Why Detecting a Pathogen Does Not Always Explain Bovine Perinatal Mortality

Perinatal mortality remains one of the most challenging areas of cattle practice. When a full-term fetus or a newborn calf dies around the time of birth, the immediate assumption is often that an infectious agent was responsible. However, identifying a pathogen and proving that it caused death are not always the same thing. 

Understanding this distinction can significantly improve diagnostic accuracy and help producers make informed herd health decisions. 

Exposure, Infection and Cause of Death Are Not Identical 

One of the most important principles in investigating bovine perinatal mortality is differentiating exposure, infection and causality. 

A fetus may have been exposed to a pathogen during gestation, which can be indicated by the presence of fetal antibodies. A pathogen may also be detected in fetal tissues or placental samples, confirming infection. However, neither finding automatically proves that infection caused the calf's death. 

For infection to be considered the cause of perinatal mortality, the pathogen should ideally be associated with inflammatory lesions in fetal tissues or the placenta, while other likely causes of death have been excluded^1,2,3. 

This distinction is particularly important because non-infectious causes such as dystocia and asphyxia remain more common causes of perinatal mortality than infectious disease. 

Primary and Secondary Pathogens: Why the Difference Matters 

Not all infectious agents behave in the same way. 

Primary pathogens possess the ability to cross an intact placenta and infect the fetus because of their inherent virulence. They can induce placentitis, fetal disease or reproductive failure even in otherwise healthy cows^1,4. 

Secondary pathogens, often referred to as opportunistic pathogens or pathobionts, typically require compromised conditions before infection occurs. Placental damage, altered reproductive tract microflora, or maternal immunosuppression may create opportunities for these organisms to invade the fetoplacental unit⁵. 

Examples of secondary pathogens include organisms that are commonly present in the environment or within the normal microbiome of cattle. Their detection often raises diagnostic questions because they may represent contamination, opportunistic infection or a true contributor to fetal disease¹. 

The Challenge of Co-Infections 

Although a single pathogen is commonly detected in infectious cases, dual infections are not unusual^1,6. 

The presence of multiple pathogens may indicate substantial infectious pressure within the herd environment. In some situations, endemic infections such as Neospora caninum or Coxiella burnetii may coexist with additional infectious agents, creating more complex disease patterns^2,3. 

When multiple pathogens are detected, interpreting their relative contribution to the death becomes more difficult. A pathogen identified in laboratory testing may have been present, but not necessarily responsible for the fatal outcome. 

Infection May Be Present Without a Detectable Pathogen 

A common misconception is that failure to isolate a pathogen excludes infection. 

In reality, autolysis, environmental contamination, scavenging, delayed sample submission or incomplete sampling can all reduce the likelihood of pathogen detection¹. 

In some cases, inflammatory lesions provide the strongest evidence that infection occurred. Findings such as pneumonia, pericarditis, hepatitis, encephalitis, peritonitis or omphalophlebitis may indicate a significant infectious process even when a specific organism cannot be identified¹. 

For this reason, histopathology remains a critical component of a complete investigation. 

Practical Take-Home Points 

When evaluating bovine perinatal mortality cases, avoid making conclusions based solely on a positive laboratory result. 

Consider three key questions: 

• Was the fetus merely exposed to the pathogen? 

• Is there evidence of active infection? 

• Is there convincing evidence that infection actually caused death? 

A heavy or pure growth of a pathogen, compatible inflammatory lesions, and exclusion of alternative causes together provide the strongest support for an infectious diagnosis. 

A calf may die with an infection rather than because of an infection. Recognizing this distinction helps improve diagnostic accuracy, supports better herd-level recommendations and prevents misdirected control strategies. 

Successful investigation of bovine perinatal mortality requires combining pathology, microbiology and clinical judgment rather than relying on a single test result.

References 

1. Mee JF, Jawor P, Stefaniak T. Role of infection and immunity in bovine perinatal mortality: Part 1. Causes and current diagnostic approaches. Animals. 2021 Apr 6;11(4):1033. https://www.mdpi.com/2076-2615/11/4/1033 

2. Jawor P, Król D, Mee JF, Sołtysiak Z, Dzimira S, Larska M, Stefaniak T. Infection exposure, detection and causes of death in perinatal mortalities in Polish dairy herds. Theriogenology. 2017 Nov 1;103:130-6. https://t-stor.teagasc.ie/bitstreams/4be6e30c-e479-4287-b528-f2c24e0753e6/download 

3. Mock T, Mee JF, Dettwiler M, Rodriguez-Campos S, Hüsler J, Michel B, Häfliger IM, Drögemüller C, Bodmer M, Hirsbrunner G. Evaluation of an investigative model in dairy herds with high calf perinatal mortality rates in Switzerland. Theriogenology. 2020 May 1;148:48-59. https://t-stor.teagasc.ie/bitstreams/6dae9bce-19ef-4563-8f17-8752c5865439/download 

4. Serrano-Pérez B, Almería S, Mur R, Alabart JL, López-Helguera I, Garcia-Ispierto I, López-Gatius F. Caracterización de las citoquinas involucradas en la luteolisis del cuerpo luteo de vacas abortadas por la infección experimental con Neospora caninum. Proceedings of the XVII Jornadas Sobre Producción Animal, Zaragoza, Spain. 2017 May:30-1. https://www.aida-itea.org/aida-itea/files/jornadas/2017/comunicaciones/2017_SBA_06.pdf 

5. Di Blasio A, Traversa A, Giacometti F, Chiesa F, Piva S, Decastelli L, Dondo A, Gallina S, Zoppi S. Isolation of Arcobacter species and other neglected opportunistic agents from aborted bovine and caprine fetuses. BMC veterinary research. 2019 Jul 24;15(1):257. https://link.springer.com/content/pdf/10.1186/s12917-019-2009-3.pdf 

6. Zanatto DC, Gatto IR, Labruna MB, Jusi MM, Samara SI, Machado RZ, André MR. Coxiella burnetii associated with BVDV (Bovine Viral Diarrhea Virus), BoHV (Bovine Herpesvirus), Leptospira spp., Neospora caninum, Toxoplasma gondii and Trypanosoma vivax in reproductive disorders in cattle. Revista Brasileira de Parasitologia Veterinária. 2019 Jun 13;28(2):245-57. https://www.scielo.br/j/rbpv/a/thkh86Fw3TysWjPnj8JLyxx/?format=pdf&lang=en