Leptospirosis in Horses

Acute leptospirosis in horses is characterized by fever of 103-105° Fahrenheit for two to three days, depression or dullness, loss of appetite, and jaundice. Abortion can occur several weeks after the fever in pregnant mares, and uveitis can strike months later. Leptospires that cause leptospirosis are bacteria.

However, says William Bernard, DVM, Diplomate ACVIM, of Lexington, Ky., they are different from other bacteria in that they are spiral shaped and are motile. Bernard, a practitioner at Rood and Riddle Equine Hospital, has authored a paper on leptospirosis that was published in Veterinary Clinics of North America: Equine Practice.

Leptospires are grouped into two complexes--L. interrogans and L. biflexa. The dangerous group is L. interrogans because those are the bacteria that cause disease.

L. interrogans are divided into serovars and serogroups. The prime serovars that can cause disease in the horse are pomona, grippotyphosa, hardjo, bratislava, canicola, and icterohaemorrhagiae. Evidence indicates that pomona is the serovar most often implicated in equine abortions in the United States.

Believed to be one of the least dangerous is bratislava. L. bratislava, says Bernard, is thought to be "host adapted" to the horse. This means that it can exist in the horse without causing disease.

However, that viewpoint might be changing. John Timoney, MVD, PhD, of the Gluck Equine Research Center in Kentucky, says that information presented at the 8th Equine Infectious Disease Conference in Dubai this past March points to bratislava as more of a culprit than previously believed. He said that bratislava has been implicated in many leptospirosis cases in Europe. A presentation at the conference by William Ellis, PhD, of Northern Ireland, Timoney said, disclosed evidence that bratislava also is a serovar implicated in leptospirosis in horses in other parts of the world, including the United States.

In his paper, Bernard describes the pathogenesis of leptospirosis this way:

"Leptospires can enter the vascular space through conjunctival, nasopharyngeal, oral, esophageal, small intestine, and genital mucous membranes. Abraded or soft, moist skin also can be a portal of entry. The outcome of infection is dependent upon the pathogenicity of the offending serovar and the host's immune response.

"Bacteremia (bacteria in the blood) occurs four to 10 days after initial infection. The invasion of internal organs occurs during bacteremia. The severity of organ involvement depends upon the presence of a humoral immune response or the ability and rapidity of the host to mount one.

"If the antibody response is not adequate, then severe tissue invasion and damage may occur. Once tissue invasion occurs, infection can persist despite the presence of circulating antibodies. Localized infection may involve tissues and fluids of such areas as the eyes, proximal kidney tubules, genital tract, and central nervous system.

"The persistence of infection is dependent upon host serovar adaptation. If the infection is accidental (i.e., non-host adapted), then infection is short and urinary shedding brief to non-existent. If the serovar is a host-adapted organism, then colonization is often persistent and may continue for the life of the host.

"Fetal infection occurs subsequent to localization in the pregnant uterus. Fetal infection (dependent upon the stage of gestation) results in abortion, stillbirth, or weak neonates.

"Generally, the pathogenic serovars of L. interrogans localize in the kidney or genital tract. Transmission is generally through urine; however, all body secretions may contain leptospires during the acute stage of the disease. Survival in the environment is usually not long, although under favorable conditions, leptospires may survive up to several weeks. Moderate temperature, moisture, and contaminated water sources provide optimal conditions for transmission."

One way the leptospire life cycle is broken, according to M. J. Donahue, PhD, a professor at the University of Kentucky who is affiliated with the Livestock Disease Diagnostic Laboratory in Lexington, is freezing followed by a thaw.

Thus, it can be assumed, leptospirosis would be less apt to prevail in a cold climate and would be more apt to flourish where there is year-round warmth and a good deal of moisture.

Yet, even this correlation can be baffling. This past spring was unusually wet in Kentucky, says Donahue; yet only three cases of abortion due to leptospirosis was recorded in the diagnostic laboratory as of early May.

Leptospirosis respects neither geographical nor environmental boundaries. Horses in the northeastern section of the United States, for example, have tested positive for the presence of serovars.

