Histophilus somni is a bacterial pathogen and a major component of the Bovine Respiratory Disease (BRD) complex, causing significant economic losses in the cattle industry. The bacterium is opportunistic, meaning it can live harmlessly in an animal but cause disease when the immune system is compromised. Its ability to cause a wide range of diseases makes it a persistent challenge for producers.
The Histophilus Somni Disease Complex
Histophilus somni is responsible for a “disease complex” because it can infect multiple organ systems. The bacterium enters the body through the respiratory tract and can spread through the bloodstream to other parts of the body, a condition known as septicemia. This systemic spread allows the bacterium to attack different tissues, leading to a range of clinical problems.
One of the most severe manifestations is thrombotic meningoencephalitis (TME), a neurological disease that affects feedlot cattle. TME occurs when the bacteria cause inflammation and blood clots in the vessels of the brain and spinal cord. Animals with TME may show signs of depression, fever, blindness, and lack of coordination, and the disease often progresses rapidly to death.
In addition to TME, H. somni is a cause of respiratory disease and pneumonia within the BRD complex. Clinical signs include high fever, coughing, and labored breathing. The bacterium can also invade the heart muscle, causing myocarditis, which can lead to sudden death. Other conditions caused by H. somni include polyarthritis (inflammation of multiple joints), which causes lameness and swelling.
Immunity and Vaccine Mechanisms
The bovine immune system recognizes H. somni as a foreign invader, triggering a defensive response. When the bacteria enter the body, specialized immune cells identify proteins and other molecules on the bacterial surface, known as antigens. This recognition initiates the production of antibodies, which are proteins designed to attach to the bacteria and mark them for destruction. A successful immune response creates memory cells that allow for a faster and more effective defense upon future exposure.
Vaccines are designed to stimulate this immune response without causing the actual disease. The most common types of vaccines for H. somni are killed vaccines, also known as bacterins. These products contain whole H. somni bacteria that have been inactivated, meaning they can no longer replicate or cause illness. When injected, the inactivated bacteria present their antigens to the animal’s immune system, prompting it to produce specific antibodies.
Many H. somni vaccines are combination products that also protect against other pathogens involved in the BRD complex. These formulations often include adjuvants, which are substances that enhance the body’s immune response to the antigens in the vaccine. Adjuvants work by helping to keep the antigens at the injection site for a longer period, allowing more immune cells to respond and generate stronger immunity.
Vaccination Strategies for Cattle Herds
Effective vaccination against Histophilus somni depends on a well-planned strategy tailored to the specific type of cattle operation. The timing of vaccine administration is an important element in building a strong immune foundation. Different protocols are recommended for cow-calf herds versus feedlot operations.
For cow-calf producers, vaccination protocols often begin before weaning. Calves can receive their initial dose of an H. somni vaccine between two and four months of age, often in combination with other vaccines. A second, or booster, dose is recommended at or just before weaning, a high-stress period for calves. This two-dose approach helps prime the immune system and solidify a more robust and lasting immunity.
In feedlot settings, cattle arriving from various sources are at high risk for BRD. Vaccination upon arrival is a common practice to protect them from H. somni and other respiratory pathogens. Following the vaccine manufacturer’s label instructions is important for any operation type.
Factors Influencing Vaccine Performance
The success of a vaccination program is not guaranteed by the injection alone, as several factors influence vaccine performance. These elements can be related to the animal itself or to how the vaccine is handled and administered. Understanding these variables is part of a comprehensive herd health plan.
Animal-related factors play a significant role. High levels of stress from transportation, weaning, or commingling with new animals can suppress the immune system. A stressed animal may not mount an effective immune response to a vaccine. Nutritional status is also important, as cattle with deficiencies in energy, protein, or certain minerals may have a compromised ability to produce antibodies. Concurrent infections with other pathogens like Bovine Viral Diarrhea Virus (BVDV) can also hinder vaccine efficacy.
Human error in vaccine management can undermine a protocol. Improper storage, such as exposing vaccines to extreme heat or freezing, can damage the antigens and render the product ineffective. It is also important to use proper administration techniques. This includes following the manufacturer’s specifications for dosage, timing, and delivering the vaccine to the correct location with clean needles.