"But we've never had rotavirus in our foals." Read on and consider yourself lucky if your foals have never had rotavirus and you have breezed through the past foaling seasons sans diarrhea. With the increasing transportation of horses comes the increasing risk of any infectious disease occurring on your farm. As you will see, once you have rotavirus, you've obtained the "gift" that keeps on giving! Consider these real situations:

  • A farm manager has 36 foals which need to be medicated, monitored, and washed several times a day because of rotavirus diarrhea. Five foals are on IV fluids; two need to be hospitalized. Forty more foals are at-risk.
  • A small animal veterinarian owns one mare and foal on five acres in Arkansas. At two weeks of age, the foal breaks with diarrhea. Diagnosis: rotavirus.
  • A Standardbred farm in Italy has 70% of its 130 foals develop rotavirus diarrhea after not having a problem with foal diarrhea for at least 10 years.

The foaling season should be a time of anticipation, hard work, and fun at seeing healthy foals running in the fields with their dams. Spring also can bring months of sheer drudgery, disappointment, and despair if a disease outbreak hits the farm, especially if it is caused by rotavirus.

The enemy rotavirus is a round RNA virus that can infect most any mammalian species, including humans (see photos page 58). Under natural circumstances, the virus is species-specific, meaning that only foals get the horse strain of rotavirus; calves get the bovine form of rotavirus; and children (especially under two years of age) are affected by human rotavirus. The vast majority of rotaviral diarrhea is caused by Group A rotaviruses; however, other groups of rotaviruses (B-G) have been described in species besides horses.

Rotaviral diarrhea is a worldwide disease, with outbreaks occurring in most major horse breeding areas, including England, Ireland, France, Japan, and Australia. Foals are exposed to the disease by a fecal-oral route, meaning they ingest fecally contaminated material, or lick surfaces that are contaminated with manure. If a foal ingests enough contaminated material, it usually develops diarrhea in 12-24 hours.

The virus passes into the duodenum, part of the small intestine attached to the stomach. This portion of the intestine is developed for digestion and absorption of food materials. Tiny finger-like projections called villi cover the inside of the intestine, increasing the surface area for digestion to take place (see illustration page 61). Cells at the tips of the villi are responsible for digestion and absorption of nutrients, whereas cells in the base secrete fluids to aid in the digestive process. These crypt cells also reproduce and migrate up the villus "finger" to replace the villus tip cells as they become worn.

The villus tip cells are attacked and killed by rotavirus, causing food not to be digested and absorbed fully. The body responds to this by flooding the intestinal tract with fluids to flush the undigested food out of the body. Not all of this fluid can be resorbed by the large intestine, and the result is diarrhea, ranging from cow-pile consistency to a squirting waterfall of liquid feces. The virus also multiplies exponentially in cells of the villi. One teaspoon of rotavirus-infected feces from a foal can contain more than 10 million virus particles, enough to infect whole herds of foals, thus explaining the highly contagious nature of the disease.

Not all foals which are exposed to rotavirus in their environment develop diarrhea. In extensive field studies conducted in Central Kentucky by the author and David Powell, BVSc, of the University of Kentucky's Gluck Equine Research Center, it was discovered that some healthy foals shed rotavirus while their pasturemates became ill with diarrhea. Foals on farms without active rotaviral disease did not shed the virus in detectable amounts.

The Disease

Actually, diarrhea is not always the first sign of the disease. Often a dam's full udder is noticed by the owner, indicating that the foal is not nursing. Within hours, young foals will be depressed and have soft to watery feces that can range in color from green/gray to yellow. Bloody feces have not been associated with rotavirus infection, and fever is not a consistent clinical sign. Foals less than 14 days of age often are more severely affected and potentially can die from the dehydration and electrolyte imbalances within six hours.

