Hendra virus infection

Hendra virus infection

M M WILLIAMSON

Introduction

In 1994 in Hendra, a suburb of Brisbane, Australia, there was an outbreak of acute viral pneumonia in horses and humans. Initially the virus that caused the disease was known as equine morbillivirus,11, 17 but subsequent studies resulted in the virus being renamed Hendra virus (HeV).19, 28 Thirteen months after the initial outbreak a farmer developed neurological disease as a result of HeV infection;15 two horses had died on his property and retrospectively were shown to have been infected with HeV.4 Nevertheless, infection is uncommon in horses and there has been only one further reported case in a horse, which occurred in January 1999.7 However, fruit bats (Pteropodidae) have a high prevalence of antibody to HeV and are a likely wildlife reservoir of the virus.10, 27

The zoonotic potential of HeV must be seriously considered when undertaking the collection of samples from suspect HeV field cases. Consequently, work at the Australian Animal Health Laboratory (AAHL) has been undertaken at biosecurity level 4.1

Aetiology

In the Hendra outbreak, a virus was isolated from lungs and kidneys of horses and the kidneys of the human case.11 The virus had the ultrastructural characteristics of members of the family Paramyxoviridae and limited sequence data indicated that it was a member of this family. Phylogenetic analyses on the matrix protein sequences indicated that it had a closer relationship to viruses in the genus Morbillivirus than to other genera in the Paramyxoviridae family.2, 11 Thus, the virus was provisionally called equine morbillivirus.

Sequence analysis of the untranslated regions of the F and P genes and the size of the genomic RNA indicate that HeV is not a classical member of the family Paramyxoviridae. Over the complete genome the virus displays only low-level homology with viruses of the genera Morbillivirus, Respirovirus and Rubulavirus.8, 19, 28 The virus also shows negligible immunological cross-reactivity with a range of antisera to other members of the Paramyxoviridae.8, 13 Therefore it cannot be easily classified in any of the existing genera in the subfamily Paramyxovirinae and has been renamed HeV after the location where it was first reported.13, 19

Epidemiology

Reports of disease caused by HeV have been confined to the Australian state of Queensland. The first reported outbreak occurred in September 1994, when a stablehand in the Brisbane suburb of Hendra developed influenza-like disease, remaining ill for six weeks before gradually recovering.12, 17 A horse trainer from the same horse stable developed symptoms similar to those of the stablehand and died within six days of developing clinical illness. Fourteen Thoroughbred horses on the property suffering from acute respiratory disease died or were euthanased. Seven other horses developed HeV-specific antibodies and therefore had been exposed to HeV, but these horses recovered from the illness. These horses were destroyed at a later date but were not necropsied because of the potential human health risk. A further nine horses from the Hendra stables remained unaffected and were seronegative.12

In September 1995, a 35-year-old farmer from Mackay, Queensland, was admitted to a hospital with neurological deficits. Serum and cerebrospinal fluid from the farmer were tested for specific neutralizing antibody to HeV and there was a low but significant antibody titre of 1:16 that rose over the next 11 days to 1:4 096.15 A nested polymerase chain reaction (PCR) amplified a 500 nucleotide region of the matrix gene and its sequence was found to be identical to that of HeV isolated from horses and humans from the Hendra outbreak.9, 11 When the patient died five weeks after admission to a hospital he had a serum antibody titre of 1:5 792.15

It became evident in retrospect that the farmer had assisted his wife, a veterinarian, in performing necropsies on two horses that had died on their property in August 1995.

This property is 800 km north of Hendra. The first horse had developed severe respiratory distress and died the same day. The second horse, which was said to have licked the face of the dead horse, died 11 days after the first.16 The farmer is reported to have had close contact with blood and blood-stained foam from the horses’ respiratory tracts, particularly those from the first case. Thus, it is likely that he became ill after these necropsies. His wife developed no clinical disease nor did she have antibodies to HeV.15 Formalin-fixed tissues from the horses necropsied at Mackay were located and indirect immunofluorescence tests and polymerase chain reaction assays (PCR) confirmed the presence of HeV viral antigen in tissues from both horses.4

Hendra virus infection of horses has been diagnosed on only one other occasion since the Mackay outbreak, although during this time over 100 equine samples have been examined for HeV exclusion. In January 1999 a nine-yearold Thoroughbred mare died following a clinical illness of 24 hours’ duration in Cairns, Queensland. The horse had severe pulmonary oedema and acute interstitial pneumonia, with evidence of vascular necrosis. Numerous syncytial cells were visible in the endothelium of blood vessels. An indirect immunoperoxidase test, electron microscopy and PCR confirmed the presence of HeV viral antigen in tissues from the horse.7 The...

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