Older Horses Part 3: Vaccinations and Deworming

Older Horses Part 3: Vaccinations and Deworming

Photo: Anne M. Eberhardt/The Horse

Experts share opinions and research on guidelines for deworming and vaccinating aging equines.

Do older horses build immunity over the years, or lose it? Do they build a resistance to worms as time passes, or to deworming agents? With more equines than ever living long into their golden years, these are questions an increasing number of horse owners are having to face. Due to a lack of hard scientific research on older equids, the answers aren't cut-and-dry.

In this article, we'll share some of the more significant findings to date, as well as insights and opinions from veterinarians experienced in the field of equine geriatrics: Joseph Bertone, DVM, MS, Dipl. ACVIM, a professor of equine medicine at the Western University of Health Sciences' College of Veterinary Medicine, and Nicholas Frank, DVM, PhD, Dipl. ACVIM, an associate professor of large animal internal medicine at the University of Tennessee's College of Veterinary Medicine.

Does Aging Impact Immunity?

In humans, it's well-established that some aspects of immune response decline after 65 years of age, says Frank. This has been tied to increased mortality from influenza infections in the elderly. What's more, says Frank, a report from the Centers for Disease Control stated that infection was prevented in 70-90% of subjects less than age 65, but in just 30-40% of people over 65.1

How does this tie to horses? It's been suggested that a 25-year-old horse corresponds, in age-related changes, to a 71-year-old human.2 So, might horses also experience some declines in immunity as they age? Some research suggests the answer is yes.

For instance, one study comparing younger and older horses found that there was "a decreased gross response to vaccination with equine influenza"3 in the older horses. Bertone, in his book Equine Geriatric Medicine and Surgery, says that, "Old horses tend to lose efficiency in immune function with age." 3

A research report by Margaret M. Brosnahan, DVM, and Mary Rose Paradis, DVM, MS, Dipl. ACVIM, section head in Large Animal Medicine and Surgery at Tufts Cumming's School of Veterinary Medicine, states, "Immune system function of geriatric equids differs from that of younger horses at rest and after exercise."4

Brosnahan and Paradis go on to cite another study that found horses greater than 20 years old had lower total lymphocyte (a type of white blood cell that pays a central role in directing and coordinating immune response) concentrations than younger horses. Again, this suggests that these older horses might have an increased susceptibility to infection.4

Bertone and Frank agree that loss of immunity can sometimes be caused by diseases such as Cushing's syndrome (pituitary pars intermedia dysfunction), which tends to affect older horses more than younger ones. Years of exposure to inhaled allergens (dust, pollens, and molds) can also compromise the respiratory tract's ability to fend off infectious agents.3

Similarly, Bertone in his book cites the 2001 outbreak of West Nile virus, noting that analysis of the data showed a statistically higher number of older horses were confirmed as having the virus. "In addition, a statistically higher mortality rate occurred in these horses," he states in the book.3 Data from the 2002 outbreak also suggests that vaccine "breaks" (vaccinated horses becoming ill) might be more likely to occur in older horses.

Still, Bertone points out that there is no way to know if the results would have differed had the older horses been given an additional booster vaccination against the disease. Furthermore, both he and Frank emphasize that not enough research has been conducted on the topic of geriatric horse immunodeficiency overall. Thus, while findings so far might suggest reduced immune system function occurs with aging, research results are not substantial enough at this point to confirm it.

Vaccinating: Is More Better?

As a horse owner, how do you approach vaccination of your older horse? Frank suggests that you simply follow the same basic protocol you'd follow for any other adult horse: Vaccinate according to anticipated exposure.

"I do not increase or decrease the frequency of vaccination if the patient is older," Frank explains.

Adds Bertone, "The important thing to realize is that increasing the number of vaccines may actually compromise an older animal's immune function. So don't assume that more is necessarily better."

So, an older horse traveling to shows or kept in a boarding barn with horses that travel should probably be vaccinated against contagious respiratory diseases, including influenza and strangles. But one kept alone or in a small, closed herd might need just the basics.

Likewise, horses in southern states are at greater risk for mosquito-borne diseases such as West Nile virus throughout the year, because the mosquito season is longer. Therefore those horses might need to be vaccinated more often than what is generally recommended; the current regimen for the available WNV vaccines is either an initial one- or two-shot series, followed by yearly revaccination (one shot). It's important to note that the current WNV vaccines all have shown in severe challenge models to protect against the disease at 12 months, and there is no published evidence that they need to be vaccinated more frequently.

Frank believes Cushing's disease "may suppress their response to vaccination, so it may be justified to vaccinate more frequently in these cases."

