Equid Blog : Horse Health & Diseases : University of Guelph : Ontario Veterinary College : Equine Infectious Diseases

Rotavirus is an important cause of diarrhea in young horses. (It's also a major cause of diarrhea in infants, but a different rotavirus is involved). Exposure to rotavirus is very common - most, if not all, horses are exposed to it early in life. Disease only occurs in foals, but not every foal that gets exposed becomes sick. Most often, rotavirus causes diarrhea if foals between 1 and 6 months of age, with most cases occurring between 1 and 3 months of age. Foals less than 1 month of age can be affected, but it's less common.

Foals become infected by swallowing the virus, which they usually pick up from the feces of other horses (including their mares) in their environment. The exact source of infection is rarely identified. It is likely that some healthy adult horses are the reservoirs and shed the virus in their manure.

Like other types of diarrhea, rotaviral diarrhea in foals can range from mild diarrhea alone to severe diarrhea with weakness, loss of appetite and dehydration. Colic can occur because of intestinal cramping associated with diarrhea. You cannot differentiate diarrhea due to rotavirus from other types of diarrhea by appearance alone. The diarrhea must be tested to identify the cause.

There is no specific treatment for rotavirus infection, but "supportive care" is often provided. One of the primary components of supportive care is fluid therapy, which may be needed for foals that start to become weak and dehydrated. Aggressive intravenous fluid therapy may be needed in some cases, depending on the severity.

Rotavirus vaccines are available in some regions, although there is limited evidence that they have much of an effect. Few people recommend vaccination. Rather, good attention to foal management practices, hygiene and early diagnosis of infected foals is the key. Foaling mares outside on pasture has been an effective practice in some outbreaks, presumably because there is less rotavirus contamination on pasture.

Image: Colourized TEM of rotavirus particles (source: CDC PHIL 173)

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Equine infectious anemia (EIA) is a rare disease but one that a lot of time, effort and money are put into avoiding. Most people know about this disease by way of the most common test (formerly) used to diagnose it: the Coggin's test (see image).

Equine infectious anemia is caused by a virus of the same name ( equine infectiious anemia virus, EIAV). EIAV is a lentivirus which, like all other lentiviruses, causes persistent infection. Unlike most other lentiviruses that cause slow, gradual preogression of disease, EIAV infection usually causes a sudden onset of disease (acute phase) followed by recurrent disease. After the initial (acute) phase, horses can appear normal, which means they can be a silent reservoir of the disease. That's why routine testing for EIAV is required in many circumstances: to detect silent carriers so they cannot continue to transmit infection.

Signs of acute EIA vary, but usually include fever, lethargy and decreased appetite. Anemia (decreased red blood cell count) and thrombocytopenia (decreased platelet count) can be detected. Anemia is more common and pronounced with recurrent infections. Intermittent illness often develops after the first acute episode. Affected horses may experience short (3-5 day) periods of fever, lethargy and decreased appetite. The severity of the anemia often correlates with the severity and frequency of these disease episodes. In some horses, these episodes are very common, long and severe, and these horses often have severe weight loss and anemia. However, most infected horses stop developing obvious signs of disease after a year and seem perfectly healthy.  This may be good for the one horse, but it's bad for other horses in the area to which the virus may be transmitted.

EIAV is a bloodborne virus that can be transmitted by blood-feeding insects, especially tabanid flies (horseflies, deerflies). Stable flies can also transmit EIAV but do so less effectively. Contaminated medical supplies such as reused needles and syringes can also transmit EIAV.

Fortunately, EIA is now rare in most regions and positive tests are quite uncommon. Routine testing for EIA is usually required for shows, sales, transportation and other situations where horses are mixed, in order to detect and remove carriers. Unfortunately, identification of a horse as a carrier is not good for the horse or owner (or other horses in the area) - EIA is not treatable, and horses that have positive tests ("reactors") are quarantined, as are all other horses that are housed within 200 yards. Horses living close to reactors are tested (usually 30 and 60 days after removal of the reactor) and only released from quarantine after getting negative test results at least 60 days after the last reactor was removed. Reactors are usually euthanized. If prompt euthanasia is not chosen, reactors are usually prominently branded or tattooed. They must be kept under quarantine for the rest of their lives, and at least 200 yards from other horses. (200 yards is used because the flies that transmit the disease don't usually travel that far.)

