Court: District Court, N.D. California; February 23, 2019; Federal District Court
Honorable Vince Chhabria, United States District Judge, addressed the issue of specific causation in the case involving Monsanto and three plaintiffs—Edwin Hardeman, Sioum Gebeyehou, and Elaine Stevick—claiming that glyphosate exposure caused their non-Hodgkin's lymphoma (NHL). Previously, the Court allowed the admissibility of the plaintiffs' expert opinions regarding general causation, which were deemed shaky but acceptable. To counter Monsanto's motion for summary judgment concerning specific causation, the plaintiffs needed to present at least one admissible expert opinion. The Court determined that the plaintiffs' experts could testify at trial, despite some limitations on their opinions.
The plaintiffs' experts employed a differential diagnosis approach, a method that identifies the most likely cause of a disease by ruling in and ruling out potential causes. The Court affirmed that Monsanto does not dispute the applicability of this method but argues the reliability of the experts' analysis in both ruling in and ruling out causes. During the ruling-in stage, the Court noted that the plaintiffs had already provided admissible expert opinions establishing that glyphosate is capable of causing NHL. Monsanto's challenge regarding the selection of epidemiological studies was deemed irrelevant, as the plaintiffs' experts can rely on the broader admissible general causation opinions rather than needing to reanalyze every study.
Furthermore, while Monsanto contended that the specific causation experts failed to show that glyphosate had an adjusted odds ratio greater than 2.0, the Court clarified that establishing a risk factor as a potential cause does not require this specific threshold. It highlighted that the general causation opinions cited, particularly the De Roos study, which indicated an adjusted odds ratio of 2.1, support the experts' decisions to rule in glyphosate exposure. The ruling underscores that the specific causation experts aim to build upon the general causation findings rather than independently validate them.
Experts must thoroughly evaluate potential causes of plaintiffs' non-Hodgkin lymphoma (NHL) and exclude factors other than glyphosate, while not dismissing glyphosate itself. A critical concern is the possibility of idiopathy, where NHL may arise from unknown causes, making it essential for experts to differentiate between Roundup users who developed NHL due to glyphosate and those who would have developed it regardless. While biomarkers or genetic signatures can assist in this differentiation, no such evidence exists in this case. Under strict Daubert standards, this could undermine the plaintiffs' claims; however, Ninth Circuit precedents suggest a more lenient approach. The Ninth Circuit's rulings indicate that district courts should often admit causation opinions that lean towards the 'art' of medicine, recognizing that emerging toxic tort cases should not be dismissed solely due to incomplete medical literature. Although the decisions in Wendell and Messick have distinct contexts, they collectively signal that expert opinions facing borderline scrutiny are more likely to be accepted in the Ninth Circuit compared to other jurisdictions. District judges retain discretion in admitting evidence but must acknowledge the broader admissibility of expert opinions in this circuit. Experts with significant clinical experience and qualifications can rely on their expertise and review of medical records to assert that a recognized risk factor is the cause of a plaintiff's disease, thereby meeting the Daubert standard.
Specific causation experts concluded that glyphosate was a substantial factor in causing the plaintiffs' non-Hodgkin lymphoma (NHL) based on admissible general causation opinions connecting glyphosate to NHL, particularly referencing the McDuffie (2001) and Eriksson (2008) studies that indicated a dose-response relationship. The experts emphasized the plaintiffs' significant exposure to Roundup, asserting that their NHL was not idiopathic due to the absence of other significant risk factors. Although the court may question the assumptions behind the experts' conclusions, particularly regarding the epidemiological evidence's strength, their core opinions were deemed admissible.
During cross-examination at the Daubert hearings, Monsanto probed the experts with hypothetical scenarios concerning the severity of exposure and its impact on causation conclusions. The experts maintained that the plaintiffs' exposure levels were sufficiently significant to attribute their NHL to glyphosate. Nevertheless, some aspects of their testimony were identified as veering into unreliable science, which could be excluded unless used for impeachment.
Dr. Nabhan was prohibited from claiming that the McDuffie and Eriksson studies imply a doubling of NHL risk for individuals using Roundup more than two days per year or ten days in their lifetime, as these studies did not account for other pesticide use. Similarly, Drs. Nabhan and Shustov were restricted from stating that glyphosate is a substantial causative factor for anyone exceeding those usage thresholds, as their conclusions were based on unadjusted data.
General causation opinions may indicate an increased risk of non-Hodgkin lymphoma (NHL) with higher glyphosate exposure; however, quantifying this risk for an individual plaintiff using unadjusted data from the McDuffie and Eriksson studies is scientifically unsound. Dr. Weisenburger cannot claim that Mr. Hardeman's risk of developing NHL more than doubled due to his extensive use of Roundup, as the studies do not support such a specific assertion. Eriksson's and McDuffie's studies reveal unadjusted odds ratios of 2.36 and 2.12, respectively, but do not clarify exposure levels sufficiently to link them to higher risks for individuals exceeding exposure thresholds. Additionally, Dr. Nabhan’s comparison of glyphosate risks to smoking is deemed speculative and lacks probative value, rendering it inadmissible under Rules 403 and 702.
The court largely granted the plaintiffs' motion to exclude parts of the opinions from Monsanto's specific causation experts. It was agreed that neither Dr. Sullivan (Monsanto's exposure expert) nor Dr. Sawyer (Mr. Hardeman's exposure expert) would testify in Phase 1 of the trial, with future challenges to experts in Phase 2 to be addressed before that phase begins. The ruling assumes familiarity with prior expert testimonies and general causation findings. The court differentiates between "differential diagnosis" (identifying a disease) and "differential etiology" (determining the cause), noting that the analysis here pertains to the causative factors of NHL. The three plaintiffs are confirmed to have NHL, with Dr. Shustov providing the only specific causation opinion, while Dr. Nabhan's general causation opinion was excluded for lacking independent analysis. Dr. Weisenburger's general causation opinion was admitted. An additional risk factor for Mr. Hardeman identified is hepatitis C.
Experts indicated that although active hepatitis C is a recognized risk factor for non-Hodgkin lymphoma (NHL), it is improbable that Mr. Hardeman's NHL was linked to his previous hepatitis C infection, as he had achieved a sustained virologic response nearly a decade prior, indicating the virus was undetectable in his blood. Although the experts could have provided a more thorough analysis of the mechanisms by which hepatitis C could lead to cancer, they had substantial scientific backing for their conclusions. Monsanto may contest their interpretation, but the experts' methodology remains valid. Under California law, a doubling of risk signifies a 50% likelihood that a specific factor caused an individual's disease, allowing studies showing a relative risk over 2.0 to be used independently to establish causation. However, California law does not necessitate that a study demonstrate a doubling of risk for an expert to opine that a risk factor caused a plaintiff's disease. The discussion around this matter relates to the sufficiency of evidence rather than strict adherence to the Daubert standard. A plaintiff may prove their case through a single compelling piece of evidence or multiple pieces that, individually, do not meet the burden of proof. While a study showing a risk factor greater than 2.0 can support the case if scientifically sound, no absolute requirement exists for such evidence to prevail in cases of medical causation. Adequate general causation evidence should allow courts to enable parties to demonstrate whether the plaintiff's disease was more likely than not caused by the implicated agent.
