NEWS STORIES
Horse Diseases and Humans
by: Roberta Dwyer, DVM, MS, Dipl. ACVPM
What can you catch from your horse?
If you work around horses long enough, you'll be handling ones that have lameness, diarrhea, abortions, skin diseases, and even neurologic signs. Our first concern is generally for the horse's well-being, and the last for our own health, since, after all, who gets sick from a horse?
Zoonotic diseases are those that are transmissible between animals and people. This article is going to address only selected diseases that humans can obtain from horses by direct or indirect contact (see sidebar "Infectious or Contagious?" below).
Rabies Characteristics
* Transmitted via the saliva of rabid animals, passed
on through bites or scratches
* Virtually 100% fatal in domestic animals and people
* Clinical signs vary from colic to lameness to neurologic disease
Rabies is known as "the great imitator" because it can present with a variety of initial clinical signs. Just ask Ann Dwyer, DVM, of the Genesee Valley Equine Clinic in Scottsville, N.Y., who has seen three rabid horses while in private practice (see page 24 for more information). She describes, "One was a juvenile Quarter Horse that presented with colic, but rapidly progressed to neurologic signs of stumbling and an unsteady gait. Another horse was crazed and had ropes of saliva coming from her mouth. She attacked anything that was near her. (This is often referred to as the furious form of rabies.) And the last horse presented with fever and being off feed. By the next day it was having difficulty swallowing and was lethargic."
Because of the profound neurologic disease, or progression of further neurologic signs, all horses were euthanized and tested for rabies--and all were positive. Dwyer, her technicians, and several people who had contact with the horses had to receive post-exposure rabies vaccinations.
The common factor: All three horses were not vaccinated for rabies, a vaccination that is affordable and is considered a core vaccination by the American Association of Equine Practitioners. Always consider rabies as a possibility, even in vaccinated horses.--Roberta Dwyer, DVM, Dipl. ACVPM
Rabies
Rabies is a viral disease transmitted via the saliva of a rabid animal and usually passed on through a bite or scratch. Infected saliva contacting mucous membranes or a break in the skin constitutes an exposure, as does direct contact with nervous system tissue (necropsy or laboratory exposure). The disease is virtually 100% fatal in domestic animals and people (only two people have been documented to have partially recovered from the disease).
While annually about 40 to 50 equine rabies cases are documented in the United States and Puerto Rico, it is a disease that warrants precaution since the clinical signs can vary from colic to lameness to neurologic disease, all which progress to death. It is always prudent to consider any horse with neurological signs as being possibly rabid (isolate and avoid all contact, especially with his mouth and saliva), and consult a veterinarian promptly.
In 2006, a horse housed at the Tennessee Walking Horse National Celebration developed neurological signs and was diagnosed with rabies after euthanasia (TheHorse.com article #7589). This resulted in the Tennessee Department of Health, with the Centers for Disease Control and Prevention (CDC), attempting to contact 150,000 people who were at the event about possible rabies exposure (article #7611 on TheHorse.com). Just one rabid horse can cause an enormous response (see sidebar, "Rabies!").
Salmonellosis
Out of more than 2,400 different types of zoonotic Salmonella bacteria, just one is transmitted only between people; the rest have animals involved. S. typhimurium is a serovar commonly detected in people, and it also is commonly found in horses.
Salmonellosis in horses causes diarrhea, septicemia, and localized infections (joints, eyes, etc.). People get exposed by fecal-oral contact (generally due to ineffective hand washing) or oral contact with contaminated materials. Any horse or foal with diarrhea should be isolated, and proper protective clothing should be worn when handling the animal, with further precautions assessed by a veterinarian.
Cryptosporidiosis
Cryptosporidium parvum is a protozoal intestinal parasite that can cause significant diarrhea in humans and young calves. Foals have been shown to shed C. parvum in feces, but generally they don't appear sick, and people in contact with affected foals have developed cryptosporidiosis (the disease associated with this parasite). Therefore, good hygiene, such as thorough hand washing, should be practiced when handling foals.
