Brucellosis

C. A. MANTHEI, A. K. KUTTLER, AND E. R. GOODE, JR.

BRUCELLOSIS is an infectious, widespread, and costly disease that mainly affects cattle, swine, and goats. It may affect other species of domestic and wild animals and can be transmitted to people.

Brucellosis is caused by bacteria of the genus Brucella, which includes three principal species: Brucella melitensis, Brucella abortus, and Brucella suis.

Br. melitensis, the first of the three species to be identified, was reported by David Bruce of England in 1887 on the Island of Malta and is the main cause of brucellosis in goats. Br. abortus, discovered by Bernard Bang of Denmark in 1897, is the principal cause of brucellosis in cattle. Br. suis, isolated by Jacob Traum of the United States in 1914, is primarily responsible for the disease in swine.

Although each of the three species of Brucella is relatively specific for individual species of animal, all can produce infection in other species and people. Brucellosis therefore must be considered as a single problem and not as three individual control problems.

Brucellosis of cattle was formerly known as Bang's disease and infectious abortion. It is prevalent in every cattle-raising country of the world.

The most frequent cause of brucellosis of cattle is Br. abortus, but Br. suis and Br. melitensis occasionally may infect cattle.

Brucellosis commonly is transmitted from infected animals and a contaminated environment to susceptible cattle by close association. Infection is believed to take place mainly through the alimentary tract.

Aborted fetuses, placental membranes, placental fluids, and the vaginal discharge that persists for several weeks after an animal has aborted teem with virulent Brucella. Cows may lick those materials and the genital organs of other cows and so are exposed to the infective agent.

Exposure to infection also takes place by ingestion of contaminated grass, dry roughage, and water. Brucella also may enter through the skin and the mucous membranes of the body openings, such as the eyes and nostrils.

All our evidence indicates that the disease is not transmitted directly from infected bulls to cows by natural service. It has been transmitted to susceptible females, however, by the intrauterine method of artificial insemination with Brucella-contaminated semen from an infected bull. Transmission has not been successful by the intracervical method of artificial insemination with semen from the same infected bull. Probably the greatest potential danger of spreading infection by bulls is by contaminating the surroundings with semen and urine or the discharge of infectious semen from the vagina of cows soon after breeding.

The course of brucellosis is related to the susceptibility of the individual and the number and virulence of Br. abortus organisms that gain entrance to the body. The incubation period the time organisms enter the body until manifestations of the disease appear may be 7 days to 7 months or longer.

Cattle generally are resistant to Br. abortus before reaching sexual maturity and become increasingly susceptible as they approach breeding age. Pregnant cattle usually are highly susceptible to infection. The disease is more progressive and severe in them than in nonpregnant females. Sexually mature bulls usually are more resistant to infection than cows and sexually mature heifers.

A factor, other than natural immunity, that also could be responsible for the lower incidence of brucellosis in males than in females is management. Because most dairy bulls and many beef bulls are maintained separately from the herd, their exposure to infection is lessened.

If an animal is susceptible and adequate numbers of virulent Br. abortus enter the body, a progressive infection takes place. The organisms are taken up by the blood and carried to the various tissues and organs, where multiplication occurs. The udder, uterus, testicles, seminal vesicles, lymph glands, and spleen are affected oftenest.

The first manifestation of infection usually is development of Brucella antibodies in the blood stream. The one antibody most commonly recognized is called an agglutinin and is measured by the blood serum agglutination test, which is used for establishing the brucellosis status of cattle.

The predominant symptoms of brucellosis in pregnant females are abortions, birth of weak calves, retained placentas, and vaginal discharge, often followed by temporary or permanent infertility. Milk production is reduced approximately 25 percent because of the alteration of the normal lactation period by abortions and delayed conceptions.

Symptoms are generally absent in mature females that are infected while open. Cattle infected by intrauterine insemination with infectious semen, however, show a pattern of repeat breeding similar to that observed in other venereal diseases vibriosis and trichomoniasis. In either case udder infection occurs frequently.

Infected bulls may or may not show clinical evidence of brucellosis. The most apparent symptoms are enlargement of one or both testicles, loss of sexual desire, and infertility. There is usually involvement of one or more accessory genital organs, but that can be determined only by rectal examination.

If an animal is relatively resistant to brucellosis, or the number and virulence of the invading organism are low, or several of these factors are present, the infection may not develop or it may be temporary or become localized in some part of the body without clinical manifestations.

Br. abortus infection that becomes localized, particularly in the udder and supramammary lymph glands, has a tendency to persist for the productive life of most cattle.

Infection of cattle with Br. suis is infrequent. This infection is usually temporary, but when it becomes permanently established, it localizes most frequently in the udder and adjacent lymph glands, but clinical symptoms usually are absent.

The prevalence and significance of Br. melitensis infection in cattle in this country are not clearly defined. Results of limited studies indicate that it is a relatively benign disease in cattle, causing localized udder and uterine infection with an occasional abortion.

RELIABLE AND PRACTICAL diagnostic procedures are essential for the successful control of diseases. The principal procedures employed for the diagnosis of brucellosis of cattle are the tube and plate seroagglutination tests and the milk-ring test. Herd history and clinical symptoms are used as supplemental evidence.

Isolation of Brucella from the animal is the most accurate diagnostic procedure, but it has limited practical value in the field. All diagnostic tests have definite limitations of accuracy and practicability, but the limitations can be held to a minimum by standardizing the Brucella antigens, reference serums, equipment, techniques, and interpretations.

Studies were begun in 1938 by the Bureau of Animal Industry to standardize the tube and plate seroagglutination tests at the request of the United States Livestock Sanitary Association, an organization made up of all segments of the livestock industry. The studies included selection of a highly antigenic strain of Br. abortus, development of procedures for the selection and maintenance of smooth colony forms, production of adequate quantities of organisms, and development of standard methods and equipment for preparing antigens with a constant sensitivity.

The first standard Brucella tube and plate antigens were made available in 1940 for the diagnosis of brucellosis in livestock. This was followed by adoption of standard equipment in all State-Federal testing laboratories. Procedures were then inaugurated to standardize the interpretation of the tests. Reference serum samples, along with both tube and plate antigens, were submitted annually or semiannually to each official testing laboratory in this country. Results of tests in each laboratory are returned to the central antigen laboratory at Beltsville, Md., for compilation and evaluation. From 1948 to 1955, 91.88 percent of the tube-test and 91.07 percent of the plate-test interpretations agreed with the standard interpretation. Furthermore, there was 99.12 percent agreement between the tube and the plate test.

The tube and plate seroagglutination tests are the most practical, accurate methods of diagnosing brucellosis in the individual animal. Both are used principally to measure the concentration of Brucella agglutinins in blood serum but may also be used to detect the same antibody in milk whey and semen plasma.

Specific agglutinins are produced in the body of animals by exposure to Brucella. As the disease progresses, the concentration of Brucella agglutinins increases. The concentration of agglutinins is measured by mixing increasing dilutions of serum with a standard amount of Brucella antigen (suspension of Brucella cells). When agglutinins are present, they cause the Brucella cells to clump and settle out of suspension. Complete agglutination (clumping) in the 1:100 dilution of serum is considered evidence of infection in non-vaccinated cattle more than 6 months of age.