Strangles is caused be a bacterium called Streptococcus equi. It is usually transmitted via nasal secretions and can survive in water for 1-2 months. It has an incubation period of 7-12 days in the horse, so after being infected it will take this long before symptoms will be seen. Horses aged 1-5 years are mainly affected.
The source of an outbreak can be a clinically infected horse or a symptomless carrier (this may be an infected horse that has recovered). Most horses recover after a month but some of them will go on to become carriers.
The symptoms are swelling and infection of the throat region. There is usually a fever of 102- 106 degrees. The horse may have nasal discharge and a cough. They may have depression and be unwilling to eat. It also causes enlarged lymph nodes in the pharyngeal region (in the throat) and other complications. The horse may find it difficult to swallow. After 7-10 days the lymph glands can rupture release thick yellow pus.
The diagnosis can't rely on the symptoms along as they are not specific to strangles. They send away swabs to the laboratory to get confirmation. the swabs are taken from the nasal cavity or from an abscess. A toxin is produced by the bacteria that breaks red blood cells down, in the lab they may look at the ability to ferment other sugars to tell it apart from other bacteria.
PCR may be used which is a diagnostic test that looks at the DNA of the bacteria to tell it apart from others. However, there is another species that is 90% the same. The primers used in this process are also important. This method can also tell the bacteria apart from the type the animal is given in a vaccine.
ELISA is another method which could be used. This looks at horses and the immune response against organism. This is quick and easy to do lots of horses.However, it takes 14 days for the immune response to develop so it may not work before this time. It may also give false positives from horses that were infected but are not any more.
The treatment is to use penicillin but the abscesses must be drained. There is a low mortality. They will need to take repeated swabs from the horse and need 3 negatives before they get the all clear. A guttural pouch wash may be done to detect carrier horses as this is where they carry the infection. Antibiotics may be given to healthy horses that have come into contact with infected horses. Rest is important along with the prevention of spread of the disease to other horses.If an abscess bursts, it is important to keep the wound clean. New horses moving onto a farm should be isolated for 4-6 weeks. Infected horses should immediately be isolated and stables should be infected along with water and feed buckets.
Vaccination is possible if the disease is persistent. The vaccine is given in the upper lip. Both intramuscular and intranasal vaccines are available. Most horses develop an immunity during recovery from strangles, which persists in over 75% of animals for 5 years or longer. This shows that immunity from a vaccine is possible if the right vaccine is developed.
Hope that was interesting! This information is from a variety of sources and unfortunately I have not written in my notes where it came from!
On our farm horses are not isolated when they move on which is an important factor and I think should be carried out. After our farm became infected with strangles, some isolation stables were made and used for a while but as the farm got busier these stables became full time stables for horses that had been there for a while.
The riding school horses were mainly affected when we had the infection. One of the new ponies they had could have been a carrier. We had a field with infected horses in and another with the non-infected horses. We thought Trooper's neck was swollen one day so they vet said to put him in with the infected horses. It turned out he was fine but he wasn't allowed to move back into the other field after he had been in with the infected ones. He never caught it though so maybe he was already immune.