Acknowledgements

HIV Transmission and Natural History

How HIV is

transmitted

Both HIV-1 and HIV-2 are transmitted in the same ways. Figure 2.1 details the primary methods of HIV transmission in the Caribbean.

• 
  The predominant route of transmission in countries with generalised epidemics (that is, an epidemic where HIV is firmly established in the general population) is through unprotected heterosexual intercourse or homosexual intercourse between men. There are no documented cases of sexual transmission between women. Sexual transmission accounts for the majority of HIV transmission in the Caribbean.
  

• 
  HIV is also transmitted through blood, blood products and donated organs or semen. Blood-borne transmission, also known as parenteral transmission, occurs primarily through the use of inadequately sterilised needles, syringes or other skin-piercing instruments and through the transfusion of infected blood.
  

• 
  HIV may be transmitted from an infected mother to her foetus or infant during pregnancy, delivery or when breastfeeding.
  

Figure 2.1. Category of transmission in reported AIDS cases in CMCs, 1982-2002.

Source: Status and Trends: Analysis of the Caribbean HIV/AIDS Epidemic, 1982-2002. CAREC/PAHO 2004

Discussing

the figure

Look at Figure 2.1 and answer the following questions.

1. 
   What is the most common mode of transmission in the Caribbean?
   

2. 
   Which type accounts for 11% of HIV transmission in the Caribbean?
   

transmitted

Figure 2.2 shows the HIV infection process.

• 
  HIV infection begins when specific attachment proteins on the envelope of the HIV virus (known as glycoprotein (gp) 120 and gp 160) bind to CD4 receptors located on the cell membranes of human T-helper lymphocytes and macrophages (two types of white blood cells) and a few other types of cells.
  
• 
  This binding reaction allows HIV to fuse with the T-helper lymphocyte or macrophage.
  
• 
  The HIV genetic material is reverse-transcribed and integrates with the cell’s DNA. Reverse-transcription is the process by which HIV’s genetic material (RNA) is transformed into DNA, which allows it to fuse with the host’s genetic material (DNA).
  
• 
  The cell produces the components of new HIV viruses, which are then released to infect additional cells.
  

Figure 2.2. HIV infection process of a T-helper lymphocyte.

Discussing

the figure

Look at Figure 2.2, on the previous page, and answer the following questions:

1. 
   What process enables the HIV virus to enter the T-lymphocyte?
   

2. 
   Where are the new HIV components produced?
   

Factors

transmission

HIV transmission depends on a variety of biological, behavioural and epidemiological factors. Together these factors are usually referred to as infectivity. Infectivity refers to the probability of an organism being transmitted from an infected person to an uninfected person. We will consider these two essential questions:

• 
  What are the inter-relationships between HIV and STIs?
  

• 
  What risk factors determine whether an STI or HIV will be transmitted through a sexual exposure?
  

STI and HIV

inter-

relationship

Risk factors

for HIV

infection

• 
  the amount of virus to which an uninfected person is exposed
  

• 
  the type/duration of exposure
  

• 
  host factors that protect uninfected persons against infection.
  

1. Amount of virus

• 
  Viral load in blood can be measured with an HIV viral-load blood test.
  

• 
  Higher viral loads in the blood generally correspond to higher viral loads in genital fluids (semen and vaginal secretions).
  

• 
  The viral load varies during the clinical course of disease. It is highest in the early and late stages of disease.
  

• 
  Effective treatment with antiretroviral therapy (that is, drugs used to fight HIV infection) lowers the viral load in blood and may lower it in genital fluids.
  

Risk factors for HIV infection, continued

• 
  Direct exposure to HIV-infected blood (for example, through blood transfusions) carries a greater chance of transmission than exposure to an infected person’s semen or vaginal secretions.
  

• 
  Sexual exposure risk is increased with the following:
  

• 
  the presence of white blood cells and inflammation
  
• 
  the duration of exposure.
  

• 
  The risk of acquiring HIV is proportional to a person’s risk of being exposed sexually to HIV. This means that a person’s risk of HIV is determined primarily by the risk of having an infected partner.
  

• 
  Persons whose primary sexual partner is infected have the greatest probability of infection through repeated sexual exposure to this primary partner.
  

• 
  The risk of acquiring HIV infection increases with the number of exposures to an infected person. An uninfected person repeatedly exposed to an infected person is at greater risk, over time, of being infected, than someone exposed only once. For example, an infected wife’s uninfected husband is at greater risk if he has sex with her 100 times over a year than if he has sex with an infected sex worker one time during that year.
  

