- Email: [email protected]
Infectious upper respiratory disease in U.S. Horses
INFECTIOUS UPPER RESPIRATORY DISEASE IN U.S. HORSES Edited from USDA/APIS NAHMS Disease Frequency Equine infectious upper respiratory disease (IURD) occurs in horses worldwide. This acute clinical syndrome is generally contagious and can be caused by several viruses and bacteria, such as equine influenza virus, equine herpes viruses, equine arteritis virus, and the bacteria Streptococcus equi subspecies equi. Usually, horses with IURD develop a fever, cough, and nasal discharge. They also may demonstrate lethargy, reduced feed intake, and enlarged lymph nodes of the head and neck.
Strangles is a specific type of IURD caused by the bacterium Streptococcus equi subspecies equi. In horses, strangles is characterized most often by purulent nasal discharge, and swelling of the lymph nodes of the head and neck. These lymph nodes may rupture and drain pus. Infected horses also may develop other signs such as fever and breathing and eating difficulties. The USDA's National Animal Health Monitoring System (NAHMS) collected data on equine health and management practices from a representative sample of equine operations in 28 states from 4 regions, a For this study, horses were defined as full-size breeds, usually standing at least 14 hands tall (56 inches) at the withers
when mature. Horses were considered residents of an operation if they spent more time at that operation than at any other site. Overall, 1,034 operations with 3 or more resident horses as of January 1, 1998, and at least 1 horse at the time of the initial interview completed the entire Equine '98 study. Horses housed at racetracks were excluded from this portion of the study. Participants monitored their resiaWestern Region: California, Colorado, Montana, New Mexico, Oregon, Washington, Wyoming. Northeast Region: New Jersey, Now York, Oluo, Pennsylvania. Southern Region: Alabama, Florida, Georgia, Kentucky, Louisiana, Maryland, Oldahoma, Tennessee, Texas, Virginia. Central Region: Illinois, Indiana, Kansas, Michigan, Minnesota, Missouri, Wisconsin.
As a veterinarian owned company, we strive to provide both you, the practitioner, and your client with: User Friendly Compounded Prescriptions Topical Ointments Antibiotics Immune Enhancement Oral Pastes Liquid Suspensions Performance Nutraceuticals
P.O. Box 164 9 St. Matthews, SO 29135 w Fax 803-655-7791 www.equi-fab.com 576
~1~ ~/f(
JOURNAL OF EQUINE VETERINARY SCIENCE
3 P 2.5 O 3: 2 "5 1.5 1
Equine '98 and the methodology used is available in NAHMS Equine '98 Part
0 March1- June1May 31 Aug. 31 Spring
Sept. 1- Dec. I Nov. 30 Feb. 28
Summer Fall Quarter
Winter
Figure 1. Percent of horses with IURD by season/quarter. dent horses for signs of IURD from March 1, 1998 to February 28, 1999. Diagnoses of IURD by a veterinarian were not required. A horse was considered to have IURD if it had a cough and/or nasal discharge with at least one of the following: fever, depression, decreased appetite, cloudy nasal discharge, or enlarged lymph nodes of the head and neck.
Strangles was considered the cause of IURD if the horse developed an opaque (mucopurulent) nasal discharge with swollen lymph nodes of the head and upper neck. The number of horses that developed IURD during each quarter (3 months) was recorded, as well as the number of horses that had strangles. More detailed information on
~4 (9
i "52--
n
0 <6 6-17 18mo.- 6-19 Months Months 5 Years Years Age of Horse
20 years or Older
*Averaged over 4 Quarters
Figure 2. Percent of horses with IURD (per quarter*) by age of horses.
