|
Elephant endotheliotropic herpesviruses (EEHV) are a type of herpesvirus, which can cause a highly fatal hemorrhagic disease when transmitted to young Asian elephants. In African elephants, related forms of these viruses, which have been identified in wild populations, are generally benign, occasionally surfacing to cause small growths or lesions. However, some types of EEHV can cause a highly fatal disease in Asian elephants, which kills up to 80% of severely affected individuals. The disease can be treated with the rapid application of antiviral drugs, but this has only been effective in around a third of cases. The first case of a fatal form of the disease was documented in 1995, though tissue samples from as early as the 1980s have since tested positive for the virus, and localized skin lesions in wild African elephants were recorded in the 1970s. Since 1995, there have been over fifty documented disease cases in North America and Europe, of which only nine have been successfully cured. Those affected are mostly young animals born in captivity, though a small number of older wild-born adults held in zoos have died, and a number of cases caused by the same pathogenic type of EEHV have been identified in both orphan and wild calves in Asian elephant populations. == Virus and transmission == The EEHVs are members of the Proboscivirus genus, a novel clade most closely related to the mammalian betaherpesviruses that have been responsible for as many as 70 deaths of both zoo and wild Asian elephants worldwide, especially in young calves.〔〔 There are currently six known species/types of the probosciviruses, and the most commonly encountered and most pathogenic form EEHV1 also has two chimeric subtypes, 1A and 1B.,〔〔〔〔〔 as well as numerous distinct strains.〔 EEHV1A (originally just known as EEHV1) was the first species/type identified, which causes an acute hemorrhagic disease with a very high mortality rate in Asian elephants.〔〔〔 This form of the virus was originally believed to occur naturally in African elephants (occasionally producing skin nodules), and to be transmitted to Asian elephants within captivity, but more extensive studies have since largely disproved this concept because several other species/types of EEHV (e.g. EEHV2, EEHV3 and EEHV6) instead have been identified in African elephants.〔〔 A second lethal subtype, EEHV1B, was identified in Asian elephants in 2001.〔 EEHV3, EEHV4 and EEHV5 have also each been responsible for the deaths of at least one Asian elephant calf.〔〔 EEHV2 has caused hemorrhagic disease in several African elephants as reported by.〔〔 Along with EEHV3 and EEHV6 it has also been found in pulmonary lymph nodes of several autopsied wild and zoo African elephants.〔Zong ''et al'', 2010〕〔 There are a further five or six species/types and varieties of elephant gammaherpesvirus (EGHVs) found in eye and genital secretions of many healthy Asian and African zoo elephants - e.g. EGHV1, EGHV2, EGHV3A, EGHV3B, EGHV4 and EGHV5 (also called Elephantid herpesviruses EIHV3, EIHV4, EIHV5, EIHV6, ElHV9 and ElHV10 - which are gamma herpesviruses. But these are neither closely related to the EEHVs, nor the known cause of any disease symptoms.〔〔 Studies have also detected both EEHV1A and EEHV1B as being shed in trunk wash secretions by healthy asymptomatic Asian zoo elephant herdmates of calves that previously had disease.〔〔 and similarly for EEHV5.〔 Thus, similar to saliva for most human and animal herpesviruses, the trunk secretions may be a source of transmission for EEHVs. Because of concerns about the origins of EEHV1 and evidence of cross-species transmission for EEHV3, long-term contact between Asian and African elephants has been discouraged,〔 along with avoiding new contacts between young captive-born Asian elephants and wild-born Asian elephants, as the latter may be carriers of the disease.〔 An analysis of a number of North American cases, which ruled out the direct transmission of the virus between any of the affected facilities studied, strongly supported the idea of a significant Asian carrier population.〔 This is supported by the fact that some of the deceased calves had no possible contact with any African elephants.〔 Artificial insemination is not believed to be a transmission factor,〔 notes that when multiple calves of a single bull or cow elephant died at different times (even at the same facility) they nearly all had distinct strains of the virus, yet when two calves died or became infected at the same time at the same facility the pairs have always had identical virus strains.〔 A program of enzyme-linked immunosorbent assay testing began in 2005, aiming to test blood samples from around a thousand elephants for antibodies to the virus in order to help identify carriers and possible transmission patterns.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「elephant endotheliotropic herpesvirus」の詳細全文を読む スポンサード リンク
|