Often when diagnosing this disease the first question is "How could my horse have gotten this?" Well, you couldn't have picked a better way than to run into your friend and her horse at the fair.
Strangles is a bacterial disease that causes abscesses in lymph nodes in the head, jaw and throat areas. Occasionally the bacteria can travel into the body and cause abscesses in the chest or abdominal cavity, which is termed "bastard strangles." The bacteria that causes strangles is called Streptococcus equi. S. equi has a cousin called Streptococcus zooepidemicus that can also cause abscesses but is not nearly as potent. this is important because some horses can develop what looks like strangles but is really just a watered down version caused by S. zooepidemicus.
Strangles is characterized by swelling and abscesses of the intermandibular and retropharyngeal lymph nodes, fever, depression, anorexia, draining pus from the abscesses or from the nose and possibly swollen legs. The route of infection is through the nose. The disease is commonly transmitted by horses touching nose to nose, getting the bacteria from an object and then touching another horse, drinking from contaminated waterers, or via flies. Once the bacteria gets inside the horse's nose, the lymph nodes (glands) that protect the head from infection can become overwhelmed by the bacteria forming abscesses in between the jawbones, underneath the throatlatch or internally around the pharynx (voice box area). The abscesses get very large before rupturing. The term strangles was derived from abscesses around the voice box becoming so large that they would literally strangle the horse. Luckily, this rarely happens.
Early signs of strangles include a lack of appetite with fever and a small, painful swelling under the jaw. The swelling under the jaw is the abscesses forming, so heat in the form of a hot compress is applied to the area to encourage bacterial growth.
As the abscesses mature, the scar tissue that covers them becomes thinner. A mature abscess that is ready to lance starts to feel soft in the center, the hair covering the area falls out and the skin often dies, and pus starts to leak out. At this point, an abscess can be lanced without much pain to the horse.
Since abscesses are usually walled off with a thick layer of scar tissue, it is difficult for antibiotics to fully penetrate and kill all the bacteria, so typically antibiotics are not started until the abscesses have matured, open and are draining. Once an abscess is opened, it is flushed to get all the bacteria out and antibiotics are started. Penicillin or penicillin-type drugs have been the traditional treatment of choice for this bacteria. Sometimes multiple abscesses are developing at different stages, so antibiotics are not started until all the abscesses are open. The length of treatment and disease varies greatly depending on the immune status of the horse, strength of the bacterial infection and the quality of care. Often the disease runs its course in two to four weeks, but it's not uncommon for abscesses to linger or reform weeks later.
Strangles is also highly contagious. The most infective stage for transmission is when the abscesses are draining, but horses are contagious before that as well. Often the disease will spread throughout a barn. Sometimes a horse can have a subclinical infection, meaning he carries the disease but doesn't show signs of it. These horses are the ones that spread the disease most because no illness is apparent, so precautions aren't taken.
This is where we get on the soapbox about your friend and her horses. She should definitely quarantine all horses at her facility, as well as minimize her exposure to other horses. If it's necessary for her to meet with other horses, she should decontaminate herself by wearing clean clothes, washing her hands with Betadine scrub, and changing her shoes (shoes are a great way to spread the disease).
Vaccines are available and work best if given before an outbreak. During an outbreak, vaccination is debatable since it will make some horses sicker if they are trying to fight off the disease. Subclinical horses can be tested by checking antibody levels in the blood. High antibody levels in an apparently healthy horse indicate that he has been exposed to the disease and is either fighting it off and shouldn't get the vaccine, or has enough antibodies to ward off illness.