Bertone agrees that using standard adult horse vaccination guidelines is a good starting point. But he also suggests that owners of older horses have their veterinarians take and test a blood sample before vaccination and one month after vaccination to gauge the horse's reaction to vaccination by looking at antibody titer levels. If the veterinarian thinks antibody levels are too low, he or she might decide that an additional booster could benefit your horse. But researchers caution that little is known about the relationship between antibody measured and protection in horses given annual boosters, and that they're not sure what protective levels are in most cases.

Bertone emphasizes exercising caution when considering increasing vaccination frequency, as more frequent vaccinations are not appropriate for every horse, nor are vaccines innocuous.

Does Age Affect Parasite Load?

Unfortunately, the question of deworming needs in older horses has been studied even less than vaccination requirements.

Frank notes that older horses are less likely to carry heavy intestinal parasite burdens. But, he adds, this might be more a reflection of housing situation than age, since many senior equines are housed individually or in small groups.

"There is no scientific evidence that resistance (to internal parasites) develops in old age," Frank says. In fact, he adds, "Some older horses have difficulty maintaining body weight, so we should try to minimize the parasite burden."

Furthermore, Brosnahan and Paradis, after an examination of horses aged 20 to 35, noted that these elderly equines "had a notable reduction in the ability to digest crude protein, phosphorus, and fiber, compared with findings in a group of younger horses." They speculated that tissue damage caused by intestinal parasites could have contributed to decreased digestive or absorptive processes in the large intestine.5

Cyathostomes (encysted strongyles, or bloodworms) are a specific threat for older horses, says Bertone. These worms can live for years in the horse's gut. Thus, he explains, if a horse is on a preventive deworming program that does not actually kill existing parasites, the cyathostomes can accumulate over a horse's lifetime, potentially leaving an older horse with a significant burden.

Should Your Deworming Schedule Change?

As with vaccinations, Frank recommends that you deworm your older horse on the same schedule you would follow for younger adult horses, or about every four to eight weeks. Unfortunately, Brosnahan and Paradis found that 15% of older horses were dewormed less than four times per year. However, they noted, "It was difficult to assess whether this deworming schedule was adequate because of the lack of information on parasite burden in those horses." 2

For cyathostome burdens, Bertone recommends using either Intervet's Panacur (fenbendazole) six-pack or Fort Dodge's Quest (active ingredient moxidectin) as an agent to eliminate the parasites. Then he recommends following up with Pfizer's daily Strongid C (pyrantel tartrate) or routine deworming with moxidectin to prevent future infestation.

A Need For More Research

Simply put, says Frank, "We need more information about older horses."

Brosnahan and Paradis agree, noting in one report that, "Demographic data and information regarding common diseases of horses that are greater than or equal to 20 years of age are limited, but will become increasingly important as this geriatric population increases."

Unfortunately, there are currently no good estimates of just how large that population is, and that is a limiting factor in securing research funding. "We don't even have an idea of how many horses over 16 there are in the country," says Bertone. Even registry figures aren't reliable, he adds, because owners aren't required to report a horse's death.

"It's difficult for pharmaceutical companies to fund research or drug development in this area because they don't know what the market is or what will be the return on investment," says Bertone. "If they put millions into drug research for older horses, how much will be purchased?"

Take-Home Message

The one undeniable fact is that the number of older horses in the United States is increasing, even if the total hasn't yet been tallied. Hopefully this trend will be a catalyst to spur additional research and, ultimately, shed more definitive light on the vaccination and deworming needs of our valued older horses. Meanwhile, the best advice is to work closely with your veterinarian to safeguard your senior's comfort and good health.


  1. Castle, Steven C. "Clinical Relevance of Age-Related Immune Dysfunction." Clinical Infectious Diseases, Vol. 31: 578-585, 2000.
  2. Brosnahan, M; Paradis, MR. "Assessment of clinical characteristics, management practices, and activities of geriatric horses." Journal of the American Veterinary Medical Association, Vol. 223, No. 1, 99-103, 2003.
  3. Bertone, J. Equine Geriatric Medicine and Surgery. Saunders, 2005.
  4. Brosnahan, M.; Paradis, M.R. "Demographic and clinical characteristics of geriatric horses: 467 cases (1989-1999)." Journal of the American Veterinary Medical Association, Vol. 223, No. 1, 93-98, 2003.
  5. McFartone, D; Sellon, D; Gibbs, S. "Age-related quantitative alterations in lymphocyte subsets and immunoglobulin isotypes in healthy horses." American Journal of Veterinary Research, Vol. 62, No. 9, 1413-1417, 2001.

About the Author

Sushil Dulai Wenholz

Sushil Dulai Wenholz is a free-lance writer based in Lakewood, Colo. Her work appears in a number of leading equine publications, and she has earned awards from the American Horse Publications and the Western Fairs Association.

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