Bacterial diseases-brucellosis, Biology


Brucellosis among cattle, buffaloes, sheep, goats, pigs produces heavy economic losses due to abortions in late pregnancy, infertility and reduction in productivity. Human beings are also affected.

Causative organisms:
Brucella abortus - Cattle, buffalo (bovine brucellosis, Bang’s disease)

Brucella melitensis - Sheep and goat
Brucella suis - Pig
Brucella ovis -Sheep and goat
Brucells canis - Dog

The organisms are gram-negative intracellular and non-motile having the appearance of a bacillus or cocco-bacillus. Some require increased carbon dioxide (CO 2) for initial isolation. The organisms are destroyed by common disinfectants, sunlight and
desiccation. They are also killed during pasteurisation of milk. The organisms survive for a long time in damp shady places.
Transmission: In a herd the disease is introduced by the introduction of infected cows or less commonly by infected materials or very rarely by a horse with fistulous wither. Under natural conditions the cattle get infection by ingestion of food and water  contaminated with uterine discharges of aborted animals, via vagina during coitus with infected bull or with infected semen, through the skin or conjunctiva and by inhalation. Non-infected bulls usually don’t get infected through servicing an infected cow. Aborted foetuses, foetal membranes, vaginal discharges, milk and faeces of infected animals contain the organisms, which contaminate the environment, feed and water. Infected cattle continue excreting the organisms even though they may not abort.

Symptoms: The organism has a predilection for the gravid uterus and causes varying degrees of placentitis and abortion. The young ones are relatively less susceptible. In non-pregnant animals, the udder and supramammary lymph glands get infected. Mastitis may be observed. Among bulls, the common sites of infection are testicles, seminal vesicles and epididymis.
The characteristic symptom among majority of cattle in a herd is abortion after the fifth month of pregnancy, usually seventh or eighth month. In subsequent pregnancies, the foetus is usually carried to full term but second or third abortion may take place in the same cow. In herds with endemic infections, retention of placenta and metritis are common sequelae to abortions or full-term parturition. In the bull, orchitis and epididymitis occur. One or both scrotal sacs may be affected with acute, painful swelling.
Lesions: The lesions in the gravid bovine uterus develop as necrotic placentitis. Some of the cotyledons become swollen and hyperaemic; the inter cotyledonary spaces become thickened and have leathery appearance.

Without laboratory facilities, diagnosis is difficult. When abortion takes place, the aborted foetus should be taken in a sealed container over ice to the laboratory. Brucella organisms can be seen in films prepared from fresh samples of foetal stomach contents and uterine discharges. The organisms can be isolated from uterine discharges of aborted animals, stomach contents and heart blood of an aborted foetus. Guinea- pigs are susceptible to infection when suspected material is inoculated subcutaneously or intraperitonealy develop “orchitis and epididmyitis” (Strauss reaction). Lesions are examined after 3-6 weeks in the spleen, liver and lymphatic glands.A variety of serological tests are applied for the diagnosis of infected animals. The antibodies develop in the infected animals. The agglutination observed in paired sera at 1:40 (80 IU) and above is considered as positive in bovines (except breeding bulls) and other animals including human. Agglutination at 1:20 dilution is considered positive for breeding bulls, sheep and goats. These antibodies are detected by agglutination test. Macroscopic agglutination tests, microscopic agglutination tests, tube agglutination tests could be used. The antibodies in the milk are detected by milk ring test. Immunoblotting techniques and ELISA techniques have also been standardised and commercial kits are now available.

Treatment: The antibiotics are effective in making the organisms disappear, but they invariably reappear when the treatment is stopped. Moreover, treatment is carried out for a prolonged period. Therefore, the treatment is not recommended.Prevention and control: The general basis for the control and prevention of  brucellosis in cattle are hygienic measures, identification and elimination of infected animals from herds, and vaccination to increase the resistance of animals to natural infection. Hygienic measures include isolation or disposal of infected animals, disposal of aborted foetuses, placental and uterine discharges and disinfection of contaminated areas. All the animals brought at the farm should be tested. The pregnant cows brought at the farm should be kept in isolation until parturition and tested after three weeks by agglutination test. If negative, only then the animal should be introduced in the herd.

In India, vaccination with Br. abortus strain 19 is practiced for the control of brucellosis in a herd. The vaccine is inoculated in calves aged 91-180 days. It stimulates the development of high level of immunity and transient agglutinins which disappear by the time of first calving. One dose of vaccine gives an immunity which lasts till fifth pregnancy. With concurrent serological tests for the detection and removal of positive animals from herd, the vaccine has enabled the incidence of abortion to be low. Adult cattle are not usually vaccinated, but it may be of value in reducing the effects of abortion storm. The vaccine strain 19 could be virulent for human beings thus care should be taken during the vaccination. The organisms in the vaccine become localized in udder and are excreted in milk. The milk from vaccinated animals should be properly pasteurized or boiled before use. Male animals are not vaccinated as it may cause infertility. This vaccine may be safely used in pregnant animals.

Posted Date: 9/17/2012 5:42:08 AM | Location : United States

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