Definition
Equine morbillivirus pneumonia (EMP) is an acute febrile
respiratory infection of horses characterized by fever, increased respiratory and heart
rates, respiratory distress, and death.
Etiology
The cause of EMP is a newly recognized virus in the genus Morbillivirus and was named equine morbillivirus (EMV). When EMV was tested against antisera to a range
of paramyxoviruses, morbilliviruses, and pneumoviruses there was a very weak reaction with
only rinderpest virus (4).
History
Equine morbillivirus pneumonia has been reported only in
Australia. The disease was first recognized on one property in Hendra, Australia, in
September 1994. In this outbreak, 20 horses were sick and 13 died, and 2 humans were
infected and 1 died (4). A retrospective diagnosis of EMP was made for horses that died in
August 1994 on a property in Mackay about 1,100 km from the first reported case of EMP. In
this outbreak four horses were infected and two died; the farmer became ill, recovered,
had a relapse, and died of EMV infection 13 months later. There was no evidence of
epidemiological association between these outbreaks (5,6).
Host Range
Equine morbillivirus has naturally caused disease in horses and
humans. Experimentally, cats and guinea pigs have been infected (3). The disease in cats
is very similar to the disease in horses (8). The recent detection of neutralizing
antibody to EMV in the native Australian fruit-bats suggests a possible resevoir for EMV
(10).
Geographic Distribution
Equine morbillivirus pneumonia has been reported only in
Australia.
Transmission
On the basis of epidemiologic findings and experimental results
with cats, EMV is not readily transmissible. In the field, very few horses in contact with
infected horses became ill, and even horses placed in uncleaned stalls previously occupied
by an infected horse did not become ill (8). For both animals and humans, EMV seems to
require direct contact with respiratory secretions of infected animals (6). If a fruit-bat
is the reservoir, the mechanism for spread to the horse is not known.
Incubation Period
The incubation period in horses is 8 to 14 days (2).
Clinical Signs
The initial case of EMP in both outbreaks was a pregnant mare on
pasture. Clinical signs in field cases at Hendra were fever up to 105.8o F (41o C), severe respiratory distress, and death. Two horses that recovered had mild myoclonic
twitching (5). In the Mackay outbreak, the pregnant mare had "severe respiratory
distress, ataxia, and marked swelling of the head — particularly of the infraorbital
fossa and cheeks. The second horse, a stallion (horse B), reportedly showed aimless
pacing, muscle trembling and a haemorrhagic nasal discharge" and died (5).
In experimentally infected horses, the first sign of disease was
fever that ranged from 102.2 to106o F (39 to 41.2o C). In some
animals, as the disease progressed, heart and respiratory rates increased up to 72 and 60
per minute, respectively, breathing became labored, and the animal's demeanor varied from
somnolence to mild agitation (2).
Gross Lesions
The most distinctive gross lesion in EMP is a bilateral
"pulmonary edema characterized by gelatinous distention of subpleural
lymphatics." The lungs are heavy and congested; the edema is more prominent in the
ventral parts and the lungs vary from a mottled yellowish-brown to dark blue. The major
air passages are essentially normal. In one experimental horse, there was also a bilateral
dilation of the pulmonary lymphatics and a very enlarged dark spleen (2).
Morbidity and Mortality
The number of animals that have been infected naturally and
experimentally is small, but using these numbers, mortality is high in animals infected.
Owing to the low transmissibility of EMV, morbidity has been low.
Diagnosis
Field Diagnosis
Equine morbillivirus pneumonia should be suspected when a horse
that dies has been febrile and the necropsy findings include a pulmonary edema
characterized by gelatinous distention of subpleural lymphatics.
Specimens for Laboratory
Specimens sent to the laboratory for culture should consist of
pieces of lung, liver, spleen, kidney, lymph nodes, brain, and heparinized blood. For
serology, serum samples from acute and convalescent animals should be submitted. For
histopathologic examination, send a complete set of tissues in 10 percent formalin.
Laboratory Diagnosis
To confirm a suspected diagnosis of EMP, the virus has to be
isolated and identified. The disease can tentatively be diagnosed by histopathology as
well as histochemical and molecular biological techniques.
