from the Vet Corner Archives

Part of Horse Previews Magazine website. Posted on 12/01/2000; 2:00:00PM.

Veterinary Corner 12/00: Coughs and Colds in Horses

by Sylvia Miller, DVM
Edgecliff Equine Hospital
S. 1322 Park Road, Spokane, WA 99212 * 509/924-6069

It seems that winter has arrived a little early in the Inland Northwest, and with it come the coughs and colds that cold weather seems to bring. What are the organisms responsible and what are the most likely types of respiratory syndromes occurring in horses?

Perhaps the most dreaded and common infectious disease affecting horses is strangles. This disease is caused by a bacterial organism, Streptococcus equi, which now appears to be a biovar or clone of Strep zooepidemicus, as identified by gene sequence studies. It may affect horses of all ages, but is most common in those younger than two years old, except foals. Some immunity may be conferred by having the disease, but it is also possible to have the disease two to three times. It is highly contagious and transmitted by oral-nasal routes, both direct and indirect, as in contaminated water, feed, and nose-to-nose contact. The organism can survive for several weeks in water troughs but not the soil or pasture. It can also survive in frozen nasal discharge.

Risk factors for this disease are a concentration of horses and a highly mobile population with the stress of transport, overcrowding, and very hot/very cold weather exacerbating the risk. The severity of the disease appears to be directly proportional to the number of organisms a horse is exposed to.

S. equi attaches to cells of the tonsils and soft palate (in the mouth or oral cavity) and is detectable in the mandibular or retropharyngeal lymph nodes. It is not present until 24-48 hours after the onset of fever, and begins to be shed 2 days afterward. The incubation period varies from 4-5 days up to 12-14 days from exposure.

It is diagnosed by clinical signs and definitively by nasal swabs or nasal washes. Clinical signs may include a high fever, high white blood cell count, high fibrinogen, and local edema or swelling. Serology is generally not useful except in the case of bastard strangles, abscesses occurring in sites other than the mandibular or retropharyngeal lymph nodes. The gutteral pouch is commonly infected during the early stages of strangles and may cause persistent shedding for many months. Most new outbreaks of the disease are thought to be initiated by horses in the incubating or late convalescent phase of the disease. Carrier animals, which are intermittent shedders of small numbers of the organism, don't appear to be a significant cause of outbreaks.

Prevention involves quarantining new arrivals for 2 weeks, isolating those individuals affected, vaccinating broodmares in the last 3 months of pregnancy, monitoring the status of recovering animals via nasal swabs/nasal washes, and taking the rectal temperatures of those horses in contact with affected individuals daily for 2-3 weeks to identify susceptible animals with a 1.50F rise in temperature.

Now on to the next big one, the Herpes family of viruses. There are five types, of which EHV-1 and -4 are the most significant. (Most horses have been exposed to EHV-2 and -5, and they are the mildest of the five. EHV-3 causes coital exanthema, a venereal disease.)

EHV-1 is the more virulent virus, spreads rapidly, causes abortion and rarely, an acute onset of neurologic signs, particularly hindlimb ataxia or even quadriplegia, urinary incontinence, and loss of tail tone. This usually occurs ten days after exposure. It is diagnosed by submitting blood for paired titers (taking blood samples 2 weeks apart and looking for a four-fold increase), which is also a means of differentiating it from EPM (equine protozoal myeloencephalitis). However, EHV-1 and -4 are closely related and can only be differentiated through virus isolation techniques in the laboratory. The recovery period may be exacerbated by Rhinovirus, tracheitis, or bronchitis from a secondary bacterial infection and results in long term immunosuppression.

EHV-4 is the most commonly known herpes disease, causing respiratory infection and occasionally abortion. The clinical signs are often a high temperature, loss of appetite, lethargy, swelling of the hind limbs, serous (clear) nasal discharge which becomes thick and mucoid if a secondary bacterial infection develops, enlarged lymph nodes (submandibular and retropharyngeal), and maybe a cough. It is often seen in young animals, particularly yearlings, as older horses may have a subclinical (not detectable) or mild form of the disease.

Again, paired titers are recommended for diagnosis, along with nasopharyngeal swabs and blood for virus isolation during the acute stages. Antibody titers peak approximately 10-14 days after exposure but may not return to the original levels for 3 months.

If this disease is manifested as an abortion, then the fetus should be submitted to a diagnostic lab. Mares may abort fetuses during the last third of pregnancy without any prior signs or problems, or the foal may be born weak and suffer from acute respiratory distress. In either case, the fetus is heavily contaminated with the virus and precautions should be instigated to limit exposure to other horses and the environment.

For both EHV-1 and -4, natural immunity acquired by the disease is relatively short-lived, less than 3 months. The best method of controlling outbreaks is to vaccinate susceptible populations at strategic times to induce a protective response and reduce the severity of the disease if exposed, and decrease virus shedding. And since recovered horses may be viremic for several weeks, horses exposed to EHV-1 should not travel to stud farms or be transported with breeding animals. Their titers can be monitored through serology.

Equine influenza usually appears where large groups of horses are transiently congregated, such as horse shows, races, and sales. This is a highly contagious virus of two prevalent subtypes that is spread by aerosol (inhaled) and can also be dispersed on the wind.

Clinical signs appear 1-5 days from exposure and may include a dry hacking cough, increased body temperature, clear nasal discharge, loss of appetite, muscular soreness, depression, swelling of the limbs, and enlarged submandibular lymph nodes. Secondary infections, usually bacterial, are common and can develop into bronchopneumonia.

To diagnose this disease, nasopharyngeal swabs should be submitted for virus isolation or blood titers during the acute stage.

Prevention involves more frequent vaccination, especially in young stock or mares during late pregnancy, as vaccination can decrease severe clinical signs and secondary bacterial infection is less likely.

Treatment varies, but the general rule of thumb is to afford affected horses one week of rest for each day of increased temperature, followed by a gradual return to work.

Now that you're totally asleep, we'll briefly cover Rhinovirus. Think "common cold." It is spread by aerosol (inhalation) and direct contact. The three types are prevalent during the first half of the year and especially affect young horses. Through repeated exposure, older horses usually develop a subclinical (not really noticeable) case. Clinical signs include fever, pharyngitis, loss of appetite, and a nasal discharge. Again, paired serum samples are submitted for diagnosis. However, this virus rarely causes a problem and is slow to spread through the population.

To summarize, management and proper vaccination (where available) should be used to control these respiratory conditions. Diagnostic lab work is the only definite way to determine what organism is involved. Treatment varies by condition, but generally, isolation, rest, and follow-up lab work or an endoscopic exam (into the gutteral pouch), along with a clinical evaluation by the trainer should determine when the horse can return to work. Follow good hygiene practices and clean the common environment with the proper virucidal products or chlorine-based products, and feed sick horses last.

Have a Merry Christmas and Happy New Year-and don't let the flu grinch get you!

P.S. If anyone has shoes they'd like to donate to some barefooted children in South America, please drop them by Edgecliff Equine before the end of January! Thanks!

Sylvia Miller DVM

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