INFECTIOUS DISEASE FORUM


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.

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 of 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 (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 was a hot topic at this forum. Attendees had the following suggestions for prevention of this rapidly spreading disease.

Vaccination--Out of 463 cases in Florida, only 44 had received two or more doses of the vaccine, indicating a high success rate for the vaccine. Further investigation of these 44 cases is under way 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 which had not had both doses of the vaccine. Vaccination still remains an important way of preventing WNV.

Controlling Mosquitoes--The mosquito viral burden for all encephalitis diseases was higher in 2002 than in any year since 1930. Researchers expect high 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 veterinarians 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 disease instead of a foreign one, testing is no longer free for all submissions (see article #4052 online). 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 Scott Weese, DVM, DVSc, Dipl. ACVIM.)

Testing is important since horses act as a predictor for human cases. It was clarified 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 be high enough to cause a positive test.

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

Strangles Prevention

There have been advances in veterinarians' knowledge and options for the control of strangles. Some horses which recover 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 might be moved from one farm to another. Because these shedding horses appear normal, there is 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 S. 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 the farm, a screening test for chronic shedders of the causative bacteria might be considered. Owners should consult with their veterinarians about screening tests.

Veterinarians in the forum discusse ways to get samples for the test, and they also emphasized that using proper procedures was important 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. (See complete article #4046 online.)

SUBDUED, NOT ERADICATED


The focus of the talk by John R. Irby, DVM, was that anthrax, screwworms, and equine piroplasmosis are not currently present in the United States, but could be re-introduced. Irby formerly worked for the Texas Animal Health Commission and now is with the Florida Department of Agriculture and Consumer Services. He noted that, "In the post-September 11th atmosphere, there is the concern that livestock could be the target of a bioterrorist attack using a disease agent available in another country."

In discussing anthrax, screwworms, and piroplasmosis, Irby said that the three diseases are related in that each "can affect equids, has the potential to devastate domestic livestock, is present in many foreign countries, and may first be detected by veterinarians in private practice."

Irby reiterated that it is imperative that private practice veterinarians realize they are the sentinels for any unusual disease conditions or exotic pests affecting animals in the United States.

"I was fortunate to be part of the first group of U.S. veterinarians detailed to the United Kingdom in 2001 to assist with the foot and mouth disease outbreak," he said. "I witnessed just how easily a contagious disease can be spread through routine marketing channels and how devastating it can be to the livestock industry of an entire nation. Even more impressive was the realization that a non-zoonotic disease involving food animals can also impact businesses seemingly unrelated to agriculture. Tourism, probably the number one industry in the U.K., suffered terrible economic losses during the foot and mouth disease outbreak. Tourism is important to the U.S. economy. Our livestock are marketed on a much larger scale and subjected to less movement restrictions compared with those in the United Kingdom. We must never lose sight of our part in providing a strong national defense." (See complete article #4118 online.)

EQUINE ZOONOTIC DISEASES


Zoonotic diseases are those that can be passed between animals and humans, and, noted Scott Weese, DVM, DVSc, Dipl. ACVIM, of the Department of Clinical Studies at the University of Guelph, they are an ever-present threat to veterinarians. "A number of zoonotic agents may be encountered in veterinary practice and the severity of human disease can range from mild to fatal," he said. "Prompt recognition of zoonotic agents, the use of protective clothing, identification of potential fomites and, most importantly, close attention to personal hygiene can help reduce the risk of developing a zoonotic illness."

He added that veterinarians need to recognize these diseases not only to protect themselves, but others who deal with affected horses. He gave the following clinical checklist:

  • Is it potentially zoonotic?
  • How is it transmitted?
  • What human disease can occur (including symptoms)?
  • Have I been exposed?
  • What precautions should I take?
  • What precautions should owners/handlers take?
  • Should it be reported?

Rabies is probably the best-known zoonotic disease, although the number of equine cases are relatively low when compared to small animal and wildlife, noted Weese. There were 82 reported cases of equine rabies in the United States in 1998; 65 cases in 1999; and 52 in 2000.

