Bovine Salmonellosis Study Guide

Overview and Clinical Importance

Salmonellosis is an infectious bacterial disease caused by Salmonella enterica that affects cattle of all ages, causing enteritis, septicemia, pneumonia, abortion, and significant economic losses. It is one of the most important gastrointestinal diseases in bovine practice and a major focus on the NAVLE due to its clinical variability, zoonotic potential, and public health significance.

Salmonellosis rivals only Bovine Viral Diarrhea Virus (BVDV) in its ability to cause such diverse clinical presentations in cattle. The increasing prevalence of Salmonella Dublin, a host-adapted serotype with multidrug-resistant characteristics, has become a major challenge for producers and veterinarians in recent years.

Etiology

Salmonella enterica is a gram-negative, rod-shaped, facultatively anaerobic bacterium belonging to the family Enterobacteriaceae. Over 2,000 serotypes exist, but cattle are clinically infected by fewer than 10 serotypes. The organism possesses peritrichous flagella for motility and fimbriae for attachment to intestinal epithelium.

Major Serotypes in Cattle

Epidemiology

Transmission

The primary route is fecal-oral transmission. Cattle become infected by ingesting organisms from contaminated feed, water, milk, or environment. Secondary routes include aerosol transmission (especially S. Dublin in closely confined calves), ocular/nasal mucous membranes, and transplacental infection leading to abortion.

Sources of Infection

• Carrier animals: Clinically normal cattle shedding organisms; 5.4% of milk cows and 18% of cull cows shed Salmonella
• Contaminated feed: Feed supplements containing animal by-products
• Wildlife reservoirs: Rodents, wild birds, reptiles
• Environmental contamination: Survives months in wet, warm areas; persists in dried feces/dust for years
• Purchased animals: Major risk factor; 28% of dairy farms have Salmonella-positive cattle

Risk Factors

Pathogenesis

After oral ingestion, Salmonella colonizes the digestive tract by attaching to mucosal cells via fimbrial appendages. The organism invades through M-cells overlying Peyer's patches in the distal small intestine and colon. Using a Type III Secretion System, bacteria inject effector proteins that trigger enterocyte cytoskeletal rearrangement and internalization.

Once in the lamina propria, Salmonella triggers a robust neutrophilic inflammatory response. The organisms are engulfed by macrophages but survive intracellularly, allowing systemic dissemination to mesenteric lymph nodes, liver, spleen, and gallbladder. Disease results from a combination of enterotoxins (causing secretory diarrhea), cytotoxins (destroying enterocytes), and endotoxin/LPS (causing systemic inflammatory response and shock).

Pathogenesis of S. Dublin vs. S. Typhimurium

Clinical Signs

Clinical Presentations by Age and Serotype

Neonatal Calves (Peracute Septicemia)

• Rapid progression to death within 6-12 hours
• Dull, depressed, failure to suck
• Diarrhea may be terminal sign or absent
• High fever (105-107°F / 40.5-41.7°C)
• Associated with inadequate colostrum intake

Calves 2-6 Weeks (Acute Enteritis)

• High morbidity; mortality may exceed 60%
• Dullness, anorexia, elevated rectal temperature
• Grey pasty feces progressing to watery, foul-smelling dysentery
• Fresh blood, mucus, and mucosal casts (intestinal shreds)
• Progressive dehydration, gaunt "tucked-up" appearance
• Surviving calves often grow poorly

S. Dublin-Specific Manifestations

• Pneumonia: Respiratory distress, hyperthermia, insidious onset suppurative pneumonia
• Joint infection: Polyarthritis affecting growth plates of long bones
• Dry gangrene: Sloughing of extremities (ears, tail tip, limbs) - "Terminal dry gangrene"
• Meningitis: CNS signs following septicemia
• Vertebral osteomyelitis: Infection of neck vertebrae causing weakness of all four legs in 2-4 month old calves

Adult Cattle

• Fever (104-106°F / 40-41.1°C) - rises 12-24 hours BEFORE diarrhea
• Depression, off feed
• Foul-smelling diarrhea with blood, mucus, and intestinal shreds - "septic tank odor"
• Severe drop in milk production
• Abortion (mid to late gestation, 5-8 months)
• Clinical illness lasts 7-10 days with recovery in 2-3 weeks
• Some animals never resume full production

Exam Focus: Classic NAVLE scenario: "3-year-old dairy cow, freshened 3 days ago, fever 106°F, foul-smelling watery diarrhea with blood, decreased milk production" = SALMONELLOSIS until proven otherwise. Fresh cows are the most susceptible group!

Differential Diagnosis

Diagnosis

Clinical Diagnosis

Clinical signs (fever, hemorrhagic enteritis, anorexia) are suggestive but not pathognomonic. Definitive diagnosis requires bacterial isolation via aerobic culture or detection of genetic material via PCR.