Anthrax Characteristics
* Humans can become ill from dermal exposure to dead
infected animals
* Fatal in horses; can be treated in humans if caught early
* Clinical signs aren’t usually caught before the horse dies, but if
blood comes from orifices of the dead animal, the owner should prevent other
animals from accessing the carcass
Anthrax
The sudden death of a horse gets the owner's attention immediately, and so do colic and severe diarrhea leading to death. If blood comes from the nose, mouth, or other orifices of the dead animal, the owner should prevent other animals from accessing the carcass, avoid all contact with the carcass, and call a veterinarian or state health official. The cause of death might be the spore-forming bacteria Bacillus anthracis that causes illness and death in livestock in several U.S. areas. However, unusual weather conditions, floods, and alkaline soils can potentially stir up spores deep in the soil and cause anthrax cases in other geographic areas. Anthrax is usually rapidly fatal in horses, so it's unlikely there would be time to treat horses with clinical signs, but you could potentially treat suspected cases very early on or prophylactically treat exposed horses. Special diagnostic sampling techniques and disposal methods are necessary for suspected anthrax cases; in some jurisdictions it might be a reportable disease.
People can become ill from dead animals by dermal exposure (through the skin, called cutaneous anthrax) to infected animals, tissues, and soils; inhalation anthrax is possible if spores are aerosolized, which can happen while processing livestock hides or wool.
Rhodococcus equi
Rhodococcus equi is commonly found in the soil and in feces of horses and other herbivores. This Gram-positive bacterium has long been recognized as a pulmonary pathogen of foals and a cause of other localized infections in foals and occasionally adults. R. equi is now considered a pathogen of importance to immunocompromised people, especially those with human immunodeficiency virus (HIV) infection. The majority of human patients with R. equi infections are immunocompromised, with approximately half of those being HIV patients (see sidebar "Immunosuppressed Humans and Horses" on page 32). Researchers are studying the epidemiology of human R. equi infection, as less than half of infected patients have a history of exposure to horses or pigs (another source of R. equi).
Brucellosis Characteristics
* Direct contact with discharges and possible aerosol
exposure in enclosed spaces are some routes of transmission to humans
* Can cause fistulous withers and poll evil in horses
* Clinical signs in humans can include undulant fever, sweating, weakness,
malaise, weight loss, and depression
Brucellosis
Testing and eradication efforts have nearly eliminated brucellosis from U.S. livestock herds. However, brucellosis still exists in wildlife that can be a source of infection to livestock. Many species of Brucellosis bacteria exist, but it's Brucella abortus and B. suis that affect horses. These Gram-negative bacteria can cause fistulous withers and poll evil. The former is an inflammation and infection of a bursa (fluid-filled sac) over the withers; poll evil is an inflammation and infection of the bursa over the first and second cervical vertebrae, close to the horse's poll. Swelling can occur in these areas, and when bursae burst, purulent material drains from the tracts. Brucella is only one of several bacteria that can cause these conditions, but it should be first on the minds of handlers because of its zoonotic potential.
Direct contact with discharges and possible aerosol exposure in enclosed spaces can serve as routes of transmission to people. While documented cases of human brucellosis are rare, the severity of the disease in people (called undulant fever) and the risk of transmission warrant prompt veterinary services for your horse if you notice an abscesslike swelling occurring in the poll or withers area. Strict hygiene measures should be applied.
Immunosuppressed Humans and Horses
With advances in early disease detection and new pharmaceuticals, cancer treatment, and amazing organ transplants, more people are alive and leading active lives, but with suppressed immune systems. Many diseases and conditions are controlled by medications that impede the immune system, such as corticosteroids, autoimmune disease drugs (e.g., some rheumatoid arthritis medications), cancer therapies, and transplant anti-rejection drugs. Infections (such as HIV) and diseases can also cause immunosuppression. Very young children, the elderly, and pregnant women (fetal risk) are also at higher risk of obtaining infections. These adult populations need to consult with their health care professionals in making proper decisions about precautions to take when working with horses.
--Roberta Dwyer, DVM, Dipl. ACVPM
Foreign Animal Diseases
Of the zoonotic encephalitic diseases of horses (Eastern and Western equine encephalitis and West Nile virus), none is transmitted via mosquito directly from the horse to human. However, high enough levels of Venezuelan equine encephalomyelitis (VEE) in horses' blood can be a source of infection to humans via mosquito bites. Direct transmission from horses to humans does not occur. VEE has been detected in South America and Central America; the last U. S. outbreak was 1969-1972, when it crossed over from Mexico. Other foreign animal diseases that can affect humans are glanders and Hendra virus.
