Feral pig hunters and farmers are at risk from a re-emerging communicable disease

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In early June over 320 pig hunters gathered in Jambin, about 450km north west of Brisbane, for the King & Queen of CQ Big Boar Competition - claimed to be Australia’s largest hunting competition. Farmers say that feral pigs need eradication as they carry disease, foul waterways and destroy crops. Recreational pig hunting in rural Australia is a widespread control method for the roughly 24 million feral pigs who call Australia home. With around 600 pigs caught during the weekend-long event it brought some temporary relief to the local agriculture industry and gave farmers something to smile about. 

But are those pig hunters and farmers at risk from the very diseases the farmers want gone?

Feral pigs. Photo by Invasive Animals Co-operative Research Centre. 
Photo by Invasive Animals Co-operative Research Centre

Queensland Murray Darling Committee (QMDC) thinks so, and has issued a warning for pig hunters throughout Queensland. QMDC regional co-ordinator for feral animals, Darren Marshall, said a huge number of wild pigs in Australia were believed to be infected with leptospirosis

Leptospirosis is an infectious disease of humans and animals that is considered the most common zoonosis in the world

Leptospirosis is caused by pathogenic bacterial spirochetes of the genus Leptospira a very common disease-causing microorganism for both humans and animals. Typically humans will become infected after contact with the urine or urine-contaminated media (e.g. water, food, soil) of infected animals. Leptospires gain entry into the body through the mucosa or open wounds in the skin and quickly acquire access to the bloodstream where they disseminate in various body tissues and organ systems.

Mr Marshall said the disease was a risk to pigs hunters, their dogs and cattle. "Blood tests have uncovered that a quarter of all feral pigs are carrying leptospirosis. Obviously when people hunt pigs they are exposed to blood and all different parts of the animal and they can contract leptospirosis from those animals," he said.

In Queensland the disease is growing, with a 57% increase in cases since 2013

The World Health Organization has recognised the disease as a re-emerging communicable disease, responsible for about one million human severe cases and 59,000 deaths per year worldwide. In Queensland the disease is growing, with a 57% increase in cases since 2013 and 50 cases reported so far this year. Queensland has the greatest infection burden in Australia, especially in the Cairns and Hinterland region. The average notification rate in Queensland from 2007 to 2016 was 1.85 cases per 100,000 population per year, however misdiagnosis and under-reporting is likely to mean that the disease is more common than this. 

Associate Professor Dr Bruce Chater (OAM), head of UQ’s Rural Clinical School, and Theodore GP, said “leptospirosis is a pretty nasty illness. You might think you have the flu - that's the difficulty. You've got a headache. You've got aches and pains and you've got fevers. But if you let it go on it can actually give quite serious complications, including liver failure, kidney failure, and heart attacks and heart failure.”

More severe presentations of the disease are referred to as Weil’s disease, or icteric leptospirosis, and include life-threatening internal haemorrhaging and multiple organ failure - affecting up to ten per cent of infected people, with an overall case fatality rate of five to ten per cent. 

Leptospirosis can be difficult to clinically distinguish from other illnesses

About 90% of affected people will experience flu-like symptoms and leptospirosis can be difficult to clinically distinguish from other causes of febrile illnesses. Its nonspecific signs and symptoms (e.g. fever, headache, nausea, vomiting) are often confused with viral illness, and diagnosis may be delayed or missd completely.

Differential diagnoses of leptospirosis include influenza, pneumonia, arboviral infections (i.e. dengue, chikungunya, Zika virus, Ross River virus), malaria, typhoid, rickettsia, Q fever, acute viral hepatitis, pyelonephritis and meningitis. Haemorrhagic complications can also mimic meningococcaemia or severe dengue.

The disease is endemic in most areas where the more common dengue virus is transmitted and may be mistaken for it. It’s important to consider leptospirosis when dengue is diagnosed in a severely ill patient, because early antibiotics are beneficial in the treatment of leptospirosis and are not given for dengue virus infection. Co-infections have also been reported, particularly in post-flooding settings, when outbreaks of multiple diseases (e.g. leptospirosis and dengue) can occur at the same time. 

In Australia, occupational exposure is the predominant source of infection, particularly in hunters, livestock and dairy farmers, veterinarians, pet shop owners, field agricultural workers, abattoir workers and meat handlers, in the fishing industry, military troops, plumbers, coal miners, sewer workers and banana workers. A detailed history is important for identifying patients who have undertaken activities that place them at risk of exposure to infection – especially for those who come into frequent contact with the urine or urine-contaminated media (e.g. water, food, soil, mud, animal bedding, hay and aborted tissue) of infected animals, such as rodents, cattle, cats, dogs, and wild animals. 

While leptospirosis is predominantly an occupational disease it is increasingly becoming a disease of recreation

But it’s not only a danger to these occupations. 

In recent decades it has also increasingly been recognised as a disease of recreation. The growth in ecotourism and outdoor adventure sports, such as “mud runs”, has the risk of bringing people into contact with contaminated water and mud. Leptospirosis is possible to contract from swimming, kayaking, wading, and rafting in contaminated rivers, lakes, and ponds. 

International travel, particularly to tropical, developing countries, is an important source of occupational and recreational exposure, so a travel history is also important for delineating possible diagnoses. Leptospirosis is found all over the world, but has a higher prevalence in countries that are located in tropical and subtropical regions. 

With advancing climate change and related trends Leptospirosis has the potential to become even more prevalent, favouring climatic conditions that are humid and hot. Cyclones in subtropical and tropical areas pose a risk of outbreaks, and widespread flooding can also lead to an epidemic in large populations. That occurred in 2011 with a spike in cases after unprecedented flooding in Queensland, and in Puerto Rico following Hurricane Maria in 2017.

Possibly typical of many people, when asked about taking safety precautions and possible leptospirosis infection when hunting feral pigs, Big Boar competition winner, Josh Allen said: “it probably doesn't really cross my mind. But it is out there in some pigs. And it can get quite serious. I suppose it's just a risk you take.”

Dr Chater said “a lot of the cases are from recreational stuff, so it's people being out in the bush. There needs to be more awareness of leptospirosis amongst rural people and also amongst rural doctors.” 

Leptospirosis Resources

Case Study, Australian Journal of General Practice, March 2018 

DynaMed Plus Topic – Leptospirosis

Queensland Health - Health Conditions - Leptospirosis   

Queensland Health - Communicable Disease Control Guidelines 

WHO Collaborating Centre for Reference and Research on Leptospirosis - Brisbane