• 
  Among persons with multiple sexual partners, the greater the number of partners an individual has, the greater his or her likelihood of having intercourse with someone with HIV infection.
  

• 
  Anal intercourse carries a higher risk of infection than vaginal intercourse, and vaginal intercourse is substantially riskier than oral intercourse.
  

• 
  Uncircumcised men have a higher risk of acquiring HIV infection than circumcised men because:
  

• 
  infected vaginal fluids are held in contact with mucosal surfaces under the uncircumcised foreskin longer
  
• 
  the foreskin often contains large numbers of white blood cells that HIV can infect.
  

Risk factors for HIV infection, continued

• 
  The risk of transmission from an infected man to an uninfected woman during intercourse may be slightly higher than the risk of transmission from an infected woman to an uninfected man. Factors that may contribute to a higher risk of transmission from a man to a woman include the following:
  

• 
  STIs and HIV in semen can remain viable for up to 72 hours following ejaculation.
  

• 
  In contrast, very small amounts of cervico-vaginal fluids are introduced into the male partner’s urethra during sex. Thus, the male is only exposed during the act of intercourse, an average of 20 minutes.
  

• 
  The cervix and the opening of the cervical canal have greater surface area than the male urethra.
  

• 
  Persons with genetic mutations for the HIV co-receptor are less susceptible to HIV infection.
  

• 
  The co-existence of inflammatory (such as gonorrhoea or chlamydia) or ulcerative (such as syphilis, chancroid or herpes simplex type 2) sexually transmitted infections (STIs) increases the risk for acquiring HIV infection.
  

HIV risk

• 
  STIs destroy cells in the epithelium at the site of infection in the genital tract (for instance, the urethra in men or the cervix in women).
  

• 
  This partially or completely exposes the sub-mucosal layer of the mucous membrane and forms ulcerations. The ulcerations can be seen on examination of the patient or can be detected microscopically.
  

• 
  The ulceration results in an inflammatory immune response. The immune system cells that come in response to the infection include, among others, cells that HIV can target. These cells are:
  

• 
  T-helper lymphocytes
  
• 
  macrophages.
  

These cells contain CD4 receptors and are present in the sub-mucosa and on the surface of the mucous membranes.

the figure

Look at Figure 2.3, and answer the following questions:

1. 
   How do STIs increase the risk of HIV transmission?
   

2. 
   What does HIV do once it has entered the sub-mucosal membrane?
   

of acquiring and

transmitting HIV
As shown in Table 2.2, STIs affect both HIV-infected and HIV-uninfected persons.

Table 2.2. The effect of STIs on HIV-infected and HIV-uninfected patients.

Discussing

the table

Looking at Table 2.2, discuss the following questions:

1. 
   How would ulceration increase the risk of acquiring HIV?
   

2. 
   What causes the recruitment of HIV target cells to the surface of mucous membranes?
   

STIs decrease

HIV transmission?
Three large randomised controlled community trials have been conducted in Africa. The purpose of the trials was to assess the impact of intensive STI control and treatment programmes on HIV transmission.

• 
  In the Mwanza district of Tanzania, improved STI syndromic management and public health programmes led to a decrease in HIV transmission compared to usual care. This study was conducted when the STI prevalence was high and the HIV prevalence was low but increasing rapidly in Tanzania. Overall, HIV incidence was 42% lower in communities that received improved STI services compared to those that continued with the usual levels of STI services.
  

• 
  Two other STI control trials were conducted in Uganda, in areas of high but stable or declining HIV incidence and moderate or lower STI prevalence. These found different results from the trial in Tanzania.
  

• 
  In the Rakai study, azithromycin, ciprofloxacin and metronidazole were used to control STIs, but this did not lead to a decrease in HIV transmission.
  

• 
  In the Masaka trial, improved syndromic case management and public health STI management also failed to decrease HIV transmission.
  

Both of the Ugandan studies were conducted during the time of stable or declining HIV transmission in Uganda.

for discrepancy

• 
  Large-scale improvements in STI control programmes are likely to have the greatest impact on HIV during the early phase of an HIV epidemic in a given community or population. During this time period, STI control programmes may lead to substantial reductions in new HIV infections.
  

Possible reasons for discrepancy, continued

• 
  When HIV infection rates are stable or declining, STI control programmes may be less effective in decreasing HIV transmission in a community or population.
  

implications

• 
  provide HIV behavioural risk-reduction counselling and testing to all patients diagnosed with an STI
  

• 
  screen HIV-infected patients for STIs
  
• 
  treat both symptomatic and asymptomatic STIs
  
• 
  provide treatment to sexual partners.