An Assessment of the Hendra virus Situation in Victoria

Aim

The objective of this document¹ is to assemble data pertinent to Hendra virus in Victoria and to draw conclusions from it with respect to the level of risk of a Hendra outbreak in the State.

Summary

Surveillance of both the flying fox and horse populations in Victoria shows that while a proportion of the flying fox population has been exposed to Hendra virus, the horse population apparently remains free of the disease. The epidemiology of the disease in Victoria differs significantly from that in Queensland/New South Wales and it is likely that the introduction of Hendra virus into Victoria – should it occur in the near future – would be through the importation of an incubating horse rather than via transmission from flying foxes.

Background and Introduction

The first known occurrence of Hendra virus disease was in Queensland in September 1994. The virus was isolated and typed at AAHL – first called equine morbillivirus and then Hendra virus (named after the Brisbane suburb in Queensland where the first cases occurred).

Since that time, 20 outbreaks have been reported (to 12 July 2011). Nearly all have been confined to a strip of land approximately 50 km wide along the eastern coast of Queensland and the northernmost part of New South Wales.

Hendra virus is carried by flying foxes (also known as "fruit bats"). So far, four species have been incriminated, but their epidemiological significance is yet to be fully understood. Flying fox populations are known to be extremely dense in the areas where the disease has been reported.

The means of transmission to horses would appear to be via bodily excretions and secretions from flying foxes (saliva, urine, birthing fluids). It appears that a high level of exposure to flying foxes is commonly required, particularly during their pregnancy and birthing season. In horses, the disease is characterised by rapid onset, fever, neurological and/or respiratory signs with rapid deterioration, usually leading to recumbency and death.

Most affected horses die from the disease, but direct transmission from horse to horse is not considered to be a common transmission pathway; typically only one or two horses in a group will be affected [There is however some evidence to show that heavily contaminated instrumentation or equipment may carry the virus between horses].

Humans may become infected as a result of direct and close exposure to droplets originating from affected horses' respiratory tracts or other bodily fluids, especially during post mortem examination. While the overall incidence of human disease is extremely low, with an estimated attack rate of 10% in people receiving high risk exposures, the case fatality rate is high (over 50%).

Hendra virus disease has never occurred outside of eastern Queensland and northern New South Wales.

The disease has never been reported in Victoria. The grey-headed flying fox (Pteropus poliocephalus) occurs in Victoria, but populations are scattered. Most colonies are found in the Melbourne metropolitan area.

Data Collected in Victoria

In assessing the situation in Victoria, the following sources of data were used:

• Serological and other samples from flying foxes

From 1996 to 2008, a total of 152 opportunistically collected bat sera were submitted to AAHL for antibody testing. During the same period, 331 other samples (mainly urine and faecal material collected from beneath bat roosting areas and a few tissue samples) were submitted for agent detection. Most serum samples originated from Melbourne Zoo and Healesville Sanctuary; most of the samples for agent detection were collected in Melbourne Botanical Gardens.

• Serological testing of horses for export

Many importing countries require horses from Australia to be tested negative for Hendra virus prior to departure. Results of all sampling from January 2009 to June 2011 were reviewed. Samples are typically subject first to an ELISA test; any that give inconclusive results are re-tested using a SNT.

In 2009, 55 horses were tested for export purposes. During 2010, 110 horses for export were tested; a further 19 were tested during the period January-June 2011.

Note: comparable negative data (and absence of positives) exists for export testing in the years between 1995 and 2009, but have not been compiled for this assessment.

• Serological testing of horses at Laverton Knackery

From October to December 2010, a total of 104 sera from horses sent to the Laverton knackery were tested for Hendra virus using the same regime as described above.

• Investigations of cases showing signs suggestive of Hendra virus disease

All suspect Hendra cases are investigated either by DPI or private practitioners and appropriate samples are taken for the purposes of exclusion. Investigations have been sporadic, but have increased in recent years (probably as awareness of the disease has increased). Records dating back to 2003 show one investigation in that year, one in 2006, one in 2007, and five in 2008. Four properties involving five horses (of which 3 died) were investigated in 2009. Eight properties (15 sick animals of which 5 died) were investigated during 2010 and a further 21 properties (27 sick animals of which 9 died) from January to June 2011 (the high number of suspect cases being due to the outbreak of arboviral disease in horses during the first semester of 2011; many of these cases mimicked Hendra).

Data ruling out Hendra in all suspect cases prior to 2003 exists, but has not been compiled in this assessment.

Results

• Flying fox samples

During the 12 years that sera were tested, several were found positive (the overall seroprevalence was 10.53% by SNT and 15.8% by ELISA – see table).

Although a percentage of the flying fox population had seroconverted to Hendra virus, it was not possible to isolate the virus from any of the 331 samples for agent detection.

• Export testing

None of the 184 samples tested were positive for Hendra virus.

• Knackery Serology

All of the 104 samples tested were negative.

• Investigation of diseased horses

None of the 57 animals investigated were found to have had Hendra virus.

Discussion

Serology results from the 184 export-tested horses are helpful, but not as helpful as those from the knackery. Horses subject to export screening are itinerant and their periods of residence within Victoria is not known; i.e. the opportunity for potential exposure to Hendra virus within the State is unknown. The knackery horses, on the other hand, represent the peri-urban pleasure horse population with an arguably higher exposure risk.

The testing of the 104 horses at the Laverton knackery is of more significance, as these horses were all resident in the area prior to being despatched to the knackery.

