To date, six viral families with single-stranded RNA genomes in sense-strand orientation are known: picornavirus, calicivirus, togavirus, coronavirus, flavivirus, and retrovirus common orthography: picornavirus, retrovirus.
Picornaviruses
Transmission of enteroviruses, parechoviruses, and hepatoviruses is by the fecal-oral route. The viruses first replicate in the intestine, from which location they reach their target organ in the bloodstream. Large numbers of inapparent infections are typical of this group. Rhinoviruses are transmitted by droplet infection and remain restricted to the upper respiratory mucosa.
Diagnosis
enteroviruses and parechoviruses are diagnosed by isolation in cell cultures or with PCR, hepatitis A serologically (IgM), and rhinoviruses, if at all, by isolation
Prevention
basic polio immunization with the dead or live vaccine; hepatitis A with the dead vaccine; exposure prophylaxis with rhinoviruses. & The picornaviruses are among the most thoroughly studied viruses of all. The name picon is an abbreviation that stands for two characteristics of this family: they are small(pico) viruses with an RNA genome. The RNA is polyadenylated at its 30 end and has no cap at the 50 ends, but instead a virus-coded, basic protein about 2 kDa in size, the VPN
Enteroviruses (Poliovirus, Coxsackievirus, Echovirus) and Parechoviruses
. The enteroviruses and parechoviruses are transmitted per os and replicate at first in the lymphoid tissue of the pharyngeal space, later mainly in the intestinal wall. They then reach their “target organs” via the bloodstream (e.g., CNS, muscles, heart, liver), followed by manifest organ infection, which, however, only develops in a small percentage of cases.
Most infections run an asymptomatic course. Viremia is always present so that even asymptomatic enterovirus and parechovirus infections confer effective immunity. The cases of manifest infection frequently run atypical courses with mild clinical symptoms. The same viral type can cause different symptoms and several different viral types can cause a given clinical symptom. Recently, severe complications have been described, mainly as a sequel to hand, foot, and mouth disease (HFMD, Table 8.2) in Southeast Asia. The following clinical pictures have been described for enteroviruses and parechoviruses.
Salk introduced a dead vaccine in 1954 for poliomyelitis prophylaxis (IPV, inactivated polio vaccine) consisting of three poliovirus types inactivated by formalin. Five years later, the live vaccine (OPV, oral polio vaccine according to Sabin) was introduced, which contains three live but no longer neurovirulent poliovirus strains, either singly or in combination.
The WHO plan to eradicate poliomyelitis worldwide would seem feasible with this vaccine as demonstrated by its eradication in several countries including all of South Ameri
Polio Vaccines: Pros and Cons
The dead or inactivated vaccine has the advantage of a long stability period and practically foolproof application safety. The disadvantages of this vaccine form are its high cost, the requirement for three injections and weaker or at least shorter lived protection than is provided by the attenuated form. Work is ongoing on development of enhanced (IPV) vaccines of this type.
The advantages of the live vaccine are its oral application route, low price and high level of efficiency. One disadvantage is its thermolability, resulting in reduced numbers of seroconversions (more nonresponders) in tropical countries.
Last word
Another difficulty is presented by the (1 in 1 ! 106) cases of paralysis (vaccination-associated paralytic poliomyelitis, VAPP) resulting from a vaccination. VAPP shows a higher level of incidence than infections by the wildtype poliovirus in industrialized countries, which has led practically all these countries to return to using IPV.
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