Why we should worry about zoonoses

Zoonoses are a growing global problem requiring a global surveillance and control system, reports Graeme O'Neill.

The list of zoonoses -- diseases of animal origin that can also infect humans -- has grown apace in recent decades, as jet travel has dissolved the barrier of distance.

Today, an obscure but potentially deadly virus from some remote corner of the globe can vault into the world's major cities in less than 48 hours.

Canadian Public Health Agency virologist Dr Mike Drebot told last month's annual conference of the Australian Society of Microbiology in Canberra that zoonoses are a growing global problem, requiring a global surveillance and control system.

Most human diseases originate as zoonoses. The list of ancient and emergent viral nemeses includes avian influenza, HIV, SARS virus, Nipah virus, Hendra virus, rabies, Australian bat lyssavirus, West Nile virus, Ebola and Marburg viruses, and hantavirus.

Ross River resurgence

Drebot, head of the National Microbiology Laboratory's Viral Zoonoses Department in Winnipeg, recounted an episode in which two Canadians, holidaying separately in Fiji late in 2003 and early 2004, returned home with an unwelcome Australian import.

The 39-year-old man and 33-year-old woman both developed a skin rash, headache, swollen joints, sore muscles, painful tendons, and overwhelming lethargy. They were witnesses to the re-emergence of Ross River virus in Fiji after a 25-year interval. Drebot said the episode illustrated how little-known zoonoses -- animal-borne infectious diseases -- can spread around the world.

He said surveillance of airline passengers returning or visiting from overseas is an important strategy for detecting zoonoses.

There had been no cases of Ross River virus infections in Fiji since 1980. A year earlier, an explosive Ross River virus epidemic spread through Melanesia and Polynesia, infecting nearly half a million people in Fiji, American Samoa, and the Wallis and Cook islands Nearly 50,000 individuals developed symptoms of endemic polyarthritis.

The mosquito-transmitted virus' primary hosts are kangaroos and wallabies in Australia, New Guinea, New Ireland and New Britain. Ross River virus infection is debilitating, but not lethal in humans.

Dead end hosts

Two other zoonoses that have triggered public health alerts in Canada in recent years are not so benign: West Nile Virus, and severe acute respiratory syndrome (SARS).

Birds are the primary hosts for West Nile virus; mosquitoes that ingest infected blood from birds transmit the virus to humans and other mammals. Most individuals remain asymptomatic, but Drebot said around one per cent of humans infected -- mainly aged or immunosuppressed individuals -- develop a potentially lethal brain infection, meningoencephalitis.

The first human cases recognised in North America were detected in New York in 1999, after local crows and zoo birds began succumbing to the virus. Genetic analyses of West Nile virus revealed it had a close relationship with a strain from Israel, implicating air travel as the mode of the virus' entry into the US.

Drebot said that the first documentation of West Nile virus activity in Canada occurred in August of 2001 when infected crows and mosquito pools were detected in Ontario, just over the border from New York. In 2002 the first human cases of viral-associated disease were diagnosed in Ontario and Quebec with almost 1000 cases identified, a significant portion of these being neurological.

The virus continued its expansion across the country in 2003 resulting in the one of the largest arbovirus outbreaks ever recorded in Canada.

Manitoba is prime mosquito country. Its largest lake, Lake Winnipeg was far more extensive in prehistoric times -- the flat landscapes are dotted with hundreds of lakes and extensive marshland suitable for mosquito breeding.

Drebot said the initial outbreak spread at 'phenomenal speed', and West Nile virus activity has now been reported throughout most of Canada. However, non-travel related human cases are primarily identified in the western prairies and Ontario where circulation of infected Culex mosquitoes occurs at higher levels in late summer.

Yet seroprevalance surveys indicate that only three to four per cent of Canadians have been exposed to date. "Unlike dengue virus, West Nile virus doesn't appear to grow very well in humans, who make poor reservoirs," Drebot said.

"So despite the rapid spread of West Nile virus, more than 95 per cent of Canadians are still immunologically naive."

West Nile virus infects some 50 to 60 mammal species -- a West Nile-like virus has even been isolated from whales.

Drebot said humans are an incidental or dead-end hosts. Many of the 300-odd bird species that are its primary hosts are prolific amplifiers -- typically, an infected bird has a million times more virus in its blood than an infected human.

Global warming plays a part

Drebot's team also maintains surveillance on hantavirus, which can cause a fatal respiratory disease called hantavirus pulmonary syndrome. Deer mice, the primary hosts, are found throughout North America. Other rodent species carry similar types of hantaviruses in Latin and South America, which have recorded several hundred cases of hantavirus pulmonary syndrome.

Drebot said that, oddly, eastern and western Canada have different strains of hantavirus, and of the 59 recorded cases of human infection, 58 were due to a western Sin Nombre hantavirus strain. It is not clear whether the eastern strain is less virulent, or whether eastern deer mice species excrete less virus.

The mosquito-transmitted dengue virus -- an occasional problem in far north Queensland -- is also on Canada's watch list.

"A lot of infected tourists bring dengue into Canada, but it does not get established in this country due to the fact that the vectors that normally spread the virus are not currently present here," Drebot said. "However, it's possible that global warming could contribute to the expansion of carrier mosquito species into northern areas of North America, resulting in the virus becoming endemic.

Drebot said importation of a virulent West Nile virus (such as the North American strain) is a potential problem for Australia. The country has mosquito species that could transmit the virus and other competent vectors, such as the Asian tiger mosquito Aedes albopictus, could become established here in the near future.

Canada has kept watch for incursions by exotic mosquito species capable of transmitting virus diseases.

Global warming is more pronounced at higher latitudes, and Drebot said a change of a few degrees could make the difference between an emerging virus becoming established or not.

"The Canadian environment will be changed more dramatically by global warming than, say, the southern US, where the mosquito vectors already exist, so they'll have more impact.

"The more hot weather you have over an extended period, the greater the chance of more mosquitoes and their progeny becoming infected.

"One thing people don't realise is that viruses replicate more rapidly when mosquitoes are in warm temperatures, so if the climate warms, you not only get more mosquito vectors, you get much more virus."

Drebot said that, to prepare for the possibility that new mosquito vectors and disease agents could emerge in areas of Canada, the Viral Zoonoses Laboratory in Winnipeg has established three Level 4 and Level 3 containment facilities where researchers can study exotic viruses like Rift Valley virus -- possibly the "the next big thing" in zoonoses -- and whether they can replicate successfully in native mosquitoes or foreign interlopers.

Bacterial zoonoses

The laboratory is not limited to viral diseases; it also maintains surveillance on bacterial zoonoses. Its major interest is in the bacterium Borrellia burgodorferi, the tick-borne agent of Lyme disease.

"We have two regions where the tick that transmits Lyme disease, Lxodes scapularius, is proliferating -- one in southern Ontario, the other in Nova Scotia.

More recently, the same tick has been shown to transmit a rickettsia, Ehrlichia phagocytophila, which causes the Lyme-like disease, ehrlichiosis.

Bacteria-infected ticks have also been detected in Canadian provinces that don't appear to have reproducing populations of the arthropod. Since the ticks feed on the blood of migratory birds, they probably find their way to various regions in Canada by being carried on birds that migrate from areas of the US where the tick is endemic.