Strangles
D. Craig Barnett, DVM
Intervet, Inc.
Also called distemper, barn fever, and equine shipping fever, strangles is caused by the highly contagious
bacterium Streptococcus equi (S. equi). This organism is similar to the bacteria that causes strep throat in people but affects only members of the horse family. This highly contagious disease of the horse’s upper respiratory tract is one of the first horse diseases described in early veterinary science publications. Today
strangles continues to rank among the three most significant respiratory diseases of the horse. Although the dreaded word “strangles” brings fear to all who hear it, in the majority of cases recovery is uncomplicated and horses seldom die.
Strangles is however a very contagious disease with a high percentage of horse on an infected farm becoming sick. The disease is especially aggressive in populations of foals and young horses. Although strangles can occur in horses of any age, it is most common in horses less than 5 years of age with a heightened incidence in
individuals from 4 months to 2 years of age. Foals less than 4 months of age are often protected from the disease by antibodies contained in the colostrum they receive from the mare. In addition to age, other risk factors include increased population density and increased movement of horses on and off the premises.
TRANSMISSION AND INFECTION
The disease is transmitted by ingestion or inhalation of the S. equi organism. Transmission can occur by direct contact via an infected horse shedding the organism from the nasal cavity, oral cavity, or abscessed lymph nodes. Transmission may also occur indirectly through contaminated feed and water buckets, grooming
equipment, tack, stall surfaces, bedding, etc. In addition, flies may serve as mechanical vectors and transmit infection. Individuals such as veterinarians, farriers, trainers, and barn help working in an infected facility may also transmit the organism.
Contrary to popular belief, the organism does not survive very long in most environments. The organism requires water, protection from sunlight, and protection from other organisms to survive. The organism may however
remain viable for longer periods of time (weeks) in frozen mucoid discharge. A study conducted by Dr. Hugh Townsend of the Vaccine and Infectious Disease Organization at the University of Saskatchewan revealed that the organism lived on average of 3 days in the dirt, 7 days on a fence and for 30 days in water. These time frames are obviously quite variable depending upon the temperature, humidity, the amount of sunlight, and competition with other organisms. A dark, cold, moist environment free of other organisms is the optimum conditions for the bacteria to survive. Under ideal circumstances, the organism survives 7 to 9 weeks in the environment. In contrast, S. equi is highly unlikely to survive for very long in a hot, dry environment with ample sunlight. With this in mind, one should consider that communal drinking sources play an important role in the transmission of infection not only because of contamination by nasal discharges, but also because of the ability of the organism to survive for longer periods of time in the water. In most situations, with the exception of drinking water, the environment is probably not a significant source of S. equi except during the outbreak and for a few days thereafter.
However, since the organism can live for up to 7 - 9 weeks under ideal circumstances, it is probably best to consider facilities occupied by infected horses to be a potential source of contamination for 2 months after resolution of the outbreak. Fortunately, S. equi is susceptible to most antiseptic cleaning agents. Cleaning facilities and equipment with phenolic compounds such as Lysol or Tektrol will help kill the bacteria and reduce transmission of the disease.
Although the S. equi organism has only a short survival time once it enters the environment, horses that are “asymptomatic carriers” of the disease (horses that are infected and shedding the organism but show no signs of the disease) can continually shed the organism in the environment and to other horses. In a study evaluating
horses from twenty two farms with outbreaks, it was determined that following an outbreak, 68% of the farms had at least one horse that shed S. equi for 4 weeks or longer after clinical signs of disease had resolved. After recovery from strangles, a significant number of horses continue to harbor and shed S. equi. for several weeks. For this reason, exposed horses or horses that have recovered from clinical disease should not travel to other
facilities for at least 1 month after exposure or recovery from disease. In a small percentage of these asymptomatic carrier animals, the infection may persist for many months with intermittent nasal shedding of the infective organism with no apparent outward signs of infection.
These asymptomatic carriers may be very important with regards to maintaining an infection on an endemic premises. If you have a situation where strangles continues to recur, you very well may have an asymptomatic carrier somewhere in the herd and should seek veterinarian assistance to identify this individual. Asymptomatic carriers may also be very important with regards to introducing S. equi onto a premises previously free of the disease. Outbreaks are often initiated by introduction of an asymptomatic carrier that is incubating the disease or shedding organism during the recovery phase.
CLINICAL SIGNS
Clinical signs of strangles infection will vary depending upon the immune status of the horse and the number of organisms the horse is exposed to. Clinical signs ofstrangles may be seen as early as three days or as late as 14 days following exposure. The earliest signs of strangles are fever, depression, and loss of appetite. Once the infective organism enters the horse’s body through the nose or mouth, it moves to the back of the throat and
invades the lymph nodes in that area. At this point the horse will have a sore throat and may have some difficulty eating. The organism continues to multiply and invade lymph nodes of the throat, head, neck, and jaw. The infected lymph nodes enlarge and become hot, swollen and painful. The swollen lymph nodes in the back of the throat (retropharyngeal lymph nodes) may become so swollen that the horse has trouble breathing and appears
to be “strangling”, thus the name strangles.
These swollen lymph nodes may also inhibit the horse from eating or swallowing and the horse may salivate excessively. As the disease progresses the lymph nodes abscess and rupture releasing thick yellow pus consisting of dead white blood cells and the S. equi bacteria. The lymph nodes under the jaw (submandibular
lymph nodes) generally rupture to the outside with subsequent drainage under the jaw while the retropharyngeal lymph nodes in the back of the throat rupture into the throat area with subsequent drainage of the thick, yellow pus from the nose. Abscesses usually rupture and drain within 1 to 2 weeks after the onset of dis--ease. In the
majority of cases, horses recover quickly and completely after rupture of the abscesses with the entire course of the disease lasting three to six weeks. Unfortunately, complications can occur in a smallpercentage of infected horses.