|
r,0
O
-I- 3 '5
#.1 Pleasure Competition Breeding Racing FamVRanch O#~er Primary Use of Hcxses
*Averaged over 4 Quarters
Figure 3. Percent of horses with IURD (per quarter*) by primary use of horses. Volume 21, Number 12, 2001
L Baseline Reference of 1998 Equine Health and Management. Age was associated with the likelihood of a horse developing IURD (Figure 2). On average, an estimated 1.1 percent [Standard Error (SE)--0.2)] of horses over 5 years of age developed IURD per quarter, compared to 3.7 percent (SE=I.0) of foals less than 6 months old. Young horses may have been at greater risk of contracting IURD because they had not yet developed adequate immunity. When comparing horses younger than 18 months with horses 18 months or older, there was no difference in the average percentage of horses that developed strangles per quarter. The percentage of operations that had at least I horse with IURD was greater for large operations (at least 20 horses) than for medium-sized (6-19 horses) or small (5 or fewer) operations. Only 12.4 percent (SE=3. 1) of small operations experienced a case of IURD during the year as compared to 38.8 percent (SE=8.0) of large operations. However, an individual horse at a large operation was not more likely to develop IUJRD or strangles than an individual horse at a small operation. The higher percentage of at least one case of IURD at large operations may be due to the higher number of horses at risk. Yet, the likelihood of an operation's horses contracting IURD may have been influenced by other factors, including management practices and the age distribution of the horses. When c o m p a r i n g horses used for different purposes, there was a difference in the average percentage of horses with IURD per quarter (Figure 3). Racehorses housed off-track had the highest rate of IURD [4.3 percent
577
o (SE= 1.9)]. Since these racehorses were not stabled at racetracks, this rate may not reflect the rate of IURD for the entire U.S. racehorse population. The function of the operation where the horses were stabled also was associated with the likelihood that at least 1 resident horse would develop IURD during the study (Figure 4). Horses developed IURD on 35.0 percent (SE=8.6) of operations used primarily for breeding and on 33.9 percent (SE-- 9.2) of boarding, training, and riding operations. Only 12.2 percent (SE=3.4) of operations where horses were used for farming or ranching and 11.4 percent (SE=2.6) of residential operations where horses were kept for personal use, reported horses with IURD. The reason that residential operations and farms/ranches reported IURD less commonly than other types is not clear. Potential reasons include: these operations may have been smaller, with fewer horses at risk; more of their horses may have been over 17 months old; or their horses traveled less and were therefore less likely to be exposed to infectious agents that cause IURD. Vaccination of horses against equine influenza virus, equine herpes virus, and Streptococcus equi subspecies equi (strangles) in the 12 months prior to the study was not associated significantly with the rate of IURD occurrence. It was not clear whether operations that administered vaccinations did so as a preventive measure or to contain a disease problem already present. Acute infectious upper respiratory disease is a problem encountered on equine operations in the U.S. Many factors may contribute to a horse's risk of developing IURD. Although there are vaccines against several common infectious agents of IURD, 578
(P~,=onattk~)
Riding
Function of Operation "Otoerl~k~ns w ~ e ~ n s i d e ~ d to l~ve lURO lf M . ~ f or~ ho~e ~ e d dini~I sf~ns d~r{r~ ~
ve~.
Figure 4. Percent of operations with IURD* by function of operation. vaccination does not prevent all cases of IURD. Laboratory results for influenza serology and nasal swab culture for S t r e p t o c o c c u s isolation Equine infectious upper respiratory disease IURD is a problem that affects horses worldwide. Infected horses often develop a fever, cough, mad nasal discharge. They also may develop swollen lymph nodes, lethargy, and decreased appetite. Usually, horses recover from IURD without suffering long-term complications. Equine influenza virus is one of the most common causes of IURD. Horses can develop antibodies to influenza virus following vaccination or after infection with the virus. Horses with
high levels of these antibodies are less likely to become ill following exposure to equine influenza virus. Bacterial infections also can cause respiratory disease in horses. Strangles is among the most serious types of these bacterial infections and is caused by Streptococcus equi subspecies equi. Often, horses with strangles develop swollen lymph nodes of the head and neck. These infected lymph nodes may rupture and dram pus. The bacteria Streptococcus equi, subspecies zooepidemicus and Streptococcus dysgalactla, subspecies equisimilis, also can infect horses. While these bacteria may cause clinical respiratory disease in horses, not all exposed horses become ill. Horses carrying these bacteria can be a source of infection for other horses, even if they are not displaying clinical signs of disease.
