"A serious epidemic of a nature still unknown is going on in the region of two of our plantations. Area of Yambuku is declared highly dangerous. All movement in and out of Yambuku is forbidden. Burn all linen in contact with the sick. Do not cross this barrier or you will die."

"The mission of Yambuku, with its schools, farms, and hospital, was still an island of efficiency and commitment in the midst of a dense, rain-soaked forest. At the end of August 1976, a lethal hemorrhagic disease, later known as Ebola fever, exploded out of the Yambuku hospital, devastating the mission and surrounding villages. An oasis of peace and order became a focus of terror and death."--William T. Close, MD

Anyone who has endured an epidemic of strangles in a group of young horses, experienced the devastation of a herpes virus-induced "abortion storm," or suffered through an outbreak of Salmonella knows the importance of understanding some of the nuances of contagious disease. The three diseases that will be discussed here are salmonellosis, herpes virus infection (rhinopneumonitis and encephalomyelitis), and Streptococcus equi infection (strangles). There are other contagious diseases, but these probably represent the three that have the greatest impact in North America. Although these three will be discussed in detail, the principles of prevention and containment are essentially the same for reducing the spread of most diseases.

It is extremely important that before discussing disease, certain medical terminology is accurately understood. There are many terms regarding the spread of disease that are used incorrectly and therefore alter the meaning, thus preventing a "true" understanding of the disease process. Words that are frequently misused are: infection vs. infectious vs. infective vs. contagious vs. contagion; and endemic vs. epidemic vs. outbreak.

The Dorland's Medical Dictionary definitions of the aforementioned words are:

  • infection--invasion and multiplication of microorganisms in body tissues
  • infectious--caused by infection
  • infective--infectious; capable of producing infection
  • contagious--capable of being transmitted from person to person (animal to animal)
  • contagion--the communication of disease from person to person (animal to animal)
  • endemic--present in a community at all times (generally in a low level)
  • epidemic--attacking many people (animals) in a geographic region at the same time; widely diffused and rapidly spreading
  • outbreak--epidemic

Note that infectious does not necessarily mean contagious.

Other words of interest to contagious disease are:

  • pathogen--any disease-producing agent or microorganism
  • fomite--an inanimate object or material on which disease-producing agents may be conveyed
  • zoonosis or zoonotic disease--disease of animals that is transmissible to man (people)

The science of studying infectious disease is called epidemiology. This science investigates the circumstances by which disease is spread throughout a population. The information is often vital in identifying the original source of disease, be it a sick animal, a carrier animal, or a contaminated feed source. It maps a pattern of the disease process and can be pivotal in the containment or prevention of a contagious disease.

There are many factors that must be considered with respect to an organism's ability to cause disease and its ability to be contagious. The ability for a microorganism to cause disease is called "virulence." The greater the virulence, the greater the ability to cause disease. Different strains of the same bacteria or virus can have a different degree of virulence. Two main factors associated with highly virulent organisms are the ability to attach and invade the body and the ability to survive the host's immune system.

How Are Diseases Spread?

The first and foremost factor in disease spread is that a potential pathogen must get inside the body. Respiratory viruses generally are spread when an infected animal breathes, coughs, or sneezes the virus into the environment, then the virus is inhaled by a healthy animal. Bacterial diseases such as Streptococcus equi and Salmonella generally are spread from an infected animal via direct contact with infective material--either pus containing bacteria from abscessed lymph nodes or respiratory secretions in the case of strangles, or fecal material in the case of Salmonella. These "contacts" also can be made from an infected horse to a healthy horse via grooming tools, bedding, water/feed buckets, people's hands, and so on.

Once contact has been made, the pathogen must attach to the tissue and invade in order to gain access to the body. To do this, it must overpower substantial barriers. Both the respiratory system and gastrointestinal system have many protective measures, but these barriers can be weakened or altered during times of stress, as will be discussed later.

If the pathogen makes it through the superficial protective barriers, it must then survive an encounter with the immune system before being successful at causing disease. Therefore, adequate vaccination can play an important role in disease prevention. (See article on vaccinations in The Horse of September 1996, page 41, and on the immune system in the August 1996 issue, page 34.)