An extensive study involving equine abortions was conducted between July 1986 and June 1993 by University of Kentucky researchers. During that seven-year timeframe, material (fetuses and stillborn foals) from 3,692 equine abortion cases was submitted to the Livestock Disease Diagnostic Center. The gestational ages ranged from 140 days to full term, with the majority of abortions occurring after 180 days of pregnancy. The mean was 250 days. Of the grand total examined, serovar pomona was "firmly associated" with 90 fetuses and 20 stillborns. The researchers reported that pomona type kennewicki was the chief culprit. In some cases, multiple serovars were isolated, but the majority--84%--of seropositive fetuses reacted only to serovar pomona.

The results of the study were published in Equine Infectious Diseases VII, with K. B. Poonacha, DVM, PhD, as the chief author.

"Gross lesions were observed in 76.1% of the fetuses, stillborn foals, and placentae," Poonacha reported. "Gross placental lesions included nodular cystic allantoic masses, edema, and necrotic mucoid exudate that coated the chorion (a placental membrane). The liver was enlarged and pale yellow; the kidneys swollen and edematous, with pale white radiating streaks in cortex and medulla. One hundred and eight of 113 cases (95.6%) had microscopic lesions in the liver, kidney, heart, lungs, brain, and placenta."

Another study, aimed at providing some specific answers concerning leptospirosis, was carried out by many of the same University of Kentucky researchers and involved three Central Kentucky farms with prior histories of leptospiral abortions. All three farms had experienced multiple leptospiral abortions in the previous foaling season. In addition to seeking more information about the serovar or serovars causing the abortions, the researchers also were attempting to determine the prevalence of leptospiral exposure and to investigate whether long-term urinary shedding occurs in horses with high leptospiral titers.

The results of that study were published in 1994 in the Equine Veterinary Journal, with the chief author as Deborah M. Williams, DVM.

The farms in the study were within a 12-mile radius of each other. Two were Thoroughbred farms and one raised Standardbreds. For identification, they were labeled by the research team as Farms A, B, and C.

During the preceding foaling season, Farm A had two leptospiral abortions, as did Farm B, while Farm C had three abortions as the result of leptospirosis.

Blood samples were taken between May and September of 1990 from all available horses on each of the three farms. More than 90% of the horses on all three farms were sampled. The animals were subdivided into three age groups--foals (nursing or weanling), yearlings, and adults two years of age and older. Urine samples also were collected.

Many of the horses at the three farms were positive reactors to leptospires. The highest percentage of positive reactors was the adult horse group (68%), followed by the yearling group (39%) and the foal group (10%).

The breakdown of positive reactors by farm and age group was as follows:

  • Foals--Farm A, seven of 144 were positive reactors; Farm B, 30 of 205; and Farm C, two of 43.
  • Yearlings--Farm A, 49 of 148 were positive reactors; Farm B, 52 of 94; and Farm C, two of 22.
  • Adults--Farm A, 85 of 129 were positive reactors; Farm B, 222 of 323; and Farm C, 48 of 70.

Of the 497 positive horses, 379 (or 76.3%) reacted against serovar bratislava. Next in line was icterohaemorrhagiae with 244 (49.1%). A total of 100 (20.1%) reacted against grippotyphosa, and eight (16.3%) against pomona. Seventy-nine (15.9%) reacted against hardjo, and five (1%) reacted against canicola.

Interestingly, the prevalence of antibodies to the leptospira serovars on the three farms were comparable to findings in an unpublished study of Central Kentucky horse farms with no histories of leptospiral abortions.

"In the present study," Williams reported, "approximately half of the seropositive horses reacted to only one serovar and half reacted to multiple serovars. A similar finding was reported by Hathaway et al. (1981) in a study in England, where 43.4% of positive horses for export and 51.3% of positive horses from diagnostic submissions reacted to multiple serovars.

"Irrespective of farm and horse age group, the greatest number of horses reacted to serovar bratislava, which is considered host-adapted to the horse."

The researchers concluded that serum antibody titers exist long after the horse is no longer shedding leptospira in the urine.

"None of the urine samples collected from horses with serum antibody titers higher than 1:800 was positive for leptospira by dark-field microscopy, fluorescent antibody technique, or culture. Based on these findings, it would appear that high antibody titers are not necessarily indicative of long-term shedding of leptospira in the urine, and that serum titers persist for a longer period than does urinary shedding of the organism."


The Kentucky researchers then came to grips with the matter of host species that harbor serovars and the role they might play in passing the bacteria on to horses.