Other "Bugs"
This is not an exhaustive list of horse-to-human zoonotic diseases. Also consider vesicular stomatitis virus, campylobacterosis, leptospirosis, ringworm (see article #5531 at TheHorse.com) and other fungal skin diseases, and methicillin-resistant Staphylococcus aureus (MRSA) infection (see article #11078 at TheHorse.com).
Take-Home Message
A few simple guidelines can reduce the risk of obtaining an infection from a horse. Here are the key points:
* Obtain appropriate vaccinations for horses and
for people!
* Isolate horses with neurologic signs and have a veterinarian out to see
the animal; avoid contact with the horse's saliva.
* Caretakers should use protective clothing and gloves when handling horses
or foals with diarrhea.
* Use mosquito and insect control strategies to reduce vector-borne diseases.
* Immunocompromised individuals should consult with a health care professional
about specific concerns with any horses and other animals in their care.
* And, like your mother said, "Wash your hands before eating or drinking!"
or after any contact with horses.
Resources
* Equine Infectious Diseases, Ed. Debra C. Sellon
and Maureen T. Long. St. Louis: Saunders 2007.
* Bender, J.B.; Tsukayama, D.T., Horses and the risk of zoonotic infections.
Veterinary Clinics of North America: Equine Practice. Vol 20, 643-453, 2004.
* Control of Communicable Diseases Manual, 18th edition, Ed. David Heyman.
Washington, D.C.: American Public Health Association, 2004.
Infectious or Contagious?
Here's some clarification for these often-confused terms!
Infection is the state of clinical disease associated with a microorganism (a virus, bacterium, parasite, or fungus). Infectious diseases are caused by infectious agents.
Noninfectious diseases include those that are metabolic, nutritional, musculoskeletal, autoimmune (lupus erythematosus), neoplastic (caused by tumors), etc.
Transmissible and contagious diseases refer to those
caused by infectious agents that can be transferred from one animal to another.
Equine influenza is transmissible or contagious between horses via respiratory
droplets, resulting in outbreaks of disease.
Zoonotic diseases (or zoonoses) are infections that are transmitted from animals
to people either by direct contact or via a vector (i.e., mosquito, tick).
West Nile virus can be transmitted from birds to people via mosquitoes, as
an example.
Direct transmission routes are primarily through inhalation, ingestion, skin or conjunctival contact, and bites or inoculation.
Indirect transmission includes transfer of the pathogen
from the horse to human via vectors such as ticks, mosquitoes, and other ectoparasites
(a parasite that lives outside the body). Indirect transmission can occur
due to contact with materials that the horse or its excretions have touched,
such as towels, bits, saddle pads, or contaminated bedding. These inanimate
objects are known as fomites.--Roberta Dwyer, DVM, Dipl. ACVPM
Article Courtesy of The Horse Magazine
Dealing with Emerging Infectious
Diseases
by: Stacey Oke, DVM, MSc
Scientists are becoming increasingly concerned about the marked and persistent
development of emerging infectious diseases--infections that are new to a
population or geographic area, have appeared more frequently, or have become
more virulent (able to cause disease).
Within the past two decades, the list of emerging infectious diseases and pathogens of clinical relevance to the equine industry has expanded and now includes: the equine herpesvirus-1 neuropathogenic mutant; equine multi-nodular pulmonary fibrosis; Clostridium difficile; equine protozoal myeloencephalopathy (EPM), and; methicillin-resistant Staphylococcus aureus (MRSA), among others.
Within the North American equine populations, likely the best example of an emerging infectious disease is West Nile virus (WNV).
"Prior to 1999, WNV was not recognized in the United States," explained Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, professor of equine medicine at Colorado State University, "Now, the WNV equine vaccine is the most commonly administered vaccine among equine operations that vaccinate against one or more diseases, which illustrates a rapid and broad response by veterinarians and equine owners that were faced with an emerging disease."
Infectious diseases are on the rise, and horse owners and veterinarians are responding to this challenge by becoming increasingly cognizant of abnormal behaviors in horses. Owners, with the assistance of their veterinarians, possess the ability to recognize new diseases.
What has caused this surge in emerging infectious diseases?
According to Barbara Bischoff, DVM, veterinary analyst from the Centers for Emerging Issues at the USDA's Animal and Plant Health Inspection Service, a variety of factors are thought to contribute to the development of an emerging disease.
"In general, shifts in the environment-host-agent triad result in the opportunity for an emerging disease to develop," said Bischoff.