Another concept that plays a role in disease prevention is that of "infective dose." Different organisms require a different "dose" or amount to cause disease. In principle, if a person ingested a drop of Salmonella, most of the time that person wouldn't get sick. But, if that same person ingested a tablespoon full--look out! Other microorganisms might require only the amount that would fit on a pinhead to cause disease.

The infective dose of a microorganism (the amount it takes to cause disease) can be considerably less in animals suffering from another illness, general stress, and/or immune system compromise.

The bottom line is that the less there is of the organism, the less chance there is of disease. So, cleaning and disinfecting and using quarantine procedures can be of significant benefit in preventing disease. (These will be discussed later.)


One disease that has killed many horses or resulted in very expensive treatment is salmonellosis (see related article on page 41). Salmonella has shut down many a riding stable, breeding farm, or veterinary clinic as it can be extremely contagious and zoonotic. The scare of having Salmonella on a property can last for years after the incident.

Salmonella is a bacterial infection that in adult animals targets the gastrointestinal system. There are more than 2,200 different strains (serovars) of salmonellae. Less than a dozen species typically cause disease in the horse, but the majority of Salmonella serovars have shown to be potential pathogens. In young animals (and occasionally adults), it can spread throughout the bloodstream (septicemia) and infect multiple organ systems. Salmonella in humans is associated with severe food poisoning, often related to improperly cooked poultry or the ingestion of raw, non-processed eggs.

The Salmonella organism itself is called a Gram negative type of bacteria. The outer wall structure of most bacteria differs between two large classifications--either Gram negative or Gram positive. The Gram nature of a bacteria is determined by the color they become when specially stained; Gram negative bacteria are red and Gram positive bacteria are purple.

One characteristic of Gram negative bacteria such as Salmonella is the presence of "endotoxin." The endotoxin is part of the bacterial cell wall and is technically called the lipopolysaccharide component or LPS. The significance of this is that the effects of endotoxin on the body are great. The main effects are to decrease blood pressure and to decrease blood flow throughout the body. When you hear from your veterinarians that your horse appears "toxic" or is suffering from "endotoxemia," it means the horse is showing clinical signs that can be attributed to the negative effects of endotoxin. Endotoxic "shock" is a potential component of Salmonella infection that can greatly complicate treatment.

Known associated "risk factors" for the development of salmonellosis in horses are co-existing gastrointestinal disease, concurrent disease and/therapy with antibiotics, transportation, and dietary change. Most "outbreaks" of Salmonella occur in summer and autumn--this might be related to the seasonal increase in risk factors (competition, transportation, and irregular feeding).

A previous report in the Journal of the American Veterinary Medical Association noted that intravenous antibiotic therapy or a combination of oral and intravenous antibiotic therapies increased the chance of developing salmonellosis by 6.4 and 10 times, respectively.

The defense mechanisms of the gastrointestinal system are complex. There is "local" immune system function, but in addition the "normal gastrointestinal microbial flora" plays an important role in protecting against invading pathogens. The horse's colon or hind gut is essentially a vat of bacteria. Much of the digestive process is based on the normal population of bacteria (flora) digesting the raw fiber (hay) that would otherwise be indigestible--without bacteria, forage-eating animals would not be able to digest plant material.

The protective nature of this normal population of digestive bacteria is called "competitive inhibition," as the few would have to compete with the many in order to survive. So, if only a few pathogenic organisms are ingested, there is a good chance they will be "competed out of existence" before causing disease. But, as the amount of pathogen ingested increases, there is an increasing chance it will establish an infection and cause disease--the pathogen thus reaches an infective dose.

As can be imagined, the bacterial "microenvironment" is very complicated and reaches a working balance dependent on numerous microorganisms and other factors. This delicate balance can easily be disrupted by sudden feed changes. As with any finely tuned fermentation process (e.g., beer, wine, pickle, or sauerkraut making), very subtle changes can really mess up the process. Changes in the carbohydrate content (e.g., sudden excessive grain feeding) can rapidly alter the fermentation process in the gastrointestinal tract and therefore alter the competitive inhibition action. These changes decrease the defense mechanisms of the normal microbial flora and can actually "select" for an increasing number of disease-causing organisms in the gastrointestinal tract.

