Worldwide, brucellosis remains a major source of disease in humans and domesticated animals. Although reported incidence and prevalence of the disease vary widely from country to country, bovine brucellosis caused mainly by B. abortus is still the most widespread form. In humans, ovine/caprine brucellosis caused by B. melitensis is by far the most important clinically apparent disease (Corbel 1997).
In 1887, British Army surgeon Sir David Bruce first isolated a bacterium he named Micrococcus melitensis, later named Brucella melitensis, from the spleens of soldiers dying of ‘Mediterranean or Malta fever.’ Bruce was able to infect monkeys, killing four of seven, produce an undulating fever in the survivors and re-isolate the bacterium. However, the source of the infection remained a puzzle (Tan and Davies 2011).
Nearly twenty years later, in 1905, Maltese doctor Themistocles Zammit found that five out of six goats reacted to the blood test for human brucellosis, still known as Mediterranean or Malta fever. This finding led to the discovery that apparently healthy goats could be carriers of the disease, a finding that has been described as 'one of the greatest advances ever made in the study of epidemiology.' It soon led to the eradication of the disease among British soldiers and ‘revolutionized ideas about animal vectors of disease’ (Wyatt 2005).
While sheep and goats are the preferred host of Brucella melitensis, it is regarded as the most pathogenic species of Brucella and can infect cattle, rodents and dogs. It is also the most common and serious zoonosis of the Brucella genus (Castrucci 2007, Aparicio 2013).
B. melitensis, the causative agent of Malta or Mediterranean fever, is, not surprisingly, prevalent in the Mediterranean countries of Spain, (and Portugal), Southern France, Italy, Greece and Turkey. It is also widespread in the Middle East, Central and South America and Asia.
While goats are the ‘classic and natural hosts,’ sheep are also favoured. B. melitensis infection is characterised by abortions or stillbirths in the last trimester of pregnancy in previously naïve does and ewes. Rams and bucks occasionally develop orchitis and epididymitis (Aparicio 2013).
Aborting and infected kidding and lambing animals shed large numbers of bacteria in the birth fluids, abortus, foetus and placenta. Bacteria are also shed in the milk and in the birth fluids of recovered animals (Aparicio 2013).
Bacteria shed by infected animals invade new cases via the nasopharynx (and potentially the skin). Bacterial spread may be halted in regional lymph nodes but if bacteria overwhelm phagocytes they spread via the circulatory system, to the udder and pregnant uterus, infecting the placenta and foetus, usually leading to abortion and chronic shedding (Castrucci 2007).
B. melitensis can survive for up to four months inside shelters, for about 35 days in the shade and for 15 days on pastures in the sun. It is killed by pasteurisation and common disinfectants (Castrucci 2007).
B. melitensis infection of cattle has emerged as a ‘serious public health problem’ in Israel, Kuwait, Saudi Arabia and some southern European countries. Infection is primarily through exposure to aborting sheep and goats. In cattle, B. melitensis abortions are rare but inapparent udder infections can lead to the shedding of large quantities of bacteria in the milk, hence the public health hazard. The problem is exacerbated because B. abortus vaccines do not adequately protect against bovine B. melitensis infection.
Of interest, an outbreak of B. melitensis infection in the 1300 head zero grazing dairy operation at the Kibbutz Revivim in the Negev Desert in Israel, in which 135 head were found to be infected and destroyed, is presumed to have originated from neighbouring flocks of sheep and goats, though the route of infection has not yet been determined (ProMED 2015).
Dogs (and presumably some other carnivores) can both become infected and spread the bacteria by translocating foetuses and placentae (Aparicio 2013).
As an aside, Corbel (1997) noted that in some areas in South America, B. suis has become established in cattle, which are now a more important source of infection than pigs.
B. melitensis is the most significant cause of human brucellosis. However, B. abortus, B. suis and B. canis are also human pathogens. B. ovis has not been shown to cause infection in humans but a case of laboratory acquired infection to B. maris suggests its zoonotic potential (Corbel 1997).
It is estimated that half a million new human cases of brucellosis develop each year throughout the world. In the acute form, symptoms include a waxing and waning (undulant) fever that may progress to the chronic form characterised by headaches, joint, muscle and abdominal pain, anorexia, weight loss and depression. The bacterium can localise in the skeletal system (Mousa et al 1987), heart valves (Al-Kasab et al 1988), urinary tract (Kelalis 1962) and nervous system (Lulu at al 1988 amd Annesley 1968).
Brucellosis is most commonly acquired when people living or travelling in endemic areas consume contaminated (unpasteurised) dairy products or contact infected animals (particulary infected vaginal discharges, foetuses or placentae). However, brucellosis can be acquired by the respiratory and conjunctival routes and has on rare occasions been (presumed sexually) transmitted from person to person (Vigeant et al 1995).
In a recent Portuguese case, 13 people were confirmed to have become infected with ‘Malta fever’ after eating cheese ‘produced in an artisan fashion,’ (presumably from unpasteurised milk) from an unauthorised home-based factory (ProMED 2014). In Algeria the number of human brucellosis cases, which has tripled since 2013 (from 113 to 363 cases annually), is attributed to the sale of ‘healthy, naturally sterile’ raw cow, goat and camel milk sold in plastic water bottles. In turn the increased incidence of brucellosis is the result of both livestock owners and consumers ignoring hygiene practices, refusing to vaccinate their livestock (fearing that vaccination leads to abortions) and the use of infected male animals (ProMED 12/18/2015).
In Australia, people have become infected after eating imported unpasteurised cheese (NSW Health 2012).
Brucellosis is regarded as a serious risk to laboratory workers and is readily acquired from cultures and infected samples.
There is an Ausvetplan for B. abortus but not for B. melitensis. Presumably given the similarity of the diseases, some of the observations on B. abortus apply equally to B. melitensis. The Ausvetplan (2005) for bovine brucellosis states that;
‘The greatest risk of introduction of bovine brucellosis would be with cattle imported from countries with endemic infection. With current import requirements, however, this method of introduction is unlikely. The disease could also be introduced with imported semen but this risk is minimised by effective import controls.’
Travellers, infected overseas, have introduced B. melitensis to Australia but a reverse zoonosis seems unlikely.
B. melitensis introduced into an Australian sheep or goat flock would be expected to cause a spectacular abortion storm in a naïve population. Such an event would be observed in flocks grazing under more intensive conditions but could pass unnoticed in flocks run under extensive conditions. Foxes and wild dogs would be expected to spread the infection locally while the transport and sale of shedding and latently infected animals would spread the disease more widely. It would pose a serious threat to people exposed to aborted foetuses and to vaginal and mammary discharges, including farmers, veterinarians and laboratory and abattoir workers.
A routine abortion investigation should detect B. melitensis, presumably initially through bacterial isolation. Because the bacterium is intracellular, it may exist without being detected by serological methods.
Many countries have eradicated B. melitensis using test and cull methods. The techniques used in Australia to eradicate B. abortus from our cattle herd would no doubt be valuable in eradicating B. melitensis from our sheep and goat (and possibly cattle) populations.