r O 30 "I~6 20
8 10 n
0
None Low High Category of Serum Antibody TIter to Equine Influenza *None = <1:10 (no detectable titer); Low = 1:10 to 1:40; High = greater than 1:40
Figure lb. Percent of horses by hemagglutination inhibition
titer
category* JOURNAL OF EQUINE VETERINARY SCIENCE
r
6O
~50 O
-r- 40
"6 3o Z0
Equine '98 study. More detailed information on the study design is available in the NAHMS Equine'98 Part I.Baseline Reference of
0 <6rt~.
8-17re~.18mo.-4ym 5-19y~. 2 0 y ~ *
Age of Horses *Serum Antibody Titer to Equine Influenza Greater Than 1:40
Figure2b. Percent of horses with high* HI titer by age. The NationalAnimal Health Monitoring System (NAHMS) collected data on equine health and management practices from a representative sample of equine operations in 28 states from 4 regions'. For this study, horses were defined as full-size breeds, usually standing at least 14 hands tall (56 inches) at the withers when mature. Horses were considered residents of an operation if they spent more time at that operation than at any other facility during the study period. Operations that completed the questionnaire phase o f the study were asked to participate in the collection of biologic samples. Blood samples from horses were obtained to determine antibody titers to equine influenza virus by using hemagglutination inhibition (HI) testing. Swabs were used to collect nasal secretions from horses and then cultured
[o identify Streptococcus bacteria. Testing was performed by the National Veterinary Services Laboratories. The number of horses tested was based on the horse inventory on the operation. Blood was collected from up to 20 horses per operation, and up to 10 of these were selected randomly for nasal swabbing. Horses were sampled during the regular site visit; they may not have shown clinical signs of IURD. Overall, blood samples from 8,265 horses Eroin 949 operation * s were tested, and nasal swabs from 5,976 horses f r o m 850 operations were cultured for Streptococcus spp. Approximately half the samples were collected from June 15 to September 23, 1998, while the remaining samples were collected from October 2, 1998, to March 3, 1999. Horses stabled at racetracks were not included in this portion of the
8O co 70 60 O 1- 50 N.o 4O 3O 2o (1)
a. 10 0
t
2
3
4
5 or more
Number vaccinations in previous 12 months *Serum Antibody Titer to Equine Influenza Greater Than 1:40
Figure 4. Percent of horses with high* HI titer by number of vaccinations in previous 12 months. Volume 21, Number 12, 2001
1998 Equine Health and Management. It is believed horses that produce an increased concentration of serum antibodies against equine influenza virus have a decreased risk of disease during outbreaks. For this study, horses were considered to have high titers if the 19 influenza antibody titer was greater than 1:40; low titers if it was 1 : 1 0 to 1:40; and undetectable titers if the antibody was less than 1: 10. An estimated 69.7 percent [Standard Error (SE)--1.9] of all horses had a detectable antibody titer to equine influenza virus (Figure lb.). An estimated 9 t.4 percent (SE=2.3) of operations had at least 1 horse with a detectable titer. There was no difference in the estimated percentage of horses with undetectable, low, or high titers and the region of the U.S. where they resided or the time of year the samples were collected. Previous studies have found that young horses have the greatest risk of disease from equine influenza virus infections. In this study the age category of the horses was associated with the likelihood that they would have a detectable equine influenza virus antibody titer. Only 20.2 percent (SE=5.9) of the young horses aged 6 to 17 months had a detectable influenza antibody titer, as compared to 89.0 percent (SE=3.5) of horses aged 20 years or more. The percentage of horses that had a high equine influenza antibody titer increased as the horse's age increased (Figure 2b). However, even in older horses, 49.9 percent (SE--4.7) had low or undetectable titers. In this study, 95.1 percent (SE=2. 1) of horses less than 18 months of age had low or undetectable titers. Young horses may have had low antibody concentrations because 579
