Porcine brucellosis: another common zoonosis in pigs

Since its identification in April 2009, an influenza A (H1N1) virus containing a unique combination of gene segments from both North American and Eurasian swine lineages has continued to circulate in humans, spreading globally. Indeed the first name the media gave to the novel virus was “swine flu virus,” though the virus is human-to-human transmitted, not pig-to-human.

One of the results of the publicized porcine history of the virus was a dramatic fall in the consumption of pork in Mexico for several weeks; another one was the recent mass-culling of pigs in Egypt, which has become a subject of heated discussions. Though its timing may be indicative of relevance to the “swine flu” scare, Egyptian authorities insisted that the culling of the Cairo pigs was planned several years ago in order to address public-health issues – including zoonoses – which are attributed to the maintenance of pigs in urban setting.

More than 30 diseases are considered common to man and pig. Most of them, such as salmonellosis, trichinellosis, toxoplasmosis, Q fever, leptospirosis etc may involve other animal species as well, while others are specifically porcine-related, such as Taenia solium (the cestode causing human cysticercosis), Streptococcus suis and Bruclla suis. The latter, one of the six recognized Brucella species (B. melitensis, B. abortus, B. suis, B. ovis, B. canis, and B. neotomae), is the subject of this review.

Porcine brucellosis

Porcine brucellosis is an infection caused by biovar 1, 2 or 3 of Brucella suis. It occurs in many countries where pigs are raised. Generally, the prevalence is low; but in some areas – such as South America and South-East Asia – the prevalence is much higher. B. suis biovars 1 and 3 are found worldwide; biovar 1 infections have been reported from feral pigs in some of the southern United States and in Queensland, Australia. In both areas, a number of human infections have been reported from people who hunt and handle material taken from feral pigs.

In studies conducted in South Carolina, the prevalence of infection in wild and feral swine ranged from 14% to 44%; in one case, the percentage of infected pigs in a location also varied over time, from 28% in 1976 to 18% in 1992 and 44% in 1999. In another study, 6% of the wild and feral pigs sampled in eleven southeastern states and Hawaii were seropositive.

Biovar 2 occurs in wild boar in much of Europe, implicated as the source of transmission to outdoor reared pigs. In addition to wild swine, the European hare (Lepus capensis) is also a reservoir for B. suis biovar 2 and has been implicated as a possible source of transmission to domestic livestock.

Transmission

Porcine brucellosis is generally transmitted by consumption of feed contaminated by birth and/or abortion products and uterine discharges. Pigs will readily eat aborted fetuses and membranes. Transmission during copulation also occurs frequently, and this has implications for those practicing artificial insemination. The initial bacteremia can persist for up to 90 days. Some animals recover from the infection, while others remain permanently infected.

B. suis can be spread on fomites, particularly feed and water. In conditions of high humidity, low temperatures, and no sunlight, these organisms can remain viable for several months in water, aborted fetuses, manure, wool, hay, equipment and clothes. Survival is longer when the temperature is low, particularly below freezing.

Clinical signs

The most common symptoms are abortion and weak or stillborn piglets. Weak piglets may die before weaning. Vaginal discharge is often minimal in pigs that abort, and abortions may be mistaken for infertility. Occasionally, some sows develop metritis. B. suis can also cause epididymitis and orchitis in boars. Swollen joints and tendon sheaths, accompanied by lameness and incoordination, can occur in both sexes. Fertility can be permanently impaired, particularly in boars.

The period between infection and the development of reproductive signs is variable; abortions can occur at any time during gestation.

The morbidity rate varies with the length of time B. suis has been in the herd. When this organism is first introduced into a herd, there may be a significant increase in returns to service, abortions and stillbirths, weak piglets, lameness/ arthritis, posterior paralysis and other signs.

Brucellosis caused by B. suis biovar 2 differs from infection caused by biovars 1 and 3 in its host range, its distribution and in its pathology.

Diagnosis

Porcine brucellosis can be difficult to diagnose and is usually recognized as a herd problem rather than a disease of individual swine. It should be suspected when reproductive failure in sows, orchitis in boars and lameness, arthritis or paralysis occur together in a herd. For differential diagnosis, diseases causing abortion, orchitis, arthritis, posterior paralysis and lameness should be considered. Porcine abortions can be caused by Aujeszky’s disease, leptospirosis, erysipelas, FMD, salmonellosis, streptococcidiosis, classical swine fever and porcine parvovirus infection.

Laboratory tests may include microscopic examination of stained smears for a presumptive diagnosis but serology is considered to be more reliable for identifying infected herds. Serological tests include indirect or competitive ELISA, rose bengal test and complement fixation. A brucellin allergic skin test is widely used to identify infected herds in some countries, but is not generally used in the United States.

A definitive diagnosis can be made if B. suis is cultured from an animal. This is done on a variety of selective media. B. suis can be identified to the species and biovar level by phage typing and cultural, biochemical and serological characteristics.

Polymerase chain reaction (PCR) techniques are available in some laboratories.

Control

B. suis is usually introduced into a herd in an infected animal. Semen can also be a source of infection. Herd additions for uninfected herds should come from brucellosis-free states, countries or accredited herds whenever possible. Animals from other sources should be isolated and tested before adding them to the herd. Since B. suis exists in wild and feral swine in some countries, including the United States, domesticated swine should always be kept from contact with these animals. In the United States, there are programs to control the spread of brucellosis in wild and feral swine populations.

Brucellosis can be eradicated from infected herds by depopulation or test and removal methods, combined with quarantine of infected herds. In the United States, B. suis has been eradicated from commercial swine, and any infected herds found are quarantined and depopulated.

No vaccine currently exists for B. suis in most countries. A B. suis strain 2 vaccine is used to immunize pigs in China, but has not gained acceptance elsewhere.

Public health

B. suis biovars 1 and 3 are serious human pathogens. Human infections are usually the result of occupational exposure in laboratory workers, abattoir workers, farmers, herders, veterinarians and others who contact infected animals or tissues. Laboratory manipulation of the cultures or contaminated material from infected animals must be done under strict biosecurity conditions to safely handle this dangerous zoonotic agent. Biosecurity containment level 3 is recommended.

In Queensland, Australia, a number of infections have been reported in hunters who handled tissues from feral pigs. Biovar 2 has been reported very rarely from humans.

Some Brucella infections are asymptomatic in humans. In symptomatic cases, the disease is extremely variable and the clinical signs may appear insidiously or abruptly. Typically, brucellosis begins as an acute febrile illness with nonspecific flu-like signs such as fever, headache, malaise, back pain, myalgia and generalized aches. Drenching sweats can occur, particularly at night. Some patients recover spontaneously, while others develop persistent symptoms that typically wax and wane. Occasionally seen complications include arthritis, spondylitis, chronic fatigue, and epididymo-orchitis. Neurologic signs (including personality changes, meningitis, uveitis and optic neuritis), anemia, internal abscesses, nephritis, endocarditis and dermatitis can also occur. Other organs and tissues can also be affected, resulting in a wide variety of syndromes. Treatment is with antibiotics; however, relapses can be seen months after the initial symptoms, even in successfully treated cases. The mortality rate is low; in untreated people, estimates of the case fatality rate vary from less than 2% to 5%. Deaths are usually caused by endocarditis or meningitis.

Arnon Shimshony, DVM, is Associate Professor at the Koret School of Veterinary Medicine Hebrew University of Jerusalem, Rehovot, and is the ProMED-mail Animal Diseases Zoonoses Moderator. Dr. Shimshony was Chief Veterinary Officer, State of Israel, from 1974 to 1999.