Page 26 - New Mexico Horse Breeder, Fall

the classic scenario is that these horses are very sick and about 30 to 40% of them develop laminitis.
Any time a horse has a systemic inflammatory process, there is risk for laminitis. “Horses with colitis in general (and diarrhea) are at higher
risk just because they absorb endotoxin from bacteria in their gut, which can trigger laminitis. But horses with PHF seem at increased risk compared with horses that have other types of colitis. Their blood vessels tend to thrombose as well, which is a common problem when we are using IV catheters in treatment.”
“Laminitis tends to be the potential long- term complication of PHF, and usually ends up being the most severe problem. The infection itself is treatable with supportive care and antibiotics,” says Wilson. Thus, it is important to be proactive in trying to head off laminitis. These treatments are not foolproof, but it’s best to err on the side of trying to prevent it rather than deal with laminitis after the fact.
Prognosis
“Published studies looking at case fatality rates vary from 5 to 30%. There are different strains
of the bacteria, and this may have something to do with severity of clinical signs. The case fatality rate may vary with the strain involved. Most of the fatalities are due to laminitis and/or severe endotoxemia,” says Wilson.
“Usually if a horse recovers from PHF (even if it was just a mild case), that horse is then immune to this disease for up to 2 years; it will have pretty good natural immunity.”
Prevention
In regions where PHF has occurred, most horse owners should consider vaccination. “There is a vaccine available, but it is not as effective as we’d like. It doesn’t have a very long duration of immunity— perhaps a couple of months. There are different strains, and the vaccine only protects against one strain. In areas where there is high prevalence of the disease, we recommend vaccinating twice—if it’s not cost prohibitive for a farm with many horses. We suggest vaccinating in late spring, before summer fly season, and again mid summer, to give protection through the fall,” says Wilson. That way, you are vaccinating the horse in time to develop immunity but before the most risky time.
“The vaccine may not necessarily prevent the disease (though we think it sometimes does), but the thought is that it might reduce the severity of clinical signs. The vaccinated horse may have just a mild case rather than a severe illness that might be life-threatening,” says Wilson.
“Unfortunately, there has not been any update of the original vaccine. I don’t know if there is any plan for that. The problem with the vaccine is that in experimental studies when the vaccine was developed, it did seem to be protective, but in the real world, in clinical cases, probably all of us clinicians have seen vaccine failures—horses that were appropriately vaccinated that still get the disease,” she says.
“Possible reasons for vaccine failure are that there are different strains of the bacteria and only one strain in the vaccine, and the vaccine may not be as protective against certain strains as others. Also, some strains may have different effects on different horses in terms of severity of the disease,” says Wilson.
Immunity to the bacteria may be best achieved through actual natural infection rather than vaccination. The process of the bacteria infecting cells may trigger stronger immunity. “The vaccine is a killed product and doesn’t actually go through the process of infecting cells and stimulating the immunity,” she explains.
Most vaccines stimulate humoral immunity, which is development of antibodies that circulate in the blood. If the bacteria are transmitted into the blood first (as from a biting insect or tick) or spend time circulating in the bloodstream, the antibodies can attack and kill those pathogens.
“If the site of entry is the GI tract and the pathogen doesn’t immediately get into the bloodstream, a vaccine that targets the immune
Proactive treatment, such as ice boots, supportive pads for feet, along with IV fluids and anti-inflammatory drugs, can help prevent laminitis problems.
system the bacteria are exposed to before they get into the blood (cell-mediated immunity) might be more effective. We don’t have very many vaccines that do this very well, however, for GI tract infections,” she says.
The vaccine recommendations haven’t really changed. Current recommendations are for twice a year, but most people actually do the two doses within 3 or 4 months (with the first dose before the typical disease season) rather than every 6 months. Since this disease is seasonal, it makes sense to vaccinate once in the spring just ahead of the fly season and again in the summer to extend immunity through the fall.
“The immunity obtained is variable and short-lived, so giving the vaccine every 6 months may not provide immunity for the whole year. The important thing that horse owners need to understand, about any vaccine, is that there are limitations to their effectiveness, especially this vaccine. It may help prevent the disease and it may help reduce the clinical signs if a horse is infected, but there is no guarantee it will do any of that for any individual horse,” says Wilson.
Horse owners are trying other methods to protect their horses, such as keeping them from grazing in areas near streams or standing water where there might be snails. Other ways the disease might be prevented is to reduce horses’ exposure
to aquatic insects. “One well-known farm was able to reduce the incidence of PHF in their horses by turning off their outside lights at night. It may also help to control insect populations around barns. It’s not an easy disease to prevent,” says Wilson.
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New Mexico Horse Breeder
since this disease is seasonal, it makes sense to vaccinate once in the spring just ahead of the fly season and again in the summer to extend immunity through the fall.