Clinical signs in foals two or more months of age can be quite different. Sometimes the only indication of the disease is cow-pile consistency feces with no other signs of sickness. Older foals have more energy reserves and can better cope with the disease. Foals from two days to five months of age can be affected, and most have diarrhea for three to four days, but the condition can last as long as a week.

What about the mares? Even in the midst of the worst farm outbreaks, mares and other adult horses are not affected by rotavirus. In two separate research studies, a small percentage of mares were shown by serology to be infected with rotavirus at the time that their foals were ill, but none showed any signs of sickness.


Although each species has its own variety of rotavirus, one of the surface proteins remains the same. Human test kits are commonly used to diagnose animal rotavirus in as little as five to 10 minutes. If one fecal test is negative, at least two other samples should be tested daily, since the concentration of rotavirus in feces, especially watery feces, might be too low for the test kits to detect. Fecal cultures for bacterial pathogens often are tested at the same time to rule out bacterial causes of foal diarrhea. Because there are many causes of diarrhea in foals, horse owners should leave the diagnosis and treatment to their veterinarian.

Blood testing is not used to diagnose rotavirus. Several research studies have shown that most foals with definite rotavirus disease do not show a significant increase in rotavirus antibody levels. Blood testing is a reliable method of diagnosis for many viral diseases, but rotavirus is not one of them.

Although once thought to be a secondary invader, rotavirus has been shown to cause diarrhea all by itself. In Central Kentucky, concurrent bacterial infections are rare in rotavirus cases, however that might not necessarily be the case in all locales.


Since this is a viral disease, antibiotics are not a cure, and can in fact complicate the situation. Treatment depends on the patient's condition, and can include medication called protectants to soothe and coat the gastrointestinal tract (e.g., Pepto-Bismol), probiotics (to replenish the "good" bacteria in the intestinal tract) that are given orally, and intravenous fluids and nutrients with intensive therapy to keep young foals alive. In some areas of the country, gastric ulcers are a concern and anti-ulcer medication might be prescribed by the veterinarian.

As with any treatments administered to animals, maintaining records on the response to treatment is imperative. Taking the foal's temperature, pulse, and respiration, looking at the gums for normal pink color and capillary refill time, and testing for dehydration using the skin pinch test (see The Horse, September 1999, page 106) only takes two to three minutes. If any of these parameters are abnormal, call your veterinarian. High fevers especially warrant a telephone call.

Pay attention to medication administration during the course of the disease. Dosing with protectants past the formation of normal feces can cause constipation in foals, resulting in anorexia, depression, and even colic. Good nursing care is essential, since foals often are recumbent more than usual, and diarrhea can scald the tender skin between their hind legs. Additional bedding should be utilized for foals kept in stalls. Mineral oil or Desitin (for baby rash) can be used on the hind legs as a barrier to the irritation from the feces. Foals which do not have adequate cleansing and treatment of the skin can develop raw, peeling sores with secondary bacterial skin infections. A little prevention can go a long way in making the foal more comfortable and additional problems less likely.


If sick foals are detected early and appropriate treatment started, rotavirus seldom is fatal. Very sick foals often recover and regain condition rapidly, assuming there are no secondary problems such as gastric ulcers.

Where does rotavirus come from? Since the virus is species specific, foals pick up the virus from materials contaminated with equine feces (you don't have to worry about opossums here). In studies conducted in swine, it has been shown that some sows shed rotavirus in feces in the time period before and after farrowing (high stress). These animals show no diarrhea and are asymptomatic carriers of the virus.

Although mares have had feces tested at the time of foaling in one research study, they were not positive for rotavirus. There could be several reasons for this. Adults might shed rotavirus sporadically or in quantities too small to detect with current test kits. Either way, at this time there is no way to detect which adults could be carrying the virus.

Another important factor is that this virus is extremely hardy, and it can survive for more than nine months at room temperature and over winter on farms. Rotavirus basically is a protein coat with viral RNA inside and is not covered by an envelope the same way as influenza, herpesvirus, and other equine viruses. Enveloped viruses generally are easy to kill with detergents and are susceptible to weather conditions. Rotavirus is a pretty tough customer, in comparison.

