Diseases: classification, causes, and management

Hep B

The Causative Agent

Hepatitis is a viral infection of the liver. The primary microbe responsible for the manifestation of the disease is the Hepatitis B virus (HBV), which is a member of the hepadnavirus family. Other members of the hepadnavirus family affect both birds and small mammals including squirrels and woodchucks, but humans are the only known mammals to be susceptible to Hepatitis B in particular (Hepatitis B; McLachlan, 1991). Hepadnaviruses are double-stranded and double-shelled DNA. The Hepatitis B virus itself is only 42-nm with an electron-dense core of 27 nm (Zuckerman, 1996). It has a small genome but also has many antigenic compounds including HBsAg, HBcAg, and HBeAg. It is therefore highly resilient and can remain without a host for up to one week (Hepatitis B). Moreover, the hepatitis B virus replicates with reverse transcription, which is unusual (Zuckerman 1996).

Demographics, Morbidity and Mortality

About 2 billion people worldwide have been infected, and 350 million people worldwide develop chronic Hepatitis B infections (Hepatitis B Green Book; Hepatitis B Pink Book). The highest rates of prevalence for the Hepatitis B virus include sub-Saharan Africa, Asia, and the Pacific Islands; whereas intermediate-prevalence regions of the world include the Amazon, the Indian subcontinent, Southern Europe, Eastern Europe, and the Middle East (Hepatitis B Green Book). North America and Western Europe have been designated as low prevalence regions by the World Health Organization (Hepatitis B Green Book). The modes of disease transmission vary depending on the rate of prevalence. High prevalence areas more prone to high rates of perinatal transmission or "horizontal transmission" among young children, but in low prevalence regions, the virus is more often transmitted sexually or via intravenous drug users (Hepatitis B Green Book).

Incubation of the disease lasts between 40 and 160 days. Infection is considered "chronic" when HBsAg is present for a period of six months or more (Hepatitis B Green Book). Chronic infections are almost certain (90% likelihood) when the virus is transmitted perinatally. On the contrary, only about 5% of previously healthy adults who exhibit the Hepatitis B antigens develop a chronic infection (Hepatitis B Green Book). Of those with a chronic Hepatitis B infection, about one fourth will develop a progressive liver disease such as cirrhosis or hepatocellular cancer (Hepatitis B Green Book). In fact, 80% of cases of hepatocellular cancer are caused by a Hepatitis B infection (Hepatitis B Pink Book).

Disease Transmission

The disease is transmitted via primarily blood and secondarily to other bodily fluids. The major route of transmission is vertical, "from the chronic carrier dam to the developing embryo," (McLachlan 1991 p. 56). Horizontal transmission may or may not be involved in the Hepatitis B disease transmission, and may indeed be irrelevant (McLachlan 1991). Research in avian populations, such as the ducks infected with the counterpart to the human strain of Hepatitis B, shows that the virus does replicate not only in the liver but also in organs in addition to the liver including the kidneys, stomach, lung, and pancreas (McLachlan 1991).

Pathogenesis and Symptoms

After the incubation period, which averages 60-90 days in humans, the disease may only be detectable in serum testing. The presence of HBsAg will denote infection, and the infection may likely resolve itself even before any symptoms are present. Further tests would reveal whether the HBsAg disappears from the serum, and the person is no longer a carrier of the Hepatitis B virus. However, a chronically infected person in whom the HBsAg presence lasts for six months or more will remain infectious and could go on to develop serious liver diseases like cirrhosis.

When symptoms do begin to manifest in chronically infected individuals, the initial phase is known as prodromal or preicteric. This early phase is characterized by jaundice lasting up to 10 days accompanied by general malaise, nausea, vomiting, abdominal pain, loss of appetite, skin rashes, and dark urine (Hepatitis B Pink Book). Many acute infections in fact result in these preicteric symptoms, and then after recovery, the adult will be immune from future infection (Hepatitis B Pink Book). Fulminant hepatitis, in which an acute infection kills the person, affects only 1-2% of the population or about 200-300 Americans each year (Hepatitis B Pink Book). Chronic infections kill 3000-4000 individuals in the United States each year, with an additional 1000-1500 dying from Hepatitis B-related liver diseases (Hepatitis B Pink Book). The liver is the primary organ that is affected, and the laboratory tests for Hepatitis B are all serum-based.