Poxviruses are the largest and most complex viruses found in humans, birds, animals, and insects. These viruses, which are members of the Poxviridae family, comprise a large group of DNA viruses that are morphologically similar and share a nucleocapsid protein.
The Poxviridae family has been divided into two subfamilies based on whether they infect insect or vertebrate hosts. These are Chordopoxvirinae and Entomopoxvirinae. The subfamily Chordopoxvirinae contains viruses that infect vertebrate hosts and consists of eight genera, at least four of which cause diseases in humans. The following are the genera:
Orthopoxvirus: Smallpox (variola), vaccinia, monkeypox, cowpox, buffalopox, rabbitpox, mousepox, and camelpox viruses are all members of the genus Orthopoxvirus.
Parapoxviruses: These include ungulate viruses, which can cause infections in humans on occasion. These viruses are known as orf viruses, pseudocowpox viruses, deerpox viruses, and bovine papular stomatitis viruses.
Yatapoxviruses: These include tanapox and yabapox viruses, which are mostly found in Africa. Molluscipoxviruses, such as the molluscum contagiosum virus, are examples of molluscipoxviruses.
The majority of poxviruses that cause human disease belong to the genera Orthopoxvirus and Parapoxvirus. The viruses that cause smallpox and molluscum contagiosum are specific human pathogens, whereas other poxviruses cause rare zoonotic infections in humans. The poxviruses that cause human infections are listed in the table below.
Human infections caused by poxviruses
|Vaccinia||Used for smallpox vaccination In patients with eczema causes eczema vaccinata and in patients with immunodeficiency causes progressive vaccinia|
|Monkeypox||A disease similar to smallpox|
|Cowpox||A localized disease, such as milker’s node|
|Orf||Localized lesion, such as a single chronic granulomatous lesion with a central ulcer|
|Molluscum contagiosum||A self-limiting condition with localized lesions of the skin Causes chronic and extensive skin lesions in patients with HIV|
|Tanapox||Localized lesions, such as a single, pock-like vesicular lesion on the skin|
Variola (Smallpox) Virus
Smallpox, caused by the variola virus, was eradicated from the world in 1980 as a result of an intensive global campaign led by the World Health Organization (WHO). The main concern right now is the potential use of smallpox virus as a bioterrorism agent.
Properties of the Virus
Variola virus exhibits the following characteristics:
- Variola virus is a large, brick-shaped virus that measures 300 200 200 nm and is almost visible under light microscopy.
- The virion is made up of 90% protein, 5% lipid, and 5% DNA (3 percent ).
- The viral genome is made up of a long strand of double-stranded, linear DNA that is fused at both ends. The DNA is 30–375 kbp in length and has a terminal loop.
- The extracellular virion has two envelopes, whereas the intracellular virus has only one. The extracellular virion’s outer envelope is made up of host cell lipids and several virus-specific proteins, including hemagglutinins. These virions are made up of a large number of proteins (over 100), at least ten of which have the enzymatic activity required for genome replication.
Poxvirus replication is difficult. They are unique among DNA viruses in that the entire replication cycle of the virus occurs in the cytoplasm of the host cell. The virus encodes the enzymes required for mRNA and DNA synthesis, both of which are required for genetic replication. The virus encodes and transports all proteins required for mRNA synthesis. The virus also encodes proteins that serve other purposes, such as DNA synthesis and immune escape mechanisms. Finally, as the virions are released from the cell, they assemble and acquire their envelopes by budding from the cell membrane.
Physical and chemical reagent susceptibility: Variola virus is most stable when temperatures and humidity are low. It can be stored at room temperature for months, protected from sunlight and cold, or freeze-dried for years. It is resistant to the action of 50% glycerol and 1% phenol. Even though it is enveloped, the virus is not susceptible to ether and thus cannot be inactivated by it. It is sensitive to ultraviolet light and other forms of irradiation, and it is easily inactivated by formalin and oxidizing disinfectants.
Antigenic properties: The smallpox virus has a single serotype, which is responsible for the disease’s successful vaccination. A common nuclear protein antigen exists in all poxviruses, including vaccinia viruses. These poxviruses have at least 20 antigens, including the LS antigen, agglutinogen, and hemagglutinin.
Virus Isolation and Animal Susceptibility
In chick embryos, both variola and vaccinia viruses grow. Within 48–72 hours, they form pocks on the chorioallantoic membrane (CAM) of an 11–13-day-old chick embryo.
Variola virus can be cultured in chick embryo cells, monkey kidney cells, and HeLa cells. Unlike the vaccinia virus, which causes cytopathic effects (CPEs) within 24–48 hours, the variola virus causes CPEs very slowly. CPEs are made up of Guarnieri bodies, which are eosinophilic inclusion bodies made up of virus particle aggregation in matrix. These are visible in stained preparations.
