There have been three outbreaks of Hendra virus in horses (see e.g. Field et al. 2001, Westbury 2000). The first occurred near Mackay in central Queensland. A 10 year-old heavily pregnant thoroughbred mare died on August 1, 1994, less than 24 h after developing severe respiratory distress, ataxia, swelling of the cheeks and supraorbital fossa. On August 12, a two year-old colt in an adjoining paddock died after a 24 h clinical course characterized by aimless pacing, muscle trembling, and haemorrhagic nasal discharge. The colt had had less than 24 h contact with the mare during the clinical course of the disease but apparently licked her face shortly after she died (Field et al. 2001, Westbury 2000). The owners, a veterinarian and her 35 year-old husband, conducted necropsies and attributed the deaths to avocado toxicity and snake bite. On August 22[1], the husband was admitted to the hospital with aseptic meningitis, from which he soon recovered. He had cared for both horses and assisted at the necropsy without gloves, mask, or eyeware (O’Sullivan et al. 1997). In October 1995, the husband developed neurologic signs, including lower back pain, irritable mood, tonic-clonic seizures, and worsening paralysis; he died 25 days after admission to the hospital (39 days after the onset of symptoms). HeV antigen and typical HeV nucleocapsids were found in his brain, and PCR was run using cerebrospinal fluid and found homologous to the 1994 Brisbane isolate. A neutralizing HeV antibody titer of 1:4 was detected in the husband’s serum after his initial meningitis, and titers of 1:16 to 1:5792 were detected between his second hospital admission and death in October (Westbury 2000). Specific fluorescent antibody and PCR tests on stored tissue samples revealed that the two horses had been infected with HeV. His wife tested seronegative and did not become sick (Barclay & Paton 2000).
The second outbreak occurred several weeks after the first in Hendra, Brisbane, 800 km south of Mackay. The index case was a heavily pregnant thoroughbred mare at pasture, who was moved to a training stable housing 23 thoroughbreds when she was observed ill. She died two days later. Within 14 days, 19 more horses became affected, and 12 of them died acutely. Westbury (2000) suspects iatrogenic spread within the farm. Ten deaths occurred in the thoroughbred stable where the index case was housed, one death occurred in a neighboring stable, and the other occurred in a horse that had been transported from the main stable to a paddock 150 km away (Westbury 2000). The seven affected horses were all from the main stable or adjoining stable. Three of the seven affected but recovered horses displayed no signs but seroconverted; of the four symptomatic seroconverters, two were left with tonic spasms. All seven seropositive survivors were euthanized. These are the only naturally seropositive horses ever reported for Australia (Young et al. 1996). Nine horses from the premises appeared to have escaped infection. Another horse died for undetermined reasons at the paddock where the mare became ill (Barclay & Paton 2000). Five and six days after the mare’s death and before the development of symptoms in other horses, the 40-year-old stablehand and 49-year-old trainer, respectively, developed influenza-like symptoms. The stablehand recovered over the next six weeks, but the trainer, Vic Rail, died from interstitial pneumonia, respiratory failure, arterial thrombosis, and renal failure after six days in intensive care. Rail had attempted to hand-feed the dying mare while he had abrasions on his hands and arms and had considerable exposure to her fluids (Selvey et al. 1995).
The third outbreak occurred in Cairns, North Queensland. On January 18, 1999, a nine-year-old thoroughbred mare at pasture died after a 24 h illness characterized by inappetance, depression, swelling of the face, lips, and neck. Shortly before death she was discovered recumbent with copious quantities of yellow nasal discharge despite symptomatic treatment. She had lived in the paddock for six years. In August/September 1998, she had visited a thoroughbred stud and was found to be in foal on 6 November, but post-mortem examination of her uterus through a small flank incision showed she was not pregnant (Field et al. 1999). A 17 year-old pony club mare had shared her paddock and feed bin for the previous two months. She remained asymptomatic and seronegative to HeV (Westbury 2000). Field et al. (1999) describe the 1.5 ha property as urban, containing scattered fruit trees (predominantly Mangifera indica) and semi-closed forest regrowth dominated by Melaleuca spp. P. scapulatus and either P. alecto and/or P. conspicillatus were observed feeding nightly on these trees in February 1999. The paddock was also shared by poultry, including chickens, ducks, turkeys, and peacocks.
There is no apparent link between the three outbreaks, despite intensive surveillance after the Hendra outbreak (Barclay & Paton 2000, Westbury 2000).
[1] The husband was admitted “10 days after the death of the colt,” according to Westbury (2000). O’Sullivan et al. (1997) report that he had, at admission, a 12-day history of symptoms. The chronology suggests he had been infected by the mare.
Author: S. Cobey.