Pallvi Slathia, Sanmeet Kour
Introduction: Brucellosis is a contagious that has a severe economic impact on livestock. The disease is caused by bacteria from the Brucella family, which infect only certain animal species. Most Brucella species, however, can also infect other animals. It affects cattle, goats, sheep camels, swine equines, and dogs. It may also infect other ruminants and is of zoonotic importance. In animals it is characterized by reproductive failure and abortions. The infected animals usually recover and will have normal gestation period following initial abortion but may shed bacteria continuously. Several species of the genus Brucella are responsible for the zoonotic bacterial disease. Animals most frequently experience reproductive losses, whereas people can experience a crippling general sickness or targeted organ involvement. Each Brucella species typically has a specific host, although other animals can get infected as well, especially if they come into close contact. The typical host for Brucella abortus is cattle.
Brucella abortus, a Gram-negative coccobacillus or short rod of the family Brucellaceae (class Alphaproteobacteria), is the main cause of brucellosis in cattle and other members of bovidae. There are presently eight recognised B. abortus biovars (1–9), including the recently reinstated biovar 7. Other Brucella species that may be present in cattle include B. suis, B. canis and B. melitensis, which may be significant in cattle in some nations.
Cattle frequently contract B. abortus by coming into contact with bacteria found in birth products (such as the placenta, foetus, and foetal fluids from infected animals) and vaginal discharge from infected animals. Although the two main modes of transmission are believed to be ingestion and transmission through mucous membranes, germs can also enter the body through abraded skin.
In many cases, cattle remain infected for years or indefinitely. They can shed B. abortus whether they abort or carry the pregnancy to term, and reinvasion of the uterus can occur during subsequent pregnancies. B. abortus is also shed in urine, semen and milk. Shedding in milk may be intermittent. The mammary gland is usually colonized during a systemic infection; however, organisms can also enter from the environment via the teats.
Cattle frequently carry an infection for years or forever. They’re able to expel B. abortus whether or not they have an abortion and re-invasion of the uterus can happen after pregnancies. In addition, B. abortus is also excreted in milk, urine and semen. In milk, shedding is intermittent. The mammary gland colonisation typically occurs during a systemic infection, organism can also enter via the teats. B. abortus can spread to suckling calves in some cases, and some calves may be born with the infection. Until they give birth or have an abortion, young animals with persistent infections can be undetected by diagnostic procedures, including serology. Contaminated syringes are another example of an iatrogenic source. B. abortus can be ingested by humans, and it can also be spread by abraded skin, contaminated mucous membranes (such as the conjunctiva and respiratory tract).
By employing modified Ziehl-Neelsen staining, B. abortus can be found by microscopic inspection of stained smears from tissues, secretions, and exudates (such as the placenta, vaginal discharges, or the contents of the embryonic stomach). Clinical Signs:
The most common clinical signs in cattle are abortions (usually during the second half of pregnancy), stillbirths and the delivery of underdeveloped offspring. Weak calves may die soon after birth. Most animals only have one miscarriage. The majority of future pregnancies are healthy. There is reduced lactation. Mastitis typically does not exhibit any clinical symptoms despite the fact that B. abortus is shed in milk. The retention of the placenta and subsequent metritis are potential complications of reproductive losses; however, these conditions are not typically accompanied by symptoms of disease. Bulls can sometimes develop testicular abscesses, orchitis, epididymitis, or seminal vesiculitis. Metritis or orchitis/epididymitis can occasionally cause infertility or impaired fertility in both sexes. Hygromas and arthritis can also develop, especially with persistent infections. Except for the foetus or newborn, deaths are uncommon. Infections in cows that are not pregnant are typically asymptomatic.
Aborted foetuses may be autolyzed, appear normal, or show signs of a widespread bacterial infection, such as an enlarged spleen, liver, or lymph nodes, or an excess of serohemorrhagic fluid in the body cavities and subcutaneous tissues. Exudate may be seen on the placenta’s surface, and it may be oedematous and hyperemic. The placentomes can have different degrees of damage, ranging from mild necrosis and bleeding to severe necrosis and extensive lesions. Frequently, the intercotyledonary regions are thickened. Males may experience epididymitis, orchitis, and seminal vesiculitis with inflammatory lesions, abscesses, or calcified foci. Fibrosis and adhesions may cause the tunica vaginalis to thicken. The testicles may atrophy under chronic conditions. Metritis, which can cause nodules, abscesses, fibrinous necrotic exudates, and haemorrhages, might affect some females. Other organs and tissues, including the lymph nodes, liver, spleen, mammary gland, joints, tendon sheaths, and bones, can occasionally develop abscesses and granulomatous inflammation. Some animals may exhibit hygromas.