Only monkeys are experimentally infected with the Variola virus. Intranasal variola virus infection causes smallpox in monkeys with generalized skin lesions. Scarification of the rabbit’s cornea with variola virus causes keratitis with the appearance of characteristic Guarnieri bodies in stained corneal smears.
Pathogenesis and Immunity
Pathogenesis of small pox
Smallpox caused by the variola virus is transmitted through the respiratory tract via inhalation of nasal, oral, or pharyngeal droplets. Infection can also be contracted through direct contact with infected skin or fomites. The virus enters the human host through the upper respiratory tract’s mucous membranes. Poxviruses encode numerous proteins that stimulate host cell growth before leading to cell lysis and viral spread, facilitating their replication in cells, lysis of infected cells, and viral spread.
The virus replicates at the site of inoculation and spreads to lymph nodes draining the mucosal entry site. The virus replicates in lymphoid tissues, causing transient viremia and reticuloendothelial cell infection. This is followed by a secondary phase of cell multiplication, which results in secondary viremia. As a result, clinical manifestations of the disease, such as fever and other toxic manifestations, occur. The viruses then enter the skin, settle in the blood vessels, and cause the characteristic smallpox rash. The rash is caused by virus replication in the skin, which is followed by cytotoxic T cell damage to virus-infected cells.
Variola infection is distinguished by the development of both humoral and cellular immunity. Within the first to third week of infection, hemagglutination inhibition (HI), complement fixing (CF), and neutralizing (NT) antibodies appear. HI and CF antibody levels typically decline within a year, whereas NT antibodies persist for many years or decades after infection.
Humoral antibodies are not protective. Cell-mediated immunity is critical in disease control and resolution. Virus-specific T cells prevent the spread of viruses by causing lysis of infected cells in the skin’s reticuloendothelial cells. A smallpox attack provides complete protection against reinfection. Vaccination provides immunity that lasts for about ten years.
The Variola virus causes smallpox, a now-extinct disease.
The incubation period ranged between 10 and 14 days. The prodromal phase, which corresponded to the viremia phase, was the first to appear. The disease manifested itself abruptly. The symptoms included a high fever, severe headache, nausea, pharyngitis, body malaise, and backache. During the prodromal stage, an exanthematous rash would appear on the palate, tongue, and pharynx.
Unlike chickenpox, the smallpox rash was characterized by skin lesions that were at the same stage of evolution. Chickenpox lesions appear in waves and at various stages, such as vesicles, pustules, and scabs.
The skin lesions appear first on the face and extremities before spreading centrifugally to the trunk. During a 17-day period, these lesions begin as macules and progress to papules, vesicles, pustules, and finally crusts. The lesions heal with the formation of distinctive scarring.
Overwhelming toxemia was the most common cause of death in smallpox patients. Smallpox came in two varieties: variola major and variola minor. Variola major was associated with a 25–30% fatality rate, whereas variola minor was associated with a fatality rate of less than 1%. Furthermore, flat smallpox and hemorrhagic variola were uncommon manifestations of smallpox in some patients, and were usually fatal.
The world is free of smallpox.
The last naturally occurring case of smallpox was discovered in Somalia in 1977. The last human case, which was caused by an unintentional laboratory infection, was reported in England in 1978. Smallpox was declared eradicated from the world by the WHO in 1980. The only known virus isolates are currently stored in laboratories at the Centers for Disease Control and Prevention (CDC) in the United States and the Vektor Institute in Russia. However, it was discovered in the 1990s that some countries were using the smallpox virus in their biological warfare program. It’s unclear how many countries still have the virus in their laboratories.
Infection reservoir, source, and transmission
Smallpox was a contagious disease. The most common sources of infection were respiratory secretions and skin lesion exudates. Inhaling respiratory droplets from smallpox patients was the most common method of transmission. During the first week of the rash, once the fever had started, the patients were extremely contagious. The characteristics of smallpox that contributed to its abolition are summarized in the table below.
- Because of the availability of an effective vaccine, smallpox was the first disease to be eradicated through a vaccination program.
- A smallpox vaccine that was safe, stable, affordable, and simple to administer was available.
- The presence of a scar indicated that the smallpox vaccination had been successful.
- Because the variola virus had only one serotype, immunization with vaccines protected against all cases of smallpox.
- Human and animal poxviruses have antigenic determinants in common. Safe vaccines derived from animal poxviruses could also be used for human vaccination.
- Smallpox was highly specific to humans as a host. Humans were the only infection reservoirs. There is no animal research.
- Because the clinical manifestations of smallpox were consistent, cases could be easily detected and diagnosed.
- This aided in the detection of human cases of smallpox as well as the implementation of preventive measures to prevent infection from spreading to other susceptible human hosts.
