Equine Parasitology

Swine, cattle and poultry raised in total confinement often exist free of gastro-intestinal
nematode parasites, however, irregardless of how they’re raised, horses rarely if ever,
escape parasite exposure sometime in their life. Horses are said to have the largest
collection of parasites of all domestic livestock. It is not unusual for a seemingly healthy
horse to harbor over one-half million gastro-intestinal nematode parasites. These
parasites cause damage to the animals both during the infection phase when the invading
larvae are undergoing early development in various tissues of the body and then again
after these larval stages have emerged and developed fully to adult parasites living in
their final or predilection sites laying eggs back into the environment.
In the development phase, when tissue damage occurs, the immune system of the horse is
negatively affected causing a cellular response directly proportional to the number of
invading larvae. With daily exposure, the effect on the immune system can be very strong
limiting the horse’s ability to fight other disease problems at the same time trying to fight
off a continuous stream of invading larvae. The problem comes from over-wintered
larvae and worm eggs shed on the pasture early in the season that develop into infective
larvae at an increasing rate as the temperature warms and summer approaches. Large
numbers of these larvae can become infective over a short period, exposing the horse to
high levels of parasitism. These free-living parasitic infective larvae present in the
horse’s environment are the foremost problem because they serve as the source for all
new infections.
Overall, millions of dollars are spent every year for internal parasite control in horses;
however, internal parasites remain one of the most important problems affecting the
health and well-being of horses. The reason for this is that parasite control measures
recommended and practiced over the past 25 years have provide limited protection to the
horse because they remove infections after these infections have already developed and
the damage to the horse is already done with little or no effect on reducing environmental
contamination. If horse owners can develop a basic understanding of the infection
process, changes in the way treatments are administered can be accomplished where the
re-infection process that occurs in the environment can be reduced or prevented. 2
I. Gastro-intestinal Nematode Parasites Affect Horses in Many Ways.
A. Clinical Parasitism is a condition where parasite numbers have reached a point
that the negative effects of parasitism are visible. Animals with rough hair coat, potbelly,
poor body condition and colic are examples of problems due to clinical parasitism.
Clinical parasitism is complicated because it is interrelated to a number of variables
including nutrition and immune status of the animals. Horses carrying heavy worm
burdens can appear normal if nutrition levels are adequate to “feed the animal past the
parasitism.” If nutrition is inadequate, the
animal may begin to develop signs of clinical
parasitism (see Appendix II for a detail list of
parasites in horses).
Animal that are allow to overgraze the pasture
are at greatest risk for developing clinical
parasitism because late in the season when the
parasitic free-living larval populations on the
pastures are at their greatest numbers, the
nutrition of the pastures are usually at their
lowest level. If left untreated, it is not uncommon for clinically affected horses to colic 3
and die from a heavy level of parasitism. Also, because of the coprophagic nature of
horses, it is a very difficult task to maintain horses totally free of parasites without first
reducing or eliminating environmental contamination by infective larval stages.
Most clinically infected horses harbor high numbers of adult parasites but also are
carrying high numbers of encysted or
inhibited larvae imbedded in the wall of
the colon. As worm burdens build
throughout the summer months, it appears
that the physiology of the gastro-intestinal
tract changes and conditions are no longer
ideal for larval development. New
incoming infective larvae then undergo a
period of arrested development waiting in
the tissues until the physiological
condition of the gastro-intestinal tract
returns to normal at which time these
larvae resume development again. Since it
is not in the best interest of the parasites to
kill their host, the arrested development of larvae protects the host from being overwhelmed which also protects the parasites because if the host dies, the parasites also
perish. The actions that triggers the release and re-development of encysted larvae into
adult worms occurs when older worms die off naturally and are not rapidly replaced by
new larvae especially during winter months or hot dry periods. The administration of a
dewormer can also trigger the development of encysted larvae.
Once these larvae become encysted, their metabolism slows down and they become
difficult to kill with conventional treatment because they are protected in the tissues.
Since these larvae are in an inhibited state their uptake of chemicals dewormers intended
to kill them is also reduced depending somewhat upon the chemical make-up of the
deworming compound. Once encysted larvae begin development and emerge into the
lumen of the colon, clinical disease can develop if high numbers of larvae emerge all at
the same time. This is called “larval cyathostomiasis
1,2
.” It’s not unusually for some
clinically infected horses to harbor over 1,000,000 encysted larvae at one time.
