Arboviruses (arthropod borne diseases) are viruses of vertebrates biologically transmitted by hematophagous insect vectors. They multiply in blood sucking insects and are transmitted by bite to vertebrate hosts. They multiply in bloodsucking insects and are transmitted by bite to vertebrate hosts. Insect viruses and viruses of vertebrates that are sometimes are mechanically transmitted by insects do not come into this category. Inclusion in this group is based on ecological and epidemiological considerations and hence it contains members that are dissimilar in other properties.
With better understanding of the physical and chemical properties of individual viruses, they are reassigned to more defined taxonomical groups. Though taxonomically unacceptable, the name arbovirus is a useful biological concept (Paniker, 2006, p.521).
History of Arbo-viruses
For hundred of years, diseases similar to some known to be caused by arboviruses have been recognized. However, only over the past few decades have techniques been available that allowed definitive diagnosis and determination of the etiological agents. Studies with newly developed isolation techniques including virus isolation in laboratory mice, revealed the existence of many other existing viruses.
When it was shown that some of these viruses were pathogens of humans or of livestocks, the field of arbovirology was established (Lennette, & Smith, 1999, p.305).
Types of Arbo-viruses
Viruses that are transmitted from one animal host to the next by insects are known as Arboviruses. They have evolved from varieties of backgrounds, belonging to different families, and cause a wide spectrum of diseases. Some arboviruses are named after the disease the cause, for example yellow fever. Some were named after their insect vector, for example Phleboviruses named after the phlebotomus sandfly. Although there are more than 500 arboviruses contained in four viral families, namely, Flaviviridae, Togaviridae, Bunyaviridae, and orbiviridae, the name of viruses of medical importance is about 25. Many of them are rapidly evolving to fill new ecological niches. Collectively the arboviruses constitute some of the most important emerging and re-emerging pathogens and some of the greatest challenges to biomedical research (Dawood,2002, p.151).
The four most common arboviruses in the United States are eastern equine enchepalitis, western equine enchepalitis, equine enchepalitis, St. Louis enchepalitis and Colorado tick fever viruses. Colorado tick fever as the name suggest is tick borne and is the most common tick borne viral fever in the United States. The other arboviruses are carried by mosquitoes. Humans do not serve as reservoir hosts. These viruses cycle between wild animals, primarily wild birds and mosquitoes, with humans and horses serving as dead end hosts (Krasner, 2002, p. 203). Crosse viruses are the most common arboviral causes of encephalitis in the United States. Japanese encephalitis occurs more frequently in the eastern hemisphere. Eastern equine encephalitis is perhaps the most severe encephalitis due to its rapidly progressive nature (Suarez, p.522).
How arboviruses are spread?
Athropods that may transmit abroviurses include ticks and dipterous insects of three families namely mosquitoes, ceratopogonid midges, and flabotomine flies (sandflies). The life cycle of the virus in the arthropod is characterized by action replication in the salivary glands, which allows the athropod to transmit the infection by biting the vertebrate host. Some arboviruses infect the ovary of the invertebrate host, allowing the transovarian infection and thus amplifying infection through the progeny. Although generally birds are often involved in life cycles of arboviruses, outbreaks of arbovirus infections in domestic poultry appear to be rather infrequent. Nevertheless, the emergence of epidemics has occurred on several occasions mainly related to climatic conditions that favor an increase of vector population (Jordan, p.364). The virus after entering the body through the bite of insect vector multiplies. After multiplication in the reticuloendothelial system, viremia of varying duration ensures and in some cases the virus is transported to the target organs, such as the central nervous system in encephalitides, the liver in yellow fever and the capillary endothelium in hemorrhagic fevers (Paniker, 2006, p.521).
Affect and prevalence of arbovirus
There are considerable numbers of arboviruses that affect humans. In these infections, the main clinical manifestation is fever, chills, joint pain, and a maculopapular rash. Mild forms of the diseases are frequent (Souhami and Moxham,2002, p.298). However in case of serious arboviruses like meningitis, the viruses invade the spinal chord, and the meninges (wrapping around the spinal chord and brain) become inflamed and may result in coma, convulsion, tremors, paralysis, memory deficits, and possibly permanent brain damage (Krasner, 2002, p.203). Other arbovirus led infection diseases like Japanese encephalitis tends to occur in seasonal peaks in the winter and summer when certain pathogens are more common. Arboviral encephalitis cases peak in late summer and early fall and it affects all age groups. Most arboviruses affect persons of all ages, with notable exception being La cross virus, which tend to afflict children and St. Louis virus, which afflicts adults older than 50 years of age. St. Louis virus has the ability to cause coma and alteration of mental status (Suarez, p. 521).
Protection from tick infestations can be by personal protection which means wearing clothes impregnated with suitable repellents. Use of netting and window and door screening can prevent contact of mosquito and other fly vectors with humans. Some other important measures include removing and destroying mosquito breeding sites, providing piped water, maintaining liquid waste system, and managing irrigation systems which helps reducing populations of both maintenance and amplification vectors. Vaccines hold great potential for arbovirus borne disease prevention. Vaccines are available for a few of the many arboviruses that cause disease (Lennette & Smith, 1999, 331). The yellow fever vaccine was the first vaccine developed against arboviral diseases. Of the two original yellow fever vaccines developed in the 1930s, only the 17D vaccine is still in use (Eldrige and Edman, 2004, p.598).
One the global scale, the public health danger posed by the arboviruses is massive and much of the problem comes from the sheer numbers of detected viruses. The information about detected arboviruses can be organized in systematic way to facilitate diagnosis of infections by specific viruses and also to enable researchers to initiate further study.
Dawood, R.M.(2002) Traveller’s Health: How to stay healthy abroad, Oxford University Press, pp. 762
Eldridge, B.F. & Edman, J.D. (2004) Medical Entomology: A textbook on public health and veterinary problems, Springer, pp.672
Jordan, F.T.W. (2001) Poultry diseases, Elsavier Health Sciences, pp.364
Krasner, R.I. (2002) The microbial challenge: Human Microbes interaction, ASM press, pp.433
Lennette, E.H. & Smith, T.F. (1999) Laboratory diagnosis of viral infections, Informa Health care, pp.868
Paniker, C.K.J (2006) Textbook of Microbiology, Orient Longman, pp.665
Souhami, R.L. & Moxham, J (2002) Textbook of Medicine, Elsevier Health Sciences, pp.1508
Suarez, J.I (2005) Critical care neurology and neurosurgery, Humana Press, pp.636
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