The best way to reduce the risk of EIA is ensuring that all horses are regularly tested. New horses coming onto a farm must be tested BEFORE arrival or after arrival but while in quarantine at least 200 yards away from other horses. Needles and other items that might be contaminated with blood should never be re-used, both due to the EIA risk and to avoid other potential problems.

Overall, EIA is a very rare disease, but the severe implications of a positive test mean that we need to be vigilant.

Image from http://www.aht.org.uk/science_eia.html

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Tyzzer's disease is a rare but devastating disease caused by the bacterium Clostridium piliforme. This bacterium causes severe liver disease and sudden illness in young, otherwise healthy foals between the ages of 1 and 6 weeks. Usually on any particular farm only one foal is affected at a time, but small outbreaks can occur.

There is still a great deal we don't know about C. piliforme and Tyzzer's disease. It is presumed that foals become infected by ingesting the bacterium from the manure of other horses or from the environment. No one knows how often foals are exposed to the bacterium in this way - it may happen to a lot of foals, but only a few of them get sick, or it may happen very uncommonly, but make most of the exposed foals sick.  Tyzzer's disease occurs very suddenly and progresses incredibly fast.  Affected foals are often simply found dead, even though they looked completely normal only hours earlier.  If they are found alive, foals may be slightly to extremely weak and lethargic, and they may have a fever, diarrhea, and increased heart and respiratory rates. The gums and whites of the eyes may be yellowish (i.e. jaundice), which is sign of liver failure. Even if foals with Tyzzer's disease are found alive, their condition usually worsens very quickly and they often start having seizures before they die.

A diagnosis of Tyzzer's disease is usually made by post mortem (necropsy) examination. Clostridum piliforme cannot be grown on regular culture plates in the lab like most of the disease-causing bacteria with which we deal (this also makes it very difficult to study). Special stains of liver tissue (silver stain) can help identify C. piliforme under the microscope. Real-time PCR, a molecular method that detects the DNA of C. piliforme, is also available.

Unfortunately, most foals with Tyzzer's disease die before, or shortly after, they are found to be ill.  In most cases there is hardly enough time to even start treatment because the disease is so severe and progresses so rapidly. There are only three foals ever reported to have survived Tyzzer's disease, or what was strongly suspected to be Tyzzer's disease.  Very aggressive therapy (i.e. in a referral hospital) is needed immediately to try to save affected foals, but the prognosis is very grave.

There are no known measures that can be taken to help prevent Tyzzer's disease in foals.  Fortunately, the condition is rare, and there is no evidence that it is transmissible to humans.

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This post was written by guest author Dr. John Prescott, Professor, Department of Pathobiology, University of Guelph.

April’s "bug of the month" is Streptococcus equi subspecies equi, the cause of equine strangles, specifically strain 4047. This is the first strangles strain to have its entire genome (i.e. all of its DNA, including every gene) sequenced. The March 2009 issue of the open-access journal Pathogens, published by the Public Library of Science (PLoS), describes how British scientists think the strangles organism evolved to be such a scourge of horses, by comparing its genome to that of the first equine Streptococcus equi subspecies zooepidemicus strain to also have its genome sequenced, strain H70. It is believed that S. zooepidemicus was the ancestor of the S. equi that causes strangles.

Like all evolution, the story involves both loss and gain. As far as loss is concerned, the strangles organism has more genes that have been "turned-off" (inactivated genes) than does S. zooepidemicus. These genes were turned off by a large number of extra pieces of DNA called insertion sequences which S. equi picked up over time. These sequences get inserted in many different places within the bacterium's DNA (its genome) leading to inactivation or loss of various genes. Loss of genes in this manner can often result in bacteria becoming restricted to a particular type of host - S. equi essentially only infects horses and other equids, while S. zooepidemicus still has a broader host range.

