Heartwater
Veterinary Services
July 2002
Heartwater is an infectious, noncontagious, tick-borne disease of domestic
and wild ruminants, including cattle, sheep, goats, antelope, and buffalo.
The disease is caused by an intracellular rickettsial parasite, Cowdria
ruminantium, and is transmitted by a number of species of ticks in the
genus Amblyomma.
Heartwater is usually an acute disease and is commonly fatal within
a week of onset of clinical signs. The disease is widespread in most
of Africa and on several islands in the West Indies. With increased
trade and movement of animals in today's global market, heartwater may
present a significant threat to the domestic livestock industry in the
United States.
Livestock owners should monitor their animals for exotic ticks and
for clinical signs of the disease. If heartwater is suspected, owners
should immediately report these findings to a veterinarian or to a State
or Federal animal health official.
History
Heartwater was first identified in sheep in South Africa in the 1830s.
By 1898, it was shown to be a transmittable disease, and in 1900, the
tropical bont tick was identified as a vector. In 1980, heartwater was
reported for the first time in the Western Hemisphere on the Caribbean
island of Guadeloupe, although the vector tick was probably introduced
from Africa much earlier. The disease is also present on the Caribbean
islands of Marie Galante and Antigua.
The tropical bont tick has spread to several other islands in the Caribbean,
although a definitive diagnosis of heartwater has not been made to date
on those islands.
Signs
The acute form of heartwater is the most commonly observed presentation
of the disease. A sudden high fever (107° F) is followed by loss
of appetite, depression, and respiratory problems. Animals may initially
have an increased respiratory rate, followed within a few days by severe
respiratory distress. Nervous disorders often follow the respiratory
signs and can include a variety of abnormal behaviors such as excessive
chewing movements, incoordination, head tilting upward, overly rigid
posture, and walking with a high-stepping gait.
Some animals may undergo convulsions or be unable to rise. These nervous
signs usually last for no more than 24 to 48 hours, followed by the
animal's death. In some cases, the nervous signs may not be noticed
prior to death.
A mild form of the disease, known as heartwater fever, is present in
some affected regions among indigenous breeds with a natural or acquired
resistance to heartwater. The only clinical sign of the mild form of
the disease is a transient fever, and animals with this form usually
recover.
Postmortem Lesions
Heartwater derives its name from a common postmortem finding of excessive
fluid in the sac surrounding the heart. More commonly, the fluid accumulates
within the lungs, thus the lungs appear "wet" and heavy. The
fluid may also accumulate within the chest cavity itself, outside the
lungs.
Confusion With Other Diseases
The observed nervous system abnormalities suggest other diseases (such
as rabies, tetanus, meningitis, or encephalitis) or toxic poisoning.
A definitive diagnosis of heartwater is made by microscopic examination
and observation of the causative rickettsia in a brain tissue smear.
How It Spreads
Heartwater is transmitted only by ticks of the genus Amblyomma, with
the tropical bont tick as one of the most important vectors. This tick
is widely distributed throughout Africa, Yemen, the Cape Verde islands,
and several islands in the Caribbean.
The life cycle of Amblyomma ticks may take from 1 to 4 years. Thus,
the infection may persist, inside the tick for a long time. In its immature
stages, the tick will feed on a wide variety of livestock, wild ungulates,
groundbirds, small mammals, reptiles, and amphibians.
Rapid spread of the tropical bont tick in the West Indies has occurred
since the 1960s. Movement of tick-infested livestock was incriminated
in some cases, but overall, the cause of the spread of these ticks has
not been determined.
Cattle egrets became established in the region in the 1950s and have
been implicated in much of the recent spread of heartwater. Small numbers
of tick-infested cattle egrets have been shown to move among islands
in the region, but these birds are not considered to be efficient disseminators
of the tick.
Susceptible Species
Animals susceptible to heartwater include cattle, sheep, goats, and
buffalo. Some breeds of cattle (e.g., Jerseys and Brahmas) may be more
susceptible than others. Exotic ruminants can also contract the disease.
In laboratory tests in the United States, the white-tailed deer (Odocoileus
virginianus) has been shown experimentally to be highly susceptible
to heartwater. Amblyomma maculatum, another potential vector, is a common
parasite of white-tailed deer in the Southern United States. However,
there is no evidence that heartwater is present in wildlife in this
country.
Prevention and Control
Preventive measures by livestock owners should include implementation
of an effective tick-control program, including regular inspection of
animals and pastures for ticks and elimination of the vector through
the use of acaricides.
To prevent introduction of heartwater or any other foreign animal disease,
the U.S. Department of Agriculture's (USDA) Animal and Plant Health
Inspection Service (APHIS) tests imported animals for heartwater and
other diseases and ensures that all animals are free of ticks or other
potential insect vectors before entry into the United States is permitted.
What Can the U.S. Livestock Owner Do?
Livestock owners should monitor their animals regularly for clinical
signs of heartwater or tick infestation. If heartwater is suspected,
a veterinarian, State or Federal animal health official, or county agricultural
agent should be contacted at once.
The livestock owner is an essential link in the cooperative effort
to protect our Nation's livestock resources from costly and deadly foreign
animal diseases. Both early recognition of disease signs and prompt
notification of animal health officials are essential if prevention
or eradication of these diseases is to be successful.
For more information on heartwater disease, contact
USDA, APHIS, Veterinary Services
Emergency Programs
4700 River Road, Unit 41
Riverdale, MD 20737–1231
Telephone (301) 734–8073
Fax (301) 734–7817
You can also find information on VS programs by visiting the Website
at http://www.aphis.usda.gov/vs
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