they either received fewer vaccinations for equine influenza virus than adult horses, produced less antibody following vaccination, or were less likely to be naturally exposed to the virus previously. The estimated percentage of horses with a detectable titer to equine influenza virus, and the percentage with a high titer, increased as the size of the operation increased. On operations with at least 20 resident horses, 81.5 percent (SE=2.7) of horses had a detectable equine influenza virus antibody titer and 52.6 percent (SE=3.9) had a high equine influenza virus antibody titer. The percentage of horses on small operations (1 to 6 horses) with a detectable equine influenza virus antibody concentration was 63.1 percent (SE=3.9), while 30.6 percent (SE=3.5) had high titers. A larger percentage of horses on large operations may have had higher equine influenza virus antibody titers because they had a greater chance of exposure to the virus from other horses on the operation. However, the difference may have resulted from other factors, such as management practices, vaccine strategies, frequency of contact with horses from other operations, and the age distribution of resident horses. An estimated 1.3 percent (SE=0.4) of horses sampled exhibited signs of acute IURD during the 30 days prior to sampling. When comparing these horses to horses with no signs of IURD in the previous 30 days, there was no difference in the percentage of horses with a detectable equine influenza virus antibody titer or the level of the titer. This suggests that the IURD reported during the last month was not necessarily caused by equine influenza virus. Direct physical contact with horses from outside the operation in the month prior to sampling also had no apparent effect on the percentage of
580
horses with detectable equine mfluenza virus antibody titers or the level of titer. Horses in the U.S. are vaccinated routinely against equine influenza virus. An estimated 65.4 percent (SE=3.0) of horses in the study were reported to have been vaccinated previously. Horses in the study vaccinated for equine influenza virus were more likely to have a detectable equine influenza antibody titer than horses that had never been vaccinated. An estimated 76.7 percent (SE--2.0) of vaccinated horses had a detectable titer as compared to only 55.4 percent (SE=3.6) of those not vaccinated. Horses that were not vaccinated may have developed an antibody titer to equine influenza, virus following natural infection. As the number of times horses received an equine influenza vaccine during the previous year increased, the percentage of horses with a detectable titer and high equine influenza virus antibody titers also increased (Figure 3b). Note that this study was conducted before intranasal influenza vaccine became commercially available in the U.S. Streptococcus equi subspecies equi, the bacterium that causes strangles, was isolated from only 3 horses, although nasal swab samples were cultured from almost 6,000 horses. Streptococcus equi subspecies zooepidemicus was isolated from an estimated 9.2 percent (SE=I.3) of horses, and Streptococcus dysgalactia subspecies equisimilis was found in an estimated 5.2 percent (SE=I.0). Although approximately 70 percent of horses in this study had a detectable antibody titer to equine influenza virus, the majority of horses had low antibody titers. Horses vaccinated for equine influenza virus were more likely to have detectable antibodies, but vaccinated horses did not always have high antibody titers. Beyond vaccinating at-risk horses, horse owners and trainers should utilize management practices that minimize the risk of their horses contracting equine influenza virus, especially when dealing with young horses.