Veterinarians and vet technicians are recommended by the Centers for Disease Control to receive pre-exposure vaccination against rabies.

Weese went through a list of common (and not-so-common) diseases that affect horses and humans. He made an important point when discussing diarrhea in horses. "Even with aggressive diagnostic testing, an etiologic agent is typically only identified in less than 50% of cases," he said. "It is important to remember that 'idiopathic' cases (those without a known cause) still have an underlying etiology that, while undiagnosed, may be infectious and zoonotic. Therefore, all cases of diarrhea in horses should be treated as infectious and potentially zoonotic."

FOREIGN ANIMAL DISEASE INVESTIGATIONS


Veterinarians are the first line of defense against infectious and contagious animal diseases, noted Brian J. McCluskey, DVM, MS, of the USDA:APHIS Centers for Epidemiology and Animal Health in Fort Collins, Colo. "The rapid diagnosis and ultimate control of foreign animal and emerging diseases is dependent on the diagnostic abilities, experiences, and diligence of veterinary practitioners," he said.

"The number one take-home message is that veterinarians are the cornerstone of foreign animal disease surveillance in the United States," he emphasized. "Reporting the presence of suspected lesions or clinical findings is an important first step in identifying foreign animal disease."

The USDA's veterinary services have rapid response capabilities when there are suspected foreign animal disease occurrences, McCluskey said, adding that animal health officials "judiciously" use quarantine for protection of a livestock industry. He reminded that many foreign or emerging animal diseases can have clinical presentations that mimic diseases or conditions that are common or known in this country. Sometimes, he noted, there will be unusual clinical signs, or higher than expected morbidity (illness) or mortality (death), or diseases that are not usually seen in a specific area. (See complete article #4120 online.)

FOREIGN ANIMAL DISEASES WITH EQUINE POTENTIAL


"When you hear hoofbeats on the covered bridge, please think about the zebra!" said Corrie Brown, DVM, PhD, Dipl. American College of Veterinary Pathology, vice chair of the Cornell Committee for Animal Diseases. Her point: When veterinarians make diagnoses, they need to consider foreign animal diseases. "The amount of damage that a foreign animal disease will cause is directly proportional to the time between introduction and accurate diagnosis," she said. "In other words, we have to accurately recognize a foreign animal disease at first blush if we are to implement effective control measures. It is imperative that practitioners consider foreign animal diseases in their diagnostic rule-outs."

Among diseases that Brown discussed were African horse sickness and glanders.

African Horse Sickness

She said that the United States "remains at risk" for African horse sickness. While it is unknown if our insect species in the Culicoides family can transmit the virus (the research hasn't been done), she said that, "It seems likely that the virus could be transmitted by our indigenous Culicoides species." She detailed the severe impact on the Iberian peninsula (including Spain) from 1987-1990. It came in with zebras, which can have subclinical infection. During the four-year period, hundreds of horses succumbed to the disease or were euthanized. The mortality rate in horses is 70-95%.

"U.S. quarantine restrictions regarding equine species are designed to protect against introduction of an infected equine," noted Brown. "Consequently, the most likely route of entry would be from an infected insect that comes in through air travel." She added that this is a very hard disease to control once it gets in the equine and mosquito populations of an area. There is a vaccine.

Glanders

"Glanders used to be a low priority, but with bioterrorism, the threat (that it could be seen in the United States) ratcheted it up," noted Brown. She said during World War I, glanders was used to maliciously infect horses of enemy troops. The disease was eradicated from the United States in 1937, but was developed as a biological weapon during the cold war era.

The causative bacteria is Burkholderia mallei, which can also cause disease in humans. Brown said the incubation period can be from weeks to months. In non-endemic areas, glanders could be mistaken for strangles, rhodococcal pneumonia, or ulcerative lymphangitis. Affected animals usually are destroyed. Treatment is not recommended, and there is no vaccine.

In conclusion, she stated, "There are two ways a veterinarian can become 'famous' when a foreign animal disease enters the neighborhood. One way is to diagnose it; the other is to miss it!" (See complete article #4121 online.)

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