Williams had this to say: "As previously stated, prevalence of infection with certain leptospiral serovars may vary significantly by region. These differences may sometimes be explained, or at least better understood, if the maintenance host species are considered. A maintenance host acts as a continuous reservoir, with horizontal transmission taking place within the particular host species. The maintenance host is highly susceptible to infection by a specific serovar, but the organism typically causes little pathogenicity in this host. A serovar may also infect a non-maintenance host species, referred to as an accidental host, possibly causing severe pathogenic effects.

"Wooded areas were in close proximity and the horses had access to streams on all three farms included in this study. Wildlife was reported to be plentiful and assorted, including mice, rabbits, raccoons, skunks, opossums, and a few fox and coyote.

"Farm A had no cattle on the premises, but a dairy farm was upstream from one of the pastures. Steers shared the pasture with horses on Farm B. A steer calf operation was upstream from Farm C and some cattle were pastured separately on the same farm. No other species of livestock were kept on any of the three farms during the period under study.

"The three farms in this study had a cumulative average of 7.1% positive reactions to serovar pomona. In the United States, serovar pomona is found in domestic livestock and wildlife, and it has been linked to periodic ophthalmia (uveitis), jaundice, and abortions in horses in several studies.

"Serovar bratislava is maintained in pigs, dogs, horses, and wildlife in other parts of the world, but so far only pigs have been identified as maintenance hosts in the USA. The prevalence of antibody titers found in this study would suggest that horses may also act as maintenance hosts for bratislava. However, this would need to be supported by isolation data. In this study, horses had a higher seroprevalence of antibodies to bratislava than to any of the other five serovars. The seropositive rate was 33% in the horse population tested, and 53% in the adult horses."

Maintenance hosts for the other serovars were listed as follows: Canicola--Prime maintenance hosts are dogs, cattle, pigs, and skunks. In the Kentucky study, there was a very low prevalence of antibodies to canicola on the three farms.

Grippotyphosa--Hosts include the raccoon, mouse, fox, squirrel, rabbit, and bobcat.

Hardjo--Cattle are the most important maintenance host for this serovar. On many farms, horses are kept in the same pastures as cattle, thus increasing the risk of transmission. In a study in Northern Ireland, it was found that one-third of the broodmares which were pastured with cattle had high titers for hardjo. In the Kentucky study it was found that Farms B and C, which grazed cattle, had 11.75% and 2.2% positive titers to serovar hardjo, respectively. By contrast, the percentage of positive titers in horses on Farm A, which had no cattle, was 1.1%.

In her concluding comments, Williams could find little of a positive nature to report relative to control of leptospirosis.

"Control and eradication of leptospirosis are difficult because of the multiplicity of serovars and maintenance hosts involved and the existence of a carrier state in certain species," says Williams. "A vaccine for leptospirosis is not approved for use in the horse. Those available for use in cattle and dogs do not include all the appropriate serovars and pose a threat of anaphylactic reaction in the horse.

"Knowledge of the role and importance of leptospira in the serology and pathogenesis of equine disease is very limited. The epidemiology of leptospirosis and its economic impact on the horse industry are still not completely understood. The problem is exacerbated by the difficulties associated with isolating and identifying leptospira and the inconsistencies in making a diagnosis on serological basis.

"Areas for future research effort include studying the carrier state, wildlife reservoirs, leptospira-induced infertility, venereal transmission, diagnosis, and prophylaxis."

Moon Blindness

As mentioned earlier, leptospirosis also often results in the afflicted horse suffering from recurrent uveitis or moon blindness. While many researchers agree that leptospirosis is implicated in recurrent uveitis, there is no single theory as to exactly what occurs. (For more information on recurrent uveitis, see The Horse of July 1998.)

One theory, says Carole Bolin, DVM, PhD, with the USDA Disease Diagnostic Laboratory at Iowa State University, is that the horse's immune system might unwittingly play a negative role. The antibodies that rise up to fight off the invading bacteria might have an adverse effect on tissues in the eye that can bring on an attack of uveitis.

Another theory, she said, is that the bacterial organism attacks the eye, bringing on recurrent uveitis bouts that can lead to blindness.

Adding to the mystery is that uveitis frequently occurs months after the horse has suffered from a bout of leptospirosis.

Bernard, in his report, says there is a definite correlation between leptospirosis and uveitis. The specific serovar blamed for uveitis is pomona--the same one strongly implicated in abortions and stillborn foals.

