Piroplasmosis in Texas
By Pauline Singleton
About Piroplasmosis (EP) and Ticks
January 2, 2010
Equine piroplasmosis (EP) is a protozoan disease. Actually
it is two diseases: one caused by Babesia caballi,
and the other caused by closely related Theileria equi
(formerly called Babesia equi, and still frequently
called that). They produce diseases that are very similar,
but with some important differences (more about that later).
At least one person has expressed some anxiety about the
current piroplasmosis situation and asked how she can
protect her horse.
The most important thing to know about EP is that it is
a tick-borne disease. No tick—no piroplasmosis transmission.
(I assuming that you, dear reader, are not stupid enough
to go from one horse to another with the same hypodermic
needle—that is another way that it can be spread. It’s
a good way to spread certain other nasty diseases, too.)
Since it is the tick that spreads EP, it behooves the
horse owner to learn a little bit about ticks.
First of all, if there is no EP-infected horse around
to contaminate the ticks, NO ticks are dangerous. If there
IS and EP-infected horse around, only certain species
of ticks are dangerous. Most of us don’t know one tick
from another, so for now, let’s just consider some general
tick control measures.
It helps to keep your pasture mowed, and keep your horse
out of wooded areas. Ticks like tall grass and brush.
Stabled horses aren’t usually at much risk.
Keep your dog tick-free. The dog can bring ticks with
him, and the ticks can drop off in your horse pasture
or paddock, and then later seek another animal on which
to complete their life cycle—perhaps your hapless horse.
If your horse shares a pasture with other livestock, try
to keep them all tick free.
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But perhaps the most important thing for you to know is
this: if you remove the tick shortly after he
attaches to the horse (or to you, for that matter), the
chances that it will spread disease are reduced. This
is thought to be true for several diseases in humans such
as rocky mountain spotted fever (which is more common
in certain places NOT in the Rocky Mountains, by the way)
and Lyme disease. It seems to be true for EP in horses,
too. Iowa State University produced and article on the
subject of EP which says “…a tick infected with either
organism (B. caballi or T. equi) must remain attached
to the host for a time before it becomes infective; often,
B. caballi and T. equi are transmitted after the tick
has been attached for a few days.” There seems
to be something about the tick’s blood meal that stimulates
the development of the disease-causing microorganism.
The importance of this is obvious. If you go out tomorrow
to work cattle or ride the trails, and pick up a few ticks—GET
THEM OFF at the end of the day. That alone will go a long
way toward protecting the horse. And he’ll be much more
comfortable, too.
Ticks often seek out sheltered places, such as in the
hair of the mane or tail, in the ears, under the tail
around the anus or vulva, and in wrinkles such as behind
the horse’s elbow. In hot weather, it would seem that
a bath would not only lead to a cleaner horse, but would
make it easier to see ticks.
We probably all get advice about the best way to remove
ticks. The experts seem to think that the best method
is to grasp the tick with tweezers and gently pull until
it comes loose. Doing this probably won’t kill the tick.
Make sure that you destroy the tick. Throw it in the campfire,
flush it down the commode, toss it into a bucket of water,
or beat it to death with a hammer—but don’t let it get
away. Ticks are tough--if you just throw it on the ground,
it likely will crawl away and look for another victim.
The above advice may be useful to the suburban horse owner
who has one or two horses and only occasionally sees ticks.
If you own fifty head of broodmares and they run in a
tick-infested pasture, your situation is more difficult.
Check with your veterinarian about how best to control
ticks.
Remember, no tick can spread EP unless it has fed on an
EP-infected horse, and most have not. |
More About Ticks
The Animal and Plant Health Inspection Service (APHIS),
a department of the U. S. Department of Agriculture, earlier
stated that the tropical horse tick is the only known
natural vector of EP in the United States. It listed 3
more species as having been shown to be capable of transmitting
EP in an experimental setting.
And as I indicated earlier, the south Texas EP outbreak
has provided a surprise, because the species implicated
in this outbreak is none of the four ticks on APHIS’s
list, but rather is the Cayenne tick.
What is the range of each of these ticks? Are we likely
to meet them in our part of the state? In researching
these questions, I encountered a very interesting web
site maintained by the Tick Research Laboratory at Texas A & M University. It contains some
interesting information about ticks and some of the diseases
they transmit. I highly recommend that anyone seeking
more tick information visit that web site.
Among other things, it provides maps that show where ticks
of a number of different tick species have been found
around the state, from 1986 to 1994.
The tropical horse tick, Dermacentor nitens, was
only reported in far south Texas, near the Rio Grande,
and it was not even found there every year.
One of the other possible tick vectors, the southern cattle
tick, Boophilus microplus, is found in several
counties in far south Texas. This tick is the vector for
Texas cattle fever. It is the pest that was exterminated
during the first half of the twentieth century. It is
the reason for the dipping vats that can still be found
in a few locations in our area (Harris and surrounding
counties). It caused huge economic losses when it was
prevalent across the southern United States. Actually,
it never completely stopped being a drain on the economy,
for the USDA and the Border States have continuously spent
money to keep this tick south of the border. Unfortunately
this tick has moved back across the Rio Grande and seems
to be expanding its range northward again. Considerable
effort is being expended to contain this pest. While it
is troublesome to know that it has returned to south Texas,
as least it is being monitored on an on-going basis.
