus is subject to continuous genetic changes (mutations), making it even harder to design and test vaccine candidates.
A novel strategy for STD prevention is the use of vaccines delivered in the genital mucosa. This approach would ideally block the action of microorganisms upon entry, preventing proliferation and spread to other body sites. Current development and testing of mucosal vaccines is promising, but solely experimental.
The design of safe and effective vaccines against HIV and other STDs is an important step to improve prevention, although research is still needed to develop novel candidates and understand immunity in the genital tract. The use of vaccination should not exclude the importance of disease screening, treatment and changes in sexual behavior.
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