Bernard says: "Positive correlations between leptospiral titers to L. interrogans serovar pomona and uveitis have been reported. Experimental and natural infections with leptospires have resulted in clinical uveitis 12 to 24 months after infection. Leptospires have been isolated from the aqueous humor of both experimentally and naturally infected horses. The etiology is not clear, but it is likely to be a result of the persistence of leptospire infection or antigen in the eye or of an induced autoimmune recognition of self-antigen."

Timoney says that more research is needed concerning the role played by serovars in equine recurrent uveitis. Cultures of fluid from afflicted eyes, he says, demonstrate that the serovar is definitely present in the eye. What isn't known is the way in which it ignites a reaction that results in recurrent uveitis.

Bernard also reports that leptospirosis has been reported as a cause of renal dysfunction in the horse.

It doesn't stop there.

"Circumstantial evidence of jaundice and liver dysfunction has been associated with leptospiral infection in horses," Bernard reports. "Leptospires have been cultured from the liver of some of these cases, and other cases have reported rising titers to specific serovars."

However, he adds, evidence of liver damage as the result of leptospirosis is not considered common.


The next logical topic to consider in a discussion of this frustrating disease is treatment. The consensus seems to be that administering antibiotics is the most effective approach, although there appears to be no definitive protocol to be followed.

Bernard reports on antibiotics as follows: "Recommendations for antibiotic therapy in horses appear to be extrapolated from other species. Oxytetracycline, streptomycin, and penicillin have been recommended for a period of one week. Tetracycline, penicillin G, and dihydrostreptomycin were not found to eliminate shedding in horses, although dosages and the length of treatment may not have been adequate.

"Potassium penicillin G has been used in pregnant mares with rising leptospiral titers in late gestation. Normal foals were delivered by treated mares. Ricarcillin-clavulonic acid has been used successfully in the treatment of renal leptospirosis in horses.

"The recent success of penicillin G given intravenously in humans suggests that this antibiotic used in high doses may be a reasonable alternative for treatment of leptospirosis in horses. The previously poor efficiency of penicillin may be related to inappropriate dosages or course of therapy.

"The success of erythromycin and tetracycline in humans and laboratory animals suggests that these antibiotics should be considered for use in the horse.

"Antibiotic therapy is unlikely to be effective in the treatment of recurrent uveitis, as the disease is likely to be immune-mediated."


Bernard notes that approved vaccines for horses are not available. "Where environmental changes can be made," he says, "limiting exposure to stagnant water, cattle, swine, and wild animals that are potential carriers can be attempted. Abortions in horses have been coincidentally related to cattle exposure. Disinfection of stalls that have been occupied by infected animals should be commonplace, as should isolation of infected animals."

The final subject to be broached involves the question of determining when the infected animal reaches a "safety" stage as far as contaminating others is concerned. Horses have been documented as shedding serovars for 110 days after infection.

The rule of thumb, says Bolin, is to obtain three serovar-free urine samples over a three-week period. If the horse's urine is checked once a week for three weeks with no serovars present, the animal then can be considered a non-shedder.

One thing is certain, a great deal more research is needed before sure-fire preventative and treatment approaches are found.

To that end, Timoney is working on sophisticated research to identify specific proteins involved in the serovars as an initial step toward the goal of finding a vaccine that will be effective in preventing leptospirosis.

Research also is needed, he says, to develop an effective anti-microbial for treating recurrent uveitis that results from the disease.

Much of the research on leptospirosis has been conducted on horses in Kentucky, but, as already mentioned, the disease can be found in horses all across the country. It also does not respect breed barriers. While the seven-year study in Kentucky revealed that the majority of abortions occurred in Thoroughbreds, the researchers reported that this was not a significant finding. "In Central Kentucky," they reported, "this breed predominates and there was no indication of a breed predilection."

The disease also was found in Standardbreds, a Quarter Horse, a Saddlebred, an Arabian, and a miniature horse.

There is one final negative about leptospirosis. It is a zoonotic disease, which means it can affect many types of animals, including humans. While transmission of the disease from cattle to man has been reported, there have been no reported cases of the disease being transmitted from horses to humans. However, it is recommended that human exposure to uterine fluids and to urine should be minimized when leptospiral infection has been identified in a horse.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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