For more information on NAHMS or the Equine '98 study, contact: Centers for Epidemiology and Animal Health USDA:APHIS:VS, attn. NAHMS 555 South Howes; Fort Collins, CO 80521 Telephone: (970) 490-8000 Email: NAHMSweb @aphis.usda.gov
http://www.aphis.usda.gov/vs/ceah/cahm
JEVS back issues on-line www.j-evs.com From the beginning of 1997, every article and paper from each issue of JEVS is on our website. That is 5 years of articles that can be down-loaded. Check your password and username on your journal name label. If there is no password listed there, your subscription has expired and you will need to renew. Everyone can see the content page of each of these issues without a password. For more information contact Paula at: [email protected] (909) 678-1889
JOURNAL OF EQUINE VETERINARY SCIENCE
Strangles is a specific type of IURD caused by the bacterium Streptococcus equi subspecies equi. In horses, strangles is characterized most often by purulent nasal discharge, and swelling of the lymph nodes of the head and neck. These lymph nodes may rupture and drain pus. Infected horses also may develop other signs such as fever and breathing and eating difficulties. The USDA's National Animal Health Monitoring System (NAHMS) collected data on equine health and management practices from a representative sample of equine operations in 28 states from 4 regions, a For this study, horses were defined as full-size breeds, usually standing at least 14 hands tall (56 inches) at the withers
when mature. Horses were considered residents of an operation if they spent more time at that operation than at any other site. Overall, 1,034 operations with 3 or more resident horses as of January 1, 1998, and at least 1 horse at the time of the initial interview completed the entire Equine '98 study. Horses housed at racetracks were excluded from this portion of the study. Participants monitored their resiaWestern Region: California, Colorado, Montana, New Mexico, Oregon, Washington, Wyoming. Northeast Region: New Jersey, Now York, Oluo, Pennsylvania. Southern Region: Alabama, Florida, Georgia, Kentucky, Louisiana, Maryland, Oldahoma, Tennessee, Texas, Virginia. Central Region: Illinois, Indiana, Kansas, Michigan, Minnesota, Missouri, Wisconsin.
As a veterinarian owned company, we strive to provide both you, the practitioner, and your client with: User Friendly Compounded Prescriptions Topical Ointments Antibiotics Immune Enhancement Oral Pastes Liquid Suspensions Performance Nutraceuticals
P.O. Box 164 9 St. Matthews, SO 29135 w Fax 803-655-7791 www.equi-fab.com 576
~1~ ~/f(
JOURNAL OF EQUINE VETERINARY SCIENCE
3 P 2.5 O 3: 2 "5 1.5 1
Equine '98 and the methodology used is available in NAHMS Equine '98 Part
0 March1- June1May 31 Aug. 31 Spring
Sept. 1- Dec. I Nov. 30 Feb. 28
Summer Fall Quarter
Winter
Figure 1. Percent of horses with IURD by season/quarter. dent horses for signs of IURD from March 1, 1998 to February 28, 1999. Diagnoses of IURD by a veterinarian were not required. A horse was considered to have IURD if it had a cough and/or nasal discharge with at least one of the following: fever, depression, decreased appetite, cloudy nasal discharge, or enlarged lymph nodes of the head and neck.
Strangles was considered the cause of IURD if the horse developed an opaque (mucopurulent) nasal discharge with swollen lymph nodes of the head and upper neck. The number of horses that developed IURD during each quarter (3 months) was recorded, as well as the number of horses that had strangles. More detailed information on
~4 (9
i "52--
n
0 <6 6-17 18mo.- 6-19 Months Months 5 Years Years Age of Horse
20 years or Older
*Averaged over 4 Quarters
Figure 2. Percent of horses with IURD (per quarter*) by age of horses.
