The poxviruses (Table 1) are the largest and most complex of all viruses. The virions consist of a double stranded DNA genome. The virions appear as oval or brick-shaped structures 200 to 400 nm in length. Because of their large size, poxvirus virions may be visualized through a light microscope.

Table1. Poxviruses

One of the most feared viruses of history, smallpox, is a member of this family. Smallpox played a crucial role in demonstrating the importance of vaccination to protect against disease. In 1798 Edward Jenner recognized that milkmaids previously infected with cowpox were immune to the disease of smallpox. This discovery led to the practice of inoculating humans against small pox by using the actual organism (virus) responsible for the disease. Smallpox is known to infect only humans and exists as two distinct subtypes. Variola major, which caused the most severe disease (case fatality rate of 30%), occurred mainly in Asia; variola minor was associated with less severe disease and case fatality rates of 0.1% to 2%. As a result of an intensive vaccination campaign, WHO declared naturally occurring variola virus eradicated in 1980. The variola virus no longer circulates in nature. The virus is feared as a possible biologic weapon, and testing capability for this organism is maintained by hundreds of Laboratory Response Network (LRN) laboratories throughout the nation. All known stocks of the virus are held at two WHO collaborating laboratories: the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, and the State Center of Virology and Biotechnology (VECTOR) in Kotsovo, Russia. WHO has requested destruction of the remaining stocks of this virus, but that has been postponed to evaluate the need for developing vaccines, rapid diagnostics, and antiviral therapy. Since the eradication of smallpox in 1980, most vaccination campaigns against this virus have stopped, and most of the world’s population lacks any protective immunity against this disease or any related poxviruses.

Besides the smallpox virus, 10 other poxviruses are capable of infecting humans. Except for the smallpox virus and the molluscum contagiosum virus, most of these are zoonoses, or infections that result from contact with animals. Fortunately, other than monkeypox and the eradicated smallpox virus, none of these viruses can sustain human-to-human transmission. The viruses normally are acquired through abrasions of the skin and contact with an infected animal, or in the case of human monkeypox, through the oropharynx or nasopharynx in addition to through abrasions on the skin. Poxvirus replicates in the epidermal cells and causes change in the cellular structure, characterized by the “pocks” on the skin. Poxvirus infection can take one of two courses: it can cause a localized infection at the site of inoculation, with little spread from the original site of inoculation, or it can cause a fulminant, systemic infection with spread of the virus throughout the body. The second type of infection is associated with variola virus (smallpox) and also monkeypox, and an increased mortality rate. Monkeypox is almost indistinguishable from smallpox infection except that it lacks the same level of mortality and transmissibility. The monkeypox virus is found in the tropical rain forests of Africa, and its host reservoir is one or more rodent species.

After the individual is exposed to the virus, symptoms of fever and headache occur first, followed by the development of a rash and lymphadenopathy. The rash typically first appears on the face, beginning as macules (small, round changes in skin color), progressing to papules (slightly elevated with no fluid) to vesicles (containing a bubble of fluid) and then pustules (containing purulent material consisting of necrotic inflammatory cells). Depending on the severity of the disease, the illness can last 2 to 4 weeks. Two clades of monkeypox exist, and the Congo Basin clade has the highest fatality rate (up to 12%). In 2003 the importation of rats as pets led to an outbreak of monkeypox in the United States, proving that international travel can be a significant portal of disease from anywhere and to anywhere in the world. RT-PCR (real-time) offers a rapid diagnostic identification tool for cases of monkeypox.

Another member of the poxvirus family is the molluscum contagiosum virus, which causes single or small clusters of lesions. Its only host is humans, and infection occurs either nonsexually, through direct contact or fomites, or sexually, through intimate contact. Usually a self-limiting disease in healthy individuals, molluscum contagiosum can cause a more severe form of disease in immunocompromised patients, resulting in large lesions, especially on the face, neck, scalp, and upper body. Laboratory diagnosis of molluscum contagiosum usually is through biopsy of the lesions and histologic examination. Molecular assays, such as traditional PCR, restriction fragment length polymorphism (RFLP), and real-time-PCR, are still under development.

Orf is another member of the poxvirus family and is transmitted from sheep to humans through human direct contact with infected sheep. This virus causes single or multiple nodules, usually on the hands. These nodules may be painful and may be accompanied by symptoms such as low-grade fever and lymph node swelling. The infection usually resolves in 4 to 6 weeks without further complication, although autoinoculation of the eye can have more serious consequences. An orf diagnosis is made through direct examination of the nodule, along with epidemiologic evidence of a recent history of contact with sheep or lambs. Continued development of PCR assays for identification of parapoxviruses will aid the diagnosis and identification of these viruses.

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مواضيع ذات صلة

Congenital Rubella Syndrome

Postnatal Rubella

Hendra Virus and Nipah Virus Infections

Measles (Rubeola) Virus Infections

Rhabdoviruses

Retroviruses

Reoviruses

Role of Biomarkers in COVID-19

Mumps Virus Infections

Human Metapneumovirus Infections

Respiratory Syncytial Virus Infections

Parainfluenza Virus Infections