B. Subclinical parasitism is hard to see and measure. Subclinically infected animals
appear normal but these parasitisms are responsible for reduced growth rates in foals;
reduce reproductive rates in mares, reduced milk production for the young and a reduced
ability of the infected animal’s immune system to fight off other disease conditions. In
performance horses, subclinical levels of parasitism can be very important because even
slightly reduced performance may be very important. It only takes a few parasites to
significantly reduce performance in a finely tuned animal.
Subclinical parasitism can be very costly because the owner is often unaware of the
damage that is taking place since the parasites are not visible and lost performance can
occur unknowingly. The most important aspect of subclinical parasitism, however, is the
ability of subclinical infected animals to shed worm eggs into the environment producing 4
future infections. Subclinically infected animals, even with low worm egg counts, may be
shedding thousands of eggs back in the horse’s environment everyday.
Monitoring fecal worm egg counts is the best way to detect subclinical levels of
parasitism (see Section IX). Positive results indicate a parasitic worm burden is present
and contamination of the environment is taking place. High egg counts indicate a high
level of contamination is already occurring. Also, fecal worm egg counts in horses often
correlate better with numbers of adult parasites present than fecal worm egg counts in
most other species. Horses with fecal worm egg counts conducted by the “Modified
Wisconsin Sugar Flotation Technique” in excess of 300 eggs/3 gm sample are considered
to be heavily infected while animals with egg counts over 1,000 eggs/3gm (150,000
eggs/pound of manure) sample are often showing signs of clinical parasitism
II. Parasites Develop Differently in Horses than in Cattle:
A. Parasitism in horses is most often an individual problem while parasitism in
cattle is considered a herd disease because cattle often graze together in designated
groups on the same pastures, are all exposed to the same infection level and subsequently
develop similar parasite burdens. Domestic horses are different because they are seldom
herded or handled in large groups. Millions of horses across the country are raised in
isolation or semi-isolation where contact with other horses is limited to just a few
animals. Even in equine operations with multiple numbers of horses, the animals are
usually either maintained separately or in small groups with little cross-contamination
between animals. In both cases, cumulative worm burdens are generated from exposure
to the infective offspring found in the environment which developed from eggs shed by
the horses themselves. Because of this auto-infection, parasitism in horses is a disease
problem requiring special attention to individual animals and their immediate
environment.
B. Horses routinely develop
higher worm burdens than cattle
especially under confined conditions.
Grazing cattle can develop extremely
high levels of parasitism depending
upon their environment; however, unless
cattle are overstocked on heavily
contamination pastures, parasitism in
cattle is usually subclinical in nature and
can easily be controlled with strategic
timed dewormings. The primary way
cattle become infected is by eating
forages contaminated with infective
larvae. Feedlot cattle or mature dairy cattle on “full feed” seldom become re-infected
while held in total confinement or dry lot conditions although calves can develop a
“barnyard infection” with certain species of parasites while held in confinement
especially when housed on a manure pack or in a crowded pen. 5
Horses tend to bit, chew or nibble at their surroundings often consuming parasite infected
bedding and, therefore, can develop relatively high levels of parasitism even under arid
conditions or while in total confinement
3
. Also, horses normally graze closer to the
ground than cattle easily picking up large numbers of infective larvae while they graze.
Because of these factors, horses can develop significant worm burdens depending upon
environmental contamination whether they’re housed in the stall, pen or on pasture.
C. Fecal worm egg output in parasitized horses is often much higher than
parasitized cattle of similar age. Horses have higher average worm egg counts than
cattle for a number of reasons. One reason is that horse manure is more concentrated and
contains less moisture than cattle manure so the concentration of worm eggs shed per
gram of manure is often much greater in horses. A second reason that horses tend to have
much higher worm egg counts than cattle of similar age is that certain species of parasites
such as the small strongyles are identified as more prolific egg layers than the common
gastro-intestinal parasites (Ostertagia, Haemonchus, and Trichostrongylus) found in
cattle. Overall, the most important aspect of high egg shedding is re-contamination of the
environment leading to continued parasite exposure.