The most common causes of emerging infectious diseases include:
* Climate change or other environmental changes that can
potentially impact the life cycle of the vectors (e.g., insects and other
animals);
* Urban sprawl of humans and horses into geographic areas that are home to
animals or insects that can transmit viruses, bacteria, or parasites;
* Alterations in host factors such as immune capabilities of horses (perhaps
via changes in management practices);
* Globalization of humans and animals that provides unique opportunities to
potentially spread disease; and
* Changes in the agent (virus, bacterium, parasite) itself at a cellular level
such as the shifting or drifting of genetic material.
"Because it is challenging to predict what the next emerging diseases might be, the best way to protect horses is to contain potentially contagious diseases via hygiene and isolation measures, until targeted intervention strategies (such as vaccination) are available," suggested Traub-Dargatz. "Other control measures could include insect or rodent management to control spread of infection, depending on the mode of transmission."
While it may not be possible to predict the next "big" infectious disease in horses, recent technological advances permit the relatively rapid development of diagnostic tests as well as vaccines, as evidenced by the rapid vaccine development subsequent to the WNV outbreak.
Emerging infectious diseases also have an obvious economic impact on the horse industry. For example, in 2002 WNV-associated costs in Colorado and Nebraska alone were $4 million; these included such factors as cost of treatment, loss of use, loss of horse, and cost of vaccinations.
"Nonetheless, beneficial impacts to the equine industry
include an increased awareness of emerging diseases, increased biosecurity
measures on the owner's part, an increased co-operation between owners and
veterinarians in the face of new or abnormal clinical signs of disease, and
an impressively rapid rate of developing diagnostic tests and safe, effective
vaccines as we saw following the emergence of WNV," concluded Bischoff.
Article Courtesy of The Horse Magazine
Preparation Key in Controlling Infectious
Disease Outbreaks
by: Erin Ryder, TheHorse.com News Editor
Effective outbreak management is implemented before a single horse gets sick, according to Mary Scollay, DVM, track veterinarian at Calder Race Course and Gulfstream Park racetrack in Florida. Scollay advocated having a plan in place and tailoring that plan to the particular disease situation as needed, in her presentation at the 52nd annual American Association of Equine Practitioners Convention, held Dec. 2-6, 2006, in San Antonio, Texas.
When an infectious disease is suspected, Scollay said to first establish a perimeter to contain infected and exposed horses, communicate the plan, and only then attempt a diagnosis.
"Act decisively, then explain why you did it and go from there," Scollay said. Raising an alarm about a potential outbreak before setting the perimeter can prompt some horse owners to leave the premises, leading to the spread of the pathogen. If you cannot establish a diagnosis, Scollay recommended maintaining infection control measures for at least 21 days.
Management of resources is key before an outbreak occurs, according to Scollay. Establish a chain of command, assess the facility, and take inventory of available testing materials and biosecurity supplies (gloves, etc.) available. Horse entry requirements should always be strictly enforced. Have a disease-reporting system in place so that people know what veterinary officials should receive information on a suspected case.
When an infectious disease is suspected, Scollay said a veterinarian should remember to do no harm--do not enter a barn or stall unless you know you can leave it without carrying an infective pathogen on your person.
Scollay said a facility manager should provide clear, concise instructions for horse handlers.
"I'm not an infectious disease expert," Scollay said. "That's why I'm here to tell you: You can handle this," no matter your background as a veterinarian.
Get research and health news from the American Association
of Equine Practitioners 2006 Convention in The Horse's AAEP 2006 Wrap-Up sponsored
by OCD Equine. Files are available as free PDF downloads.
Article Courtesy of The Horse Magazine
Antibody Titers
by: David W. Horohov, PhD
You might have heard your veterinarian say, "Let's run a titer on him,"
when referring to your horse and whether he's protected against disease, or
to figure out what might be causing particular clinical signs. What exactly
does "titer" mean?
In immunological terms, titer refers to the concentration of specific antibodies in a blood serum sample. The titer is determined by serially (repeatedly) diluting the serum and assaying (quantitatively analyzing) each dilution for the activity (for example, how many virus-neutralizing antibodies are present). The last dilution of a sample that responds in the assay determines the titer.
The greater the concentration of the specific antibody you're looking for in the serum sample, the higher the titer.
