With West Nile virus (WNV) marching across the United States and affecting both horses and humans, infectious disease prevention has been a hot topic in barns, the media, veterinary clinics, and elsewhere. During the Infectious Disease forum, moderated by Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, professor of Equine Medicine at Colorado State University's Veterinary Teaching Hospital and Biomedical Sciences Department in Fort Collins, Colo.; and Steve Conboy, DVM, a private practitioner in Lexington, Ky., three infectious diseases were discussed by attendees--Clostridium difficile-associated disease, WNV, and strangles.

Clostridium difficile-associated Disease
Traub-Dargatz started the discussion with whether people recognize disease associated with C. difficile in clinical settings. C. difficile is a gram-positive anaerobic bacillus that can cause disease in both foals and adults. Typically C. perfringens and C. difficile both cause diarrhea in foals;, however, diarrhea caused by C. perfringens is more common than C. difficile and has a much higher mortality rate. C. difficile can be very difficult to culture, therefore it might not be diagnosed based on culture of feces from ill animals.

Attendees discussed methods of diagnosis for C. difficile. Attendees agreed that identifying the toxins produced by the bacterium in the feces in horses with C. difficile is the optimum diagnostic method. If a veterinarian suspects a horse has C. difficile-associated disease, he will work with a diagnostic lab to determine the most appropriate testing. Toxin testing is available through veterinary laboratories.

There is concern with testing for the actual organism because transport of the very fragile organism of longer than a few hours might prevent it from growing in the lab. C. difficile got its name because it is difficult to grow in the lab. However, if the bacteria is successfully grown, then diagnosticians can try to determine which antibiotics will kill the organism and help treat the horse. Sometimes a test can come back positive for the organism, but that particular type of C. difficile might not be producing toxins and is therefore not causing disease. So, the best test to detect C. difficile as a cause of disease appears to be the toxin assay on feces or intestinal contents. If the toxin is found , but the organism is not, then many veterinarians still consider this a definitive diagnosis for C. difficile. Another test used for further determining pathogenicity of the bacteria is a polymerase chain reaction test (PCR) test, which can be performed to identify the genes that code for toxins.

Some cases of C. difficile in equids are associated with antimicrobial use, and the group discussed which ones might be more of a concern. If a horse being treated with antibiotics experiences diarrhea or goes off feed, Traub-Dargatz recommends calling the treating veterinarian immediately so that he or she can consider whether treatment should be continued or whether further diagnostics might be needed.

West Nile Virus Prevention
The importance of WNV prevention measures was a hot topic at this forum. Attendees had the following suggestions for prevention of this rapidly spreading disease.

Vaccination--Out of 460 cases in Florida, only 44 had received both doses of the vaccine, indicating a high success rate for the vaccine. Further investigation of these 44 cases is underway by Maureen Long, DVM, PhD, assistant professor of large animal veterinary medicine at the University of Florida's College of Veterinary Medicine, to determine how long it had been from time of last vaccination to onset of disease. Most cases nationwide included horses that which had not had both doses of the vaccine. Therefore, vaccination still remains an important way of preventing WNV.

Controlling mosquitoes--It was mentioned that the mosquito viral burden for all encephalitis diseases was higher this year than in any year since 1930. Researchers expect a high amount of activity for WNV to continue for the next two to three years. Therefore, mosquito control, also called mosquito mitigation, is imperative.

Horse owners should check with their veterinarian regarding what products they should apply to their horses to control mosquitoes, and they should follow label directions closely. Other methods of mosquito control include getting rid of standing water, using mosquito dunks (safe, small, donut-shaped larvicidal products that can be placed in water), and using pesticides. One attendee mentioned that in Ohio there were 210 cases of human WNV in one county because that county didn’t want to use pesticides due to environmental concerns. When officials realized that pesticides were necessary, it was too late. All agreed that mosquito control was as important as vaccination, and being proactive early in the spring can improve mosquito eradication.

Testing suspected cases--Now that the USDA considers WNV an endemic domestic disease instead of a foreign one, testing will likely no longer be free or at a very low cost. Horse owners can expect a cost of $10-$25 per test, (although this might vary from state to state), to see if a suspected case has WNV. (Testing in Canada has cost about $81 per test, according to Dr. Scott Weese, DVM, DVSc, Dipl. ACVIM.)

However, testing is important since horses act as a predictor for human cases. It was clarified in the forum that if an initial test came back negative from the lab and the veterinarian felt that WNV was still a likely cause of disease, then the horse should be retested eight days later since by that time antibody levels might then be high enough to cause a positive test.

Several web sites were recommended for more information on West Nile virus. They included:


Strangles Prevention
There have been several advances in veterinarians’ knowledge and options for the control of strangles. Some horses that which recover from strangles can continue to intermittently shed the causative bacteria Streptococcus equi for prolonged periods without showing signs of disease. These horses are called subclinical or asymptomatic shedders.

This chronic shedding phenomenon is now considered to be a main reason the disease recurs on some farms and how the disease may might be moved from one farm to another. Because these shedding horses appear normal, there is not a no way to recognize them based on clinical signs alone. For farms whose horses are part of a mobile horse population with likely exposure to respiratory pathogens, some attendees felt that screening tests might be a valuable option for preventing strangles outbreaks beyond vaccination of all horses and isolation of new horses on a farm. However, at a cost of $50 per test (with discounts available for multiple tests), for the most sensitive test for detection of Streptococcus equi, this option could be expensive. Owners must weigh the cost of the test versus the risk of getting disease on the farm and the impact of disease if it occurs (i.e., cost of treating horses during an outbreak). If it is likely that an infected horse could be unknowingly brought onto their farm, a screening test for chronic shedders of the bacteria that causes strangles might be considered.

Owners should consult their veterinarian about whether screening tests might be an option for prevention or control of strangles on their farm.

Veterinarians in the forum discussed a variety of ways to get samples for the test, and they also emphasized that using proper procedures were was important while when getting a sample since improper techniques could affect results. For instance, a twitch used on multiple horses might have enough dead DNA to trip the test. In addition, gloves must be changed between horses and any lip chains that are used must be cleaned.

Veterinarians also discussed whether owners should notify farm owners about a recent strangles outbreak who if those owners might be going to receive horses from their farm and thoseor are bringing horses onto their farm about a recent strangles outbreak. There tends to be a negative stigma attached to a farm that has had a strangles outbreak, and this can economically hurt or cripple an equine business. However, the consensus seemed to be that it is best to have open communication so that prevention methods can be implemented. For instance, if a horse is leaving a previously infected farm, he can be tested for shedding of the bacteria that causes strangles before leaving, and then monitored at the new farm for any signs of disease.

About the Author

Sarah Evers Conrad

Sarah Evers Conrad has a bachelor’s of arts in journalism and equine science from Western Kentucky University. As a lifelong horse lover and equestrian, Conrad started her career at The Horse: Your Guide to Equine Health Care magazine. She has also worked for the United States Equestrian Federation as the managing editor of Equestrian magazine and director of e-communications and served as content manager/travel writer for a Caribbean travel agency. When she isn’t freelancing, Conrad spends her free time enjoying her family, reading, practicing photography, traveling, crocheting, and being around animals in her Lexington, Kentucky, home.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from TheHorse.com. Learn More

Free Newsletters

Sign up for the latest in:

From our partners