Hendra virus is an emerging exotic disease first described in 1994 in Hendra, a suburb of Brisbane, Australia.  In 1994, two outbreaks occurred in Queensland within one month of each other.  In one case, 21 horses were affected and 14 died or were euthanized after developing severe respiratory signs.  Two humans were affected, one fatally.  In the second case two horses were sickened and died and one person died following bouts of recurrent encephalitis.  A third case affected a 9 year old Thoroughbred mare in Queensland in January 1999.  In the field, it affects horses and humans.  Experimentally, cats and guinea pigs can be infected with the virus.  The reservoir is fruit bats, which are subclinically affected.

Infectious agent:  Formerly known as equine morbillivirus, this virus is now labeled "Hendra virus" and shares a unique genus, along with Nipah virus, in the Family Paramyxoviridae.  The Hendra virus is an enveloped ssRNA virus. 

History:  When epidemiologic factors were examined in three Australian cases, risk factors included breed (Thoroughbred), sex (female), age (< 8 yrs.) and housing (pasture).  Late term pregnancy also appears to be consistent in the three Australian cases.  Access to pasture by flying foxes or other infected mammals is important, and the existence of an undiscovered intermediate host is possible.

Clinical Signs:  In horses, there is an acute respiratory syndrome, followed by death in 1 to 3 days.  Signs include depression, anorexia, fever, dyspnea, tachycardia, tachypnea, clear to serosanguinous nasal discharge, injected mucous membranes with a cyanotic border, and occasionally icterus.  Prior to death some horses have dependent edema, head pressing and ataxia, with frothy nasal discharge exuded right at the time of death.  In humans, flu-like symptoms are present which may progress to pneumonitis, respiratory failure, renal failure, arterial thrombosis, cardiac arrest, and meningoencephalitis.

Transmission:  Oronasal routes, including direct contact between horses, via contaminated tack, ingesting material contaminated with urine from infected animals.  Flying foxes, or fruit bats (Pteropus spp.) have been identified as a reservoir for this disease.  A 1998 study from the Australian Veterinary Journal suggests that transmission may occur from infected cats to uninfected horses.  High titers of the virus are present in the kidneys, urine, and saliva of infected horses.

necropsy of infected cat - pulmonary congestion and hydrothorax	flying fox - reservoir host

Incubation period:  in field observations, incubation is thought to be 8-16 days; in experimental infections incubation is shorter, 4-10 days.

Pathogenesis:  Hendra virus produces lung and brain lesions.  The virus replicates in endothelial cells of at least these two organs, causing circulatory problems and associated necrosis.  Occasionally multinucleate giant cells may be seen in pulmonary endothelium, which are highly suggestive of Hendra virus.   Click here for histo slides.
 

Diagnosis:  As a foreign animal disease, you are not expected to diagnose this in the field, but you must report this to the proper authorities if you suspect it.  Trained individuals can diagnose Hendra using a serum neutralization test and indirect fluorescent antibody tests. High titers of Hendra virus can be found in kidneys, urine, saliva of affected horses.

Dr. Peter Hooper, pathologist for the Australian government, working with a horse infected with Hendra virus

Treatment and Prevention:  There is no treatment for this disease in horses.  Affected animals should be destroyed to prevent spread.

                 Next...