Mokola virus
| Mokola virus | |
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Group V ((−)ssRNA)
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Mokola virus
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Mokola virus (MOKV) is a RNA virus related to the Rabies virus that has been sporadically isolated from mammals across sub-Saharan Africa. The majority of isolates have come from domestic cats exhibiting symptoms characteristically associated to Rabies virus infection.[1]
There have been two reported cases of humans being infected with Mokola virus.[2][3]
Contents
Classification
Mokola virus (MOKV) is a member of the Lyssavirus genus, which belongs to the family Rhabdoviridae. MOKV is one of four Lyssavirus species found in Africa. The other three species are the classic Rabies virus, the Duvenhage virus and the Lagos bat virus.[4]
Emergence
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MOKV was first isolated in Nigeria, in 1968, from three shrews (Crocidura species) found in the Mokola forest, Ibadan, Oyo State, Nigeria. The virus was shown to be morphologically and serologically related to Rabies virus. Since the initial isolation of MOKV, the virus has been mainly isolated in domestic cats and small mammals in sub-Saharan Africa.[5]
There have been two recorded cases of human infection with MOKV, with both instances occurring in Nigeria. Both cases involved young Nigerian females. The first patient, who had been suffering from a fever and convulsions, recovered fully from infection.[6] The second case, which occurred in 1972, involved a 6-year-old girl who had been suffering from drowsiness. The second patient ultimately suffered from paralysis and a terminal coma as a result of the MOKV infection.[7]
The origin of the MOKV in both human cases of MOKV infection remains unidentified.[8]
Genome and structure
The MOKV genome comprises a non-segmented, single-stranded, negative-sense RNA molecule of 11,939 nucleotides in length. It is estimated that 90.3% of the MOKV genome consists of protein-encoding or signal-encoding sequences.[9]
The genome encodes five proteins: matrix protein M; transmembrane glycoprotein G; nucleoprotein N; phosphoprotein P; and the RNA-dependent RNA polymerase L.[10]
MOKV is an enveloped virus, with the envelope lined with spikes consisting of the viral protein G.[11] MOKV exhibits the 'bullet-shaped' morphology that is characteristic of members of the Rhabdoviridae family.[12]
Evolution
This virus appears to have evolved between 279 and 2034 years before the present.[13]
Transmission
The human cases of human MOKV infection have demonstrated the zoonotic potential of MOKV [14][15] but as a natural reservoir has yet to be identified, transmission of the virus is poorly understood. Considering the MOKV isolates from small mammals and domestic cats; it has been suggested that small mammals, which are preyed upon by domestic cats, may be the natural MOKV reservoir.[16]
Unlike other Lyssavirus species, MOKV is able to replicate inside mosquito cells in vitro, suggesting that insects may be implicated in MOKV transmission.[17]
Pathology
Symptoms provoked by MOKV infection are very similar to the symptoms associated with Rabies virus infection, such as fever and headaches for example. Ultimately both MOKV and Rabies virus infection can lead to death by encephalitis. The similarity between the symptoms associated with MOKV and Rabies virus may be the reason why there have been so few cases of recorded MOKV infections. MOKV infections may be being falsely diagnosed as Rabies virus infections.
There is one significant difference between the symptoms observed in domestic cats infected with MOKV and Rabies virus. MOKV-infected domestic cats do not demonstrate the aggressive, unprovoked behaviour traits typically associated with Rabies virus-infected mammals.
Control
At present there is no medical or veterinary vaccine that protects against MOKV infection.[18] It has long been established that in spite of the relatedness between MOKV and Rabies virus, immunisation with the rabies vaccine does not confer protection to MOKV infection. Isolation of MOKV from a number of domestic cats vaccinated with the rabies vaccine have demonstrated this fact.[19]
In order to neutralize Rabies virus, the rabies vaccine targets the transmembrane glycoprotein G on the viral envelope. Variation in the amino acid sequences of the antigenic domain III within protein G between Rabies virus and MOKV has rendered the rabies vaccine ineffective against MOKV infection.[20]
References
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