Lepto: Important but Potentially Misleading

W. Mark Hilton, DVM, Elanco Animal Health
( Credit Purdue University )

“If I’m investigating herd infertility, I’m not immediately thinking of Leptospirosis,” says W. Mark Hilton, DVM, technical consultant with Elanco technical services. Reproductive problems, he says, typically are multi-factorial and may be difficult to diagnose, but the causes usually involve non-disease issues such as bull fertility, cow nutrition, genetics, toxins or others.

“If pregnancy rates in a herd usually run 90% and drop to 84% at pregnancy check, we’ll take a close look at those non-disease factors before testing for Lepto,” Hilton says. He adds that this evaluation should include the producer, veterinarian and nutritionist if possible, covering the entire management system and looking for changes that might have affected fertility. Leptospirosis certainly is a potential cause of abortions, weak calves and open cows, but probably not as common as many producers believe.

Hilton stresses that herd history plays a key role in diagnosing fertility problems. Any sudden decline in pregnancy or calving rates deserves close attention, and if previous testing has confirmed Lepto in the herd, testing is justified. However, some producers can mistakenly assume they have a Lepto problem and focus on that while not addressing more common problems. They might, for example, blame their vaccination program and switch vaccines, then see the same problem the following year.

Paraphrasing Kansas State University veterinarian Brad White, Hilton says reproductive problems usually result from an accumulation of errors. “Eliminate the common before focusing on more difficult issues such as Lepto,” Hilton says.

Hilton says that well under half of abortions occurring in beef herds receive a diagnosis, either because producers don’t submit samples or the sampling and handling methods were inappropriate. Veterinarians can serve clients by explaining the strengths and weaknesses of Lepto tests and communicating with their diagnostic labs to ensure they collect and submit the proper samples when abortions occur. Ideally, Hilton says, veterinarians should submit the entire fetus and placenta for Lepto testing when cows abort. Paired serum samples, collected at the time a cow aborts and two weeks later, are not very useful, as the first sample ideally would take place 30 days before the cow aborts.

Vickie Cooper, DVM, PhD, at the Iowa State University diagnostics laboratory, says leptospirosis is on the differential list for diagnosing bovine abortions at ISU and other diagnostic labs. The ISU lab includes sample-submission guidelines for bovine abortions, and other samples, on its website (https://vetmed.iastate.edu/vdl), as do many other labs.

Most herds today are well-vaccinated against leptospirosis, Cooper says, and she does not see many acute cases in cows. And while it does turn up in some herds where abortions or conception problems appear, some producers retain a bias, suspecting Lepto when other problems are more likely.

Michigan State University veterinarians Dan Grooms, DVM, PhD, says Limited studies in the United States estimate herd prevalence of serovar Hardjo to be near 60% in dairies and 40% in beef herds. Multiple diagnostic lab surveys have found Leptospira associated with less than 10% of aborted fetuses. Because of the overall difficulty in making a definitive diagnosis in bovine abortions, he adds, it is likely these surveys underestimate the true prevalence of Leptospira related abortions.

Animals can be divided into maintenance hosts and incidental hosts of Leptospira, Grooms says. In a maintenance host, the disease is maintained by chronic infection of the renal tubules. Transmission of the infection among maintenance hosts is efficient and the incidence of infection is relatively high at a population or herd level. Incidental hosts may become infected by direct or indirect contact with the maintenance host, and are not important reservoirs of infection and the rate of transmission is low.

Since the introduction of vaccines that protect against the serovar Hardjo about a decade ago, Grooms says cases of leptospirosis in cattle have declined. However, once a veterinarian has ruled out other causes of a reproductive problem he encourages them to be persistent in pursuing a diagnosis. Submit samples from multiple abortions if possible, he says, noting that it sometimes takes several to find a positive result.

In diagnosing reproduction problems, Grooms stresses that veterinarians need to remember to look at the big picture first, then narrow down toward specific details, rather than vice-versa. Use all the tools available, including epidemiology, herd records and diagnostic testing.

Where lepto is diagnosed or strongly suspected, the veterinarian and client can begin looking for sources of the pathogen, such as herd introductions or exposure to other animal carriers such as dogs or rodents. Grooms cites a recent case where a high percentage of cows in a beef herd aborted at six to seven months of gestation. Diagnostic testing confirmed leptospirosis. The closed herd had not experienced problems in the past, and thus was not well vaccinated. Investigators found that two weeks before the abortions began, the producer had started feeding from a new hay pile that was contaminated with raccoon feces and urine. Even in well-vaccinated herds, Grooms points out, a high level of exposure can overcome the protection from vaccination.

Control of Leptospirosis in cattle requires a comprehensive approach that includes reducing the risk of Leptospira exposure, vaccination, and selective antimicrobial treatment to clear carriers of serovar Hardjo when indicated, Grooms says.

