Transmission routes of HIV

The main transmission routes of HIV are

1. unsafe sex with an HIV-infected partner

2. sharing injection paraphernalia with an HIV-infected partner

3. vertical transmission of HIV from the HIV+ mother to the newborn (before or at

birth; or later, due to breastfeeding)

All other transmission routes, for the most part case reports, are notably rare. Among

these are transmissions due to transfusion of blood or blood products in countries

where blood donations are not routinely screened for HIV.

Extremely rare are transmissions due to contact with HIV+ blood through open

wounds or mucosa, or transmission of HIV after a bite (Bartholomew 2008). Three

cases were reported where mothers infected their newborns probably via pre-chewed

food (Gaur 2008). These transmission routes however are of a casuistic nature. Large

case registries, in particular from the CDC, which have investigated other transmission

routes of HIV, clearly show that daily contacts of everyday life, such as the shared

use of toilets or drinking from the same glass, cannot transmit HIV. Case registries

in the health care setting, which analyze contact via saliva, urine, or infectious blood

with intact skin, did not find a single transmission of HIV (Henderson 1990).

Potentially favorable factors and risks

Sex

The most important transmission route for HIV is sexual contact. The prerequisite

for sexual transmission is direct exchange of infectious body secretions / fluids. The

highest viral concentrations are found in blood and seminal fluid. A study investigating

heterosexual transmission of HIV in female partners of HIV+ hemophiliacs

in Bonn found an HIV seroconversion rate of 10% (Rockstroh 1995). The risk for

sexual transmission was significantly higher if the HIV+ partner suffered from

advanced immunodeficiency or an advanced clinical stage of HIV infection. It is

important to note that a precise calculation of transmission risk of one individual

exposure is not possible. Various environmental factors have an influence on the

actual transmission risk, such as specific sexual practices, concurrent sexually transmitted

diseases, skin lesions, circumcision and mucosal trauma, that are difficult to

take into account. 

The correlation of transmission risk with the level of HIV viremia has important

epidemiological implications. In environments where body fluids like blood and

seminal fluid are exchanged with many persons over days or weeks, the risk of

meeting people who have been recently infected, and thus who are highly infectious,

is high. Likewise, the probability of infecting someone else between the transmission

event and the detection of HIV antibodies is high. The later stage of disease is also

a highly infectious period, as HIV infection progresses and higher viral loads are

again observed as one gets closer to falling below 200 CD4 T cells or AIDS. Sexually

transmitted diseases and infections disrupt physiological skin and mucosal barriers

and enhance the risk for HIV transmission. This is particularly true for endemic areas

with a high prevalence of other sexually transmitted diseases. Primarily genital herpes

lesions have been identified as a potential co-factor facilitating HIV transmission in

endemic areas (Mahiane 2009).

The observation that the level of HIV RNA is obviously critical in the infectiousness

of an HIV+ person initiated a discussion regarding the possibility of a sero positive

person having “safe” unprotected sex. The Swiss Commission for AIDS (“Eid -

genössische Kommission für AIDS-Fragen”, EKAF) proposed to classify HIV+ persons

who are on ART with a plasma HIV RNA below the level of detection for at least

6 months, if they are adherent to therapy, regularly come to medical examinations,

and if they do not have any signs of other sexually transmitted diseases, as persons

who most likely do not transmit HIV via sexual contact and therefore may have

unprotected sex if they want (Vernazza 2008). The intention of the EKAF

recommendation is to manage fears of HIV transmission and to enable a normal sex

life, as far as possible, between persons with and without HIV. The EKAF recommendation

is not agreed to by all HIV experts. 

A case report from Frankfurt raised

questions (Stürmer 2008), where HIV transmission occurred though HIV viral load

was not detectable and the HIV+ partner was on successful ART (see chapter 6.12 on

Prevention). It is important to highlight though that large international studies in

discordant couples with early ART initiation clearly demonstrate a dramatically

reduced risk of HIV transmission to the seronegative partner in the setting of

suppressed HIV viremia on HIV therapy (Cohen 2011). Ever since these results

became available, immediate treatment of an HIV+ individual with a seronegative

partner was possible according to most HIV treatment guidelines, with the

accompanying liberty of condom-free sex.

Sharing injection paraphernalia

Sharing injection paraphernalia is the most important HIV transmission route for

persons who use drugs intravenously. Due to the usually quite large amount of blood

that is exchanged when sharing needles, the transmission risk is high. The aspiration

of blood to control the correct intravenous position of the needle constitutes

the reservoir for transmission. With the introduction of needle exchange programs,

the installation of needle vendors, methadone substitution and multiple other preventive

measures and social programs, HIV transmission rates have significantly

decreased within intravenous drug users in Western Europe. In Eastern Europe, where

intravenous drug use constitutes a criminal offence and clean needles are not

provided, one sees an unyielding continual increase of HIV transmissions in this

Introduction 5

population. One can only hope that the success of prevention efforts in Western

Europe will lead to a more liberal management and implementation of prevention

programs in Eastern Europe.

Vertical transmission

Without intervention up to 40% of newborns born to HIV-1-positive mothers are

infected with HIV-1. The most important risk factor is viral load at the time of delivery.

Since 1995 the mother-to-child transmission rate of HIV-1-infected mothers has been

reduced to 1–2%. These low transmission rates were reached through the

combination of antiretroviral therapy / prophylaxis for the pregnant woman, elective

cesarian section prior to the start of labor (no longer necessary if the maternal HIV

viral load is successfully controlled on ART and HIV RNA is persistently undetectable),

antiretroviral post-exposition prophylaxis for the newborn and substitution for

breast feeding. For details refer to the “HIV and Pregnancy” chapter as well as to the

European AIDS Clinical Society (EACS) guidelines for the clinical management and

treatment of HIV-infected adults.

Blood

The transmission of HIV via blood and blood products has been largely reduced on

a global scale, though the risk is not completely eliminated. In Germany blood and

blood products are considered safe. Since 1985 all blood donations are tested for

HIV-1 via antibody tests, and since 1989 also against HIV-2. For a few years now

blood donations are additionally tested via PCR to identify donors who may be in

the window of seroconversion and where the HIV ELISA is still negative. Persons

with so-called risk behavior, i.e., active injection drug users, sexually active men and

women as well as immigrants from high-prevalence countries are excluded from

blood donations.

Occupationally-acquired HIV infection

The overall risk for HIV infection after a needlestick injury is estimated to be around

0.3%. The risk for HIV transmission is significantly higher if the injury occurred

using a hollow needle – e.g., during blood withdrawal – than with a surgeon’s needle.

For details on post-exposure prophylaxis (PEP) please refer to the respective chapter

in this book. On the other hand, the risk of infecting a patient with HIV when the

medical personnel is HIV+ is extremely low. In 1993 19,036 patients of 57 HIV+

physicians, dentists or medical students were screened for HIV infection (CDC

1993a). While 92 patients tested HIV-positive, none of the transmissions was related

to the health practitioner.

Non-suitable transmission routes

In general, HIV-transmission due to day-to-day contact between family members is

unlikely. It is important to avoid blood-to-blood contacts. Thus, razor blades or tooth

brushes should not be commonly shared. In cases of cannula or needle usage, these

should be safely deposited in appropriate sharps-containers and not be placed back

into the plastic cover.

Insects

All studies that have investigated the possible transmission of HIV via insects have

come to the same conclusion, that it is not possible. This holds true as well for studies

performed in Africa with a high AIDS prevalence and large insect populations (Castro

1988).

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