Bovine respiratory disease (BRD) is a syndrome that encompasses a variety of factors. Many producers would like to believe that BRD will never occur if the right set of “shots” is administered to calves, says Roger Winter, DVM, AgriLabs. Although vaccination can help prevent BRD, or at least reduce the incidence and severity of the disease that occurs, a number of other factors are involved.
Good calf health starts off with adequate passive immunity from quality colostrum. Viral vaccination helps prevent disease caused by viral infection but only helps prevent bacterial pneumonia by reducing the predisposing effects of the virus. Calves can suffer from bacterial pneumonia with or without viral involvement. Under stress conditions caused by weather, malnutrition, or management procedures, both innate and acquired immune functions can be suppressed, allowing for development of pneumonic pasteurellosis with severe lung damage caused by Mannheimmia haemolytica and the leukotoxin this bacteria produces.
M. haemolytica is frequently a normal resident of the nasal-pharyngeal area of the upper respiratory tract. Under the right conditions, usually some stress such as severe weather or poor ventilation in a confinement facility, M. haemolytica is able to move down into the lower respiratory tract and begins to proliferate. As it grows in the deeper lung tissues, it releases leukotoxin, a compound that damages and kills white blood cells. As white blood cells die, they release their toxic granules which are responsible for most of the tissue damage that takes place.
Only ruminants are susceptible to the effects of M. haemolytica leukotoxin. The ruminant leukocyte has surface receptors specific for leukotoxin that are not found on leukocytes of other animals. IBR infection increases the susceptibility of leukocytes to leukotoxin by increasing the expression of the receptor on the surface of the cell.
Adequate immunological protection against M. haemolytica involves stimulating antibody production against cellular components as well as leukotoxin. Antibodies that attach to the surface of the bacteria itself will initiate the opsonization process whereby the bacteria is engulfed and destroyed. Since dead bacteria don’t produce leukotoxin, this portion of the immune response is important.
Antibodies to leukotoxin are also vitally important in order to reduce the lung damage caused as the war takes place. For these reasons, a vaccine which contains antigenic fractions of both a cellular and leukotoxoid component is a good choice, as these dual antigens will stimulate the production of antibodies that can kill the bacteria as well as neutralize the leukotoxin that is produced.
It is important to understand what a vaccine for M. haemolytica can or cannot do. As we have all experienced in practice, sometimes producers have an unrealistic expectation that vaccinated animals are guaranteed to be free of respiratory disease. Respiratory bacterins will help reduce the incidence of disease in a group of calves and the severity of disease in an individual animal, but given the right set of stress causing management or environmental conditions, respiratory disease may still occur.
Although vaccines for pasteurellosis are often labeled for a single dose, most product labels point out that a subsequent dose may be indicated. Revaccination prior to expected stress may be worthwhile.
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