The number of worm eggs shed per gram of feces influences the contamination rate of
the environment surrounding the animals. The more eggs that are shed into the
environment, the greater the chance for re-infection to occur. Using the “Modified
Wisconsin Sugar Flotation Fecal Technique” for floating worm eggs out of fecal
material, it is not uncommon for a mature horse to have a fecal worm egg count greater
than 1,000 eggs/3gm sample (150,000 eggs/pound of manure) whereas a count of greater
than 100 eggs/3gm sample (15,000 eggs/pound of manure) is rare in mature cattle. As a
general statement, the average fecal worm egg counts from horses routinely produce a 10
fold higher contamination rate when compared to cattle and is notably one of the main
reasons for the ongoing failure to adequately control parasites in millions of horses across
the country.
D. The economic value of deworming or cost of treatment is often less important
with horse owners than with cattle producers. Cattle dewormers are often purchased
and administered to the animals based on perceived economic benefit in terms of
increased feed efficiency or growth whereas most horses are handled individually with
personal care so treatments are given based on perceived need with less concern about
cost versus benefit of the treatment given. Deworming costs for adult horses are also
regularly more expensive that the cost for deworming cattle. Horse dewormers can cost
from several dollars per dose to as high as $60.00/treatment with an average cost of
treatment around $8.00 to $9.00/horse while treatment cost for adult cattle run from $1.80
per dose to a as high as $7.00/treatment with average costs of around $3.50/mature
animal.
Many horse owners alternate dewormers to help prevent parasite resistance from
developing, whereas, cattle producers often use products that are convenient, products
that work well with their type of operation and products that match the season, i.e., cattle 6
producers may use an endectocide pour-on in late fall for lice and grub control while
administering a medicated mineral or dewormer block in the spring. Because horse
owners generally have lower concerns over product cost than cattlemen, the need to
create a “least-cost most-effective” treatment program for horses has not been one of
great concern for the horse industry. Because of this lack of economic concern, strategic
deworming programs designed to reduce or eliminate environmental contamination by
gastrointestinal internal parasites has not been widely researched or recommended.
E. The lack of scientific evidence that strategic deworming strategies are
effective is a problem for horse owners when compared to cattle producers. Most
cattle deworming programs are based on economic use data generated from carefully
conducted trials measuring such parameters as growth rate, reproductive efficiency, and
feed efficiency. These types of studies are seldom conducted with horses but rather horse
owners are exposed to hundreds of treatment recommendations from a multitude of
sources of which few provide scientific evidence that seasonal parasite control can be
achieved by following the recommended program. Most of these deworming
recommendations are confusing where one author recommends product rotation to
prevent “resistance” while another author suggests that product rotation promotes
“resistance” to all products use. Neither author provides any scientific evidence but rather
provides their recommendations because it sounds like a “good recommendation.” Horses
all across the country are meanwhile suffering from unnecessary parasitism and parasite
resistance has now become widespread such that horses can be exposed to increased
levels of parasite exposure while their owners assume they’re administrating an effective
deworming strategy to their animals.
Equine dewormers are also easily available to horse owners where the owner purchases
dewormers without knowing which products will work for their horses and which will
not. The problem here is that if a particular product provides inadequate control due to
the presence of parasite resistance, for example, millions of worm eggs can be shed into
the environment before another deworming is given. A second deworming product may
also not work. The only way owners can determine whether the products they’re using
are successful in their horses is to have fecal worm egg count exams conducted on a
regular basis.

Another problem facing the horse industry is that thousands of horses are purchased and
moved to new locations every year immediately contaminating the new location because
care is seldom taken to confirm that animals are parasite-free before being moved. These
horses usually have health records that outline deworming treatment history but animals
can still be shedding worm eggs despite a recent deworming since parasite resistance
could be present in the animals or sufficient time has elapsed allowing the animals to
become re-infected since their last treatment. Two fecal checks should be conducted
several weeks apart to determine a parasite-free status prior to moving the horse to the
new environment. Using a larvicidal of fenbendazole (10 mg/kg daily for 5 days) is also
recommended to remove all encysted small strongyle larvae

Swine, cattle and poultry raised in total confinement often exist free of gastro-intestinal
nematode parasites, however, irregardless of how they’re raised, horses rarely if ever,
escape parasite exposure sometime in their life. Horses are said to have the largest
collection of parasites of all domestic livestock. It is not unusual for a seemingly healthy
horse to harbor over one-half million gastro-intestinal nematode parasites. These
parasites cause damage to the animals both during the infection phase when the invading
larvae are undergoing early development in various tissues of the body and then again
after these larval stages have emerged and developed fully to adult parasites living in
their final or predilection sites laying eggs back into the environment.
