Equine version of strep throat can be ugly, but with proper care is seldom fatal

UTTER THE WORD "strangles" and it brings to most people's minds John Steinbeck's 1937 novel, The Red Pony, a gruesome tale of a troubled ten-year-old boy and his pony, Gabilan, who succumbs to the disease after a bloody tracheotomy fails to save his life.

It is no wonder that the recent outbreaks of strangles in Kentucky and Florida have sent horsemen into a panic, but the reality is that the disease caused by Streptococcus equi is no more threatening than strep throat is in schoolchildren. As in humans, complications may arise in some cases, but normally the disease runs its course and the horse recovers with no enduring effects if the patient receives proper veterinary care.

The greatest loss from strangles is economic--loss of use of the horse, veterinary costs, and the cost of labor and supplies to implement stepped-up biosecurity measures--so keeping the disease from spreading to healthy horses and ridding your premises of the organism that causes the disease are the key battles in the war against strangles.

The first clinical signs of strangles occur after an incubation period of three to 14 days. Sudden onset of fever to 103¡ or higher is followed by a heavy nasal discharge and swelling of the lymph nodes under the jaw. The main sites of the infection are the lymph nodes, guttural pouches, and sometimes the sinuses. Swallowing may be painful, making the horse reluctant to eat or drink. Horses treated with antibiotics--the drug of choice is penicillin--at the first sign of fever may not progress to the characteristic abscess of the swollen lymph nodes.

Abscesses that already have begun to form may resolve sooner if they are allowed to "ripen" and drain without interference from antibiotics. In this situation, the horse is given supportive care of anti-inflammatory drugs, fluids, electrolytes, and vitamins, while application of hot packs or drawing salves encourages abscesses to drain. When drainage begins, antibiotics are administered through the remaining course of the disease, which lasts about two weeks.

In extreme cases when swollen lymph nodes compress the airway and impair breathing, it may be necessary for a veterinarian to perform a tracheotomy.

In very rare cases (about 0.4%), the disease may spread to the horse's lungs, liver, kidneys, or brain. Called "bastard strangles," the condition may be caused by a particularly virulent strain.

"People shouldn't panic about this disease because it's a bacteria, not a virus, and bacteria are susceptible to antibiotics," said Robert Holland, D.V.M., Ph.D. A practicing veterinarian in Kentucky who specializes in infectious diseases, Holland has tested more than 1,500 horses for strangles and has treated more than 600 cases while helping to eradicate recurrent outbreaks of the disease on 15 large farms.

How the disease spreads

Strangles typically is not spread by inhalation, but usually enters the horse's body through its mouth. Horses mingling together are at risk. Shared water sources and common tack used near or in a horse's mouth are the easiest avenues for spreading the disease. This often occurs when horses are in the incubation period or when an asymptomatic carrier (an infected horse that does not show clinical signs) is present because the trainer or farm manager and his crew presume their horses are healthy when clinical signs are absent.

Holland recommends that horsemen adopt certain biosecurity protocols at farms and on the track, not just when disease is present, but as standard operating procedure.

"It is very common at the track to have a common water bucket hung at the end of the barn to allow the horses to drink and then move on to cool out. That could be an excellent source to spread the disease in a barn," Holland said. "I would recommend that horses that cool out walk into their individual stalls to drink from their own water buckets or that their own buckets are hung outside their stalls for them.

"Water hoses should not be placed in the bucket when filling it with water because the hose could potentially spread the disease," Holland continued. "If disease is a problem in a barn, I routinely recommend that you throw the hose away after the infection has run its course and start with a new hose that never goes into the bucket when filling water."

Gathering all buckets together for scrubbing is discouraged. Buckets should be scrubbed individually with a brush soaked in disinfectant, then rinsed and returned to the respective horses' stalls.

Holland also suggested that bridles with bits, lip chains, twitches, tongue ties, nose rags, and any tack that is exposed to the horse's saliva or nasal discharge be disinfected with chlorhexidine (Nolvasan) solution and then rinsed thoroughly before use on another horse. Veterinarians should heighten their disinfection protocol for endoscopes and other instruments that could be a source for spread of disease.

Two types of strangles vaccines are available commercially: an intramuscular, killed-bacteria vaccine and an intranasal, modified-live bacteria vaccine. John Timoney, D.Sc., M.V.B., Ph.D., a veterinary microbiologist at University of Kentucky's Maxwell H. Gluck Equine Research Center who helped develop the intranasal vaccine, said both vaccines have problems.

"The intramuscular does not afford a high level of immunity," Timoney said. "It reduces the severity of the disease, but it doesn't prevent infection. The intranasal provides mucosal immunity, but since it is a live vaccine, it can cause fever [and sometimes] abscess."

Timoney said each situation should be assessed individually by the owner's veterinarian when making a decision whether to vaccinate and, if so, which vaccine to use.

Quarantine procedures

Infection with S. equi is confirmed by use of a highly sensitive polymerase chain reaction (PCR) test, which detects the organism's DNA sequence in mucus flushed from the horse's nasal cavities, or a culture of the nasal discharge.

On premises where strangles is present, Holland urged horsemen to have their veterinarians examine and perform cultures and PCR tests on horses. The veterinarian then can determine which horses should be segregated from the general population into three quarantine groups:

• Red isolation--horses with abscesses or fevers greater than 1.5¡ above normal;
• Yellow isolation--asymptomatic carriers that test positive for S. equi; and
• Green isolation--horses that return two negative tests taken one week apart.