- There were no subclinical infections, nor was there any chronic virus carriage. The source of the infection was a smallpox patient who was easily detected and treated in the community, and preventive measures were implemented.
Clinically, smallpox was usually diagnosed. The following were the primary criteria for clinical diagnosis of smallpox:
- 1–4 days before the onset of rashes, there is a febrile prodromal phase;
- Skin lesions resembling smallpox (deep, firm, round), which could be umbilicated or confluent; and
- Skin lesions at the same stage of development can appear on any part of the body.
Skin lesions, such as vesicular fluid, were chosen as specimens.
If an electron microscope was available, it was used to show typical virus particles in clinical specimens.
It was useful in distinguishing a smallpox virus from a chickenpox virus. Light microscopy can reveal the Guarnieri bodies, the characteristic eosinophilic inclusion bodies, in stained preparations of clinical specimens.
Isolation of the virus
The virus is isolated in the laboratory through inoculation in chick embryos and cell culture. This is required for the rapid and accurate detection of poxvirus infections.
Embryonated egg: Inoculation of vesicular fluid into the chick embryo’s CAM is a reliable method for detecting and identifying variola virus. After 48–72 hours of inoculation, the virus causes characteristic pocks. Variola pocks are smaller in size, whereas vaccinia pocks are larger and have necrotic centers.
The monkeypox and cowpox viruses cause visible hemorrhagic lesions on the CAM, whereas the tenapoxvirus, molluscum contagiosum, and the Parapoxvirus do not.
Cell culture: For virus isolation from clinical specimens, human and nonhuman primate cells, such as monkey kidney and HeLa cells, are used. Viruses are identified by the presence of Guarnieri bodies, the variola virus’s distinctive CPE. Unlike the vaccinia virus, which produces CPE within 24–48 hours, the variola virus produces CPE slowly.
Tenapoxviruses and parapoxviruses grow poorly in cell cultures, whereas molluscum contagiosum does not grow at all.
Serological tests were helpful in confirming the presence of poxvirus infection. For the demonstration of serum antibodies that appear after the first week of infection, an indirect immunofluorescent antibody test as well as HI, CF, and NT tests are available.
There are no specific antiviral agents available to combat the variola virus.
Methisazone has some antiviral activity against some poxviruses. It is only recommended for chemoprophylaxis, not treatment.
Except for postvaccinated encephalitis, vaccination immune globulin is recommended for the treatment of all complications. Despite the fact that smallpox has been eradicated worldwide, there is still concern about the spread of smallpox through bioterrorism.
Prevention and Control
Smallpox was the first disease to be eradicated thanks to a successful immunization campaign. Even before the first millennium, India and China used empirical preventive measures against smallpox. In the eighteenth century, the practice of variolation spread from India to the Old World and then to East Europe by intentional inoculation with some virulent strains of variola to prevent variola infection.
This variolation procedure was very popular in Europe until it was replaced by vaccination, which Jenner introduced in 1796.
In vaccinated individuals, the variant conferred lifelong immunity. It was a risky procedure with a mortality rate that was roughly one-tenth of that seen in people with naturally occurring disease. Furthermore, for some time after variolation, inoculated individuals were capable of transmitting smallpox to susceptible individuals.
The WHO developed a concept program for global smallpox eradication in the 1960s. The disease was then present in 44 countries, with an annual global incidence of approximately 10 million cases. The disease was finally eradicated in 1977, when the last case of endemic smallpox was detected in Somalia, thanks to the use of highly potent and stable vaccine, rapid identification of outbreaks, and ring vaccination of all contacts of an infected person (refer to the box “Vaccine”).
Vaccinia virus is unique in that it is a man-made virus that does not exist in nature. The smallpox vaccine was made from the Vaccinia virus. Edward Jenner first used the cowpox virus to immunize himself against smallpox. The original virus strain was kept alive in subsequent years through arm-to-arm inoculation and as dry material on threads. The virus mysteriously underwent some permanent changes from its original cowpox form in the strain of vaccinia now used in vaccines over time. This vaccinia virus was easily distinguishable from fresh isolates of cowpox and smallpox viruses.
Vaccinia virus is now being studied in greater depth than variola virus because it is less dangerous to work with. The virus is also being used to develop recombinant vaccines as a vector.
The morphology of vaccinia and variola viruses is similar. They can, however, be distinguished by their growth characteristics and host range.
Vaccinia virus was recently discovered to be closely related to the New World orthopoxviruses Cantagalo and Aracatuba viruses.
The Vaccinia virus, which is used in vaccination, can also infect humans. Vaccinia infection is caused by unintentional skin inoculation. The lesion begins as a maculopapular rash at the site of inoculation, progressing to vesicles, pustules, and finally forming a scab. The lesion heals with prominent scarring. Vaccinia virus causes eczema vaccinata in patients with eczema and progressive vaccinia in patients with immunodeficiency.
