Diagnosing Leptospirosis in Livestock: Practical Strategies for Herd Investigation and Reproductive Disease

In livestock, leptospirosis is often more than an individual animal health concern, it is a herd health issue that can significantly affect reproductive performance and disease transmission. Unlike acute clinical cases, persistent infections frequently remain unnoticed while infected animals continue to shed leptospires, contributing to the spread of infection within the herd. For practicing veterinarians, successful diagnosis requires a herd-oriented approach that considers the clinical presentation, reproductive history, and the epidemiological status of the herd rather than relying solely on individual animal testing. 

Recognizing Herd-Level Disease Patterns 

The clinical expression of leptospirosis in livestock varies considerably between species and production systems. Animals may present with acute illness similar to that seen in humans, but more commonly the disease is associated with spontaneous abortion, uveitis, or persistent infection leading to poor reproductive performance, including early embryonic death and failure to breed in bovine, ovine, equine, and porcine species¹. 

Subclinical infections deserve particular attention because apparently healthy animals may act as long-term carriers, maintaining disease transmission within livestock populations. Therefore, the diagnostic strategy should be tailored according to whether the veterinarian is investigating acute disease, reproductive failure, or suspected herd-level infection. 

Interpreting Serology in Livestock 

The microscopic agglutination test (MAT) remains the reference standard for serological testing in livestock. During acute disease, paired acute and convalescent serum samples demonstrating a rising antibody titre support the diagnosis of active infection. 

Outside the acute phase, however, interpretation becomes more complex. A positive MAT result confirms exposure or vaccination but does not necessarily indicate active infection. Equally important, the absence of detectable antibodies does not exclude infection because seronegative livestock may continue to carry and transmit pathogenic leptospires¹. 

The MAT is also applicable to specimens beyond serum, including cerebrospinal fluid, ocular fluids in cases of equine uveitis, and fetal serum or body fluids collected during abortion investigations. Although diagnostic laboratories commonly use a screening cut-off titre of 1:100, lower dilutions may improve detection sensitivity in certain situations¹. 

Identifying Persistent Carrier Animals 

Host-adapted infections present one of the greatest diagnostic challenges in livestock practice. Certain pathogenic serovars establish long-term relationships with specific livestock species, allowing infected animals to become reservoirs without obvious clinical disease. 

An important example is Leptospira borgpetersenii serovar Hardjo in cattle. Infected animals may excrete leptospires in urine for several months despite having MAT titres below 1:100, and shedding may occur intermittently^1,2,3. As a result, relying solely on serological screening may fail to identify infected animals contributing to ongoing transmission within the herd. 

When investigating suspected Hardjo infection, herd sampling strategies should include sufficient numbers of animals to achieve a 95% confidence of detecting at least one positive animal when the expected prevalence is 20% or greater¹. 

Investigating Reproductive Losses 

Leptospirosis should remain an important differential diagnosis during investigations of abortion and reproductive failure. 

Although aborting cows may demonstrate high MAT titres, abortion often occurs well after the initial infection, meaning affected animals can also be seronegative at the time of investigation¹. Consequently, diagnosis should not depend solely on maternal serology. 

Confirmation is strengthened by demonstrating leptospires within fetal or placental tissues. Placenta, adrenal gland, lung, and kidney are recommended specimens for examination using fluorescent antibody testing (FAT), PCR, or culture¹. 

Practical Clinical Insights 

For veterinarians investigating leptospirosis in livestock, several practical principles can improve diagnostic success: 

• Adapt the diagnostic strategy to the clinical situation, whether investigating acute disease, reproductive losses, or persistent herd infection. 

• Interpret MAT results cautiously, recognising that positive titres indicate exposure while negative titres do not necessarily exclude infection. 

• Consider persistent carrier animals as an important source of ongoing herd transmission. 

• Include fetal and placental tissues during abortion investigations whenever available. 

• Develop herd sampling plans that provide adequate confidence for detecting infection, particularly when investigating host-adapted serovars such as Hardjo. 

Conclusion 

Diagnosing leptospirosis in livestock requires a broader perspective than individual patient evaluation. Understanding the role of persistent carrier animals, recognising the limitations of serology, and applying appropriate herd investigation strategies enable veterinarians to identify infection more effectively and support informed decisions for reproductive health and disease control. A herd-focused diagnostic approach remains fundamental to managing leptospirosis within livestock populations 

References 

1. Sykes JE, Reagan KL, Nally JE, Galloway RL, Haake DA. Role of diagnostics in epidemiology, management, surveillance, and control of leptospirosis. Pathogens. 2022 Mar 24;11(4):395. https://www.mdpi.com/2076-0817/11/4/395 

2. Nally JE, Ahmed AA, Putz EJ, Palmquist DE, Goris MG. Comparison of real-time PCR, bacteriologic culture and fluorescent antibody test for the detection of Leptospira borgpetersenii in urine of naturally infected cattle. Veterinary sciences. 2020 May 15;7(2):66. https://www.mdpi.com/2306-7381/7/2/66 

3. Nally JE, Hornsby RL, Alt DP, Bayles D, Wilson-Welder JH, Palmquist DE, Bauer NE. Isolation and characterization of pathogenic leptospires associated with cattle. Veterinary microbiology. 2018 May 1;218:25-30. https://www.sciencedirect.com/science/article/am/pii/S0378113517313391