This information has several important uses. In clinical practice it can be used to determine if an individual has been exposed to an infectious agent. Prior to exposure the antibody titer would be very low or undetectable. Following exposure the immune system produces antibodies, resulting in an increase in the titer. Rising antibody titers, as determined using paired sera samples collected days or weeks apart, provide evidence for exposure to the infectious agent. This is particularly useful when it is not possible to isolate the agent for identification, as is the case for Potomac horse fever (you cannot isolate Neorickettsia risticii, the bacterium that causes Potomac horse fever).
Very high antibody titers are also useful in the diagnosis of purpura hemorrhagica (an immunologically mediated condition characterized by swelling of the limbs and widespread skin hemorrhages, varying in severity from a mild transient reaction to a severe fatal condition) and metastatic abscesses caused by Streptococcus equi, the bacterium that causes strangles infection.
Another use of titer is to determine vaccine efficacy. Protection from infection often requires that a certain antibody titer be obtained following vaccination. These levels are determined experimentally. This information is then used to formulate vaccines so that they stimulate the necessary antibody response and achieve the desired titer. Since antibodies slowly disappear from the circulation over time, re-vaccination is necessary to boost the titer back to these protective levels.
The exception is strangles, for which vaccination is contraindicated in horses with high antibody titers.
Should we be vaccinating horses against other diseases without first determining their titers?
While this might seem more efficient, the time and cost associated
with determining titers prior to vaccinating makes this approach impractical
at this time. Further, the time-dependent decay in antibody titers is fairly
predictable, meaning that regular revaccination can be recommended without
having to determine titers.
Article Courtesy of The Horse Magazine
Shipping and Disease
by: Kimberly S. Brown, Editor
The welfare issues of horses in the United States, and around the world, are
of concern to all horse owners. Whether it's starving, neglected, or abused
horses in our own county, or the plight of working burros and mules halfway
around the world, we care. Sometimes we don't know how to put our caring into
action. The first thing that we want, however, is information. We need to
know more about the animals involved and why they got into in their situations.
In our cover story this month on Canadian Equine Welfare, we delve into several
of the issues that we in the United States either already have concerns about,
or might need to recognize as problems that could affect our horses. For example,
if, or when, the commercial slaughter of horses is ended in the United States,
how will U.S. horses be protected from being shipped to Canada or Mexico for
slaughter? When planning for our story on horse welfare in Mexico (April 2005;
see www.TheHorse.com/ViewArticle.aspx?id=5636), an international veterinarian
friend of mine gave me a pitying look when he heard me say that horses wouldn't
be allowed by law to be shipped for slaughter in Mexico. "The slaughter
horse pens and pleasure horse pens are right next to each other," he
said. So where does free trade end and federal law begin?
According to the USDA Economic Research Service, the number of live horses shipped to Mexico from the United States was: 2001/4,608, 2002/3,467, 2003/3,755, 2004/17,951, 2005/23,309.
Do we really think Mexico had a 400% increase in the number of pleasure horses needed for riders in those four years?
The number of live horses shipped from the United States to Canada has remained fairly steady at just under 30,000. USDA numbers showed: 2001/29,344, 2002/29,395, 2003/26,040, 2004/26,324, 2005/29,701.
So that's more than 53,000 U.S. horses that were shipped across our neighboring borders to unknown destinies in 2005. Sure, some are probably around today living useful, healthy lives. My bet is that most aren't.
When planning to stop slaughter, let's make sure these unwanted horses are kept in mind while still keeping open the borders for normal trade.
Ship Some Disease, Please
Equine viral arteritis (EVA) isn't something you hear about every day. In fact, most of you have probably never even known someone who has seen a case of EVA. But there are some Quarter Horse owners in New Mexico who now know more than they wanted to about this easily passed virus that causes respiratory disease, stocking up, abortion, and a persistent carrier state in stallions. That's a pretty versatile virus.
One of the major problems with this disease is how easily it can be transferred from one horse to another. Of course there is the respiratory route, where nasal secretions spread the virus from horse to horse through a barn or training stable. Then there are the contaminated fluids from an aborted fetus, which are loaded with the virus.
As mentioned before, stallions can become persistent shedders of the virus. But the most incredible way this virus is spread is through overnight mail. That's right, you can FedEx a container of semen anywhere in the world and take the unwanted virus right along with the desired sperm. Think about that next time you order semen. Has that stallion been tested for EVA? If it were my farm and my mare, you bet those tests would be negative, or my mares would be vaccinated and protected.
Currently, the United States has no semen import restrictions for EVA, which means infected semen can be imported for breeding purposes and thus expose resident horses.
