Page 30 - AIDSBK23C

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mononuclear cells capable of either harboring or becoming infected by HIV in blood than are
present in other body fluids or secretions. The number of infectious HIV particles free in
peripheral blood can range from undetectable to well over a million per mL.[142]
The primary risk group for HIV transmission via blood exposure is injection drug users
sharing infected needles. The estimates of infectivity per intravenous drug injection range from
0.63 to 2.4%. If needles are not shared, then this form of transmission will not occur. Less
common practices of blood co-mingling, or use of instruments such as tattoo needles not
properly disinfected, also carries a potential risk for HIV infection (Table 2). Health care
workers with percutaneous exposures to HIV-containing blood, however, have an average rate of
infection of only 0.3%.[159,160,161]
Before laboratory tests were developed to detect HIV, persons who received blood or
blood products by transfusion were also at risk. Now when rigorous testing of donor blood is
routinely done, this form of infection is extremely rare-- with a risk for occurrence of 1 case for 1
900 000 single donor units of screened blood for persons receiving transfusions of blood
products in the U.S.[162] In a Canadian study encompassing years 2006 to 2009, the risk was 1
occurrence in 8 000 000 donations.[163] However, in developing nations where economic and
political problems interfere with screening programs for blood products, 5 to 10% of HIV
infections may occur from exposure to infected blood products.[164]
Even though HIV has been found in a variety of body fluids such as saliva, urine, feces,
and tears, non-sexual transmission of HIV by these body fluids is improbable.[160,165,166]
There is no evidence for HIV transmission by the aerosol route.[161] The lack of transmission is
related in part to the paucity of HIV-infected cells in such secretions. Oral transmission of HIV
by seminal fluid, milk, and colostrum may be due to their isotonicity, which overcomes
hypotonic salivary inactivation. Even though the amount of virus is small in body secretions and
presents a very small risk with routine household contact, prolonged contact or contact in
sexually intimate situations with such fluids should be avoided.[167]
Transmission of HIV through anal receptive intercourse remains a significant factor
driving prevalence of HIV infection, particularly in men having sex with men. A single layer of
columnar epithelium lines the rectum, and the underlying lamina propria contains abundant
lymphoid cells. Rates of transmission average 0.3% to 5% per encounter.[130]
Oral transmission of HIV via oral-genital, oral-anal, or oral-oral routes is uncommon,
with rates of transmission averaging just 0.04% per encounter.[130] Though there is a non-
keratinized stratified squamous epithelium lining oropharynx, reduced rates of transmission are
likely the result of fewer CD4+ lymphocytes in oral mucosa, presence of IgA antibodies in
saliva, and endogenous salivary antiviral factors including lysozyme, defensins, thrombospondin
and secretory leukocyte protease inhibitor (SLPI). However, the presence of erosions, ulcers or
inflammation with bleeding (gingivitis or periodontitis) within the oral cavity may increase the
risk of HIV transmission.[168]
Routine transmission of HIV occurs only through semen, vaginal fluid, blood or blood
components, and breast milk.[22,169] In a liquid environment at room temperature, the virus
can survive for at least 15 days, but despite HIV presence and survival in such an environment,
infection through casual household and institutional contacts is rare, even when hepatitis is
transmitted in the same setting.[165,170,171] Significantly, HIV transmission by insect vectors
such as mosquitoes appears highly improbable.[166]
HIV infection can also be acquired as a congenital infection perinatally or in infancy
(Table 2). Mothers with HIV infection can pass the virus to their babies transplacentally, at the