Monkeypox can infect humans and cause a disease similar to smallpox. This virus was first isolated from captive monkeys in Copenhagen in 1958. In the early 1970s, the first human infection with this virus was described.
Monkeypox has been identified as a rare zoonotic infection in West and Central Africa, particularly in Zaire. Following cases of monkeypox were reported in North America in 2003. The infection was traced back to exotic rodents imported from Africa. Handling infected animals, such as monkeys and squirrels, can lead to monkeypox infection. Transmission from person to person is unusual.
Monkeypox and smallpox cannot be distinguished clinically. The condition is characterized by the appearance of a generalized pustule, rash, fever, and toxemia. Electron microscopy can be used to identify virion particles in clinical specimens and aid in the diagnosis of the condition.
Buffalopox, which is thought to be caused by the vaccinia virus, is found among buffaloes in India. The viruses cause pustular lesions on the teats and udders of lactating buffaloes in infected buffaloes. Individuals who come into contact with infected buffaloes, such as milkmen, become infected. The milkmen’s hands and faces are commonly affected by lesions.
The virus that causes buffalopox appears to be distinct from the viruses that cause variola and vaccinia. The smallpox vaccine does not protect against buffalopox infection.
Human cowpox infection is typically an occupational hazard. Cowpox virus is antigenically similar to variola and vaccinia viruses, but it can be distinguished by the hemorrhagic lesions it causes on CAM and rabbit skin.
Cowpox infection causes ulcers on the teats and udders of cows. During the milking process, it has the potential to spread to other cows and humans. Natural cowpox infection has also been observed in zoo animals such as cheetahs and elephants, as well as domestic cats. The primary host of cowpox infection has been suggested to be rodents or cats rather than cows.
Milker’s node, also known as paravaccinia, is an occupational disease contracted by humans while milking infected cows. Small ulcerating nodules are common lesions. The condition is characterized by painful and hemorrhagic lesions on the hands and face, as well as pyrexia and other constitutional disturbances. The virus differs from cowpox in that it grows in bovine kidney cell culture but not in embryonated eggs.
Differences between variola virus and vaccinia virus
|Property||Variola virus||Vaccinia virus|
|Isolated from||Humans||Origin unknown|
|Pocks on CAM||Small, white||Large, grayish, and hemorrhagic|
|Ceiling temperature on CAM||37.5–38.5°C||41°C|
|Growth on rabbit skin||–||+ or ++|
|Thymidine kinase sensitivity||+||–|
|Pathogenicity for baby mice||Low||High|
|Polypeptide pattern||Character of variola||Character of vaccinia|
|Host range||Narrow host range (only humans and monkeys)||Broad host range (includes rabbits and mice)|
Orf is a human infection caused by the virus of contagious pustular dermatitis in sheep and goats. The disease manifests itself in humans as a single chronic granulomatous lesion with a central ulcer. The lesions are most commonly found on the hands, forearms, and, on rare occasions, the face. Lesions heal without leaving scars. In terms of morphology, the virus resembles paravaccinia virus.
Molluscum contagiosum is a human-specific poxvirus. The virus is spread through close contact, most commonly through sexual contact.
The virus causes a skin disease that primarily affects children and young adults. It causes small, pink, papular pearl-like benign skin or mucous membrane tumors. These lesions are found on the epidermis and can be found on almost any part of the body except the soles and palms. Lesions on the trunk and anogenital areas are more common. This is a self-limiting condition that usually resolves on its own over time.
Molluscum contagiosum can cause chronic and extensive skin lesions in HIV patients. Sections of the nodular lesions reveal molluscum bodies, which are hyaline acidophilic inclusion bodies. Their bodies are large, measuring 20–30 m in length, and are made up of a large number of virion particles embedded in a protein matrix. These inclusion bodies can be seen in stratum corneum and stratum granulosum cells.
Humans are the only hosts who can be infected. Neither embryonated eggs, tissue cultures, nor animals can support the virus’s growth.
Tanapox virus was first isolated from cases of febrile illness along Kenya’s Tana River in the 1950s. The virus causes a single, pock-like vesicular lesion on the skin, which does not usually progress to form pustules. This lesion is most commonly found on the upper body. Insect bites from monkeys and other wild animal reservoirs spread the virus. It has no antigenetic resemblance to other poxviruses.
The virus multiplies in human and monkey tissue cultures but not in embryonated eggs.
In monkeys, the Yabapox virus causes large benign tumors. It causes benign histiocytomas in monkeys 5–20 days after a muscular or subcutaneous inoculation. Such lesions have been reported in people who have come into contact with affected monkeys.
Textbook of Microbiology and Immunology, 2/e. Subhash Chandra Parija