In 2002, a study was conducted in California to compare the seroprevalence of equine arteritis virus (EAV) in California horses and horses imported from other countries. Serum samples from 364 horses from 44 farms in California were compared to 226 samples from imported horses. The results indicated only 1.9% of resident horses were seropositive for EAV, compared to 18.6% of foreign horses (16% of which were stallions). Certain breeds appear to be more susceptible to EAV infection, most notably warmbloods.
It is clear that importation restrictions on EAV-positive
horses and semen should be considered as a means of preventing this virus
from spreading in the United States.
Article Courtesy of The Horse Magazine
Antibiotics
by: Nancy S. Loving, DVM
Most owners forget that antibiotics are drugs, and, if misused, they can cause
short- and long-term problems for horses and, potentially, humans.
Growing up in an era of modern medicine, the availability of antibiotics has
been taken for granted. How often have you been asked if you are taking antibiotics
for a cough or a sniffle? How many times have you seen a horse being administered
antibiotics for a nonspecific fever or for a relatively superficial wound?
Antibiotics have been impressive tools for improving health, yet they also pose one of the most pressing public health problems facing the medical community today. With increased antibiotic use comes increasing emergence of resistant strains of bacteria; such bacteria have developed ways to avoid being eliminated by drugs.
Some organisms that infect animals also infect people, and these bacteria make no distinction between types of mammalian tissue; to bacteria, it is a warm body with nutrients to support growth and multiplication of their numbers. And, similar antibiotics are used in many animal species and in people. The more exposure bacteria have to antibiotics, the more opportunities there are to develop drug resistance, with the potential for disastrous consequences: humans with serious infections can die, and in the veterinary world, there might be treatment failure or life-threatening diarrhea in horses.
Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, professor of equine medicine at Colorado State University's College of Veterinary Medicine and Biomedical Sciences in the Animal Population Health Institute, is dedicated to educating horse owners about the proper use of antibiotics. She remarks, "Antimicrobial drugs are critical in the treatment of bacterial infections in horses, and we need to think of them as tools to be taken care of just as we would take care of any valuable piece of equipment that we want to keep working for us. In order to use these tools most effectively, education is the key."
Prevention
Traub-Dargatz endorses a proactive disease-prevention approach: "Infection prevention strategies can be implemented that reduce, if not entirely eliminate, the need for some antimicrobial drug use. For example, we can promote biosecurity practices that emphasize strategies that limit risk of exposure along with a prophylactic vaccination program to protect against disease. A thorough examination should be performed on any horse that shows signs of being ill to identify if there is a bacterial infection that may require antimicrobial treatment. Early detection of ill horses should be followed with isolation and good hygiene on the premises to reduce spread of disease. Ensuring that foals receive adequate passive transfer is an effective strategy to prevent systemic infections that might require antibiotics. Neonatal foals must receive adequate transfer of antibodies from the colostrum to help protect them from infection, and owners should work with their veterinarian to have foals tested to be sure they have gotten adequate antibodies and, if not, then work on a plan for intervention."
Louise Southwood, BVSc, PhD, Dipl. ACVS, assistant professor of large animal emergency and critical care at the University of Pennsylvania's New Bolton Center, also urges veterinarians to implement strategies besides antimicrobials to curtail infection. She suggests, "Antimicrobials are not needed for minor superficial wounds and should never be used in place of appropriate wound cleansing and topical treatment. Use of antimicrobial drugs for deep wounds affecting bone and joints/tendon sheaths is, on the other hand, an important part of treatment. In appropriate cases, use of local antimicrobials may reduce the risk for selection of resistant bacteria in sites like the digestive tract that occur when antimicrobial drugs are given orally or by systemic injection." Local treatment includes regional limb perfusion through a vein or bone in an infected lower limb region, constant rate infusion delivery to a joint via an indwelling catheter, or implants such as antimicrobial- impregnated beads inserted into infected sites.
She notes the equine immune system deals with contamination and fights infection effectively, "and we should also focus on providing appropriate nutrition and other support for our sick equine patients."
Antibiotics in Veterinary Medicine
By definition, an antimicrobial is a substance that inhibits the growth of bacteria. Antibiotics are derived from living organisms, while antimicrobial drugs also include synthetic or combination medications that suppress bacterial growth. Just as there are many different types of bacteria, there are different antimicrobial drugs that possess properties that allow them to target specific microorganisms.