It is therefore extremely important to avoid sudden feed changes--especially with respect to concentrates (grain). A horse's diet should be maintained as consistently as possible and include an adequate amount of fiber (hay or grass). Any changes in feeding habit should be made gradually and a great effort should be made to avoid playing "musical feeds," especially during times of other concurrent stresses. Bringing your horse's normal feed with him to shows and events might reduce one of the overall stress factors. Many horses which are ultimately confirmed to have Salmonella have a history that includes transportation in the week preceding the development of disease and/or a recent change in feeding habit.

In the majority of circumstances, Salmonella is spread via "fecal-oral" transmission, i.e., any fecal material from an infected horse is a potential source of infection for other horses--and people too. As was mentioned in the basic principles section of this article, there is a "required" amount of Salmonella organism that must be ingested in order to cause disease. In normal, healthy adult horses, the "infective dose" is relatively high. But, in high-risk patients such as foals, sick horses, horses receiving antibiotic therapy, or horses suffering from transportation stress where the immune and other defense mechanisms might be compromised, the amount of organism necessary to cause disease is reduced.

One factor to consider with Salmonella is its persistence in the host. It has been shown that after infection the bacteria can be carried in the gastrointestinal tract and associated lymph nodes for months, thus creating a potential "carrier" state. During times of stress, that animal might develop diarrhea related to Salmonella infection or asymptomatically shed the organism in its feces. Given this, the most cautious approach would be to maintain a horse which was confirmed as having had Salmonella in some degree of isolation for several months. Repeated culturing of the feces could be helpful in determining when that animal stops shedding Salmonella organisms.

Many veterinary schools and private referral practices routinely culture feces from all hospitalized horses as part of a routine Salmonella control program. The reported number of horses which were "shedding" Salmonella organisms range from 1.4% to greater than 20%, with survey culturing of farms and stables indicating that 1-3% of horses could be expected to shed Salmonella. (None of these horses had diarrhea.)

Noah Cohen, VMD, PhD, Diplomate ACVIM, reported at the 1996 ACVIM meeting in San Antonio, Texas, on the use of polymerase chain reaction (PCR) technology to identify the presence of salmonellae in the feces of horses observed as either outpatients or hospitalized animals at Texas A&M University. Of 110 hospitalized horses which had fecal samples submitted to the clinical laboratory because the clinician caring for the case considered salmonellosis as a possible problem, 71 (64.5%) were positive by PCR. Of these 110 horses, 102 (92.7%) were admitted for colic or diarrhea. Cohen demonstrated the "the PCR was more rapid and sensitive than microbiologic culture for detecting salmonellae in fecal and environmental samples."

Routine microbiologic procedures generally require 48-96 hours to obtain a preliminary result and up to seven days for confirmation. In addition to the time factor, it is more difficult to culture Salmonella from diarrhea than from firm feces due to the dilutional effect of the large volume of water being secreted into the gastrointestinal tract. Given the increased sensitivity of PCR and the potential for less than a 24-hour turnaround time, Cohen's work in this area could be a very important advance in the ability to detect and control Salmonella.

Another factor to be considered regarding Salmonella is the organism's ability to persist in the environment. It is known that bacterial survival in manure for up to one year is possible, with cool, dark, and damp conditions being best. Complete removal and isolated disposal from an environment of a horse which has been confirmed to have Salmonella or has had Salmonella can be very important.

Any horse which develops diarrhea has the potential to have Salmonella and should be treated as such until confirmed otherwise. The animal should be isolated as much as possible from other stock--especially foals--and reduction in environmental contamination should be attempted. (See section on Prevention And Containment.)

