2 2017 1 C. pseudotub em cavalos Pigeon Fever

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Once found mainly
in regions with
arid ciimes, this
bacterial infection
i is becoming
more common in
other parts of the
country. Here's
what you need
to know.
By Heather Smith Thomas
with Laurie Bonner
I i you don't live in California or theSouthwest, you probably haven'tseen many ca.ses of pigeon fever. Ifcurrent trends continue, however,
\ou are likely to encounter the condition
more often in the future.
First identified in San Mateo County,
Caiifornia, in 1915, pigeon fever is an
i nfection caused by Corynebacterium
pseiidotuberculosis bacteria. One of the
most common signs of the disease is
the formation of large abscesses, often
on the front of the chest, that give the
iiorse the look of a pigeon.
Aiso known as dryland distemper,
false strangles or Colorado strangles,
[)igeon fever has iong been one of the
tnost common bacterial infections
among horses living in the southwest-
ern United States. But, says Sharon
I. Spier, DVM, PhD, of the University
of California-Davis, "The incidence is
increasing in other regions and maybe
associated with climate change."
Over the past two decades, Chris
Mcllmoil, DVM, of the Country Animai
Clinic in La Grande, Oregon, has seen
a striking increase in pigeon fever
cases. "My colieague has |)racticed
here since 1988 and had never seen a
case here in the valley," sa>^ Mcllmoii.
"Then, in 2004, we saw more than 20.
This year |by November 2007], we've
seen more than 50 horses with it." In
recent years, outbreaks have occurred
in Washington, Wyoming, Colorado
and Kentucky.
Aithough pigeon fever is usually
iittie more than an inconvenience, it can
keep a horse out of work for weeks or
months, and prompt veterinary attention
is critical to reduce the risk of complica-
tions. So, no matter where you live, it's
wise to iearn what pigeon fever looks
iike and how it progresses, just in case.
How the infection begins
C. pseudotuberculosis lives in the
soil, where, studies suggest, it can sur-
vive more than eight months. The exact
conditions that turn this normal soil
resident into a pathogen are not yet
entirely understood. It is known, how-
ever, that pigeon fever outbreaks tend
to peak during the driest months—late
summer and fall in many parts of the
country. And more outbreaks occur
during drought years. Nonetheless,
individual cases of pigeon fever can
crop up under any weather conditions at
any time of year.
Regardless ofthe climate, once pi-
geon fever has occurred in a particular
region, local horses tend to remain free
of the disease for several years, which
implies that at least temporary immu-
nity can be naturally acquired.
Spier is currently investigating
how C. pseudotuberculosis bacteria
behave in the soil. Preliminary results
suggest that the organisms iiroliferate
readily in drier soil and in the presence
of manure. Spier speculates that in
hot, dry weather, horses tend to congre-
gate in shady areas, and their hooves
churn theirmanure into the dry soil,
.stirring up clouds of bacteria-laden
dust. C. f).seudotubercuio.sis may then
invade the horse's body when airborne
bacteria come into contact with wounds,
abrasions, insect bites or other breaks
in the skin. If the horse inhales the
pathogen, infection can also form in
the iungs.
Flies may also transmit pigeon fever.
As many as 20 percent of houseflies
were found to carry C. pseudotubercu-
losis on three farms with pigeon fever
outbreaks, according to a 2004 study
fiom UC-Davis, and a significant
E Q U U S 3 6 8 31
percentage of stable flies and horn flies
were also found to harbor the bacteria.
In contrast, in years when no cases of
pigeon fever were reported, none ofthe
flies from the same farms were found to
carry C. pseudotuhercuiosis. These find-
ings, says Spier, suggest that the tiies
do not act as a reservoir for C. pseudo-
tuberculosis but can become carriers
(orvectors) when the pathogen is pres-
ent in the soil in high numbers.
Once a horse Is infected with pigeon
fever, he can pass it to others who come
in contact with pus frnm draining
abscesses. The number of subsequent
cases within a local area can climb
quickly inlo the hundreds—spurred
|)erhii|)s by flying insects as well as
environmental conditions.
Three forms of
infection
Once the C. pseudotuberculosis
l)acteria gain entry through a breach
in tbe skin, tbey begin to multiply. In
le.sponse, tbe borse's immune system
produces white blood cells called mac-
rophages whose job it is to absorb and
destroy the invaders.
