Sexually transmitted infections (STIs) are spread predominantly by unprotected sexual contact. Some STIs can also be transmitted during pregnancy, childbirth and breastfeeding and through infected blood or blood products.

STIs have a profound impact on health. If untreated, they can lead to serious consequences including neurological and cardiovascular disease, infertility, ectopic pregnancy, stillbirths, and increased risk of Human Immunodeficiency Virus (HIV). They are also associated with stigma, domestic violence, and affects quality of life.

The majority of STIs have no symptoms. When they are present common symptoms of STIs are vaginal or urethral discharge, genital ulcer and lower abdominal pain.

The most common and curable STIs are trichomonas, chlamydia, gonorrhoea and syphilis. Rapidly increasing antimicrobial resistance is a growing threat for untreatable gonorrhoea.

Viral STIs including HIV, genital herpes simplex virus (HSV), viral hepatitis B, human papillomavirus (HPV) and human T-lymphotropic virus type 1 (HTLV-1) lack or have limited treatment options. Vaccines are available for hepatitis B to prevent infection that can lead to liver cancer and for HPV to prevent cervical cancer. HIV, HSV and HTLV-1 are lifelong infections: for HIV and HSV there are treatments that can suppress the virus, but currently there are no cures for any of these viral STIs.

Condoms used correctly and consistently are effective methods to protect against STIs and HIV. Screening with early diagnosis of people with STIs and their sexual partners offers the best opportunity for effective treatment and for preventing complications and further transmission.

A person can have an STI without having obvious symptoms of disease. When present, common symptoms of STIs include: abnormal vaginal discharge, urethral discharge, genital ulcers and lumps, and lower abdominal pain.

Symptoms of specific STIs are:

Gonorrhoea and chlamydial infection

These STIs cause cervicitis in women, urethritis in men and extra-genital infections, including rectal and oropharyngeal manifestations. Common symptoms include vaginal or penile discharge and burning with urination. Infants of infected mothers can contract neonatal conjunctivitis (red eyes) due to exposure to the STIs during vaginal delivery. Rectal and pharyngeal infections can be asymptomatic.

Syphilis

Syphilis is often asymptomatic, when symptoms occur, primary syphilis presents as a solitary, painless ulcer. Secondary syphilis may manifest as generalized lesions affecting skin, mucous membranes and lymph-node including a classic rash on the palms of the hands and soles of the feet. Latent syphilis is asymptomatic and characterized by positive syphilis serology.

Trichomoniasis

The predominant symptoms include abnormal vaginal discharge with redness of the vulva, itching and painful intercourse.

Genital herpes simplex virus (HSV)

HSV most commonly presents as painful sores, vesicles or ulcerations on the external genitalia and mouth. Symptomatic genital HSV is a lifelong condition that can be characterized by frequent symptomatic recurrences.

Human T-lymphotropic virus type 1 (HTLV-1)

Generally asymptomatic, the chronic form of HTLV-1 can cause severe disease, including adult T-cell leukaemia/lymphoma (ATL) and a progressive nervous system condition known as HTLV-1-associated myelopathy or tropical spastic paraparesis (HAM/TSP).

Effective treatment is currently available for several STIs.

Three bacterial STIs (chlamydia, gonorrhoea and syphilis) and one parasitic STI (trichomoniasis) are usually curable with existing, effective single-dose or multiple-dose regimens of antibiotics.

For viral STIs (HIV, HSV and HTLV-1), the most effective medications available are antivirals or anti-cancer drugs (in the case of HTLV-1), that can modulate the course of the diseases, though they cannot cure these three diseases.

Antimicrobial resistance (AMR) to antibiotics used to treat STIs, in particular gonorrhoea, has increased rapidly in recent years and has reduced successful treatment outcomes. Results from the current Gonococcal AMR Surveillance Programme (GASP) show trends of high rates of quinolone resistance, increasing azithromycin resistance and emerging resistance of extended-spectrum cephalosporins.

The emergence of decreased susceptibility of gonorrhoea to extended-spectrum cephalosporins, together with established high-levels of resistance to penicillins, sulphonamides, tetracyclines, quinolones and macrolides make gonorrhoea a multidrug-resistant organism. AMR for other STIs, though less common, also exists, making prevention and prompt treatment critical.

To adequately treat STIs it is important to take the appropriate antimicrobials, with the correct dose and duration for the specific STI to ensure adequate treatment or cure, and to prevent the development of antimicrobial resistance.