Each horse in the red group should be housed in solitary with strict biosecurity that includes people using disinfectant foot baths, coveralls, and disposable gloves. Equipment used with these horses should be confined to the red area, and manure should be disposed of separately and composted to kill the organism, not spread on fields.

Horses in the yellow isolation area may be housed in adjoining stalls, but strict biosecurity should be observed. Horses in the green area are permitted to mingle freely.

Holland urged that each groom be restricted to caring for horses within a particular isolation group, or if that is not practicable, grooms should care for sick horses only after they are finished caring for healthy ones.

"If you have sick horses, don't start moving around all the horses that don't appear to be sick because an asymptomatic horse may not show signs yet [incubation period], and when you move the carrier, you could spread the disease to other susceptible animals," he cautioned. "I recommend taking temperatures two to three times a day on horses that are near sick ones, as this is an early indicator of infection."

Disinfection of premises

A common misconception about strangles is that once an infection occurs, S. equi persists on the premises for ten years. But, according to Holland, recent studies have shown that the bacteria is viable for just three days in soil, seven days on a porous surface, such as a wooden fence, and 30 days in water. Periodic outbreaks of strangles on the same premises have been attributed to asymptomatic carriers that harbor S. equi, usually in their guttural pouches and sometimes in their sinuses, and then begin to shed the bacteria when they are under stress, sometimes for as long as five years after they are first infected with strangles.

Holland offered a plan to eradicate S. equi from premises that includes disinfection of water sources, housing, and equipment, plus testing to uncover and treat asymptomatic carriers.

Disinfection should begin with water sources--buckets, waterers, and troughs, which Holland calls "strep Jacuzzis." He recommends using a phenolic disinfectant (Tektrol, One-Stroke Environ) because phenolics have the ability to penetrate organic matter, such as globs of nasal discharge sneezed onto a surface.

When using a phenolic disinfectant, it is imperative that the surface be rinsed thoroughly afterward because this type of solution is harmful if ingested. Water sources should be emptied and scrubbed out using disinfectant at the dilution percentage recommended on the label. Rinse thoroughly before putting water sources back into use. Stalls should be thoroughly cleaned out and the manure disposed of separately and composted. When disinfecting stalls, start by rinsing the walls with water to remove surface debris. It is not advisable to use a pressure washer when disinfecting a stall since it will aerosolize bacteria into lights, ceiling, and hard-to-reach surfaces, according to Roberta Dwyer, D.V.M., M.S., who specializes in veterinary preventive medicine at the Gluck Center.

Next, scrub the walls with a brush and phenolic disinfectant to penetrate organic matter. Rinse, then scrub with a soapy detergent. Rinse again. Allow the stall to remain open and vacant for seven days.

Anything that came into contact with the infected animal--feed tubs, hay racks or nets, stall toys, gates, webbings, chains, tack, and equipment--also should be disinfected with a phenolic disinfectant and rinsed thoroughly.

On the farm, although it may be a Herculean task, Holland recommends spraying fences with disinfectant, as well, since some horses may become reinfected by chewing wood or cribbing. Rinse thoroughly after disinfection.

S. equi are destroyed by exposure to the sun, so fields that housed infected horses should be mowed and allowed to remain vacant for seven days.

Detection of carriers

All horses that may have come into contact with ones confirmed to have strangles should be tested for the presence of S. equi, even if they are not showing clinical signs. These include horses that shared a groom, a rider, or equipment, or were transported in the same trailer or van. Holland reports that problem farms that have undertaken this task have discovered 30% to 69% of their horses were asymptomatic carriers.

"We are now learning about asymptomatic carriers, thanks to work done by Dr. [J. Richard] Newton at Animal Health Trust in Newmarket," Holland said. "Most of the time these horses go through an acute phase where they have swollen lymph nodes, fever, and nasal discharge. But you can have a horse that has a lymph node that bursts into the guttural pouch and not down, so you don't see much from the outside of the animal. That's what happens in a lot of these outbreaks. People move horses that they don't think are showing any signs when, in actuality, they are moving the carrier."

Targeting the guttural pouches is the most effective way to detect carriers. A veterinarian will use an endoscope to visualize the guttural pouch and obtain a sample for testing. If the horse is found to have S. equi in one or both of its guttural pouches, the veterinarian will flush out the mucus with saline solution and infuse an antibiotic. Horses are then treated systemically with penicillin for seven to ten days, followed by sulfa antibiotics (SMZ, Tucoprim, Tribrissin) for 14 days.

Horses should be retested after treatment has been completed and weekly thereafter until three negative PCR tests confirm the horse is free from S. equi.

Holland said it is a good practice for farms with recurrent outbreaks of strangles to quarantine all newcomers to the farm, including horses that are returning from other premises, for a minimum of two weeks during which time they should be tested for S. equi. Administration of antibiotics should be suspended for one week before testing because antibiotics could suppress the bacteria enough to cause a negative test result.

"What has happened to me in the past is that I have cleaned up a farm, but maybe they haven't tested all the mares that have returned from their owners the next year," Holland said. "Sometimes the carrier is in that group of returning mares. So all horses coming onto the farm need to be tested."