Complex Diagnosis

Grooms, along with his colleague Carole A. Bolin, DVM, PhD, published a report titled “Diagnosis of Fetal Loss Caused by Bovine Viral Diarrhea Virus and Leptospira spp” in the journal Veterinary Clinics: Food Animal Practice. In that article, they note that diagnostic tests for leptospirosis include those designed to detect antibodies against the organism and those designed to detect the organism or its DNA in tissues or body fluids. Each of the diagnostic procedures, for detection of the organism or for antibodies directed against the organisms, has advantages and disadvantages.

Some of the assays suffer from a lack of sensitivity, while others fall short on specificity. Therefore, no single technique can be recommended for use in each clinical situation. Use of a combination of tests allows maximum sensitivity and specificity in establishing the diagnosis. Serological testing is recommended in each case, combined with one or more techniques to identify the organism in tissue or body fluids.

Serologic assays are the most commonly used technique for diagnosing Leptospirosis in animals. Serology is inexpensive, reasonably sensitive, and widely available. The microscopic agglutination test involves mixing appropriate dilutions of serum with live leptospires of relevant serovars. Presence of antibodies is indicated by the agglutination of the leptospires.

The authors note that Interpretation of leptospiral serologic results is complicated by a number of factors. Antibodies produced in an animal in response to infection with a given serovar of Leptospira often cross-react with other serovars of leptospires. An infected cow is likely to have antibodies against more than one serovar in an agglutination test. In general, however, the infecting serovar is assumed to be the one for which that animal develops the highest titer.

Widespread vaccination of cattle for leptospirosis also complicates the interpretation of leptospiral serology, Grooms and Bolin note. In general, cattle develop relatively low agglutinating antibody titers (100–400) in response to vaccination, and these titers persist for one to three months after vaccination. Some animals, however, develop high titers after vaccination, and although these high vaccination titers decrease with time, they may persist for 6 months or more after vaccination.

The third complication of interpretation of leptospiral serological testing is caused by a lack of consensus as to what titer is significant for the diagnosis of leptospiral infection, the authors say. An agglutinating antibody titer of 100 is considered significant by many; however, this cutoff level may be exceeded in vaccinated animals, and may not be reached in Hardjo infections. A low antibody titer does not necessarily rule out a diagnosis of leptospirosis. Antibody titers are often at a peak at the time of abortion and persist for months following infection, making paired serology around the time of abortion unlikely to demonstrate significant changes in titers.

Diagnostic take-home points

Noting that Leptospirosis remains an important economic disease in livestock, Cooper offers these key points:

  • The disease is zoonotic.
  • In cattle, Leptospirosis primarily causes abortions, stillbirths, infertility, loss of production, and less commonly, acute systemic infection in neonates.
  • Cattle are the maintenance host for serovar Hardjo. Leptospira borgpetersenni serovar hardj-bovis is most common in North America.
  • Transmission among maintenance hosts is typically direct with contact with urine, placental fluid, milk, or venereal/trans-placental transmission most common.

Features of Infection with serovar Hardjo include:

  • Generally mild to no acute clinical signs.
  • Persistent infection in the reproductive tract is a prominent feature.
  • Abortions, still births or weak born calves can occur but typically only when a cow is infected for the first time when pregnant.
  • Abortions due to Hardjo tend to be sporadic and not “storms.”
  • Most economically significant aspect is infertility.

Factors in diagnosing Leptospirosis include:

  • Highly dependent on clinical history, vaccination history.
  • Microscopic agglutination test (MAT) is most common serologic tests.
  • High titers cause by incidental host infections are sufficient diagnostically.
  • Serovar Hardjo may have low or negative titers, often population/herd screening, becomes more valuable in these situations, along with detection of the organism in tissues or fluids.

Serology interpretation is complicated by:

  • Cross-reactivity of antibodies.
  • Antibody titers induced by vaccination.
  • Lack of agreement among laboratories on what titers indicate active infection.

Detection of leptospires

  • Diagnostic laboratories infrequently use dark field microscopy or fluorescent antibody detection systems today.
  • Immunohistochemistry is still used on formalin fixed tissues.
  • Polymerase chain reaction (PCR] has become the most frequently used detections system for tissues, body fluids, or urine samples.
  • While highly sensitive, some formats of PCR detection can lack specificity as they tend to be sensitive and can be prone to false positive results.

“This is one of the components of discrepancies practitioners can see between laboratories when submitting samples,” Cooper says. “Some laboratories use 16S real-time PCR which is highly sensitive, but may not be as specific, while other use LipL32 real-time PCR which is slightly less sensitive but tends to be more specific. As a consequence PCR results need to be interpreted both in context of the clinical findings, as well as with sample collection quality.”


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