Google Map and Directions
Document Actions
Liver Flukes in Michigan
Dan Grooms, DVM, Ph.D
The first group of calves raised from heifers purchased under the Michigan Beef Improvement Program were slaughtered this summer with exceptional results. More than 90% of these calves graded mid choice or better. However, as with most good news, there is usually some bad news. Nearly 100% of the livers from these calves which were raised in the Upper Peninsula of Michigan were condemned due to infestation with liver flukes. The presence of one fluke is cause for condemnation of livers in U.S.D.A. inspected slaughter plants. This is a significant problem for both the packer, who loses money from not being able to market the livers, and the producer, who may lose profits due to reduced performance subsequent to liver damage.
There are two liver flukes that commonly infect cattle in the United States: Fasciola hepatica and Fasciola magna. F. hepatica is the most common liver fluke of cattle. It is found mainly in the Gulf States, California and the Pacific Northwest. It is not considered a problem in cattle raised in Michigan. F. magna is found throughout North America primarily in the deer population which is its natural host. It is the most common liver fluke found in cattle raised in the Great Lakes region, and is the liver fluke which was found in the beef improvement calves this past summer. Based on slaughter plant surveys, the areas in Michigan where F. magna appear to be most prevalent include portions of the Upper Peninsula and Northern Lower Michigan, especially northeast of the Saginaw Bay. The following discussion will focus on F. magna as this is the most common liver fluke found in Michigan.
The natural host for F. magna is the white-tailed deer. F. magna can infect cattle, sheep and goats while grazing areas shared with deer. Since snails are needed as intermediate hosts during their development, liver flukes are most commonly found in areas with abundant moisture such as swampy or poorly drained pastures. The life cycle of liver flukes begins when eggs are shed in feces of the natural host (white-tailed deer in the case of F. magna). When deposited into warm, moist environments, the eggs develop into free-swimming organisms called miracidium. These are able to penetrate snails which serve as intermediate hosts and are necessary for further development of the fluke. Snails shed a tadpole like form of the liver fluke called a cercaria which migrates onto the leaves of green plants where they form a cyst and are eventually ingested by grazing animals. In the small intestines, young liver flukes penetrate the gut wall and migrate to the liver. Once in the liver, young flukes migrate throughout the liver tissue. Eventually they become encysted within the liver. In the natural host (deer), they mature and produce eggs. Since cattle are an abnormal host, they react more intensely to the parasite, forming an impermeable cyst around the flukes which effectively prevents the release of eggs. Thus, cattle are a dead end host for F. magna. This creates a problem in detecting cattle with F. magna as most parasitic diseases are diagnosed by identifying eggs in the feces of infected animals. Interestingly, in sheep, F. magna does not become encysted. This allows flukes to migrate through the liver unabated causing severe damage and eventual death of the infected sheep.
Besides liver condemnation, F. magna it appears to cause few, if any, signs of disease in cattle. However, little is known about the effects that this parasite may have on production efficiency. In contrast, F. magna infection is highly fatal in sheep and limits sheep-raising in areas heavily infested with this parasite. Cattle infected with F. hepatica typically show signs of chronic parasitism including weight loss, anemia and edema due to low serum protein. Production losses from reduced feed efficiency and daily weight gain, have been reported in cattle chronically infected with F. hepatica. Bacillary hemoglobinuria and Black disease, two highly fatal diseases cause by clostridial bacterium, may be a secondary complication of liver flukes migrating through the liver.
Specific recommendations on the control of F. magna infections are difficult to make because of the lack of effective drugs and the potential for constant exposure to the parasite during the grazing season. Because of a lack of knowledge on the production and health effects that F. magna have on cattle, the costs/benefits of fluke control is unknown. Two drugs are available in the United States for the treatment of liver flukes: Clorsulon (CuratremR and Ivomec PlusR) and Albendazole (ValbazenR). These drugs are approved only for the treatment of F. hepatica. Albendazole is effective only against adult liver flukes. Clorsulon is very effective against adult flukes and somewhat effective against later stages of immature migrating flukes. In limited studies, both albendazole and clorsulon appear to be less effective at eliminating F. magna infections. Although there is no specific supporting evidence, treatment of pastured calves or yearlings when entering the feedlot may be of benefit in reducing liver condemnation and improving performance. The benefit of treating adult animals is unknown. As with most internal parasites, it is likely that adults develop some degree of age related resistance to infection with F. magna, thus decreasing the benefit of treatment. Unlike control programs for other parasites, strategic treatment of cattle aimed at reducing pasture contamination are of no value for F. magna since deer are the major source of contamination. Another approach to controlling infection with F. magna is to prevent exposure to snails. This can be done in several ways including fencing off areas which might be natural habitats of snails, draining swamps or wet pastures, or treating snail habitats with molluscicides. Unfortunately, these measures are often impractical and uneconomical. Control of white-tailed deer grazing cattle pastures could also serve to reduce exposure to F. magna. Given the estimated two million free ranging deer in Michigan, this also seems impractical, especially in the northern regions of the state.
With the current understanding of liver fluke infections in Michigan, control strategies should be evaluated carefully. Consult with your veterinarian to decide what is appropriate for your operation.