In the development phase, when tissue damage occurs, the immune system of the horse is
negatively affected causing a cellular response directly proportional to the number of
invading larvae. With daily exposure, the effect on the immune system can be very strong
limiting the horse’s ability to fight other disease problems at the same time trying to fight
off a continuous stream of invading larvae. The problem comes from over-wintered
larvae and worm eggs shed on the pasture early in the season that develop into infective
larvae at an increasing rate as the temperature warms and summer approaches. Large
numbers of these larvae can become infective over a short period, exposing the horse to
high levels of parasitism. These free-living parasitic infective larvae present in the
horse’s environment are the foremost problem because they serve as the source for all
new infections.
Overall, millions of dollars are spent every year for internal parasite control in horses;
however, internal parasites remain one of the most important problems affecting the
health and well-being of horses. The reason for this is that parasite control measures
recommended and practiced over the past 25 years have provide limited protection to the
horse because they remove infections after these infections have already developed and
the damage to the horse is already done with little or no effect on reducing environmental
contamination. If horse owners can develop a basic understanding of the infection
process, changes in the way treatments are administered can be accomplished where the
re-infection process that occurs in the environment can be reduced or prevented. 2
I. Gastro-intestinal Nematode Parasites Affect Horses in Many Ways.
A. Clinical Parasitism is a condition where parasite numbers have reached a point
that the negative effects of parasitism are visible. Animals with rough hair coat, potbelly,
poor body condition and colic are examples of problems due to clinical parasitism.
Clinical parasitism is complicated because it is interrelated to a number of variables
including nutrition and immune status of the animals. Horses carrying heavy worm
burdens can appear normal if nutrition levels are adequate to “feed the animal past the
parasitism.” If nutrition is inadequate, the
animal may begin to develop signs of clinical
parasitism (see Appendix II for a detail list of
parasites in horses).
Animal that are allow to overgraze the pasture
are at greatest risk for developing clinical
parasitism because late in the season when the
parasitic free-living larval populations on the
pastures are at their greatest numbers, the
nutrition of the pastures are usually at their
lowest level. If left untreated, it is not uncommon for clinically affected horses to colic 3
and die from a heavy level of parasitism. Also, because of the coprophagic nature of
horses, it is a very difficult task to maintain horses totally free of parasites without first
reducing or eliminating environmental contamination by infective larval stages.
Most clinically infected horses harbor high numbers of adult parasites but also are
carrying high numbers of encysted or
inhibited larvae imbedded in the wall of
the colon. As worm burdens build
throughout the summer months, it appears
that the physiology of the gastro-intestinal
tract changes and conditions are no longer
ideal for larval development. New
incoming infective larvae then undergo a
period of arrested development waiting in
the tissues until the physiological
condition of the gastro-intestinal tract
returns to normal at which time these
larvae resume development again. Since it
is not in the best interest of the parasites to
kill their host, the arrested development of larvae protects the host from being overwhelmed which also protects the parasites because if the host dies, the parasites also
perish. The actions that triggers the release and re-development of encysted larvae into
adult worms occurs when older worms die off naturally and are not rapidly replaced by
new larvae especially during winter months or hot dry periods. The administration of a
dewormer can also trigger the development of encysted larvae.
Once these larvae become encysted, their metabolism slows down and they become
difficult to kill with conventional treatment because they are protected in the tissues.
Since these larvae are in an inhibited state their uptake of chemicals dewormers intended
to kill them is also reduced depending somewhat upon the chemical make-up of the
deworming compound. Once encysted larvae begin development and emerge into the
lumen of the colon, clinical disease can develop if high numbers of larvae emerge all at
the same time. This is called “larval cyathostomiasis
1,2
.” It’s not unusually for some
clinically infected horses to harbor over 1,000,000 encysted larvae at one time.