|
r,0
O
-I- 3 '5
#.1 Pleasure Competition Breeding Racing FamVRanch O#~er Primary Use of Hcxses
*Averaged over 4 Quarters
Figure 3. Percent of horses with IURD (per quarter*) by primary use of horses. Volume 21, Number 12, 2001
L Baseline Reference of 1998 Equine Health and Management. Age was associated with the likelihood of a horse developing IURD (Figure 2). On average, an estimated 1.1 percent [Standard Error (SE)--0.2)] of horses over 5 years of age developed IURD per quarter, compared to 3.7 percent (SE=I.0) of foals less than 6 months old. Young horses may have been at greater risk of contracting IURD because they had not yet developed adequate immunity. When comparing horses younger than 18 months with horses 18 months or older, there was no difference in the average percentage of horses that developed strangles per quarter. The percentage of operations that had at least I horse with IURD was greater for large operations (at least 20 horses) than for medium-sized (6-19 horses) or small (5 or fewer) operations. Only 12.4 percent (SE=3. 1) of small operations experienced a case of IURD during the year as compared to 38.8 percent (SE=8.0) of large operations. However, an individual horse at a large operation was not more likely to develop IUJRD or strangles than an individual horse at a small operation. The higher percentage of at least one case of IURD at large operations may be due to the higher number of horses at risk. Yet, the likelihood of an operation's horses contracting IURD may have been influenced by other factors, including management practices and the age distribution of the horses. When c o m p a r i n g horses used for different purposes, there was a difference in the average percentage of horses with IURD per quarter (Figure 3). Racehorses housed off-track had the highest rate of IURD [4.3 percent
577
o (SE= 1.9)]. Since these racehorses were not stabled at racetracks, this rate may not reflect the rate of IURD for the entire U.S. racehorse population. The function of the operation where the horses were stabled also was associated with the likelihood that at least 1 resident horse would develop IURD during the study (Figure 4). Horses developed IURD on 35.0 percent (SE=8.6) of operations used primarily for breeding and on 33.9 percent (SE-- 9.2) of boarding, training, and riding operations. Only 12.2 percent (SE=3.4) of operations where horses were used for farming or ranching and 11.4 percent (SE=2.6) of residential operations where horses were kept for personal use, reported horses with IURD. The reason that residential operations and farms/ranches reported IURD less commonly than other types is not clear. Potential reasons include: these operations may have been smaller, with fewer horses at risk; more of their horses may have been over 17 months old; or their horses traveled less and were therefore less likely to be exposed to infectious agents that cause IURD. Vaccination of horses against equine influenza virus, equine herpes virus, and Streptococcus equi subspecies equi (strangles) in the 12 months prior to the study was not associated significantly with the rate of IURD occurrence. It was not clear whether operations that administered vaccinations did so as a preventive measure or to contain a disease problem already present. Acute infectious upper respiratory disease is a problem encountered on equine operations in the U.S. Many factors may contribute to a horse's risk of developing IURD. Although there are vaccines against several common infectious agents of IURD, 578
(P~,=onattk~)
Riding
Function of Operation "Otoerl~k~ns w ~ e ~ n s i d e ~ d to l~ve lURO lf M . ~ f or~ ho~e ~ e d dini~I sf~ns d~r{r~ ~
ve~.
Figure 4. Percent of operations with IURD* by function of operation. vaccination does not prevent all cases of IURD. Laboratory results for influenza serology and nasal swab culture for S t r e p t o c o c c u s isolation Equine infectious upper respiratory disease IURD is a problem that affects horses worldwide. Infected horses often develop a fever, cough, mad nasal discharge. They also may develop swollen lymph nodes, lethargy, and decreased appetite. Usually, horses recover from IURD without suffering long-term complications. Equine influenza virus is one of the most common causes of IURD. Horses can develop antibodies to influenza virus following vaccination or after infection with the virus. Horses with
high levels of these antibodies are less likely to become ill following exposure to equine influenza virus. Bacterial infections also can cause respiratory disease in horses. Strangles is among the most serious types of these bacterial infections and is caused by Streptococcus equi subspecies equi. Often, horses with strangles develop swollen lymph nodes of the head and neck. These infected lymph nodes may rupture and dram pus. The bacteria Streptococcus equi, subspecies zooepidemicus and Streptococcus dysgalactla, subspecies equisimilis, also can infect horses. While these bacteria may cause clinical respiratory disease in horses, not all exposed horses become ill. Horses carrying these bacteria can be a source of infection for other horses, even if they are not displaying clinical signs of disease.