They resemble coccobacilli or short rods and are often grouped alone, though occasionally pair up or form tiny clusters. B. abortus may be isolated from the placenta, milk, colostrum, the secretions of nonlactating udders, semen, the testis or epididymis, and sites of clinical localization like infected joints or hygroma fluids. It can also be isolated from aborted foetuses (stomach contents, spleen, and lung), the placenta, vaginal swabs, milk, spleen, various lymph nodes (such as the supramammary, retropharyngeal, and vaginal lymph nodes), the pregnant or early postparturient uterus, the udder, and male reproductive organs are recommended specimens to take at necropsy. Molecular techniques such as PCR can be used for identification of Brucella in samples. Loop-mediated isothermal amplification (LAMP) assays, immunostaining/ immunohistochemistry (antigen detection techniques) can also be used. Serological test such as Rose Bengal Plate Test (RBPT), complement fixation, fluorescence polarization assay (FPA) and indirect or competitive ELISA can be used for diagnosis.
Control, eradication and management of brucellosis
Strict biosecurity measures on farms, test and slaughter policies and immunization of the susceptible population should be followed as control and eradication strategies. The most effective course of action will rely on a number of factors, including the epidemiological context, and resource accessibility.
In order to prevent Brucella infection, management and hygienic actions must also be directed at reducing the likelihood of interaction with live Brucella, including affected animals and contaminated environments. These measures include implementing quarantine before the introduction of new animals, separating animals whose status is unknown or uncertain, controlling animal movements, managing replacements appropriately, isolating pregnant females before parturition and strictly enforcing sanitary and quality standards for semen. Avoiding or restricting contact between cattle and wildlife during artificial insemination in areas where wild animals have been suspected of being a source of infection. Nomadism, grazing with animals from diverse origins and the usage of shared pastures are some management/farming techniques that may favor the spread of the bacteria and reduce the efficiency of control strategies.
Test and slaughter policy
This strategy’s primary goal is the early identification and elimination of potentially infectious animals in order to stop the spread of Brucella. Despite the efficiency of the diagnostic method employed, there is always a possibility of having sick animals that may continue to act as silent carriers , preserving the virus in the flock and, if there is a decline in the herd’s immunity, may result in an abortion storm. This tactic works best in low-prevalence areas where there are financial resources and veterinary knowledge to support it. When the quantity of animals involved makes the adoption of stamping-out procedures impractical, test and slaughter strategies may also be helpful for managing outbreaks.
In some instances, the stamping out of the flock, followed by a thorough cleaning and disinfection, and replacement with animals free of Brucella, is the only method that completely eradicates the bacterium of the flock.
The most widely used vaccine for the prevention of brucellosis in cattle is the B. abortus S19 vaccine. It is typically administered as a single subcutaneous dose to female calves between the ages of 3 and 6 months as a live vaccination. Brucella abortus S19 and Brucella melitensis Rev. 1 vaccines have been widely used in some developed countries because it is crucial to control brucellosis in the animal population. However, both vaccines cause abortions in pregnant animals and are dangerous to humans; additionally, they cause the production of anti-Brucella antibodies, which obstruct sero-diagnosis. Abortus strain 45/20 is a rough strain that can prevent Brucella infection in guinea pigs and cattle, but its application as a live vaccine has been constrained by reversions to the wild smooth form, B. abortus strain S19 has been replaced by B. abortus strain RB51, a rough attenuated bacterium that was originally produced from a rifampicin-resistant mutant of B. abortus strain. In addition to being extremely stable, strain RB51 has very little to no ability to cause abortions.
Several subunit vaccinations have been studied due to safety concerns. Subunit vaccinations also offer the benefit of being effective against all strains of the Brucella virus since a candidate protein with high homology can be chosen.