HIV STRUCTURE
HIV as the name suggests stands for “Human Immunodeficiency Virus” a virus that leads to a state of immune deficiency in humans. HIV is a retrovirus consisting of two single-strands of RNA. It is similar to the SIV (Simian Immunodeficiency Virus) that leads to similar disease condition in Chimpanzees and Monkeys. It is probably derived from SIV and is a part of the same family of viruses. There are two types of HIV viruses: HIV-1 and HIV-2. Both appear to cause clinically indistinguishable AIDS. However, it seems that HIV-2 is less easily transmitted, and the period between initial infection and illness is longer in the case of HIV-2. Worldwide, the predominant virus is HIV-1. HIV-2 type is concentrated in West Africa and is rarely found elsewhere.
TRANSMISSION OF HIV
In children, the mode of transmission is predominantly mother to child (vertical transmission) with transmission rate varying from 25% to 40%. The transmission can occur intrauterine (materno-fetal transfusion, placental inflammation), intrapartum (on exposure to body fluids and cervical secretions at the time of labour) and through breast-feeding. Infact transmission through breast-feeding has been reported to vary from 14% to 29% in various studies. Thus, the ideal way to prevent vertical transmission of HIV would be to avoid intrauterine, intrapartum and postpartum (breast-feeding) transmission of HIV.
Transmission of HIV by transfusion of blood and blood products is seen in patients with Thalessemia and patients with Hemophilia or leukemia who require repeated transfusions.
Sexual transmission of HIV is rare and is seen in patients who have been sexually abused by HIV infected adults.
Transmission of HIV is found to be more during high viremia, recent infection, low CD4 count state and advanced stage of disease and thus regular monitoring for progression of the disease is essential.
HIV – A “FAMILIAL” DISEASE
HIV in children being predominantly a vertical disease is also a “familial” disease. Though not a chromosomal or a genetic disease, it still tends to run in families and if one person in the family is affected, usually the entire family is affected by the disease.
Thus, HIV is an “infectious familial disease” in children with no cure at the moment (just as in most genetic diseases) but with a high preventable rate.
PATHOGENESIS
HIV virus when it enters into a susceptible host zooms to the lymphocytes (predominantly CD4 T cells) and infects the cells. After initial infection, there is a state of high replication leading to acute viral like illness. But the body’s immune system subsequently recognizes the virus as foreign and builds an immune response by a complex interplay of various cytokines and proteins and leads to a latent phase whereby a balance is struck between the acute replication and immune response. This is a phase when the infected individual would be asymptomatic and just a laboratory investigation would reveal the infectivity status.
During the latent phase, the HIV virus lives predominantly in the lymphoid tissue and multiplies. Peripheral viremia is less. The person may just present with generalized lymphadenopathy. Once the fine balance between viral replication and immune response is overcome, the virus leads to peripheral blood over-pooling and viremia with destruction of CD4 T cells and immunosuppression. CD4 T cells also called as helper cells are the cells that lead to activation of CD8 T cells for cell mediated immunity and also help in conversion of B cells to plasma cells and generation of humoral immunity. With destruction of CD4 cells, both cell-mediated immunity and humoral immunity is affected and a person becomes susceptible to opportunistic infections including viral, fungal, bacterial and parasitic and also has increased susceptibility to malignancies.
With increased destruction of CD4 T cells, the HIV virus replicates extensively and can also confer damage to the various organs of the body directly. Thus, a patient may have in addition various organ dysfunctions such as encephalopathy, nephropathy, cardiomyopathy, pulmonary disease, gastrointestinal disease, hepatitis and suppression of bone marrow.
In children, the latent phase is usually not seen, as there are several differences as compared to adults.
1. Children usually acquire the infection vertically whereas predominant mode of transmission in adults is sexual.
2. Children have an immature immune system at the time of acquisition of the virus. Thus, with initial exposure to the virus, children have a prolonged viremia phase, the immune system is not able to mount an adequate immune response and thus progression to AIDS is faster.
