Picornaviruses are of the family Picornaviradae. These include large numbers of viruses that infect humans and animals, and are among the smallest of viruses (thus, the name pico which means small). Picornaviruses contain ss-RNA and have two genera responsible for human disease:
- the Rhinovirus (dealt with separately) primarily inhabits the nasal cavity (dealt with separately);
- the Enterovirus found mainly in the alimentary tract.
The picornaviruses are stable and able to survive for long periods of time in sewage, water, and foods. They are not phased by antibiotics (no viruses are) or disinfectants, can be frozen for years without effect, and can withstand heat to 50°C (122°F). Chlorination will inactivate them,but the slightest presence of organic matter protects them from many common treatment methods. They love human tissue.
The Enteroviruses have traditionally been subdivided into the poliovirus, coxsackievirus, and echovirus groups, but, since 1970, newly discovered members have just been designated as enteroviruses followed by a number. Generally, they produce mild or asymptomatic infections of the intestinal tract, but they can also cause respiratory or systemic infection. They also can be responsible for such infections of the central nervous system, as viral meningitis. Viral meningitis is a mild illness compared to the bacterial form ,and most people over the age of forty seem to be immune. The echoviruses and coxsackie viruses are responsible for the majority of cases. These viruses infect humans only and are spread by the fecal/oral route. Since polio is also an enterovirus, other related viruses are sometimes referred to as non-polio enteroviruses.
Coxsackieviruses have some thirrty members that cause illnesses, ranging from intestinal disorders to meningitis and throat ulcers with fever and flu-like symptoms. Generally, they are noted for producing diseases resembling poliomyelitis, but without paralysis. Scientists have been able to prove that the polio virus initially infects cells of the stomach. The coxsackieviruses were so named because they were first isolated in 1948 from children living in the town of Coxsackie, New York, whom authorities thought had mild cases of polio. Other members cause myositis (an inflammation of the muscles), as well as being responsible for heart attacks – if there is an inflammation of the heart muscle. In 1997, a strain of the virus caused the deaths of about thirty children in Malaysia from myocarditis. There is also a strong connection between the Coxsackie virus and insulin-dependent diabetes since the virus appears to destroy insulin-secreting cells in the pancreas.
The first echoviruses were discovered several years after the coxsackieviruses during field trials of the killed polio vaccine. It was customary in these trials to examine many vaccinated children to determine if they were shedding the polioviruses. Besides finding some polioviruses and coxsackieviruses, a group of unidentified viruses was discovered. Because these new isolates were from the intestinal tract, they were not associated with any particular disease. They were then given the name of echo which is an acronym for “enteric cytopathic human orphan viruses.” Since that time, however, many of the echoviruses have been shown to cause a variety of diseases. About thirty different serotypes of coxsackieviruses and thirty-three serotypes of echoviruses have been identified, but the differences between the two groups are slight. These viruses are transmitted by both the fecal/oral route and the airborne route. Some of the diseases they produce include the following: common colds, fevers with skin rashes, pharyngitis, pneumonitis, meningitis, encephalitis, carditis, and diarrhea, with the highest rates of infection occurring in late summer.
The Polio virus is a simple ss-RNA virus, of the Enterovirus family, consisting of only twelve genes. It replicates in the oropharynx or small intestine. There are three distinct types (numbered 1, 2, and 3) with varying degrees of potency. Those exposed to one type only can also become infected with the other two. All types are capable of causing paralysis, but type 1 is generally responsible for epidemics. The “trivalent” oral polio vaccine means that it protects against all three types.
The virus is usually spread via the fecal/oral route, from mouth secretions, and directly from person to person, and not through water, food, or insects. It multiplies in the digestive tract and then enters the bloodstream. In rare cases, it goes on to invade the spinal cord, brain, and certain nerve cells, causing paralysis. Ninety-five percent of the time, it is spread from healthy people who show no signs of any disease, and it is through these asymptomatic people that polio is spread through an unimmunized community. Currently, every year there are about 100,000 cases with 10,000 deaths worldwide. These occur mostly in Asia and Africa.
In addition to the poliovirus, there are a number of other water-borne viruses – rotaviruses, caliciviruses, certain adenoviruses, and the Norwalk agent. They all cause diarrhea and vomiting. Some are difficult to study because they do not grow in tissue cultures.
Dr. Salk developed a vaccine from killed polio viruses in 1956, and Dr. Sabin developed the oral vaccine from attenuated live viruses in 1960. Killed virus vaccine has not been manufactured in the US since 1965. The live vaccine is cheaper to manufacture and is now used exclusively. However, because the live vaccine contains a weakened type of poliovirus that could mutate into more virulent forms, it is not given to people with impaired immune systems or to members of their households.
The pathogenesis of poliomyelitis is as follows:
- Ingestion of the virus by the oral route.
- Some multiplication may occur in the tonsils and lymph nodes of the upper respiratory tract, with common cold-type symptoms.
- The virus enters the stomach, where it is resistant to stomach acids and digestive fluids.
- Multiplication of the virus occurs in Peyer’s patches and other lymphoid cells along the intestinal tract with clinical symptoms of enteritis.
- Viruses drain into regional lymph nodes, which stimulates antibody response.
- Viruses may pass into the blood.
- In a small percentage of cases, the viruses may cross the blood-brain barrier into the CNS. Motor nerves (anterior horn cells) are specifically infected and result in paralysis of certain muscles.
- Lower brain centers may be infected causing bulbar polio.
- Motor cortex may be infected causing widespread paralysis.
- Large amounts of virus is shed in the feces.