C. pseudotuberculosis has two char-
acteristics that help defend against the
immune system. One is a waxy llpld
coating on its outer surface that resists
the macrophages' attempts to destroy
the cells they've absorbed. The other is
the ability to release a toxin, phosphoti-
pase D (PLD), that degrades cell mem-
branes. Tbe PLD then causes leakage
and cell dealh among tbe endothelial
cells, whicb line tbe blood and lymph
vessels,leading to localized swelling.
I \('nluallV, the body r-esponds ''"i '"i''^'.
by building a tbick-walled abscess
around the site of tbis leakage. '
Tbe incubation period tor jiigeon '
fever—the time between initial
infection and the formation ofthe
abscess—is about three or four
weeks. The infection may occur in
three different forms:
• Located just under the skin or
within tbe musculature, external al>-
scesses are by far tbe most common
manifestation of pigeon fever. In a
retrospective study of 538 cases con-
ducted by UC-Oavis, 91 percent ofthe
horses had this form ofthe infection
and nearly 60 percent ofthe abscesses
were located In the chest area. How-
ever, the external abscesses can occur
anywhere, usually near lymph nodes.
"We've seen mammary gland abscesses,
preputJal abscesses |in the sheath of
male horses}, abscesses on witbers,
hips, tbe backs ofthe ears, just about
anywhere," says McIImoil. A horse may
have a single abscess or a cluster.
Pigeon fever abscesses, wbicb may
grow up to eight inches in diameter,
mature over a few days to a few weeks.
Left on Iheir own, tbey eventually erupt
through the skin, draining a thick, tan,
odor-free pus. Most horses recover fully
once the wound left by the drained ath
scess has heyled.
Recovery times vary, but after a
1997 outbreak in Colorado, an epide-
miologicat study of 40 affected borsex
showed that abscesses resolved in as
little as seven days or as many a.s 180
days, but the mean recovery time was
77 days. Abscesses on the genitals
were slowest to beal. About 43 percent
iif Ilic horses experienced recurring
v\ ilhin the vcnr.
J*K«..
T-'.-ijii***.--—
• Internal abscesses occur when C.
pseudotuberculosis are carried to sites
within the liver, lungs, kidneys or other
organs. If not detected and treated,
these abscesses enlarge and cause
organ failure, with catastrophic results.
This form ofthe disease is less com-
mon but its mortality rate is compara-
tively high: Only 8 percent (42 of 538) of
the horses in the UC-Davis study devel-
oped internal abscesses but 40 percent
of those died. Some horses can bave
both internal and external abscesses.
• Even less common than internal
abscesses is ulcerative lymphangitis,
wbich accounted for only 1 percent of
the pigeon fever cases in the UC-Davis
study. This type of pigeon fever, wbich
is associated witb poor hygiene or skin
abrasions, along with insect vectors,
begins wben tbe bacteria contaminate
wounds on tbe lower leg and affects the
lymphatic system at tbat site, causing
.swelling and cellulitis".
Eventually, abscesses form over the
lymph channels, whicb then ulcerate
and drain. The horse is feverisb, stops
eating and becomes very lame and le-
tbargic. Although individual abscesses
may clearwithin weeks, new ones are
likely to form, and drainage may persist
for as long as a yearor more.
Diagnosis and
treatment
Early warning signs of pigeon fever
vary, depending on wbere tbe infection
takes hold. Usually, the Hrst noticeable
sign is a distinct, hard swelling forming
on t^e chest or other part of the body.
Only about a quarter of borses witb ex-
ternal abscesses have a fever, and most
sbow no other signs of illness. If the
swelling inhibits movement, tbe borse
may seem sore or lame. Often, a borse
developing external pigeon fever ab-
scesse.s looks like be bas been kicked
by a pasturemate.
Deeper abscesses and those that
form near internal organs are even
more difficult to diagnose initially.
Depending on the location ofthe
^Definition: bacterial infection that
creates severe, purulent abscesses.
The disease takes three forms: External
abscesses form under the skin or in the
musculature. Internal abscesses form
within the internal organs, usually the
lungs, liver, spleen or kidneys, Ulcerative
lymphangitis 3i1ects the lower legs,
^Stgns: External abscesses create
distinct large swellings, often in the
pectoral region, but also under the belly,
on the udder or genitals, and on virtually
any other part of the body; only about 25
percent of horses vi/ith this form show
fever or other signs of systemic illness.