The most meaningful results were those obtained from investigations of sick horses. These animals all exhibited signs suggestive of Hendra virus disease, namely fever and neurological and/or respiratory signs. Additionally, there were usually only one or a few horses affected within a group, typical of the field manifestation of Hendra. Despite this, in all the cases investigated Hendra virus was ruled out as the cause of disease.

The map below shows the distribution of horse-bearing properties (a proxy for horse distribution), the distribution of flying fox colonies (as reported by the public – since 1970) and the locations of Hendra investigations since January 2010.

The majority of the known flying fox colonies are not in close proximity to large concentrations of horses; although a few of the Hendra investigations occurred near to flying fox colonies, many did not. The investigations were widely distributed across the State.

Fig. 1. The distribution of horse properties, Hendra investigations and known bat colonies in Victoria. Sources: Biosecurity databases and Museum Victoria.

The dichotomy of findings is of interest. Laboratory data support exposure of flying foxes to infection in Victoria, but not horses. While it appears that a percentage of the flying foxes sampled had had contact with Hendra virus at some point in their lives, the inability to detect either virus excretion from flying foxes or cases in horses provides evidence to suggest that transmission is not occurring in Victoria. It must be noted however, that the bat testing data are some years old, and that this work would need repeating with more modern diagnostic techniques.

Why clinical cases of Hendra virus disease should occur in Queensland but not in Victoria can only be attributed to clear differences in horse/flying fox ecology. Some differences are easily observable and probably constitute the reason for the lack of Hendra disease detection in Victoria.

• Flying fox populations are nowhere near as dense in Victoria as they are in coastal Queensland. It would appear from all the evidence that Hendra virus is not easily transmissible: a very high level of exposure to large numbers of flying foxes for some period of time would appear to be necessary for transmission to occur². While these parameters are not fully understood, it appears they are currently not being achieved in Victoria.
• Another issue worth considering is whether the grey-headed flying fox – the species prevalent in Victoria – plays a key role in transmission events to horses. While it is apparent that they are capable of harbouring Hendra virus, it is worth noting that very few Hendra cases have occurred within the range of grey-headed flying fox, and only in the part of its range that overlaps with the range of the black flying fox. However. all of the recorded Hendra cases have occurred within the normal range of the black flying fox (Pteropus alecto); and also in the areas where black flying fox populations would appear to be at their most dense (see fig. 2 for a map of flying fox distributions). [Note: aberrant isolated detections of a single black flying fox have been reported on rare occasions within a grey headed flying fox colony within Victoria].

Conclusion

Hendra virus disease of horses has never been identified in Victoria, despite regular investigation of horses exhibiting clinical signs consistent with HeV infection.

The virus has not been isolated from samples obtained from flying foxes, although serological evidence indicates that a percentage of the flying fox population has been exposed to the virus.

There are clear differences in epidemiological conditions between Victoria (where the disease has not been identified) and coastal Queensland (where its occurrence has been sporadic) that may provide an environment at lower risk of spill-over. Flying fox populations in Victoria are smaller and less numerous, and the opportunities for bat to horse contacts and thus transmission may be fewer.

The flying fox species within whose natural range all HeV outbreaks have occurred – the black flying fox – is not native to Victoria.

However, it would be unwise to conclude that the disease will never be diagnosed here. The horse population in Australia is highly mobile, and it is possible that a horse that is incubating Hendra virus disease could be transported from Queensland or northern New South Wales to Victoria. In addition, the typical range of the black flying fox appears to be slowly expanding southwards over decades. It is thus important that vigilance be maintained.

References

Australian Animal Health Laboratories. Laboratory results for Hendra diagnosis collated by R. Suter, Victoria DPI.

Hall, L. S. & Richards, G. C. (2000). Flying-foxes: fruit and blossom bats of Australia. Sydney: University of New South Wales Press.

Halpin, K., Young P.L, H. E. Field & Mackenzie, J.S. (2000) Isolation of Hendra virus from pteropid bats: a natural reservoir of Hendra virus. Journal of General Virology (2000), 81, 1927–1932.

Halpin, K., Young P.L, H. Field & Mackenzie, J.S. (1999) Newly discovered viruses of flying foxes . Veterinary Microbiology 68 (1999) 83±87

Museum Victoria. Online mammal species mapper Accessed 12 July 2011.

Plowright, R.K., Foley, P., Field, H.E., Dobson, A.P., Foley, J. E., Eby, P., & Daszak, P. (2011). Urban habituation, ecological connectivity and epidemic dampening: the emergence of Hendra virus from flying foxes (Pteropus spp.) Proceedings of the Royal Society. doi:10.1098/rspb.2011.0522

Roberts, BJ, Catterall, CP, Eby, P and Kanowski, J. Latitudinal range shifts in Australian flying foxes: A re-evaluation. Australian Ecology (2011).

Acronyms

AAHL: Australian Animal Health Laboratories.
DPI: Department of Primary Industries (Victoria).
ELISA: Enzyme-linked Immunosorbent Assay
SNT: Serum Neutralisation Test.

Footnotes

¹ This report was compiled on 12 July 2011

²There are two possibilities here. Either bats secrete small amounts of virus and many are required to build up an infective dose, or "super shedder' bats are required to deliver the correct dose of virus. Such super shedders, if they exist, are thought to form a tiny percentage of the population, and it would require the presence of a large number of bats to ensure that at least one super shedder is present. In either case, exposure to a large number of bats is required to ensure transmission.