Equine Herpesvirus

Equine herpesvirus 1 and 4 (EHV-1 and EHV-4) are the most clinically significant of the eight different equine herpes viruses that have been identified. EHV-1 infections can result in respiratory disease, abortion ("abortion storms"), fatal neonatal illness, and neurologic syndromes. EHV-4 primarily causes respiratory disease and sporadic abortions. Commercially available vaccines can protect against EHV-1 and EHV-4.

The respiratory form of EHV-1 and EHV-4 is usually characterized by fever (generally 102-106� Fahrenheit), nasal discharge, and coughing. (Coughing is not a consistent sign and might be completely absent.) The neurologic form of EHV-1 can either be associated with the respiratory form or can occur in absence of other EHV-1 syndromes. Horses afflicted with the neurologic syndrome typically show weakness and incoordination that starts in the hind end. Other signs often include loss of tail tone and a paralyzed bladder.

Abortion caused by EHV-1 infection can be either a sporadic single abortion or an epidemic "abortion storm" affecting all unprotected (unvaccinated) pregnant mares on a farm. The abortions are usually in late gestation (seven to 11 months) and typically occur in the absence of any respiratory signs. The aborted fetus shows characteristic pathology associated with herpes abortion, and virus can be isolated from many of the fetal organs.

Nasal secretions and aerosolized respiratory secretions (a sneeze) and anything that comes into contact with them are a major source of virus. Horses suspected of having equine herpesvirus should be isolated as much as possible and handled after all other horses. (See section on Prevention And Containment.)

With respect to abortion, it is actually the aborted fetus and the placental membranes and fluids that are a major source of the virus--these should be bagged (along with all contaminated bedding) and appropriately disposed of along with adequate cleaning and disinfection of the environment.

Streptococcus equi

Streptococcus equi is a bacteria that causes pharyngitis and lymph node infection of the upper respiratory tract only in horses, donkeys, and mules. Also known as strangles, the disease is most common in young horses, but horses of all ages can be affected. The bacteria does not survive well in the environment for long periods of time. The disease is highly contagious and is transmitted directly by nose or mouth contact or aerosol. Direct transmission can occur via any contaminated surface (water/feed buckets, people's hands, grooming tools, etc.).

Clinical signs generally consist of depression, fever, nasal discharge, cough, inappetence, difficulty in swallowing, and subsequent swelling and tenderness of lymph nodes about the head. Complications occasionally associated with strangles are: obstruction of airway from lymph node swelling necessitating an emergency tracheotomy; infection of the guttural pouch; and the spread of infection to other lymph nodes within the body (bastard strangles).

Horses suspected of having strangles should be isolated as quickly as possible. (See section on Prevention And Containment.)

Prevention and Containment

Obviously, adequate vaccination programs for diseases that have a highly contagious nature are a paramount first step in the prevention process. This becomes increasingly more important as your animals or farm have increasing contact with other animals. Horses on the road traveling through many new and foreign environments or resident animals being exposed to such horses are at greater risk.

Farms that have a high volume of horses passing through, for either sale, training, or breeding purposes, have a greater risk for the resident horses to be exposed to contagious disease. Many larger farms, especially farms housing pregnant mares and/or foals, set up fairly strict isolation procedures for newcomers on the farm. The ideal situation is to have a separate barn so that there is a significant amount of "physical" space separating new animals. The incoming horses then can be monitored in their "quarantine" area for any signs of disease.

For up to two weeks they should be monitored for any coughing, sneezing, nasal or ocular discharge, swelling of lymph nodes about the head, development of fever, and/or development of diarrhea. During this quarantine time, they can also be brought up to speed on your vaccination and deworming programs before entering the general population.

The above "ideal" quarantine set-up is great if you have the extra barn to devote specifically to that purpose. But even if space is limited, some type of basic quarantine should be performed. In a one-barn stable, a single stall can be devoted to incoming horses, if possible in a corner without direct nose-to-nose contact with resident horses. I have seen a single, temporary stall placed in the corner of an indoor riding arena for this purpose. This will be less effective for respiratory diseases, such as influenza or rhinopneumonitis, that can be spread over substantial distances via aerosolizing of virus particles in nasal secretions, but it can help prevent the spread of more direct-contact diseases.

