Rocky Bay Equine Newsletter, January 2019

a bug with a very long rap sheet

Strangles, or equine distemper, is a bacterial infection in horses caused by Streptococcus equi subspecies equi generally known as S.equi. These bacteria have infected horses of all ages, all over the world, for centuries. First reports can be found as far back as 1251. Wow, that's a long time! Who is this bacteria and what makes it so masterful as an infective agent? What can we do in 2019 to minimize it’s impact on our own animals?

The S.equi bacterium predominantly affects the respiratory and lymphatic systems. It produces fever, pharyngitis and nasal discharge. It is common for submandibular lymph nodes (located in the space between the lower jaw bones) and retropharyngeal lymph nodes to become swollen and form abscesses which may or may not drain externally. The name, “strangles” comes from these enlarged lymph nodes pressing against and obstructing the upper airway. While most horses recover without problems, the bad news is that the disease is highly contagious and can spread rapidly to affect lots of animals. Other bad news is that there are significant variations and complications of the disease that can occur, making this infection one to avoid if at all possible. Variations of the disease include infection of deep lymph nodes in the throat, in the abdomen or in other organs that may abscess within the body where they cannot drain externally. This bacteria may also cause chronic guttural pouch infection and it can affect the immune system causing purpura hemorrhagica, lymphangitis (inflammation of blood or lymph vessels with head or limb swelling) or myositis (muscle pain).

Transmission of the bacterium occurs via direct (horse to horse) or indirect contact with infected nasal/ oral/ ocular discharges. Indirect methods of spread include sharing contaminated buckets, tack, brushes etc. and humans handling both sick and non-infected horses in the barn. Incubation, from exposure to first signs of disease (fever) is between 3 and 14 days. Infected horses can transmit disease within a day or two of the first onset of fever and may continue to be infectious for 6 weeks or even longer if they have a residual guttural pouch infection and/or if they become subclinical carriers. The bacteria can remain active in water for 4 to 6 weeks but it survives only a few days in soil or feces. 

Uh oh, it's too quiet, Bubba the buckskin isn't banging his feed bucket. He's hanging his head over there in the corner. He won't even look at this carrot, this is bad. You take his rectal temperature and discover that he's running a fever of 103. Now what? While you've got a call in to your veterinarian, the number one, most important thing for you to do as a farm manager is to immediately isolate this sick animal from any other horses and employ pre arranged bio security measures. (You have those already set for your barn, right? If not, make it your first new year’s resolution to develop a bio security protocol.) The next step is to call your veterinarian to make a definitive diagnosis and to implement individualized treatment and management plans for poor Bubba. Treatment is not always straightforward with strangles. Antibiotics given at certain stages of the infection may actually be contraindicated. They also may prevent the horse from developing it’s own immune response to the organism, leaving it at risk for reinfection. Strangles is a reportable disease in the United States. Movement of horses on or off the farm should be stopped and healthy animals should have their rectal temperature monitored twice daily. Infected animals generally don't start shedding bacteria until about 24 hours after the first fever episode, so this close monitoring allows any new cases to be identified and quarantined before exposing even more stablemates.

Yikes! Can’t I just vaccinate for this disease? Yes, there are a couple of vaccines against S.equi. However, it has proven tricky to administer them effectively without undue side effects. The intranasal modified live vaccine provides best local immunity but it does not guarantee disease prevention, only some amelioration of disease severity. Vaccination is usually contraindicated in the face of active infection or in horses with high antibody levels due to increased risk of adverse reactions. Deciding whether or not to vaccinate your own healthy horse depends on the odds of exposure, so it’s best to talk with your veterinarian for specific recommendations. Natural immunity, lasting from 2 to 5 years, occurs in about 75% of horses that have recovered from strangles. Oh, and up to a whopping 50% of horses that have had strangles may become asymptomatic carriers of the disease. These animals can intermittently shed bacteria any old time, causing new infection seemingly “out of the blue”. Ah hah, that’s it, the master bacterial survival strategy and why S.equ continues to afflict horses over and over and over, for centuries!

So, come on it’s 2019, a long way from 1251, there must be something else we can do to thwart this diabolical bacteria? Well, yes there are a couple of newish approaches to strangles management. The first concerns establishment of a definitive diagnosis. Bacterial culture used to be the method of choice for identification of active infection but it’s relatively slow and problematic to collect adequate samples in a timely fashion. A new approach includes testing for the DNA of S.equi bacteria through a technique called polymerase chain reaction, or PCR test. This test is very sensitive (about 3x more sensitive than culture) but it doesn’t differentiate between dead and live bacteria so false positive results are possible when trying to confirm active infection. (Note; This technology is ongoing in development, with qPCR the current best assay.) The second development is really very exciting, it involves serologically sleuthing out those sneaky subclinical shedders! (Seriously!?) Yes, serology has given us a tool for identification of carrier animals using the SeM Antibody Titer (an indirect ELISA test) which measures antibody reaction to a specific protein unique to S.equi. It’s a huge advantage for the veterinary detective to be able to use a simple blood sample to identify early infection or carrier animals. Identifying healthy carrier horses that may pose an unexpected infection risk to other horses is a game changer for effective management of this disease. The testing is currently in use at some farms when any new horses are to be introduced to an existing herd.

When it comes to strangles, the bottom line is still all about prevention as the best medicine. Limiting exposure is the best preventative. That means biosecurity measures including quarantine, disinfection, hygiene and screening of new arrivals are the gold standards of good horse keeping. It’s understandable that these measures must be developed specifically for each unique farm/stable situation. They may not be perfect, but any plan is better than no plan for both prevention and for management should a strangles (or any other contagious disease) outbreak occur in your barn. Strangles will be with us for the foreseeable future but in 2019 we do have a few new tools to limit it’s impact and spread.


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