COMPLICATIONS
In a small percentage of horses, the S. equi organism spreads or metastasizes past the lymph nodes of the throat and localizes in other organs or lymph nodes elsewhere in the horse’s body. This is referred to as “bastard strangles”, the presence of S. equi abscesses at locations other than the head and throat areas. In these cases the abscess may locate in the lungs, the brain, the joints, or internal organs such as the intestine, spleen and
kidney. Depending upon the location of these abscesses, these horses may show signs of colic, recurrent fever, lameness, localized pain, chronic weight loss, respiratory distress, or neurological signs.
Another potential complication of strangles is Purpura hemorrhagica. Purpura hemorrhagica is an allergic type
of disease that develops in certain predisposed horses after exposure to the S. equi organism or after vaccination with a strangles vaccine. When predisposed animals are exposed to the vaccine or the bacteria, their immune system produces an abundance of S. equi antibodies. The antibodies combine with the bacterial components and lodge in small blood vessels with subsequent damage to the blood vessels causes them to become “leaky”, resulting in extensive edema and swelling (often of the pharynx or inner throat, legs, sternum or midline, and the
head). This condition is most common in horses that have had previous exposure to the S.equi organism via infection or vaccination. The condition is also more common in asymptomatic carrier horses. Clinical signs of Purpura generally appear 4 to 6 weeks after subsequent exposure to the bacteria or vaccination. Although very rare, this condition is very serious and potentially life threatening. If your horse starts showing clinical signs of
Purpura after exposure to strangles or after vaccination with a strangles vaccine, you should seek veterinarian attention immediately.
DIAGNOSIS
The majority of strangles cases are diagnosed based on clinical signs (fever, anorexia, painful swollen lymph nodes of the throat or under the jaw with subsequent formation of abscesses, andnasal discharge). The diagnosis is confirmed by culture and identification of the S. equi organism from samples taken from the nose, the pharynx (back of the throat), or an abscess. In order for asymptomatic carrier animals to be identified, your veterinarian may take nasal or pharyngeal (throat) swabs from all seemingly healthy horses and have the samples tested for the presence of S. equi.
TREATMENT
Infected horses should be kept in a warm, dry and dust free environment. Nonsteroidal anti-inflammatory drugs
(Bute; Banamine) are often used to help control the fever, take down inflammation, and reduce pain. Lymph
nodes that are abscessed can be hotpacked or poulticed to encourage rupture and drainage. If the abscesses
don’t rupture, your veterinarian may elect to lance the abscess to hasten the progression of the disease.
Ruptured or lanced abscesses should be flushed with dilute iodine solution for several days. Antibiotic therapy remains controversial. Some veterinarians prefer to aggressively treat with antibiotics while others prefer to let the disease run its course. Before giving antibiotics to an infected horse it is highly recommended that you contact
your veterinarian. Appropriate administration of antibiotics in the early stages of infection will generally clear the
infection. However, it should be recognized that because the antibiotics kills the bacteria, that the horse’s
immune system will not be exposed to the S. equi organism in a sufficient manner to stimulate a protective immune response and the horse will be susceptible to infection after cessation of the antibiotics.
IMMUNITY & VACCINATION
Immunity following recover from natural infection is strong but does not provide life long protection. Solid protective
immunity occurs in approximately 70-75% of horses after natural infection with these horses generally being protected for 2 - 4 years after recovery. Even if a recovered horse does not have complete immunity after infection, upon subsequent exposure the horse will generally have a milder form of the disease. This is also the case in horses that have been vaccinated. Although vaccination cannot completely prevent strangles, vaccination generally reduces the severity of disease if the horse does become infected. For at-risk horses, even partial protection is better than none. In other cases where the risk of disease is moderate to low, vaccination may not be recommended. You and your veterinarian should determine your horse’s risk of exposure and the need to
vaccinate.
MANAGEMENT AND CONTROL MEASURES
The following are guidelines for management and control of strangles;
Newly arriving animals should be isolated and observed for strangles for 4 weeks prior to admission to the resident population.
Horses coming from a known infected premises, should be isolated and have pharyngeal cultures taken. Three negative cultures at weekly intervals should be obtained prior to the horse being released from quarantine.
During an outbreak, affected horses should be promptly isolated.
Clean and disinfect the environment and contaminated equipment (tack, grooming equipment, water & feed buckets, stall area, etc.). Remember, S. equi lives the longest in water so special care should be taken to disinfect water sources such as buckets and troughs.
Rectal temperatures of incontact, exposed horses should be taken twice daily for 2-3 weeks. Horses with temperature elevations of 1.5*F (body temp of 101.5*F) or greater should be isolated. Because shedding of S. equi does not begin until about 2 days after the onset of fever, this measure is often very effective to help prevent further spread and transmission of strangles.
In an ideal situation, recovered horses should be cultured 4 weeks after recovery to determine if they may be asymptomatic carriers and still shedding bacteria.
Consult with your veterinarian regarding treating asymptomatic carriers.
Consult with your veterinarian regarding vaccination. Vaccinating during an outbreak is controversial and there may be an increased risk of post vaccination purpura hemorrhagica.
Vaccination of pregnant mares 4 weeks prior to foaling may be beneficial on endemic farms as this practice significantly increases S. equi antibodies in the colostrum and provides protective immunity to the foal for the first few months of life. Strangles will continue to remain a serious threat to horses. Although the disease is highly contagious with most horses on an infected premises becoming sick, in most situations sick horses will recover without death or complications. Good management practices and control measures can dramatically help decrease spread of the disease. Farms that have recurrent outbreaks of strangles should consider the possibility that they may by housing asymptomatic carriers and should seek veterinarian assistance.
