Bunyaviruses

The bunyavirus and phlebovirus species are transmitted by arthropods. They cause benign, febrile infections, more rarely infections of the CNS and hemorrhagic fever. All bunyaviruses feature a single-stranded antisense RNA genome with three segments. Certain types of hantaviruses are the patho­gens responsible for “hemorrhagic fever with renal syndrome” (HFRS), other types cause the “hantavirus pulmonary syndrome” (HPS). The hantavirus species are transmitted from mouse species to humans aerogenically.

Diagnosis: serological.

Prevention: exposure prophylaxis.

Pathogen. The family Bunyaviridae comprises over 200 viral species, among them four human pathogen genera: Bunyavirus, Nairovirus, Phlebovirus, and Hantavirus. The bunyaviruses are spherical, 80-110 nm in size, and posses envelopes with spikes formed on membranes of the smooth endoplasmic retic­ulum. The genome consists of three antisense-strand RNA segments, whereby each segment produces a separate ribonucleoprotein complex, resulting in a unique feature of the virion: it contains three helical nucleocapsids.

Pathogenesis and Clinical Picture.

-Genus Bunyavirus: these viruses are transmitted by arthropods. They cause benign forms of encephalitis such as California encephalitis and La­Crosse virus infections, both endemic to the USA, and the Oropouche virus in Brazil.

-Genus Nairovirus: the main human pathogen in this group is the Crimean-Congo hemorrhagic fever virus, with a lethality rate as high as 50%. The virus is endemic to southeastern Europe, Central Asia, China, Saudi Arabia, and Africa and is transmitted by ticks as well as by direct contact with infected animals or patients.

-Genus Phlebovirus: this group includes the pathogens that cause the be­nign Pappataci or phlebotomus fever (“sandfly fever”), which occurs in Europe (Italy, Yugoslavia), North Africa, Asia, and South America and is trans­mitted by the phlebotomus sandfly.

Rift Valley fever (RVF), an acute, febrile disease, rarely also involving hemor­rhagic fever, is transmitted by mosquitoes and is endemic to Africa, usually following epizooties in livestock, in which case aerosol infection occurs ⁸ (slaughtering). Epidemics have been reported with over 200 000 cases in Egypt and 25 000 cases in Senegal. Further epidemics have occurred in Somalia, Kenya, and Sudan.

-Genus Hantavirus: this genus includes several viral species (or serotypes)

(Table 1) that can be classified in two groups according to the clinical symp­toms they cause:

-the pathogens of nephropathica epidemica (NE) and hemorrhagic fever with renal syndrome (HFRS),

-and the hantavirus pulmonary syndrome (HPS).

The sources of infection are rodents (mice and rats). The infection is acquired by inhaling aerosols of urine, feces, and animal saliva. In NE and HFRS a renal dysfunction follows the influenza like symptoms. HPS infection results in a rapidly progressive, acute dyspnea with pulmonary edema and is lethal in 60% of cases.

Table 1 Serotypes of Hantaviruses

Diagnosis. It is possible to isolate the virus from blood, but the procedure is too drawn-out and costly for routine diagnostics. Serology (IgM detection) is the method of choice, although the results can be difficult to interpret with bunyaviruses due to the rapidly changing antigenic variants produced in many of the viral species.

Epidemiology and prevention. The bunyaviruses and phleboviruses are transmitted by bloodsucking arthropods, whereby the cycle involves either human and vector only or, as with the togaviruses and flaviviruses, a mam- mal-arthropod-mammal cycle actually independent of humans, and in which human victims represent a dead end for the infectious agent. Hantaviruses are transmitted aerogenically to humans from rodents, in which the viruses persist a pathogenically for the lifespan of the animal. The most recent isolates of the HPS pathogens have also apparently persisted in the reservoir animals

for a long time, with occasional human infections as shown by retrospective analysis of blood and tissue specimens. Viral outbreaks are explained by sud­den plagues of mice. Preventive measures include exposure prophylaxis (avoidance of insect bites and contact with rodents). An active vaccination is available for protection against Rift Valley fever.