BLOOD-BORNE DISEASES

The blood-borne diseases include a broad spectrum of infectious conditions that reach their target tissue through the circulation of blood. For the purposes of this discussion, a blood-borne disease is one that is transmitted from one person's blood to another's (often by an insect vector) and that manifests itself prominently in the blood elements.

Depending upon the nature of the causative agents, blood-borne diseases fall into four categories of which parasites and viruses are far more prevalent than are bacteria and prions. Malaria is one of the most common blood-borne diseases on earth, infecting nearly a half-billion people, primarily in the tropics. Of the four forms of human malaria (numerous animal pathogens exist) Plasmodium falciparum is the most deadly. The malaria parasite lives and grows in circulating red blood cells during one phase of its life cycle (liver involvement occurs during another stage). Mosquitoes are the primary vectors for malaria, disseminating the parasite as they take blood meals from different people. The advent and spread of malaria parasites that are immune to common treatments, such as chloroquine, makes malaria one of the world's major public health challenges. Intense efforts are aimed at developing vaccine treatments to complement drug therapy regimens.

The human immunodeficiency virus (HIV) is the most deadly blood-borne condition of viral etiology. The virus has infected hundreds of millions of people, most of whom live in sub-Saharan Africa, India, and Southeast Asia. HIV is, in fact, primarily a sexually transmitted disease in which blood transmission (often by transfusion) is much less frequent than sexual transmission. The pathological manifestations of HIV reflect the fact that it destroys a category of blood cells called lymphocytes that are vital to normal immune function. Infection due to impaired immune activity is the leading cause of death in people affected with the HIV virus. Through 2001, effective therapies were far too expensive for people in the developing world, though this situation is changing due to pressure on drug companies and the manufacture of generic versions of copyrighted drugs in some countries. The magnitude of the problem hits home with the prediction that the population of sub-Saharan Africa will decline over the next ten years due to the HIV epidemic.

The Epstein-Barr virus (EBV) is a herpes virus that is transmitted primarily by saliva and less commonly by blood transfusion. Manifestations of EBV infection are variable, but infectious mononucleosis is the most common. Like HIV, EBV infects lymphocytes in the blood. Unlike HIV, which destroys lymphocytes, EBV only disrupts their function. Although EBV infections are lifelong, most people develop an immune balance so that their lymphocyte function returns to normal.

Blood-borne bacterial infections are uncommon in people with normal immune function. Insect bites often seed the blood with bacteria. The body's bactericidal response is so powerful that a small number of bacteria entering the bloodstream are quickly cleared. However, bacteria that contaminate blood or blood products can multiply to the point that transfusion introduces an overwhelming number of organisms. Chills, high fever, shock, and death can occur quickly following transfusion of a contaminated blood product. The bacterium Yersina enterocolitica is a common culprit in this scenario. Only scrupulous sterility during blood collection, processing, and storage can eliminate this potentially deadly problem.

Prions are a category of pathogen differing from any previously discussed. Most cells, including parasites, viruses, and bacteria, require nucleic acid for replication (deoxyribonucleic acid [DNA] or ribonucleic acid [RNA]). Prions consist solely of protein. These protein molecules somehow commandeer the control machinery of cells, producing disease. Creutzfeldt-Jakob disease in humans results from a prion that can be transmitted by blood transfusion. The condition produces brain deterioration and death. "Mad cow disease" results from a similar prion-induced disorder in cows. Whether the bovine condition can be transmitted to humans is a point of controversy. In the United States, people who may have been exposed to the bovine prion currently are exempted from the blood donor pool.

Kenneth R. Bridges

(see also: Bovine Spongiform Encephalopathy; Communicable Disease Control; HIV/AIDS; Malaria; Prions; Vector-Borne Diseases )

Bibliography

Greenberg, S. B., and Lahart, C. J. (2000). "HIV/AIDS: Clinical Considerations." In Kelly's Textbook of Internal Medicine, 4th edition, eds. H. D. Humes, H. L. Dupont, L. B. Garder et al. Philadelphia, PA: Lippincott Williams and Wilkins.

Krogstad, D. J. (2000). "Malaria." In Kelly's Textbook of Internal Medicine, 4th edition, eds. H. D. Humes, H. L. Dupont, L. B. Garder et al. Philadelphia, PA: Lippincott Williams and Wilkins.