~ Chapter 4 ~ HIV Transmission Seble G Kassaye and Vivian Levy GLOBAL EPIDEMIOLOGY OF HIV The HIV epidemic was initially identified among men who have sex with men (MSM) in the United States in 1981, leading to the epidemiologic connection between unprotected anal intercourse and HIV infection. As the epidemic progressed, it became apparent that other modes of transmission were also possible. HIV transmission occurs through percutaneous or mucous membrane exposure to infected blood, genital secretions, and breast milk, with the efficiency of HIV transmission varying based on the type of exposure (Table 1). HIV transmission occurs through sexual contact, vertical transmission from mothers to infants, and among injection drug users sharing infected needles, as well as through transfusion of infected blood products. Nonsexual casual contact has not been associated with HIV transmission. Table 1. Estimated per act risk of acquisition of HIV. Source: Centers for Disease Control and Prevention, 2005. MMWR. January 21, 2005 / 54(RR02), 1-20. Exposure route Risk per 10,000 exposures Blood transfusion 9,000 Needle-sharing injection drug use 67 Percutaneous needle stick 30 Receptive penile vaginal intercourse 10 Insertive penile vaginal intercourse 5 Receptive anal intercourse 50 Insertive anal intercourse 6.5 Receptive oral intercourse 1 Insertive oral intercourse 0.5 Globally, the HIV incidence is believed to have peaked in the late 1990s, and to have stabilized thereafter. In 2007, the WHO estimated that 33.2 million individuals were living with HIV world-wide, with 2.5 million people newly infected in 2007. Women are estimated to account for approximately half of infected adults, and approximately 2.1 million children under the age of 15 years are thought to be living with HIV. Sub-Saharan Africa is disproportionately affected, with approximately two thirds of the global persons (approximately 22.5 million individuals) living with HIV residing in sub- Saharan Africa. HIV epidemics in sub-Saharan Africa are highly diverse and still expanding in some parts of the region, with Fundamentals of Global HIV Medicine © 2009 by the American Academy of HIV Medicine. All rights reserved. Printed in the USA. 40 HIV Transmission a female to male ratio of 3:1 among 15–24 year olds. In some regions of southern Africa, the leveling off of the epidemic reflects realities where the number of persons newly infected with HIV roughly match the number of people dying of HIV-related illnesses. Structural interventions will need to address gender inequalities, the necessary continuum between HIV transmission prevention and treatment and reducing stigma and discrimination. In addition to biological factors to be discussed in this chapter, social and contextual factors such as global migration patterns, violence against women and commercial sex work clearly influence and drive HIV transmission within and between different global regions. In this chapter, we will review the biology of HIV transmission, the risk of transmission via the various known modes of HIV transmission, and cofactors which can modulate the risk of HIV transmission. PRINCIPLES OF TRANSMISSION The different interactions between host and pathogen dynamics, reflected in the equation Ro=B x D x C, will determine whether an epidemic will grow or slow, where Ro (or the reproductive ratio) is the number of secondary cases produced by a single index case in a population of susceptible persons, B is the transmission efficiency of the pathogen or infectiousness, D is the duration of infectiousness and C is the rate of partner change. When the Ro is less than one, each infected individual will infect less than one person, and the infection is expected to die out. When Ro is greater than one, the infection will continue to be propagated through the population. HIV is a pathogen of low infectiousness, but has a long duration of infectiousness. Co-factors, such as genital ulcer disease and high HIV viremia, increase the transmission efficiency of the pathogen. Sexually transmitted epidemics can also be classified as early or late and concentrated or generalized. Early in an epidemic, sexually transmitted infections (STIs) are transmitted between high risk persons with high rates of infection and frequent partner change (core groups). Examples of core groups are sex workers and their clients, mobile populations such as long distance truck drivers, MSM with multiple sex partners who engage in unprotected anal intercourse, MSM who have sex with women, members of the uniformed services, recreational sex tourists, substance users, incarcerated persons especially juveniles, persons who experience sexual or gender based violence, and children and young people who are living on the street. In a