Once a foal breaks with diarrhea, its feces can cause contamination of a whole barn full of foals, with all becoming ill within 36 hours of the first case. Once the first case breaks, you have to assume that all foals which are in contact with the sick foal have been exposed.

For example, if half of the foals in a barn are pastured with the now-sick foal, those are the foals most likely to become ill. The other foals also are at risk, since foals can start shedding rotavirus two days prior to becoming ill.

Hands, shoes, pitchforks, and other things that have been used in common between foals can efficiently spread the virus throughout the barn.

How To Stop The Spread

Start disinfection and isolation measures immediately when a foal is noticed with diarrhea which is not considered to be foal heat diarrhea (see The Horse of February 2000). This can be done while waiting for the veterinarian to arrive at the farm. Do not move mares and foals to other areas of the farm, since you could be spreading the virus by in-contact foals incubating the disease and just waiting their 18 hours before scouring.

Disinfection using phenolic compounds (e.g., Tek-Trol, Bio-Tek Industries Inc., Atlanta, Ga.) can kill rotavirus on equine facilities, on clothing, towels, and other washables used on sick animals. (An in-depth article on disinfection and isolation can be found in The Horse, November 1999.) These techniques have consistently proven useful in rotavirus outbreaks.

An Ounce Of Prevention

Effective farm management is important in controlling and preventing any infectious disease. Vaccines are important in the arsenal of defenses, and fortunately there is a rotavirus vaccine (Equine Rotavirus, Fort Dodge Animal Health, Fort Dodge, Iowa) that is available through veterinarians. This vaccine is administered intramuscularly to mares at eight, nine, and 10 months of gestation to enhance the antibody levels in colostrum. The vaccine has been shown to be extremely safe, and on many farms where rotavirus was a major yearly problem, the disease has been effectively slowed, if not stopped.

The antibodies obtained from the colostrum are absorbed into the foal's bloodstream, along with all of the other colostral antibodies. Just like all antibodies obtained from the mare, these will decrease in concentration over the first two months of life, until the foal begins making antibodies itself. Some farms have noticed that two-month-old foals (from vaccinated mares) develop pasty feces, but do not stop nursing or act sick. This is in sharp contrast to pre-vaccination days when foals were consistently getting sick at two to five days of age, with some foals dying from dehydration and electrolyte imbalances. Other farms that have used the vaccine as directed have had no cases of rotavirus since the vaccine became available to the horse-owning public through veterinarians.

Adding another vaccine to your arsenal might seem like an unnecessary cost, but the economics show otherwise. Compared to the cost of the vaccine and preventive farm management techniques used to combat all infectious diseases (routine disinfection, isolation protocols, etc.), the expense of an outbreak can economically ruin a farm's foaling season. The "seen" costs of rotavirus outbreaks include veterinary calls, diagnostics, medication prescribed by veterinarians, skin care products to prevent/treat skin scalding, preventive medication for gastric ulcers if indicated, and hospitalization for severe dehydration. Total cost could range from $100 to $1,000 or more!

Then you have the "hidden" costs of rotavirus. These include additional disinfection of stalls, equipment, and clothing, labor costs for disinfection and isolation, possible loss of time in re-breeding because the stud farm won't accept a mare from a farm with an ongoing infectious disease, disposable gloves, boots, and clothing, additional labor for nursing care, medication, and monitoring, additional bedding for keeping a sick foal stalled 24 hours/day for an average of six days (three sick days; three convalescent but virus-shedding days). This could total $150 or more!

Risk reduction against rotavirus would include vaccination of pregnant mares (three doses) at a total cost of $45-$55, routine disinfection, isolation protocols for new mares and foals, and preventive farm management techniques (which reduces the risk for all infectious diseases).