r O 30 "I~6 20
8 10 n
0
None Low High Category of Serum Antibody TIter to Equine Influenza *None = <1:10 (no detectable titer); Low = 1:10 to 1:40; High = greater than 1:40
Figure lb. Percent of horses by hemagglutination inhibition
titer
category* JOURNAL OF EQUINE VETERINARY SCIENCE
r
6O
~50 O
-r- 40
"6 3o Z0
Equine '98 study. More detailed information on the study design is available in the NAHMS Equine'98 Part I.Baseline Reference of
0 <6rt~.
8-17re~.18mo.-4ym 5-19y~. 2 0 y ~ *
Age of Horses *Serum Antibody Titer to Equine Influenza Greater Than 1:40
Figure2b. Percent of horses with high* HI titer by age. The NationalAnimal Health Monitoring System (NAHMS) collected data on equine health and management practices from a representative sample of equine operations in 28 states from 4 regions'. For this study, horses were defined as full-size breeds, usually standing at least 14 hands tall (56 inches) at the withers when mature. Horses were considered residents of an operation if they spent more time at that operation than at any other facility during the study period. Operations that completed the questionnaire phase o f the study were asked to participate in the collection of biologic samples. Blood samples from horses were obtained to determine antibody titers to equine influenza virus by using hemagglutination inhibition (HI) testing. Swabs were used to collect nasal secretions from horses and then cultured
[o identify Streptococcus bacteria. Testing was performed by the National Veterinary Services Laboratories. The number of horses tested was based on the horse inventory on the operation. Blood was collected from up to 20 horses per operation, and up to 10 of these were selected randomly for nasal swabbing. Horses were sampled during the regular site visit; they may not have shown clinical signs of IURD. Overall, blood samples from 8,265 horses Eroin 949 operation * s were tested, and nasal swabs from 5,976 horses f r o m 850 operations were cultured for Streptococcus spp. Approximately half the samples were collected from June 15 to September 23, 1998, while the remaining samples were collected from October 2, 1998, to March 3, 1999. Horses stabled at racetracks were not included in this portion of the
8O co 70 60 O 1- 50 N.o 4O 3O 2o (1)
a. 10 0
t
2
3
4
5 or more
Number vaccinations in previous 12 months *Serum Antibody Titer to Equine Influenza Greater Than 1:40
Figure 4. Percent of horses with high* HI titer by number of vaccinations in previous 12 months. Volume 21, Number 12, 2001
1998 Equine Health and Management. It is believed horses that produce an increased concentration of serum antibodies against equine influenza virus have a decreased risk of disease during outbreaks. For this study, horses were considered to have high titers if the 19 influenza antibody titer was greater than 1:40; low titers if it was 1 : 1 0 to 1:40; and undetectable titers if the antibody was less than 1: 10. An estimated 69.7 percent [Standard Error (SE)--1.9] of all horses had a detectable antibody titer to equine influenza virus (Figure lb.). An estimated 9 t.4 percent (SE=2.3) of operations had at least 1 horse with a detectable titer. There was no difference in the estimated percentage of horses with undetectable, low, or high titers and the region of the U.S. where they resided or the time of year the samples were collected. Previous studies have found that young horses have the greatest risk of disease from equine influenza virus infections. In this study the age category of the horses was associated with the likelihood that they would have a detectable equine influenza virus antibody titer. Only 20.2 percent (SE=5.9) of the young horses aged 6 to 17 months had a detectable influenza antibody titer, as compared to 89.0 percent (SE=3.5) of horses aged 20 years or more. The percentage of horses that had a high equine influenza antibody titer increased as the horse's age increased (Figure 2b). However, even in older horses, 49.9 percent (SE--4.7) had low or undetectable titers. In this study, 95.1 percent (SE=2. 1) of horses less than 18 months of age had low or undetectable titers. Young horses may have had low antibody concentrations because 579