Internal abscesses cause more generalized
signs of illness, such as fever, weight loss,
colic, coughing or lethargy. Ulcerative
lymphangitis is characterized by swelling of
the lower leg and the formation of nodules
that ultimately open and drain pus, usually
along iymph tracts,
^Causes: Corynebacterium
pseudotuberculosis bacteria penetrate
PUS POCKETS External pigeon fever
abscesses (arrow) can occur anywhere
but usually form near lymph nodes.
the horse's skin, either through physical
contact between contaminated dirt and
open wounds or via bites from flies carrying
the pathogen.
^Diagnosis: observation of developing
abscesses that are visible under the skin
Of vi/ithin the muscles. Culture of fluids
extracted from an abscess are needed to
positively identify the organism. Ultrasound
may be needed to locate abscesses hidden
deep within the musculature or internal
organs, A blood test is available that can
aid in detection of internal infections,
^Treatment: External abscesses that do
not interfere vi'ith breathing, swallowing
or walking can be left to mature and drain
on their own; a veterinarian may also
choose to lance them. Hot compresses
may speed maturation of the abscess,
and anti-inflammatory medications may
be administered to relieve pain and
swelling. Internal abscesses and ulcerative
lymphangitis require aggressive treatment
with antibiotics.
The toxin phospholipase D (PLD) released by
Corynebacterium pseudotuberculosis is similar to the
PLD contained in the bite of the brown recluse spicier.
Both produce pain and edema at the site of infection.
M .°i Y .; n 0 8 t Q . U U S 3 6 a 33
DIAGNOSIS: Ultrasound imaging can
be useful in locating hidden pigeon
fever abscesses.
abscesses, potential indicators range
from lethargy, poor appetite and colic
to sorene.ss antl/or lanienes.s. If an al>-
scess forms within muscle, "sometimes
lameness is the first thing you see,"
says Debra Sellon, DVM, PhD, of
Washington State University. "The
abscess is so deep you don't see it, but
the horse feels it." Ultrasound" can be
useful in locating hidden aliscesses
and a blood antihody test may help in
the diagnosis.
Treatment of external abscesses
is fairly straightforward. "Most ofthe
time we treat uncomplicated cases like
any other abscess," says Sellon. "When
ready, they can be opened and drained.
Then the draining area is kept clean,
and we let it heal on its own." It can
take seven to 30 days or more for an
abscess to mature and drain spontane-
ously, but hot compresses applied daily
can hfilp bring Ihe abscess to a head
more (]uickly. Once the abscess is ma-
ture—indicated by a softening spot on
the skin—it can be lanced by a veteri-
narian, and the opening flushed with
TREATMEim In most cases, a
veterinarian will wait until external
abscesses mature, then open and
drain them.
saline ora mild antise|)tic solution.
To lance abscesses located under
muscles, Christian Comeau, DVM, of
Bakersfield Veterinary Hospital in
California, use.s ultrasound to guide his
scalpel: "Ultrasound is helpful because
you want to find the lowest spot to lance
to drain it most effectively. In deep ab-
scesses, I often sew in a drain so the
owner can continue to flush it regularly
until there is no more pus coming out,"
Anti-inflammatory medications
may be administered throughout treat-
ment of external abscesses, primarily
to minimize discomfort, but antibiotics
are generally avoided. Not only are
ilii^ y ineffective against external ab-
scesses, but they can delay healing or
worsen a horse's condition. "Antibiotics
tend to drive abscesses in deeper, and
this is a potential way to get internal
abscesses, which are more dangerous,"
says Comeau.
On the other hand, antibiotics are
standard treatment for Internal ab-
scesses and also may be used in more
complicated cases of external pigenn fe-
ver abscesses. "Every horse is different,
and we may or may not use antibiotics,
depending on how bad the Infection is,
where the abscess is located, how well
it's draining, and whether the horse is
systemicaily ill," says Sellon. "We don't
use antibiotics in many cases, but most
of the horses tbat come bere to the uni-
versity to be treated are pretty serious
and end up on antibiotics."