"In both veterinary medicine and human medicine, we have an obligation to use antimicrobial drugs judiciously and responsibly, administering these drugs where there is a clear indication for use and in situations where we believe treatment is likely to result in resolution of a bacterial infection," Traub-Dargatz says. This objective calls for evidence-based science rather than guesswork.
"The key to identifying the infection problem is in taking a good medical history on not only the individual horse, but on other horses in the facility as well as horses exposed to the ill horse," Traub-Dargatz emphasizes.
Follow this with a thorough physical examination and possibly collection of samples for diagnostic testing, she adds. This gives critical information to suggest if a bacterial infection exists, and if an antimicrobial drug is needed.
"In some patients," Traub-Dargatz says, "a veterinarian will recommend an antimicrobial drug based on past experience with similar-type infections; this is called empirical treatment. This approach is an accepted standard of care, as it allows for immediate initiation of treatment." Certain bacteria are more commonly associated with some diseases than others, and an antimicrobial drug can be selected empirically, based on the type of bacteria that's most likely causing the infection.
"However, if a patient fails to respond to the antimicrobial treatment, or if the animal has severe disease or is not a typical case, then material should be collected from the infection site with subsequent culture and sensitivity testing in a laboratory," Traub-Dargatz elaborates.
Southwood explains, "Different antimicrobial drugs are effective against different bacteria; therefore, bacterial culture and sensitivity testing are important to isolate the type of infecting bacteria, and to identify the best possible antimicrobial to suppress bacterial growth," she states.
"With increasing emergence of resistance to commonly used antimicrobial drugs, bacterial culture and sensitivity testing are becoming more and more important to treat accurately and to monitor antimicrobial drug resistance and sensitivity patterns," she concludes. You can't determine which bacteria are causing the infection by just examining the patient.
Traub-Dargatz says to accurately monitor change in antibiotic resistance patterns, it would be ideal if results from multiple laboratories or veterinary clinics could be collected and summarized. "However, funding to do this kind of research is difficult to attain," she states.
Drug Availability
"There are only a limited number of antimicrobial drugs approved for use in equine patients at the dose or route by which we use them," Traub-Dargatz explains. "However, veterinarians can prescribe the use of drugs by what is called 'for extra-label use' when there is no labeled drug that will work as well or better than the one prescribed for the medical condition. In addition, there must be an active veterinary-client-patient relationship that allows us to diagnose and treat the animal."
Veterinarians are limited in their choices of drugs labeled specifically for horse use. One reason is it's very costly for drug companies to research, develop, and complete the necessary approval process.
Some well-conducted studies of the behavior of various antimicrobial agents in horses have determined the duration of blood levels of the drug as well as the concentration achieved in various tissues. However, Traub-Dargatz points out that these studies are often conducted on healthy adult animals. She notes, "The drugs do not necessarily perform the same in ill or neonatal animals, so we don't always have all the information available that would allow us to most effectively dose the antimicrobial drug in all patients. Often we must use data that was generated in laboratory animals or humans."
Inappropriate Use?
Southwood explains why antimicrobial drugs might be "inappropriately used." She mentions antimicrobials are often reached for when there is a concern of a serious or life-threatening infection, and, similarly, their use has become something of a habit, especially since antimicrobials have been proven effective in preventing and treating bacterial infections. In addition, they are economical to use and readily available.
Southwood states that prolonged antimicrobial treatment is the most common inappropriate use; this has been documented in human hospitals, but there are no data for veterinary hospitals. She adds, "Another component is created by pressure from horse owners to treat with antimicrobial drugs to lower the risk of secondary infection, reduce recovery time, and in some cases to comply with stable management guidelines." This can lead to treating a virus-infected horse with antimicrobials, which are ineffective against viruses.
"Antimicrobial drugs are like any other drug," she notes. "They are very effective for treating disease, but there can be complications and consequences of misuse. Treating a horse with an antimicrobial drug just because it is easy, safe, and economical, without a sound clinical basis, is inappropriate. Equine owners should not begin treating a horse with antimicrobial drugs without first consulting their veterinarians, and owners need to become educated on the risks of using antimicrobial drugs in horses."
Southwood compares principles of antibiotic use in human medicine: "Compare the infant that attends day care that frequently has a runny nose and intermittent fever with the weanling horse that also has a runny nose and intermittent fever; we do not immediately treat the human infant with antimicrobial drugs (because the problem is most likely caused by a virus), but the weanling is often treated with a broad-spectrum oral antibiotic for seven to 10 days."