they either received fewer vaccinations for equine influenza virus than adult horses, produced less antibody following vaccination, or were less likely to be naturally exposed to the virus previously. The estimated percentage of horses with a detectable titer to equine influenza virus, and the percentage with a high titer, increased as the size of the operation increased. On operations with at least 20 resident horses, 81.5 percent (SE=2.7) of horses had a detectable equine influenza virus antibody titer and 52.6 percent (SE=3.9) had a high equine influenza virus antibody titer. The percentage of horses on small operations (1 to 6 horses) with a detectable equine influenza virus antibody concentration was 63.1 percent (SE=3.9), while 30.6 percent (SE=3.5) had high titers. A larger percentage of horses on large operations may have had higher equine influenza virus antibody titers because they had a greater chance of exposure to the virus from other horses on the operation. However, the difference may have resulted from other factors, such as management practices, vaccine strategies, frequency of contact with horses from other operations, and the age distribution of resident horses. An estimated 1.3 percent (SE=0.4) of horses sampled exhibited signs of acute IURD during the 30 days prior to sampling. When comparing these horses to horses with no signs of IURD in the previous 30 days, there was no difference in the percentage of horses with a detectable equine influenza virus antibody titer or the level of the titer. This suggests that the IURD reported during the last month was not necessarily caused by equine influenza virus. Direct physical contact with horses from outside the operation in the month prior to sampling also had no apparent effect on the percentage of
580
horses with detectable equine mfluenza virus antibody titers or the level of titer. Horses in the U.S. are vaccinated routinely against equine influenza virus. An estimated 65.4 percent (SE=3.0) of horses in the study were reported to have been vaccinated previously. Horses in the study vaccinated for equine influenza virus were more likely to have a detectable equine influenza antibody titer than horses that had never been vaccinated. An estimated 76.7 percent (SE--2.0) of vaccinated horses had a detectable titer as compared to only 55.4 percent (SE=3.6) of those not vaccinated. Horses that were not vaccinated may have developed an antibody titer to equine influenza, virus following natural infection. As the number of times horses received an equine influenza vaccine during the previous year increased, the percentage of horses with a detectable titer and high equine influenza virus antibody titers also increased (Figure 3b). Note that this study was conducted before intranasal influenza vaccine became commercially available in the U.S. Streptococcus equi subspecies equi, the bacterium that causes strangles, was isolated from only 3 horses, although nasal swab samples were cultured from almost 6,000 horses. Streptococcus equi subspecies zooepidemicus was isolated from an estimated 9.2 percent (SE=I.3) of horses, and Streptococcus dysgalactia subspecies equisimilis was found in an estimated 5.2 percent (SE=I.0). Although approximately 70 percent of horses in this study had a detectable antibody titer to equine influenza virus, the majority of horses had low antibody titers. Horses vaccinated for equine influenza virus were more likely to have detectable antibodies, but vaccinated horses did not always have high antibody titers. Beyond vaccinating at-risk horses, horse owners and trainers should utilize management practices that minimize the risk of their horses contracting equine influenza virus, especially when dealing with young horses.
For more information on NAHMS or the Equine '98 study, contact: Centers for Epidemiology and Animal Health USDA:APHIS:VS, attn. NAHMS 555 South Howes; Fort Collins, CO 80521 Telephone: (970) 490-8000 Email: NAHMSweb @aphis.usda.gov
http://www.aphis.usda.gov/vs/ceah/cahm
JEVS back issues on-line www.j-evs.com From the beginning of 1997, every article and paper from each issue of JEVS is on our website. That is 5 years of articles that can be down-loaded. Check your password and username on your journal name label. If there is no password listed there, your subscription has expired and you will need to renew. Everyone can see the content page of each of these issues without a password. For more information contact Paula at: [email protected] (909) 678-1889
JOURNAL OF EQUINE VETERINARY SCIENCE