Preventive measures
No vaccine currently exists to protect
a horse against a C. pseudotubercutosis
infection (see "A Vaccine on the llori
/on?" page 36). But Spier and other ex-
perts say taking the following measures
ran reduce the risk of an outbreak:
• Minimize fly populations on your
property. Compost or remove manure
and waste bedding materials from your
property regularly. Pay particular atten-
tion to paddocks: "The organism persists
in .soil contaminated with feces, .so keep
paddocks as clean as possible," Spier
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A vaccine
on the horizon?
3 6 E Q U U S 3 6 8
Sharon J. Spier, DVM, PhD, of the
University of California-Davis has done
some preliminary work on developing
a vaccine against pigeon fever. She is
testing a formulation in a mouse model
of disease, but, she says, "I do not want
to infect horses for obvious reasons
unless it is required for the final stages
of development or licensing and it would
benefit the horse population in general."
Because pigeon fever is perceived
as a regional disease, and one with
a relatively low mortality rate, it has
been difficult for researchers to secure
funding. "Vaccine companies have
not felt the market is large enough
to produce a commercial product,"
Spier says. "I have not had success
to date getting a company interested
in funding efficacy studies or marketing
the product,"
Consumer pressure, however, might
have some impact. "Piease contact your
vaccine company to tell them how urgent
the need is for a protective vaccine,"
Spier says. "Donate money for research
targeted toward this disease. Vaccine
development and safety testing are
time-consuming and expensive, and
companies need to determine the market
interest before committing R&D dollars."
says. Fly predators, flytraps and judi-
ciously applied insecticides can also
help reduce Ihenumber of flies on your
property. Feed-lh rough insect growth
regulators help reduce house and stable
fly population.s.
• Protect your horses from flies.
Spier recommends using oil-based fly
repellents on pastured borses, espe-
cially tbose prone to sweet itcb. It is
also beneficial to keep borses stabled
during peak periods of fly activity and/
or fit them witb fly .sheets that have
belly protectors and tail covers.
• Keep wounds clean. "Cbeck every
borse daily fornicks and sores, and look
closely at the underside to check for
M A Y 2 0 0 8
Pigeon fever can appear virtually anywhere, but it is much more common in endemic
states, where hundreds to thousands of cases occur each year. The disease has long been
associated vi/ith the Southwest and Calitornia but in recent years, the number ot endemic
states has grown and now includes Washington, Idaho, Oregon and Kentucky.
: swelling," says McIlmoil.Aftera wound
: is cleaned, applying a layer of ichtham-
; mol or an over-the-counter antibiotic
. paste or ointment will help speed heal-
' ing while preventing flies from contact-
ing the exposed tissues,
i Bandaging, when possible, will also
: keep insects off of sores until they
can heal. Take extra precautions for
horses who are prone to sweet itch,
an allergic dermatitis that is a reac-
tion to biting flies. In severe cases,
horses will rub their.skin raw, provid-
ing ample entryways for fly-borne C.
pseudotit berculosis.
' Isolate new horses before
introducing them to an established
herd. Carrier horses who are incubat-
ing the infection but are not yet show-
ing signs may carry pigeon fever into
new geographic areas. "Today people
are traveling fartherwith horses.
Some are tlown from coast to coast,
and there's a lot of intermingling of
horses," says Nathan Slovis, DVM, of
the Hagyard Etiuine Medical Institute
in Lexington, Kentucky.
He says his hospital began to see an
increased numberof cases over the !a.st
five or six years. Some of the first were
M A Y 2 0 0 8
horses who had just been shipped In
from the West, but others had never left
their home farms. "We don't know if we
are seeing increased incidence because
more horses are being brought to this
area for sales, racing, .shows, etc., or if
the weather was just more perfect for it
during those years," Slovis says.
When bringing new horses onto a
property, it's best to keep them isolated
from resident horses long enough to
be sure they are not incubating any
infections, including diseases such as
strangles and influenza as well as pi-
geon fever. House newcomers in a sepa-
rate harn or pen as far as possihie from
residents for at least three weeks.
Pigeon fever is not the most deadly of
equine diseases, but it's certainly one to
avoid, and it is becoming more than just
a regional concern. By learning how
this infection occurs and factors that
help it develop, you'll be better able to
protect your horses should an outbreak
ever come your way. %'
To
learn
more
... go to "Controlling Contagion"
(EQUUS 306), also available on
EquusMagazine.com.
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