B. Subclinical parasitism is hard to see and measure. Subclinically infected animals
appear normal but these parasitisms are responsible for reduced growth rates in foals;
reduce reproductive rates in mares, reduced milk production for the young and a reduced
ability of the infected animal’s immune system to fight off other disease conditions. In
performance horses, subclinical levels of parasitism can be very important because even
slightly reduced performance may be very important. It only takes a few parasites to
significantly reduce performance in a finely tuned animal.
Subclinical parasitism can be very costly because the owner is often unaware of the
damage that is taking place since the parasites are not visible and lost performance can
occur unknowingly. The most important aspect of subclinical parasitism, however, is the
ability of subclinical infected animals to shed worm eggs into the environment producing 4
future infections. Subclinically infected animals, even with low worm egg counts, may be
shedding thousands of eggs back in the horse’s environment everyday.
Monitoring fecal worm egg counts is the best way to detect subclinical levels of
parasitism (see Section IX). Positive results indicate a parasitic worm burden is present
and contamination of the environment is taking place. High egg counts indicate a high
level of contamination is already occurring. Also, fecal worm egg counts in horses often
correlate better with numbers of adult parasites present than fecal worm egg counts in
most other species. Horses with fecal worm egg counts conducted by the “Modified
Wisconsin Sugar Flotation Technique” in excess of 300 eggs/3 gm sample are considered
to be heavily infected while animals with egg counts over 1,000 eggs/3gm (150,000
eggs/pound of manure) sample are often showing signs of clinical parasitism
II. Parasites Develop Differently in Horses than in Cattle:
A. Parasitism in horses is most often an individual problem while parasitism in
cattle is considered a herd disease because cattle often graze together in designated
groups on the same pastures, are all exposed to the same infection level and subsequently
develop similar parasite burdens. Domestic horses are different because they are seldom
herded or handled in large groups. Millions of horses across the country are raised in
isolation or semi-isolation where contact with other horses is limited to just a few
animals. Even in equine operations with multiple numbers of horses, the animals are
usually either maintained separately or in small groups with little cross-contamination
between animals. In both cases, cumulative worm burdens are generated from exposure
to the infective offspring found in the environment which developed from eggs shed by
the horses themselves. Because of this auto-infection, parasitism in horses is a disease
problem requiring special attention to individual animals and their immediate
environment.
B. Horses routinely develop
higher worm burdens than cattle
especially under confined conditions.
Grazing cattle can develop extremely
high levels of parasitism depending
upon their environment; however, unless
cattle are overstocked on heavily
contamination pastures, parasitism in
cattle is usually subclinical in nature and
can easily be controlled with strategic
timed dewormings. The primary way
cattle become infected is by eating
forages contaminated with infective
larvae. Feedlot cattle or mature dairy cattle on “full feed” seldom become re-infected
while held in total confinement or dry lot conditions although calves can develop a
“barnyard infection” with certain species of parasites while held in confinement
especially when housed on a manure pack or in a crowded pen. 5
Horses tend to bit, chew or nibble at their surroundings often consuming parasite infected
bedding and, therefore, can develop relatively high levels of parasitism even under arid
conditions or while in total confinement
3
. Also, horses normally graze closer to the
ground than cattle easily picking up large numbers of infective larvae while they graze.
Because of these factors, horses can develop significant worm burdens depending upon
environmental contamination whether they’re housed in the stall, pen or on pasture.
C. Fecal worm egg output in parasitized horses is often much higher than
parasitized cattle of similar age. Horses have higher average worm egg counts than
cattle for a number of reasons. One reason is that horse manure is more concentrated and
contains less moisture than cattle manure so the concentration of worm eggs shed per
gram of manure is often much greater in horses. A second reason that horses tend to have
much higher worm egg counts than cattle of similar age is that certain species of parasites
such as the small strongyles are identified as more prolific egg layers than the common
gastro-intestinal parasites (Ostertagia, Haemonchus, and Trichostrongylus) found in
cattle. Overall, the most important aspect of high egg shedding is re-contamination of the
environment leading to continued parasite exposure.