generalized epidemic, pathogens spread into lower risk populations often through bridge populations which can serve as important links between core groups and the general population. In countries with concentrated epidemics in specific high risk groups, interventions that interrupt high transmission networks will have the greatest impact. In countries with generalized epidemics, reducing partner number is key in reducing HIV prevalence. MECHANISM OF HIV TRANSMISSION Successful HIV transmission requires contact between infectious viral particles and susceptible host immune cells. The primary target of HIV is CD4+ T-lymphocytes. HIV enters the host cell by binding to the CD4 receptor and either the CCR5 or CXCR4 chemokine co-receptors. With mucosal introduction of HIV, CD4+ dendritic cells in the mucosa are infected and fuse with local CD4+ T-lymphocytes which transport HIV to the lymphatic system. Activated CD4+ T-lymphocytes are infected by the virus, with ensuing viral replication. Systemic dissemination of HIV through the blood stream follows within a few days, and infection becomes disseminated and established within the host. BIOLOGY OF HIV TRANSMISSION Successful HIV transmission requires contact between infectious viral particles and susceptible host immune cells. The primary target of HIV is CD4+ T-lymphocytes. HIV enters the host cell by binding to the CD4 receptor and either the CCR5 or CXCR4 chemokine co-receptors. CXCR4 viruses become more abundant after several years of infection and are associated with accelerated disease. There is some evidence viruses that use CXCR4 receptors may be less transmissible than viruses using CCR5 receptors to enter target cells. Layers of stratified squamous epithelium that line female and male genital mucosa serve as a physical barrier to incoming virus. With cervical ectopy in women aged 15–24, endocervical columnar HIV Transmission 41 epithelium grows onto the exocervical surface at the top of the vaginal vault. Endocervical cells express high levels of CXCR4 and CCR5 co-receptors; cervical ectopy has been associated as a risk factor for HIV acquisition. The thin rectal epithelium provides little protection to trauma. In addition, the rectum has organized lymphoid tissue with cells that can bind and present HIV. With mucosal introduction of HIV, CD4+ dendritic cells in the mucosa are infected and fuse with local CD4+ T-lymphocytes which transport HIV to the lymphatic system. HIV-1 can stay infectious in dendritic cells for up to 5 days without infecting the cell. Dendritic cells interact with and activate CD4+ cells in T cell rich regions of lymph nodes. Activated CD4+ T-lymphocytes are infected by the virus, with ensuing viral replication. Systemic dissemination of HIV through the blood stream follows within a few days, and infection becomes disseminated and established within the host. Infection with more than one strain of HIV is possible, and several scenarios have been described. Dual infection is defined as infection with two HIV strains at or near the time of seroconversion. In a study of 31 female sex workers, 19% were dually infected. Dual infection is associated with a higher viral set point and more rapid AIDS progression. Superinfection, defined as infection with a second HIV strain after initial infection has occurred, is rare and documented in a few published case reports during early HIV infection. The relative infrequency of documented superinfection may be due to the inability of viral strains to compete given existing immune responses or to competitive pressure from existing viruses. Among 78 persons with new HIV infection, followed from primary infection, 3/78 (3.8%) became superinfected. In this cohort, superinfection was associated with an increase in plasma HIV RNA and rapid decline in CD4+ cells. HIV TRANSMISSION THROUGH INFECTED BLOOD PRODUCTS Direct inoculation of HIV-infected blood through the transfusion of infected bodily fluids is the most effective mode of HIV transmission, with one Swedish study demonstrating HIV infection among 50 of 51 infected blood product recipients. HIV antibody testing of blood donors significantly decreases the risk of transfusion associated HIV transmission, but the potential for HIV transmission from HIV-infected individuals who are HIV antibody negative but RNA positive still exists. Screening of blood products using HIV antigen (p24) or RNA (viral load) testing will detect HIV infection among individuals still in the “window-period” prior to HIV antibody seroconversion, further increasing the safety of blood products. However, this can be an expensive undertaking, with variable yield with regards to the proportion of infections detected and transmission averted; such an undertaking would be more beneficial