What S. equi lost by becoming restricted to equids, it gained by becoming a nastier bug. It did this by becoming "infected" by four unique bacterial viruses. (Bacterial viruses are viruses that only infect bacteria. Some of these viruses "transmit" (transfer) genes to the bacteria they infect but leave the bacteria alive. Other bacterial viruses can kill bacteria after infecting them). Most bacteria have anti-viral defense mechanisms to protect themselves from these viruses, but S. equi's defenses may have been damaged or turned-off by insertion sequences. Being infected by these four bacterial viruses gave S. equi many genes not found in S. zooepidemicus. One of these viruses is thought to be capable of leaving S. equi and killing S. zooepemidicus in the horse’s throat - a useful trick for S. equi to wipe out the competition in this area so it can move in more easily and cause infection. A second virus gives the strangles organism a phospholipase enzyme which is known to be involved in the bacterium's ability to cause disease. The third and fourth viruses each donated two different superantigen genes – these genes help S. equi to cause inflammation and impair the ability of the immune system to fight off the bacteria. A unique additional piece of “foreign” DNA that S. equi acquired is associated with the ability to capture iron. This allows S. equi to grow better in infections than S. zooepidemicus, because the body of the infected host will often try to bind up as much iron as possible so the bacteria can’t use it to grow. The scientists speculate that this iron-capturing gene was the key to starting S. equi on its evolutionary spiral to becoming a major horse pathogen.

A final thought is that the bacterial viruses that made S. equi what it is today have many similar DNA sequences to bacterial viruses found in Streptococcus pyogenes, a major human bacterial pathogen. Humans might have had an unwitting hand in creating this pathogen from one of their own.

More information on strangles can be found on the equIDblog Resources page.

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Actinobacillus equuli is a common Gram-negative (i.e. looks red/pink with Gram stain) bacterium, the natural host of which is (not surprisingly, based on the name "equuli",) the horse. This organism can be found quite commonly in or on healthy horses, for example, as part of the normal bacterial flora of the upper respiratory tract.  However, A. equuli can cause disease in certain situations.

Horses that most often become infected and sick from A. equuli are foals. This bacterium can cause a wide range of infections in these young animals, including septicemia (bloodstream infection), meningitis, pneumonia, omphalophlebitis (umbilical infection) and septic arthritis (joint infection). Foals can get infected from contact with other horses or even their own mares because the bacterium is so common.  However, it typically only becomes a problem in foals that do not receive adequate colostrum immediately after birth, or that do not receive proper umbilical care.

Disease in adult horses due to A. equuli is uncommon. As in foals, the bacterium can cause a variety of infections in adults, including pneumonia, wound infections and guttural pouch infections. These infections are usually opportunistic - they occur following another problem that affects the animal's normal defense mechanisms, such as viral pneumonia or lacerations. Primary infections with A. equuli, including peritonitis, pleuritis and septicemia, can occur in healthy adult horses, but are very rare.

Infection with A. equuli is diagnosed by culturing the bacterium from the infected body site.  Culture also permits identification of what antibiotics may or may not be effective for treating the infection.  Actinobacillus equuli is usually susceptible to a variety of routinely-used antibiotics.

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Streptococcus zooepidemicus (technically Streptococcus equi subspecies zooepidemicus), commonly called Strep zoo, is an important bacterium in equine medicine. It is one of the most common bacteria isolated from infections in horses. Like other streptococci, S. zooepidemicus is a Gram positive coccus, meaning that it stains purple with Gram stain and has a ball shape (coccus). Streptococci tend to stick together in chains (see picture right) which are often described as "string of pearls."

Streptococcus zooepidemicus can be found in the upper respiratory tract of healthy horses. In most instances, it lives there without causing any problems.  Strep zoo is considered an opportunistic pathogen - a bacterium that can live in harmony with its host, but can also cause disease in certain situations. Strep zoo  can cause a variety of infections, including most commonly pneumonia, lung abscesses and guttural pouch infections. More severe conditions, like bloodstream infections, joint infections and meningitis, can also occur, but are uncommon and most often affect neonatal foals.

Diagnosis of disease caused by S. zooepidemicus requires culture of appropriate samples collected from the horse. Care should be taken when interpreting cultures of samples collected from the nasal passages or upper respiratory tract, since S. zooepidemicus is often present there even in normal animals, so finding it does not necessarily mean it is causing disease.

Unlike some bacteria that cause infections in horses, S. zooepidemicus tends to be susceptible to numerous antibiotics, including penicillin and trimethoprim-sulfa. However, since some strains can be resistant to various drugs, culturing the bacterium and testing its susceptibility to antibiotics in the lab is still required to determine the optimum treatment, .

Strep zoo is of minimal risk to people. Human infections caused by this bacterium have been reported but they are considered very rare.

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