Only you and your veterinarian can decide the usefulness of vaccination on your farm. However, if rotavirus has been a problem on the farm, "You can vaccinate a whole lot of mares for what it costs to treat just one sick rotavirus foal," said Walter Zent, DVM, an equine practitioner who has had years of experience working on broodmare operations in Central Kentucky.

Because of its hardy nature, rotavirus will be in the equine industry for a very long time. Vaccination, preventive farm management techniques, disinfection, and early detection all are essential in preventing and controlling this foaling season problem. The foaling season is complicated enough without having to deal with rotavirus!

A Case In Point

“I can smell it coming,” said Sharon Wilson, breeding operations manager of Horizon Farm, Barrington, Ill. The facility has nine breeding stallions, 80 broodmares, boarding mares, and horses in training. “I have lived through many foaling seasons with rotavirus foals. Back when rotavirus was a yearly event, I dreaded going to the barns to find another foal sick,” she said. “All of our mares are now vaccinated for rotavirus, and I don’t have that problem anymore.”

She emphasized that vaccination is a vital tool, but farm management is critical. “Our foaling stalls are kept disinfected and are picked out whenever the mare defecates. Mares are washed prior to foaling, and strict isolation protocols are activated in the event a foal does develop diarrhea.”

“We have been using the rotavirus vaccine since its inception and are very pleased with the results,” stated Jeanne Cox-Owens, 1999 Kentucky Thoroughbred Farm Manager of the Year. As general manager of Cobra Farm, Lexington, Ky., she foals more than 30 mares per year. “With the rotavirus vaccine, we now have the much-needed protection against this depleting disease of our young foals, especially considering the widespread transport of bloodstock. One never knows what disease is being introduced into the herd, so it only behooves us all to be as stringent as possible with our vaccination programs.”

All Diarrhea Is Not The Same!

Rotavirus is not the only cause of foal diarrhea, although a Central Kentucky study in the late 1980s revealed that more than 90% of outbreaks were caused by the virus. Other causative agents of scours include the bacteria Salmonella, Clostridium difficile, Clostridium perfringens, Rhodococcus equi, and Gram negative bacteria causing septicemia (E. coli, Actinobacillus, etc.).

Parasites that can cause diarrhea include Strongyloides westeri, Cryptosporidium (usually in immunocompromised foals), and Giardiaz.

Non-infectious causes of scours include foal heat diarrhea, lactose intolerance, ingestion of sand, dirt, or foreign material, overfeeding, and antibiotic treatment.

Owners must remember that in some cases, the actual cause of the diarrhea cannot be determined, despite an arsenal of diagnostic tests available.

Bismuth Subsalicylate-huh?

Pepto-Bismol or GastroCote is much easier to say. Bismuth subsalicylate (commonly called “the pink stuff”) is the active ingredient in these brand name products which often are prescribed for diarrheic foals. But what does it do?

This chemical compound is split into bismuth and salicylate during digestion. The bismuth binds bacterial toxins and has some anti-microbial properties, as well as coats and soothes the lining of the stomach and intestine. The salicylate portion helps reduce symptoms of secretory diarrhea by helping to normalize fluid movement into and out of the gut. Be aware that the color of feces can change to a grayish color after dosing foals with these products, which is normal.

Rotavirus Vaccine

Since January 1996, an equine rotavirus vaccine (Fort Dodge Animal Health, Fort Dodge, Iowa) has been available to veterinarians under USDA conditional licensure, and now also has conditional licensure in Great Britain and Ireland. This intramuscular, killed virus vaccine is administered to pregnant mares at eight, nine, and 10 months of gestation. The three vaccinations are repeated yearly to pregnant mares. This vaccine is not labeled for use in foals.

About the Author

Roberta Dwyer, DVM, MS, Dipl. ACVPM

Roberta Dwyer, DVM, MS, Dipl. ACVPM, is a professor within the University of Kentucky's Maxwell H. Gluck Equine Research Center and editor of Equine Disease Quarterly.

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