Wildlife Matters

 

One more parasite before I switch to other topics. There will be more on parasites and diseases in the future as some can be very serious for people too.

The “Large American Liver Fluke” is well named as it can be three inches long, 1 ½ inches wide, and the adult lives in the liver. The normal hosts for it include white-tailed deer, elk, caribou, and maybe both mule and black-tailed deer. Although each fluke has both male and female organs, usually 2 are found encapsulated along a bile duct so that eggs and waste products can flow with the bile to the intestines and then be excreted with the feces.

The eggs hatch in water and the larvae infect aquatic snails where each multiplies to hundreds of individuals of another larval stage. These larvae are released back into the water and each forms a cyst on aquatic vegetation. When a suitable host ingests those plants, it becomes infected.

When moose, bison, cattle, sheep and goats become infected, the flukes roam around in the liver damaging liver tissue. The resulting damage is often fatal for sheep and goats. In moose and bovids, one often finds considerable scar tissue and thick-walled capsules which contain very dark material of varying consistency. Infected moose livers can become twice their normal size and appear to be lumpy. When cut open it is common for very dark and smelly matter to run out.

This map shows the the density of liver flukes found in deer and moose throughout much of the state. The bigger the black dot, the more liver fluke infections were found in moose and deer livers in those highlighted areas. Picture courtesy of Bill Peterson

In the late 1990’s, the northwest Minnesota moose population crashed. A graduate student began a study to determine the cause, but he died near the end of his study. A group of biologists from various agencies examined the data and published a paper that concluded that the liver fluke was the major mortality factor. The data showed that 89 percent of northwest Minnesota moose livers examined during the study contained liver flukes.

However, during the 1971 Minnesota moose hunting season D.N.R. personnel examined livers from northwest Minnesota moose taken by hunters and found that 87 percent of those livers contained liver flukes and that an average of 32 percent of liver tissue had been destroyed by flukes. Many of those livers were very swollen and smelly material ran out of them when cut, as was described above. Moose can live with considerable liver damage and also thrive, as the northwest Minnesota moose population increased steadily until it declined a couple of decades later.

Moose in eastern North Dakota also declined at the same time as did the northwest Minnesota moose, but the moose population in the rest of the state did not. They, too, were concerned and looked at data they had already collected. Of 190 moose livers examined at hunter check stations from 1977 to 1992, only 19.5 percent were infected with liver flukes. A North Dakota wildlife veterinarian did a complete necropsy of 32 non-hunting related moose deaths from 1983 to 1992. He found that only 18.8 percent of the moose livers contained liver flukes, but that 75 percent of the heads were infected with brain worm. They concluded that their moose decline was due to brain worm.

Moose livers were examined during the 1971 moose season in northeast Minnesota too. Only 17 percent of the livers contained liver flukes and an average of only nine percent of liver tissue had been destroyed. That is far less than had been found in northwest Minn. When the northeast MN moose population declined in the early 2000’s, the MN D.N.R. was very quick to try to blame it on liver flukes too. Even ignoring the severe problems these moose had with winter ticks and that brain worm mortality had been of concern for many decades.

I was retired when the northeast Minn. moose population declined but had kept most of my deer and moose necropsy data summaries for use in various papers yet to be published. I’ll present a quick summary of fluke prevalence in livers of adult moose from 1972 to 2000 obtained during examinations of hunter killed moose (this does not include data from the 1971 hunt) and my examinations of moose that died from other than hunting. The data is divided into four areas east to west. I divided Cook County on the 4th Principal Meridian (a north-south line approximately two miles west of downtown Grand Marais) into Cook County East (an area east of the meridian to the Grand Portage Reservation boundary) and Cook County West (an area west of the meridian to the Cook-Lake County line). Lake and St. Louis Counties are the third and fourth areas.

Prevalence of flukes in the livers of adult moose in each area: Cook County East: 5.5 percent of 127 livers, Cook County West: 14.1 percent of 213 livers, Lake County: 24.7 percent of 507 livers, St. Louis County: 32.7 percent of 52 livers. Overall northeast Minnesota infection was 19.9 percent of 899 adult moose livers. The figure below shows the prevalence of infected moose livers by township in NE Minn. No livers were examined in townships without a black dot. None of the livers examined were infected in townships with an open circle. The size of the black dots indicates which percentage grouping the prevalence of fluke infection was for that township. (From Peterson et al., 2013, Acta Theriologica 58:359-365)

I also examined deer livers for flukes since deer are the normal host. The results for livers of adult deer are as follows. Cook County East: zero percent of 184 livers, Cook County West: 2.2 percent of 548 livers, Lake County: 30.0 percent of 110 livers, no deer livers were examined from St. Louis County as it was outside of my work area.

Clearly there is an increase in prevalence of fluke infected livers in adults of both deer and moose from east to west in northeast Minnesota. This makes sense because the deer population density also increases from east to west in northeast Minnesota. The more deer (the normal host) there are, the greater the chance of liver fluke transmission. There were very few infected moose livers in the BWCAW; there are very few deer there either.

However, none of the portions of the northeast have anywhere near the prevalence of fluke infection as in northwest Minnesota moose. Liver fluke prevalence in moose livers in both northeast Minnesota (19.9 percent) and eastern North Dakota (19.5 percent in one data set and 18.8 percent in the other data set) are essentially the same.

All three moose populations declined greatly, but only northwest Minnesota had a high prevalence of liver fluke infection. North Dakota data makes it clear their eastern moose declined because of brain worm. Mild winters in northeast Minnesota have been favorable to both deer (and therefore brain worm) and winter ticks. I feel both parasites are responsible for the decline here.

I never worked in northwest Minnesota, but friends that did tell me that northwest Minnesota moose also have a history of problems with both winter ticks and brain worm. I have to question how a two percent increase in liver fluke infection (from 87 percent in 1971 to 89 percent in the late 1990’s) can cause a moose population to crash. It seems likely that something else was responsible.