Combination antiretroviral therapy (ART), also known as Highly Active Antiretroviral Therapy (HAART), is a treatment that helps prolong the life of HIV-infected patients. ART therapy consists of using multiple drugs including a combination of protease inhibitors (PI), nucleoside reverse transcriptase inhibitors (NRTI), and nonnucleoside reverse transcriptase inhibitors (NNRTI). HIV evolves a resistance to drugs over time and the use of multiple drugs helps to compensate for the loss of effectiveness of a single drug due to drug-resistance.
This represents a way in our models to compare how advantageous or detrimental the use of ART/HAART treatment is in comparison with a population that does not have any treatment. To calculate the fraction of deaths averted add up the cumulative AIDS deaths and divide this by the cumulative number of deaths of a population with no ART/HAART treatment. To find the % deaths averted use the following formula: 100.0*[1 - fraction of deaths averted].
There are two ways one can become infected with a drug-resistant pathogen. First, one can develop drug-resistance directly due to treatment from inappropriate drug regimens or noncompliance with treatment regimens. These two types of improper treatments select for the drug-resistant pathogens that have arisen by mutation (in the case of HIV, TB, or HSV-2) this is called acquired resistance. Second, one can be infected by another who has a drug-resistant pathogen, which we call primary resistance. The concept of drug resistance is incorporated into many of our models.
This refers to the state of a disease being present in a particular region or population for a long period of time without much fluctuation.
This term refers to either the rapid or extensive spread of a disease or a disease that has already become widely prevalent in an area or population.
This stands for the human immunodeficiency virus (HIV), which is the cause
of acquired immune deficiency syndrome (AIDS). There are two main types of
the human immunodeficiency virus, type one and type two, HIV-1 and
HIV-2, respectively. HIV belongs to a subset of retroviruses called
lentiviruses. Retroviruses are distinct from other viruses in that they
contain a type of enzyme named reverse transcriptase (RT), which aids in
transcribing their RNA to DNA within the host.
HIV works by attacking the immune system. By eroding our defense system the body loses the ability to fight off infections. Eventually pathogens that usually do not pose a serious threat for healthy people, can be potentially life threatening for people with AIDS (these are called opportunistic infections). The progression to AIDS generally takes several years. With the use of new drug treatments such as combination antiretroviral therapy (ART/HAART), people with HIV can live relatively long lives. HIV-infected people are diagnosed as AIDS cases on the basis of certain criteria. The main criterion on AIDS diagnosis is having CD4 (immune cell) counts of less than 200 per mL. Other AIDS defining criteria are the development of certain cancers or specific diseases known as opportunistic infections.
Two opportunistic infections that are common to people with AIDS are pneumocystis carinii, which is a protozoan that causes pneumonia and toxoplasma, which is a parasite that can cause a brain infection. Common cancers in AIDS patients are 'Kaposi's sarcoma', which causes purplish spots to develop on the skin and 'Lymphoma', which is a cancer of the lymph glands[1]
According to a global report on the AIDS epidemic at the end of 1999 there were 34.3 million people living with HIV and AIDS. That same year there were 5.4 million newly infected individuals and 2.8 million deaths. The number of people that died in 1999 accounted for about 15% of all AIDS deaths since the beginning of the epidemic. The areas that have been devastated the most by this epidemic are in developing countries. Over 70% of all HIV-infected individuals live in sub-Saharan Africa[2].
This is an acronym for herpes simplex virus. Types one and two, HSV-1 and HSV-2 are a cause of cold sores and genital ulcers. Herpes simplex viruses belong to a large group of about 100 known types of herpesviruses, eight of which are known to infect humans[1].
HSV-2 is one of the most prevalent sexually transmitted diseases (STD). In the United States approximately 22% of the general population is affected, while in developing countries the prevalence ranges from 40 to 60%[2]. The worldwide annual incidence of infection for genital herpes is 21 million people[3].
When an individual is infected with genital herpes, the disease is dormant most of the time in the dorsal root ganglia at the base of the spine. Periodically ulcers develop on the genitalia, which usually heal in under twenty days. Some people who are infected with HSV-2 are asymptomatic, however it has been shown that these people still actively shed virus and are therefore still infectious[4].
The incidence is the rate at which infections or new cases arise in a population. In epidemiology it is generally useful to keep track of both the annual incidence of infection and the annual incidence of disease. The annual incidence of infection is the number of susceptible people that are infected by a particular pathogen each year. The annual incidence of disease is the number of people that develop the specific disease per year.
A seemingly inactive period, as that between exposure of tissue to an injurious agent and the manifestation of response or that between the instant of stimulation and the beginning of response[1].
An epidemic that effects an entire population or a large geographical area.
The proportion of an infected population of the entire population at a particular time. For example, if a whole population is made up of two groups: a susceptible population (S) and an infected population (I), the equation for the prevalence is simply: I / (S + I).
Although TB prevalence is extremely low in most developed nations, it is still a serious problem in developing countries. It has been estimated that about 1/3 of the world has been infected with TB. The World Health Organization (WHO) estimates that from 2000 to 2020 there will be 200 million infections and 35 million deaths will occur as a result of TB[2].
With proper treatment TB has an extremely high cure rate, however, non-compliance and poor treatment programs have been contributing to the growth in the numbers of individuals with drug-resistant and multiple drug-resistant (MDR) strains of TB. Strains of MDR TB are potentially a serious threat in both developing and developed countries because they are extremely difficult and very expensive to treat.
This is sometimes called the 'infectious contact number' (Hethcote), 'intrinsic rate of reproduction' or 'basic reproductive rate' (Anderson & May). The term was first formally used in 1952 by George Macdonald while doing the analysis for his model on malaria transmission[1]. It represents the number of secondary infections caused by a single case of disease. If R0 evaluates to be less than one, an epidemic will be eradicated. If it evaluates to be greater than one then an epidemic will take off until it reaches an endemic equilibrium.
Kermack and McKendrick developed the classic SIR model in 1927[1]. Their model was used to quantitatively explain the dynamics of an epidemic. SIR is actually an acronym that stands for susceptible, infected, and removed or resistant. The susceptible (S) group is made up of healthy individuals who are available hosts for a disease and are assumed to have no prior immunity. The infected (I) group is made up of hosts that carry the disease. The removed (R) group is made up of individuals that have either recovered from the disease and gained immunity, individuals that have been quarantined, or individuals that have died from the disease.
Many contemporary models use the classic SIR model as a starting point. In the literature you will encounter variations on the theme such as SIS, SIRS, SEI, SEIS, SEIRS, etc (note the (E) in these models stands for exposed and usually represents a latent period in the model). Many of these models are designed very specifically for particular diseases.