Differential diagnosis
Because of the pulmonary edema, African horse sickness is a
primary consideration in a differential diagnosis.
Other causes of acute death are poisons, intoxications (botulism),
and acute bacterial diseases such as anthrax.
Treatment
There is no treatment for the primary disease.
Vaccination
There is no vaccine.
Control and Eradication
In Australia, disease control and eradication procedures consisted
of slaughter of known infected horses and extensive serological surveillance. No antibody
to EMV was detected in sera from 98 horses remaining on the Hendra property, in sera from
horses on adjacent properties, nor in sera from horses on trace-back and trace-forward
properties. No EMV antibody was detected in sera collected from the Queensland population
of nonracing horses selected using a stratified proportional approach (7).
Public Health
If one considers that there have been three human infections and
two human deaths in only two outbreaks of this disease, maximum precautions should be used
if this disease is suspected. The three individuals infected had extensive contact with
the infected horses and "assisted in their necropsies without gloves, mask, or
protective eyeware"(6). It should be noted, however, that others also had contact
with the animals and conducted or participated in necropsies and did not get infected;
thus it is suspected that "direct contact with respiratory secretions of infected
animals seems to be necessary for transmission"(6).
GUIDE TO THE LITERATURE
1. HOOPER, P.T., GOULD, A.R., RUSSELL, G.M., KATTENBELT, J.A., and
MITCHELL, G. 1996. The retrospective diagnosis of a second outbreak of equine
morbillivirus infection. Aust. Vet. J., 74(3):244-245.
2. HOOPER, P.T., KETTERER, P.J., HYATT, A.D., and RUSSEL, G.M.
1997. Lesions of experimental equine morbillivirus pneumonia in horses. Vet. Path.,
34:312-322.
3. HOOPER, P.T., WESTBURY, H.A., and RUSSELL, G.M. 1997. The
lesions of experimental equine morbillivirus disease in cats and guinea pigs. Vet. Path.,
34:323-329.
4. MURRAY, K., SELLECK, P., HOOPER, P., HYATT, A., GOULD, A.,
GLEESON, L., WESTBURY, H., HILEY, L., SELVEY, L., RODWELL, B., and KETTERER, P. 1995. A
morbillivirus that caused fatal disease in horses and humans. Science, 268:94-97.
5. ROGERS, R.J., DOUGLAS, I.C., BALDOCK, F.C., GLANVILLE, K.T.,
SEPPANEN, K.T., GLEESON, L.J., SELLECK, P.N., and DUNN, K.J. 1996. Investigation of a
second focus of equine morbillivirus infecti J., 74(3):241-243.
6. O'SULLIVAN, J.D., ALLWORTH, M.A., PATERSON, D.L., SNOW, T.M.,
BOOTS, R., GLEESON, L.J., GOULD, A.R., HYATT, A.D., and BRADFIELD, J. 1997. Fatal
encephalitis due to novel paramyxovirus transmitted from horses. The Lancet, 349:93-95.
7. WARD, M.P., BLACK, P.F., CHILDS, A.J., BALDOCK, F.C., WEBSTER,
B.J., and BROUWER, S.L. 1996. Negative findings from serological studies of equine
morbillivirus in the Queensland horse population. Aust. Vet.
8. WESTBURY, H.A., HOOPER, P.T., BROUWER, S.L., and SELLECK, P.W.
1996. Susceptibilty of cats to equine morbillivirus. Aust. Vet. J., 74(2)132-134.
9. WESTBURY, H.A., HOOPER, P.T., SELLECK, P.W., and MURRY, P.K.
1995. Equine morbillivirus pneumonia: susceptibility of laboratory animals to the virus.
Aust. Vet. J., 72(7):278-279.
10. YOUNG, P.L., HALPIN, K. SELLECK, P.W., FIELD, H., GRAVEL,
J.L., KELLY, M.A., and MACKENZIE, J.S. 1996. Serological evidence for the presence in
pteropus bats of a paramyxovirus related to equine morbillivirus. Emerg. Infect. Dis.,
2:239-240.
C. A. Mebus, D.V.M., Ph.D.,USDA, APHIS, VS, Retired, Southold, NY
|