Herd Immunity an Important Concept

The goal of herd immunity is the reduction of infection or disease in a population’s susceptible segment.

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The term “herd immunity” has come into common usage in recent months.

“This is an important term and is a concept that is critical to understand as it pertains to human and animal health,” says Gerald Stokka, North Dakota State University Extension veterinarian and livestock stewardship specialist.

The concept of herd immunity is a companion concept to individual immunity.

“Individual immunity is important to us all as immunity is critical to our very survival,” Stokka says. “We are constantly exposed to potential disease-causing organisms, and yet the vast majority of people are asymptomatic (show no obvious signs or symptoms of disease) or have only mild symptoms for a short period of time.”

A good example of the importance of individual immunity is the practice of vaccinating horses to protect them against the disease caused by West Nile virus. The West Nile virus is transmitted to horses through the mosquito. This virus cycles between mosquitoes and bird populations, with horses and humans becoming infected when bitten by an infected mosquito.

Horses, humans and other mammals are known as dead-end hosts because they cannot infect other like species. Preventing the mosquito bite is difficult and thus protection through vaccination is important. The vaccination of mosquitoes and wild birds is logistically impossible at this time; thus, the need exists for an effective vaccine in the horse population.

The three objectives of vaccination are to:

•Induce protection against disease

•Reduce susceptibility of an individual to infection

•Reduce infectivity (shortens time and severity) after the occurrence of an infection

In contrast, some disease-causing infectious agents are transmitted in a different manner. An example in cattle populations is the virus known to cause the disease bovine virus diarrhea, or BVD. This disease is misnamed because it rarely causes diarrhea, but it may play a role in a host of other disease manifestations, such as respiratory disease, by compromising the immune system of the animal, making it more susceptible to other infections.

This virus is spread from animal to animal through nose-to-nose contact, and also likely through the fecal oral route. The infection is the most harmful when the developing fetus is infected.

Exposure and infection that occur early in gestation, up to 60 days, may result in absorption or expelling of the fetus. When this occurs later in gestation, from approximately day 60 to 180, abortion may result or the virus may not be recognized as foreign by the developing fetal immune system.

Provided the calf survives this infection, the calf is born with the virus and will shed (ex