Sample Collection

• Feces: Preferred sample in live animals; collect serial samples (daily for 3-5 days) due to intermittent shedding
• Necropsy samples: Ileum, cecum, colon, mesenteric lymph nodes, gallbladder (bile excellent sample), liver, lung, spleen, kidney
• Abortion workup: Aborted fetus, placenta, uterine discharge

Laboratory Methods

Necropsy and Pathology Findings

Gross Pathology

Peracute/Septicemic Form

• May have minimal gross lesions
• Serosal and epicardial petechiae
• Pulmonary congestion
• Splenomegaly

Acute Enteric Form

• Fibrinonecrotic enteritis (most severe in ileum, cecum, and spiral colon - "ileotyphlocolitis")
• Intestinal walls red and thickened
• Mucosal surface coated with necrotic debris and fibrin (diphtheritic membrane)
• Watery, often bloody intestinal contents with mucosal casts
• Mesenteric lymph nodes enlarged, edematous, wet, sometimes hemorrhagic
• Peritoneal effusion with fibrin tags
• Characteristic "septic tank" odor

S. Dublin-Specific Findings

• Paratyphoid nodules in liver (multifocal yellow-white foci of necrosis - highly suggestive)
• Icterus (jaundice)
• Gallbladder edema with mucosal pseudomembrane
• Pneumonia with hepatization of lung lobes
• Multifocal renal cortical petechiae and white foci

Histopathology

• Severe villous blunting and loss of intestinal architecture
• Neutrophilic infiltration of lamina propria
• Fibrinoid vascular necrosis and capillary thrombi
• Hepatocellular necrosis with Kupffer cell hyperplasia ("paratyphoid nodules")
• Gram-negative coccobacilli visible in lesions

Treatment

Fluid therapy is the MAINSTAY of treatment for cattle with enteric salmonellosis. Antimicrobial therapy remains controversial and is generally reserved for septicemic cases.

Fluid Therapy

• Calves: Oral electrolytes between milk feedings; IV crystalloids for severe dehydration; bicarbonate for acidosis; dextrose for energy
• Adults: Hypertonic saline (7.2%) followed by at least 10 gallons oral fluids (via stomach tube or voluntary intake)
• Colloids: Plasma or hetastarch for hypoproteinemia due to albumin loss from GI tract

Anti-inflammatory Therapy

NSAIDs (flunixin meglumine) are essential to control the inflammatory cascade triggered by endotoxin. Flunixin also provides analgesia and may reduce fever. Use caution regarding withdrawal times in food animals.

Antimicrobial Therapy

Prevention and Control

Biosecurity Measures

• Maintain closed herd - purchasing animals is the #1 risk factor
• Quarantine all introductions for at least 4 weeks (most outbreaks occur 2-3 weeks after arrival)
• Source animals directly from farms with known health status, NOT markets
• Test purchased cattle for S. Dublin (ELISA on blood/milk)
• Control rodent and wild bird access to feed stores
• Do not allow outside vehicles (rendering truck, livestock haulers) access to cattle areas
• Double fencing at perimeters to prevent nose-to-nose contact with neighboring cattle

Calf Management

• Provide adequate colostrum (2 liters within first 2-4 hours) from vaccinated dams
• Use clean, dry calving pens; avoid group calving areas
• Remove calves from contact with adult cow feces as soon as possible after birth
• House calves individually (hutches) with no contact with other calves or adults
• Do NOT feed raw waste milk to calves; consider pasteurization
• Sanitize feeding equipment between calves

Sanitation and Disinfection

• Remove organic matter and bedding FIRST
• Wet clean with hot water/steam
• Apply disinfectant IMMEDIATELY after cleaning (Salmonella "blooms" in warm, wet environments)
• Effective disinfectants: chlorine solutions, iodine, quaternary ammonium compounds, phenolics
• AVOID power washing calf facilities - aerosolizes bacteria and facilitates spread
• Do not spread manure on pastures used for grazing within 3 weeks

Vaccination

Vaccine efficacy is variable and serotype-specific. Options include killed bacterins, autogenous vaccines, and modified-live vaccines. Vaccination of dry cows 3-4 weeks before calving can provide passive immunity to calves via colostrum.

• BOVIVAC-S: Inactivated vaccine covering S. Dublin and S. Typhimurium; calves from 3 weeks of age; 2 doses 3 weeks apart; boosters before calving
• J5/Endotox vaccines: E. coli core antigen bacterins; may provide some cross-protection against gram-negative endotoxemia
• Autogenous bacterins: Made from farm isolate; provide serotype-specific protection

Exam Focus: Vaccination ALONE will not prevent salmonellosis - it is only ONE component of a comprehensive control program. Good management, biosecurity, and hygiene are MORE important than vaccination!

Zoonotic Considerations and Public Health

Salmonellosis is a MAJOR ZOONOSIS. Human infection can occur through direct contact with infected cattle/feces, contaminated clothing/equipment, aborted material, unpasteurized milk, or contaminated beef products.

• Young children, elderly, and immunocompromised individuals at highest risk
• Human symptoms: nausea, vomiting, abdominal cramps, diarrhea within 8-48 hours
• S. Typhimurium DT104 causes severe illness in humans due to antimicrobial resistance
• Personal protective measures: Hand washing, protective clothing, facial protection during procedures that may cause splashes/aerosols
• Children should be supervised when handling cattle, especially calves

Prognosis

• Peracute septicemia: Guarded to poor; mortality can exceed 60% in calves
• Acute enteritis: Fair with aggressive fluid therapy; clinical illness 7-10 days, recovery 2-3 weeks
• S. Dublin infection: Guarded; animals become LIFELONG carriers
• Recovered animals may never return to full production
• S. Dublin-infected herds face ongoing endemic challenges