Zoonoses are 'those diseases and infections which are naturally transmitted between vertebrate animals and man' (WHO, 1959). Of the emerging human infectious diseases approximately 60%-70% are zoonoses (Jones et al, 2008). An emerging zoonosis is defined by WHO as 'a zoonosis that is newly recognised or newly evolved, or that has occurred previously but shows an increase in incidence or expansion in geographical, host or vector range' (WHO, 2004). There are more than 40 zoonotic diseases currently recognised of public health significance in NSW (Adamson et al, 2011). People with a zoonotic illness frequently present with non-specific symptoms so under-diagnosis may occur. Cross reactivity of serology and the need for acute and convalescent sera may also result in under-diagnosis. Exposure factors are important in determining the likelihood of disease.
Leptospirosis and brucellosis are emerging diseases in parts of NSW, and have human health and animal health issues that are closely linked.
Leptospirosis is a bacterial zoonotic disease caused by one of nine pathogenic Leptospira species. Humans usually become infected through broken skin or mucous membrane contact with water, food, soil or vegetation contaminated with urine from infected animals. The incubation period is 5 to 14 days (range from 2-30 days). The illness usually lasts from a few days to 3 weeks. Outbreaks are commonly associated with exposure to flood water contaminated with the urine from infected animals, but exposure during mouse plagues are also of concern (NSW Health, 2012).
Ninety-seven notifications of leptospirosis in humans were reported for the period 2009-2012 in NSW. The peak being in 2011 with 39 notifications, this coincided with a mouse plague in south-west NSW. More than 80% of notifications are in males with the peak age groups being 30-34 years and 50-54 years (NCIMS, 2013a).
Brucellosis is a zoonotic infection caused by Gram-negative aerobic coccobacilli from the Brucella genus (Bossi et al, 2004). Four Brucella species are associated with moderate to significant human pathogenicity, specifically Brucella melitensis which is found primarily in goats, Brucella suis in pigs, Brucella abortus in cattle and Brucella canis in dogs. Humans have also very infrequently been infected with Brucella species from marine mammals (Van Bressem et al, 2009). Globally B. melitensis more frequently affects humans than the other Brucella species and is the most virulent, pathogenic and invasive species, followed by B. suis, B. abortus and B. canis. B. abortus has been eradicated from Australia and B. melitensis and B. canis are not found in Australia (Newman et al, 2010).
Brucellosis symptoms in humans are non-specific and may include undulant fever, sweats, malaise, anorexia, headache and back pain. The onset can be insidious or acute, generally beginning 2-4 weeks after exposure, but can occur up to 6 months later (MMWR, 2009). Depression and chronic infection can occur (Young, 2009) with delays in diagnosis increasing the risk of complications (Eales, 2010).
Human infection with B. suis follows occupational or recreational exposure to infected animals, inhalation of infectious aerosols, laboratory exposure, or consumption of inadequately cooked contaminated meat (Young, 2009). Worldwide many cases of human infection follow contact with infected feral pigs or 'wild boars' (Meng et al, 2009).
Sixteen notifications of Brucellosis in humans were reported from NSW for the period 2009-2012. Nine of the notifications were from feral pig hunters in north-west NSW. Brucella suis has recently been identified in feral pigs in northern NSW and continues to emerge as a human and animal health issue (Ridoutt et al, 2013, DV Conference paper). Most notifications are in young males (NCIMS, 2013b; Irwin et al, 2009).
The vaccination of livestock against leptospirosis will reduce some of the risk for humans but as infection is also from rodents and feral pigs, an important level of risk remains. Currently the most important means of preventing leptospirosis and brucellosis in humans is to take precautions when coming into contact with potentially infected animals (NSW Health, 2012; Queensland Government, 2010; WHO, 2006. Many of the current strategies to reduce the risk of leptospirosis and brucellosis do not appear appropriate or acceptable for people at highest risk of disease, such as farmers and feral pig hunters (Massey et al, 2011). More acceptable strategies need to be developed and tested in consultation with the people who need to implement them.
Emerging zoonotic diseases remain an important challenge for human health and animal health practitioners to tackle together.