Prophylactic Use for Surgery
Southwood says antimicrobial drug prophylaxis for surgical procedures has probably had as big an impact as anything on improving patient survival and reducing postoperative complications. She says the favorable effects of antimicrobial drugs are achieved only if given within the two-hour window immediately before surgery. "If given postoperatively, the infection rate is no different to that when antimicrobials are not used," she says. "Antimicrobial drugs given more than two hours prior to making an incision are also not effective."
She stresses, however, "antimicrobial drugs should never replace aseptic and atraumatic surgical technique."
Traub-Dargatz says guidelines exist for using antimicrobial drugs based on the existing or likely contamination of a surgery site. Southwood expands on this: "Based on recommendations from human surgery, antimicrobial drugs are indicated in cases where the infection rate without antimicrobials is greater than 5%, such as in clean-contaminated, contaminated, and dirty procedures, when implants are used, or when infection would be devastating or life-threatening."
Traub-Dargatz explains that antimicrobial drugs are given prior to colic surgery because until the abdomen is visualized at surgery, it is difficult to know the severity of intestinal compromise or whether the bowel will need to be entered as part of the surgical intervention. Drugs should not be used as a substitute for good sterile technique during surgery.
Adverse Reactions and Resistance
Traub-Dargatz suggests the use of any drug or biologic product should be carefully considered and weighed against the risks for adverse reactions. Additionally, a growing concern is emerging antimicrobial drug-resistant bacteria in hospitals and in the community. "If a person or animal has a multidrug-resistant bacterial infection, there is a delay in treating with appropriate antimicrobial because the 'first-line' antimicrobial drugs are no longer effective," Southwood says. "This increases the death rate from serious infections."
In humans, mortality rates are higher when patients with multidrug-resistant bacterial infections receive ineffective antimicrobial drugs, as compared to such patients treated with antimicrobials that are effective against nonresistant bacteria.
What are the practices that are most likely to cause resistance? "It is possible that if we do not use the most appropriate antimicrobial or don't use it for an adequate duration, then we might see treatment failure, not necessarily due to development of resistance per se," says Traub-Dargatz. "We strive to use an antimicrobial drug that is likely to resolve the infection, while reserving use of the most potent antimicrobial drugs for those patients with demonstrated multidrug-resistant infections."
Traub-Dargatz describes the potential for antibiotic resistance: "With antimicrobial drug use we risk selecting for bacteria that are resistant or are becoming resistant to the drug used in treatment. Some bacteria are very adaptable and have multiple ways to survive exposure to an antimicrobial drug. Some bacteria are inherently resistant to certain antimicrobial drugs because of the mechanism by which the drug inhibits bacterial growth or kills that type of bacteria. Or, bacteria develop methods to resist the influence of the antimicrobial."
In addition, she points out that disinfectants are also considered antimicrobials, by definition. "We are learning that there may be cross-resistance between some disinfectants and antibiotics (meaning use of the disinfectant in an environment renders the antibiotic ineffective in horses kept there)!" she notes. "Another area to explore is whether the use of disinfectants in hospitals promotes resistance to antibiotics."
She says veterinarians have to keep in mind that "other bacteria beyond those we are trying to treat are also exposed to the drug. Thus, we can select for resistance in what we call 'bystander bacteria,' for example, those that live in the digestive tract."
It is just as important to administer the antimicrobial at the optimal dose, at appropriate dosing intervals, for the appropriate time (depending on the severity of the infection and response to treatment), and by an appropriate route.
Southwood says we don't know the appropriate equine doses for all of the therapeutic and prophylactic antimicrobials available. "In human medicine, it is becoming apparent that the duration of therapeutic antimicrobial drug use can be much shorter than that previously used," she says. Patient and physician education and clinical trials have led to improvements in human drug use. Veterinarians hope more funding and clinical studies will allow the same improvements for horses.
Take-Home Message
"Historically, people have had the attitude that giving
an antibiotic might help and can't do any harm," notes Traub- Dargatz.
"The more we know about antimicrobial drugs (or any drug), the more we
find potential for adverse outcomes from their use. Therefore, in deciding
what is best for the patient, we must consider the potential for harm along
with the potential for benefit." In addition to implementing infection
prevention strategies, all aspects of drug use should be taken into account
when reaching for this valuable tool in the fight against infection.
Article Courtesy of The Horse Magazine