The number of worm eggs shed per gram of feces influences the contamination rate of
the environment surrounding the animals. The more eggs that are shed into the
environment, the greater the chance for re-infection to occur. Using the “Modified
Wisconsin Sugar Flotation Fecal Technique” for floating worm eggs out of fecal
material, it is not uncommon for a mature horse to have a fecal worm egg count greater
than 1,000 eggs/3gm sample (150,000 eggs/pound of manure) whereas a count of greater
than 100 eggs/3gm sample (15,000 eggs/pound of manure) is rare in mature cattle. As a
general statement, the average fecal worm egg counts from horses routinely produce a 10
fold higher contamination rate when compared to cattle and is notably one of the main
reasons for the ongoing failure to adequately control parasites in millions of horses across
the country.
D. The economic value of deworming or cost of treatment is often less important
with horse owners than with cattle producers. Cattle dewormers are often purchased
and administered to the animals based on perceived economic benefit in terms of
increased feed efficiency or growth whereas most horses are handled individually with
personal care so treatments are given based on perceived need with less concern about
cost versus benefit of the treatment given. Deworming costs for adult horses are also
regularly more expensive that the cost for deworming cattle. Horse dewormers can cost
from several dollars per dose to as high as $60.00/treatment with an average cost of
treatment around $8.00 to $9.00/horse while treatment cost for adult cattle run from $1.80
per dose to a as high as $7.00/treatment with average costs of around $3.50/mature
animal.
Many horse owners alternate dewormers to help prevent parasite resistance from
developing, whereas, cattle producers often use products that are convenient, products
that work well with their type of operation and products that match the season, i.e., cattle 6
producers may use an endectocide pour-on in late fall for lice and grub control while
administering a medicated mineral or dewormer block in the spring. Because horse
owners generally have lower concerns over product cost than cattlemen, the need to
create a “least-cost most-effective” treatment program for horses has not been one of
great concern for the horse industry. Because of this lack of economic concern, strategic
deworming programs designed to reduce or eliminate environmental contamination by
gastrointestinal internal parasites has not been widely researched or recommended.
E. The lack of scientific evidence that strategic deworming strategies are
effective is a problem for horse owners when compared to cattle producers. Most
cattle deworming programs are based on economic use data generated from carefully
conducted trials measuring such parameters as growth rate, reproductive efficiency, and
feed efficiency. These types of studies are seldom conducted with horses but rather horse
owners are exposed to hundreds of treatment recommendations from a multitude of
sources of which few provide scientific evidence that seasonal parasite control can be
achieved by following the recommended program. Most of these deworming
recommendations are confusing where one author recommends product rotation to
prevent “resistance” while another author suggests that product rotation promotes
“resistance” to all products use. Neither author provides any scientific evidence but rather
provides their recommendations because it sounds like a “good recommendation.” Horses
all across the country are meanwhile suffering from unnecessary parasitism and parasite
resistance has now become widespread such that horses can be exposed to increased
levels of parasite exposure while their owners assume they’re administrating an effective
deworming strategy to their animals.
Equine dewormers are also easily available to horse owners where the owner purchases
dewormers without knowing which products will work for their horses and which will
not. The problem here is that if a particular product provides inadequate control due to
the presence of parasite resistance, for example, millions of worm eggs can be shed into
the environment before another deworming is given. A second deworming product may
also not work. The only way owners can determine whether the products they’re using
are successful in their horses is to have fecal worm egg count exams conducted on a
regular basis.

Another problem facing the horse industry is that thousands of horses are purchased and
moved to new locations every year immediately contaminating the new location because
care is seldom taken to confirm that animals are parasite-free before being moved. These
horses usually have health records that outline deworming treatment history but animals
can still be shedding worm eggs despite a recent deworming since parasite resistance
could be present in the animals or sufficient time has elapsed allowing the animals to
become re-infected since their last treatment. Two fecal checks should be conducted
several weeks apart to determine a parasite-free status prior to moving the horse to the
new environment. Using a larvicidal of fenbendazole (10 mg/kg daily for 5 days) is also
recommended to remove all encysted small strongyle larvae


يتبع>>>>>>>>>>>>>>>>>