AAEP 2002: Infectious Disease/Epidemiology Sunrise Session

There was tremendous interest in and opportunity for information exchange on the topic of infectious diseases during the AAEP Convention. Early Friday morning, Maureen Long, DVM, PhD, Dipl. ACVIM, of the University of Florida, and Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, of Colorado State University, headed a session to discuss relevant topics and answer questions.

One of the first topics was clostridia diseases (specifically Clostridium difficile and Clostridium perfringens), which have been associated with inflammation in the digestive tract that can lead to diarrhea in foals and in adult horses.

The group discussed the potential benefits of using "friendly" yeasts or probiotics to help prevent diarrhea and treat horses with diarrhea. Long said all diarrhea cases at the University of Florida hospital receive probiotics.

Part of the difficulty in using probiotics is not knowing if the organisms that they contain are active or alive, or if what is on the label is actually what is in the product. There also was some discussion that, based on information presented at last year's AAEP Convention by Scott Weese, DVM, DVSc, Dipl. ACVIM, of the University of Guelph, the type of organisms in the probiotic is very important. It was mentioned that Enterococcusspare now being avoided in probiotics for humans because they might be prone to promoting acquisition of antimicrobial resistance genes and thus should be avoided. Lactobacillus (particularly L. rhamnosis) is the preferred ingredient for horses.

Traub-Dargatz said some probiotics aren't efficacious in the horse, but there has been little research done on what works best. Also, there is no probiotic on the market that has been tested in a controlled trial or can match the complex make-up of the normal gut flora. So while the concept of having "good" bugs out-compete potentially "bad" bugs makes sense hypothetically, the industry is a long way from knowing if this will actually be feasible in the foal or adult horse. Much work needs to be done to determine if this will be feasible or not.

There also was discussion on the potential to have Clostridia become resistant to an anti-microbial used to treat it called metronidazole, and the fact that this has been reported in some settings. Further work is necessary to see how relevant this is in the effective treatment of clostridial enterocolitis in equine patients.

In discussing Salmonella, it was noted that in the clinic, a high heat index which is the combination of high environmental temperature and high humidity has been associated with an increase in equine cases. There is the possibility for "blooms" of Salmonella, a bacteria that has been associated with diarrhea in horses that increase the population of bacteria in the environment in hot weather, and there is the thought that heat stress might make horses more prone to Salmonella infections.

In discussing outbreaks of diarrhea in barns and clinics, Traub-Dargatz strongly recommended that you should do a good job of cleaning the facility and equipment used to care for horses well first, then worry about disinfecting. She said there also is concern about the environmental impact of using phenolics to disinfect so it is important whenever using disinfectants to read the label and follow the directions exactly as written for the use of the product and comply with all environmental protection agency regulations for their use

It was pointed out that there hasn't been a new antimicrobial (antibiotic) class developed since the 1980s. Traub-Dargatz said that makes it important to have judicious use of antibiotics to try to minimize the development of resistant strains of bacteria. Her recommendation: Refer to the judicious use of antimicrobial drugs guidelines developed by the AAEP and available on the AVMA website.

West Nile virus (WNV) has been an extremely important topic during the convention. One project that Colorado State University is doing is following up on this year's WNV cases in Nebraska and Colorado. Veterinary students from the university are calling owners who had confirmed cases and getting specific follow-up information on the horses, such as whether they were vaccinated against WNV and when, did they live or die, clinical signs, and if the horse became recumbent. The information will be analyzed at the university and the students are getting not only an important introduction to epidemiology but honors credit for the work (which is estimated at 900 or more hours of work on the students part).

One of the things all attendees seemed to agree upon is that there is underreporting of the number of horses which had WNV. Traub-Dargatz said that in visiting farms in Nebraska to draw blood samples during the outbreak, the owners would indicate that one or more other horses on the farm had similar symptoms but they did not have diagnostic testing done once they had one horse on the farm with similar clinical signs diagnosed as WNV.

Part of the WNV discussion centered around mosquito abatement. Long said there aren't true mosquito abatement programs in many parts of the country, and if present, are mostly aimed at urban areas to control mosquitoes around human populations.

Methods of mosquito control that don't depend on pesticides are using aerators in ponds, putting certain types of fish that eat mosquito larvae in water tanks or at least changing the water every week to 10 days, putting fans in barns so they blow over the horses (mosquitoes are poor fliers and do not like breeze or wind), and cleaning up any areas of standing water, including gutters as well as areas with organic material such as manure piles.

Even though WNV hasn't been seen in horses yet in California, it is predicted that the disease will be there shortly. Long also warned that it is possible that because of the deep canyons in some of the mountainous regions, there could be year-round mosquitoes and therefore a year-round WNV threat.

The question of whether this virus is becoming more virulent, or has different strains in different parts of the country, has been raised on numerous occasions. While the experts in this room didn't feel there were different strains in the United States, there were environmental influences that could affect the spread of the disease, and thus make it seem like it was affecting the populations of birds, horses, and humans more severely in some areas.

Long said that she thinks over time, the resident bird population will become more resistant to the virus, which in turn will give the mosquitoes fewer places to pick up the virus. She added that with vaccination and natural exposure, the population of na•ve horses will be reduced over time.

The topic of vaccination of pregnant mares and foals was brought up. Long said that mares might need to be boostered late in gestation to increase the antibody level in the colostrum. Long also said she has seen disease in foals any time after four months of age. No one knows how long maternal antibodies will protect foals, nor do they know how long a horse is protected if it has natural exposure. Traub-Dargatz will be following horses which were naturally infected with WNV in 2002 and checking antibody levels in 2003.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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