Deadly dog disease burrows into Australia

Continent was the last in the world to be free of tick-borne canine ehrlichiosis

Published: February 12, 2021
By Ross Kelly

Photo courtesy of Dr. John Beadle
Spida was brought in July to All Creatures Veterinary Hospital in Broome, Australia, with a severe nose bleed, low blood-cell counts (pancytopenia) and weight loss, signs of chronic-stage canine ehrlichiosis. He was euthanized two weeks later.

Often confronted with wildfire and snakebite victims, Australian veterinarians face their share of challenges. But they historically haven't had to contend with many of the world's worst animal diseases, thanks to the country's geographic isolation and strict animal importation rules.

Now, Australia's fortress reputation is being undermined by a worsening outbreak of canine ehrlichiosis, a serious tick-borne disease of dogs that occurs throughout the world, but until last year had never been detected on the island continent.

Veterinarians were first put on alert last May, when cases of canine ehrlichiosis were found in the remote north of the state of Western Australia. More cases soon turned up in the neighboring Northern Territory and, last month, were identified in another neighboring state: South Australia.

The outbreak took another serious turn this week, when authorities in Australia's most populous state of New South Wales said the disease had been identified in "several" dogs there. They stressed that the dogs had been brought into the state from the Northern Territory before they were diagnosed.

NSW, which contains the metropolis of Sydney, has a surveillance plan that includes testing dogs with clinical signs and retrospectively testing historical samples, the state's acting chief veterinary officer, Dr. Paul Freeman, said in a Feb. 8 press release. "To date, results from all these tests have been negative,” Freeman said at the time. A spokesperson for the NSW department that oversees biosecurity declined to comment further. 

A "small number" of infected dogs also were moved from the Northern Territory into the northeastern state of Queensland prior to their diagnosis, a spokesperson for that state's agriculture department told the VIN News Service. With the latest cases, the disease has been found in five of Australia's eight states and territories.

Of 867 dogs sampled so far in Western Australia and the Northern Territory, 323 have tested positive for the disease, with a mortality rate of about 10%, according to a spokesperson for Australia's federal agriculture department. With so many dogs infected, the opportunity for Australia to reclaim its canine ehrlichiosis-free status is long gone, said Dr. Peter Irwin, emeritus professor at Perth-based Murdoch University and an expert on vector-borne diseases. "It's here to stay," Irwin said in a telephone interview. "We'll never get rid of it now."

Nobody knows how the disease arrived in Australia or precisely when. Theories, Irwin said, range from an infected tick somehow making its way across the sea, to an infected dog getting past strict importation curbs, perhaps via an undocumented fishing boat.

"This outbreak of ehrlichiosis has heightened everyone's nervousness about rabies," Irwin added, referring to another lethal disease that has never been detected Down Under. (Others include foot-and-mouth disease and avian influenza). "If an infected dog had somehow eluded testing with ehrlichiosis, it's theoretically possible that the same thing could happen with rabies, which is dangerous to humans."

A 'nasty' pathogen

Canine ehrlichiosis is caused by the bacterium Ehrlichia canis, which is transmitted by Rhipicephalus sanguineus, commonly known as the brown dog tick. As its name suggests, the tick prefers to dine on dogs; other animals, including humans, are far less prone to infection. (Little is known about the zoonotic potential of E. canis, though one study in 2006 found that it had sickened a handful of people in Venezuela). The infection is spread by an infected tick; it doesn't pass directly from dog to dog.

E. canis infection occurs in three phases: acute, subclinical and chronic. Clinical signs of the acute phase arise one to three weeks following the bite of an infected tick, and may include listlessness, loss of appetite, enlarged lymph nodes and fever, according to an article in Veterinary Partner. If treated during the acute phase, most dogs recover. But if untreated or inadequately treated, they may enter a subclinical phase in which they appear normal. The subclinical phase can last months or even years while the pathogen is sequestered in the spleen.

Dogs that progress to the chronic phase — not all do — become sick again. Signs include severe fever, inflammation in the eyes, abnormal bleeding, including from lesions on the skin and mucosa, and nose bleeds.

Diagnosis is made by detecting genetic material of the tick (with a polymerase chain reaction, or PCR, test); or by detecting antibodies in the patient's blood (through serology). The antibiotic doxycycline is widely considered to be the frontline treatment for an E. canis infection. In the chronic phase, dogs can die even after receiving treatment.

Few veterinarians are closer to the front lines of the Australia outbreak than Dr. John Beadle at All Creatures Veterinary Hospital in the remote Western Australia town of Broome. Beadle said he's treated around 40 or 50 infected dogs in recent months. "It's absolutely endemic through here now," he told VIN News.

Beadle said some patients have shown very mild signs of illness, while others have died two days after arriving at the clinic. The worst signs he's seen include heavy bleeding from the nose and soaring temperatures, sometimes as high as 41 degrees Celsius (106 degrees Fahrenheit).

"It's a nasty disease," said Beadle, who also has observed a roughly 10% mortality rate in the dogs he's treated. That figure doesn't tell the whole story, he added, because many sick dogs might not make it to a clinic.

While the suffering of patients and their owners is difficult, Beadle said the development is intriguing scientifically. "I find it fascinating, to be honest — seeing this new disease and things that nobody had seen around here before," he said.

The likelihood of further spread

Until now, E. canis had been endemic in parts of every continent except Australia. The pathogen was first identified in the 1930s by French scientists in Algeria, and came to prominence in the 1960s and 1970s after it killed scores of American military dogs serving in the Vietnam War, according to scientific papers written at the time.

The brown dog tick, preferring warmer climates, is more likely to flourish in tropical and subtropical regions. But Irwin thinks it's only a matter of time before canine ehrlichiosis becomes more widespread in Australia, despite the country's varied climate, potentially cropping up in big cities such as Sydney, Brisbane and Perth, where brown dog ticks are present in small numbers.

"Movement of animals and the fact that the tick itself is very adaptable — it enjoys living in urban dwellings that can be well outside its tropical range — are factors that will lead to the slow and steady dissemination of this disease around the country," Irwin said.

Over the longer term, Irwin said global warming, too, potentially could expand the ticks' preferred habitat, including in northern parts of Europe and North America, particularly during the summer months. (According to a 2012 study in the United States, the highest E. canis prevalence was found in a region encompassing Arkansas, Louisiana, Oklahoma, Tennessee and Texas). 

Beadle noted that the disease is more apt to spread in locations inhabited by people of limited economic means. He and Irwin noted that poorer, overcrowded communities in parts of northern Australia, where preventive treatment for ticks and access to veterinary care is limited, so far have been the hotspots of infection.

Among available tick-control products, some are more effective at preventing E. canis infection than others. That's because the pathogen can infect a dog rapidly — in three to six hours after exposure — so it's important that the control product kill ticks fast, Irwin said.

For that reason, he said, oral products, which take the form of tablets or chews, aren't the best choice. "The treatments that rely on the tick having a drink [of blood] first to kill it don't protect against ehrlichiosis because it's transmitted so quickly," Irwin said.

Topical treatments, which are applied to the dog's skin, may be effective depending on the active ingredients, he said, pointing to a study published in 2016 that tested the efficacy of several products in protecting against E. canis infections.

Certain tick collars provide protection, as well, Irwin said. For his part, since the Australian outbreak started, Beadle has been recommending a collar containing flumethrin, a powerful acaricide, and imidacloprid, an insecticide. A published study by the collar manufacturer indicated it was effective against E. canis.

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