For farms that only have pasture and run-in sheds, a quarantine paddock can be set up for incoming horses. If two paddocks are adjacent, a "buffer" fence should be placed so that nose-to-nose contact between new and resident horses is prevented.

In all of the aforementioned quarantine setups, the incoming horses should be monitored for any coughing, sneezing, nasal or ocular discharge, swelling of lymph nodes about the head, development of fever, and/or development of diarrhea for a period of up to three weeks. Daily temperature monitoring for the development of a subtle fever might be the most sensitive "early-warning" sign as mild fever often precedes other signs of illness. All new animals should be handled after all the chores have been performed to the resident horses to minimize the potential of carrying a problem to the resident horses.

It is also important to remind everyone working with incoming horses on a farm that all the quarantine effort performed--no matter how great--is a waste of time if people, other domestic pets, brushes, sweat-scrapers, etc. carry a disease into the resident population of horses. In larger stables, a separate person can be assigned to the quarantine horses.

One problem associated with showing or eventing is that when you put your horses in their new, temporary abodes, you don't really know who was there before or how good the cleaning procedures were prior to your arrival.

When I worked with show horses, we always arrived at the grounds early enough before the horses to thoroughly wash the stalls with a detergent cleanser followed by a spray-down with a chlorine bleach solution that had been diluted 1:10 with water. This procedure is the most effective if you allow the area to dry between washings and before placing bedding. We would also do this to the trailers.

This procedure by no means "sterilizes" a stall or trailer, but can reduce any clandestine pathogens to a less significant level. As many a great veterinary professor has quoted to students: "The solution to pollution is dilution." So, at the very least, a simple stall wash-down is probably in order.

This cleansing/disinfecting is also recommended for contaminated stalls, water/feed buckets, stall cleaning utensils--essentially anything that has come in direct contact with a horse suspected of having a contagious disease. It is extremely important to cleanse an environment prior to disinfection. "Organic" material (feces) quickly inactivates most disinfectants, so it is important to minimize organic debris. Extremely porous surfaces are more difficult to disinfect because of all the nooks and crannies. Placing a smooth plaster/concrete layer over cinder-block, or a wood sealer to wood surfaces, will improve the efficiency of cleaning and disinfection.

There are a multitude of disinfectant products available commercially. Many of them will indicate on the label what they are best used for. Great care must be taken when using these as several types of disinfectant chemicals inactivate each other or can potentially produce toxic gases if mixed with chlorine bleach. In addition, there are some chemical disinfectants that are inactivated when in contact with certain detergent cleaning solutions, so adequate rinsing with water is important prior to the application of a disinfectant. Label directions should be implicitly followed and any questions should be referred to a professional.

Any horse which is sick and suspected of having a contagious disease should be immediately isolated from the healthy stock. If possible, a single person should be assigned to the care of this animal, and that person should be educated to use proper quarantine procedures. If this is not possible, the horse should be worked with last--after all other animals have been cared for. That animal should have separate feed/water buckets, grooming tools, stall cleaning utensils, etc.

In the case of strangles, where a draining abscess is involved, disposable rubber gloves should be worn. Any potentially infectious material (pus from strangles abscess, fecal material from a horse with diarrhea, etc.) should be handled with great care and disposed of appropriately.

If you have a sick horse you are caring for, or even healthy ones--especially foals--the act of hand washing can greatly reduce the spread of disease. Hand washing has been shown to significantly reduce the spread of disease in human hospitals, and it is a simple, basic procedure that should be done between the handling of different animals. In addition, this can greatly reduce the chance of spreading a disease such as Salmonella to yourself--rigorous hand washing should always be performed after contact with a horse which has diarrhea as a precaution. It is also recommended that disposable plastic boots be worn around quarantined animals in order to reduce the risk of spreading infectious material around the farm.

All of the aforementioned prevention and control actions are labor intensive and time consuming. But, when weighed against the devastation of an EHV-1 abortion "storm" or the economic losses encountered with a strangles or Salmonella outbreak at a boarding facility, they really are a small effort to put forth.

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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