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The winter tick, Dermacentor albipictus, is primarily
found in south Texas, near the Rio Grande, but there are
reports of it from several counties northwest of Dallas.
One county in deep east Texas consistently reported a
small number of these ticks.
The American dog tick, Dermacentor variabilis,
is more widespread. In 1992, it was reported in Liberty
and Montgomery Counties. In 1993 it was found in Montgomery
& Walker Counties. In 1994, it was reported from Liberty,
Austin and Colorado Counties, plus 2 dozen more counties
across east and north Texas. The Cayenne tick, the culprit
that has been implicated in the Kleberg County outbreak,
seems to be mostly found south of Corpus Christi. But
during 1993 it was reported from Harris County. There
have been a few reports from across the state, including
Bexar County, Nacogdoches County, and Wise County.
What do these reports mean? If the Cayenne tick has been
reported from south Texas as well as north Texas, but
many counties in between have never reported even one,
can we conclude that most counties are free of it? That
depends on how sampling is done.
If all counties are sampled equally, we may conclude that
the five ticks we’ve discussed are very spotty in their
occurrence. If, on the other hand, all counties are NOT
adequately sampled, the cayenne tick (the one implicated
in the Kleberg County outbreak) may be more widespread
that these maps indicate.
Regardless of their range, if the ticks don’t meet an
EP-infected horse to feed on, they can’t transmit the
disease. For most of us, the risk is low, but probably
not zero.
Last year, piroplasmosis was detected in Florida, Missouri,
and most recently, in New Mexico. In the Missouri case,
2 horses were illegally removed from quarantine and their
whereabouts has not been determined. Let us hope that
they were taken to a tick-free place, or better yet, out
of the country. So far, there is no indication that these
episodes were linked to the Kleberg County ranch. So where
did they come from? That’s a very good question. More
on that subject later. |
A Note on Tick Control
January 10, 2010
I received this communication from Floron C. Faries, Jr.,
DVM,MS, Professor and Extension Program Leader for Veterinary
Medicome. Texas Agrilife Extension Service, Texas A&M
System:
"For prevention, there are no effective tick repellents.
The wipes and sprays labeled as fly repellents for horses
are helpful. Another application is rubbing pine tar on
the lower legs. Also helpful, is avoiding infested trails
used by wildlife that have dropped ticks. Visual finding
and removal of ticks at end of daily rides is necessary."
"Spray compounds approved for treatment to kill ticks
on horses are available over-the-counter at horse supply
stores and veterinary clinics. The insecticide compound
is a concentrate solution that is diluted with water in
a pump-up sprayer and used as a total body spray as directed
on the label for horses. The active ingredient is commonly
a pyrethroid, like permethrin. For use on horses must
be stated on the label. Receive complete directions from
the place of purchase for application and intervals for
re-applications." |
EP: Two Diseases
January 10, 2010
EP is very similar to equine infectious anemia in its
presentation. On rare occasions, the first indication
that something is wrong is that the horse is found dead
or dying. This is the exception, however. More often the
horse presents with fever, anemia, loss of appetite and
malaise. And in many cases, there are no outward signs
that anything is wrong—the horse appears to be healthy.
The percentage of horses that fall into each category
varies from one geographical area to another, and from
outbreak to outbreak. Evidently in the Kleberg County
case, the horses showed no outward signs of disease. In
a way, that is the most dangerous situation. When you
find a seriously ill horse, you find out what’s wrong
with it and take steps necessary to protect your other
horses. But if the horse shows no signs of being sick,
you suspect nothing, and you take no action. And it spreads.
And spread it did. On the affected Texas ranch, more than
3/4ths of the horses are infected. That is catastrophic,
by any standard. .
Equine Piroplasmosis (EP) is actually two different diseases,
caused by closely related protozoans, Babesia caballi
and Theileria equi (called Babesia equi in
older literature). They produce diseases that are similar,
but there are some important differences. .
The version of EP that bedeviled Florida for 20 years
was caused by B. caballi. It was brought under
control and eventually eliminated by the use of drug treatment
for the horses, spraying and dipping for ticks, and quarantine.
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Drug treatment. That sounds encouraging. The drug imidocarb
seems to be capable of clearing B. caballi from
a horse’s body (Caution: some authorities seem not to
agree that imidocarb can clear B. caballi from
the horse's body). There seems to be no reliable way to
clear the horse’s body of T. equi., however. It
is not considered curable—at least not at this time. Horses
infected with T. equi likely become lifelong carriers.
Not only are drugs not as effective as we’d like them
to be, but they are toxic to the horse. According to a
paper presented at the 2002 AAEP convention by Dr. John
Irby, there have been reports of horses dying from the
treatment. Donkeys are even more susceptible to the toxic
effects. .
Historically, T. equi has produced a higher mortality
rate, although this is variable. .
A mare infected with B. caballi rarely passes it
on to her unborn foal. A mare infected with T. equi
more often does. .
So, in several important ways, T. equi is a more
serious disease. .
Unfortunately, it is T. equi that is responsible
for the Texas outbreak. |
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