Liver Flukes in Missouri: Distribution, Impact on Cattle, Control and Treatment
There are two types of liver flukes found in the United States that affect cattle. One is Fasciola hepatica, otherwise known as the common liver fluke. It is predominately located in Gulf Coast states and the Pacific Northwest and can have significant impact on bovine health and production. It is also a common cause of liver condemnations at slaughter.
The second fluke is Fascioloides magna, also known as the deer fluke or the giant liver fluke. It is found in certain areas of the Gulf Coast, southern Atlantic seaboard, the Rocky Mountain trench, Pacific Northwest and is widespread throughout the Upper Great Lakes. It has also been found in Missouri, which has raised questions and concerns about this parasite. The purpose of this article is to describe the distribution of F. magna in the state, as well as the impact it has on cattle and strategies for control and treatment.
F. magna is not new to Missouri. Records from the University of Missouri Veterinary Medical Diagnostic Laboratory show deer fluke cases in livestock as early as January 2011. By December of 2021 the total number of cases had risen to 23. All but three of them came from counties that lie north of Interstate 70 and along and east of Highway 63: Audrain, Boone, Howard, Macon, Monroe, Randolph and Shelby. The three cases from outside this area were from Moniteau, Pettis and Stoddard counties.
Considering the limited number cases on record, an attempt was made in 2021 to further characterize the distribution and prevalence of the fluke by collecting liver condemnation data from state and federal meat inspectors. However, condemnations due to fluke damage are extremely rare in Missouri and the project did not yield enough data to be useful.
Local veterinarians continue to be the best source of information on the presence and distribution of the fluke within an area. Cattle producers in northeast Missouri should visit with their herd veterinarian to determine risk for exposure. Veterinarians in areas bordering the region where flukes have been identified should be on the lookout in case the population is expanding.
A mentioned previously, F. hepatica, or the common liver fluke, is well known for the negative impact it has on cattle. While F. magna will result in liver condemnations at slaughter, cattle appear to be fairly tolerant of infection and its impact on health and production is usually minor.
There are exceptions however. With heavy infestations, significant liver damage occurs leading to production losses, poor health, loss of body condition and occasionally death. In addition, even minor liver damage associated with light infestations creates an ideal environment for growth of the bacteria Clostridium haemolyticum and Clostridium novyi. C. haemolyticum causes a syndrome known as bacillary hemoglobinuria, common name “red water” and C. novyi causes infectious necrotic hepatitis, common name “black disease.” Both syndromes have been diagnosed in northeast Missouri and C. haemolyticum in particular, has been responsible for substantial death loss in adult cattle.
The life cycle of F. magna is complex but important to understand since it serves as the basis for control and treatment recommendations. Cervid species, such as white-tailed deer and the North American elk, are considered to be definitive hosts, meaning they are required for the completion of the fluke life cycle. Cattle, on the other hand, are considered dead-end hosts for reasons described later.
A summary of the life cycle as it occurs in cervid species is provided in Figure 1. It begins with the ingestion of metacercaria which are the infective stage of the fluke. These are found on blades of grass in and around aquatic or semi-aquatic environments where fluke populations exist. Upon ingestion, the metacercaria excyst as juvenile flukes, penetrate the wall of the small intestine and migrate through the abdominal cavity to the liver. They penetrate the liver capsule and then migrate throughout the liver feeding on tissue as they seek another fluke to mate with. Upon pairing, migration ceases and the pair will mature into adults and be encapsulated by a fibrous cyst that forms as a result of an inflammatory response by the host animal. This cyst connects with a bile duct so eggs produced by the adults are passed into the bile duct, enter the intestinal tract and are excreted in the feces. The period of time from ingestion of metacercaria to when they mature into egg laying adults can be as short as 3 months or as long as 7 months.
If the eggs are deposited in a wet environment, they will hatch releasing a miracidium that must enter a snail within a few hours in order to survive. If successful, the miracidia will undergo further development resulting in multiple cercariae which emerge from the snail as free swimming cercaria 1 to 2 months later. The cercaria will swim to blades of grass, attach just below the water line and encyst forming metacercaria thus completing the cycle.
In cattle, the segment of the life cycle from ingestion of metacercaria to maturation is the same as it is in cervid species with two exceptions. First, even though the flukes can mature into reproductively active adults, there is some indication that development is hindered which negatively impacts egg production. Second, the cyst that forms around the adults is much thicker and a connection between the cyst and a bile duct is often nonexistent. Therefore, any eggs produced are trapped within the cyst and not passed in the feces. Such is the reason cattle are considered a dead-end host for this fluke. The advantage of this, is cattle do not perpetuate F. magna populations. The disadvantage is that examining fecal samples for eggs is not a reliable way to determine if F. magna infestations are present or absent in a group of cattle.
The goal of the control program is to minimize cattle exposure to the infective stage of the fluke. Eliminating deer and/or snail populations would be an effective strategy because of the crucial role they play in the life cycle. However, there are obvious challenges and limitations associated with controlling deer populations and there are no products approved to control snail populations in the U.S.
Therefore, improving drainage in low lying areas so the environment is less suitable for snails is one strategy that has been suggested. Another strategy is limiting access to low lying areas during peak snail activity such as spring and fall. Finally, hay harvested from low lying areas where fluke populations exist may contain metacercaria and therefore be a potential risk factor for exposure or dissemination of flukes to other locations. One study suggested to delay feeding of wetland hay for 3 months after harvest so metacercaria are no longer viable. However, other studies have found viable metacercaria in stored hay at 8- and 17-months post-harvest. Even though these studies involved F. hepatica, one should suspect survival time of metacercaria from F. magna will be similar and just as unpredictable.
The compound used most often in the U.S. for treatment of F. hepatica is clorsulon. It is combined with ivermectin and sold as an injectable formulation under the trade names of Ivomec Plus, Normectin Plus, Ivermectin Plus and Bimectin Plus. At label dose, these products provide 2 mg of clorsulon per kg of animal body weight (2 mg/kg). While this is effective against F. hepatica, it is not highly effective against F. magna. A study using oral clorsulon at 7mg/kg in cattle experimentally infected with F. magna showed 65% efficacy against 8-week-old flukes and only 20% efficacy against 16-week-old flukes.
Currently, the only product available in the U.S. considered to be reasonably effective against F. magna is albendazole, which is sold under the trade name Valbazen. This is an oral product that is labeled for control of intestinal parasites as well as flukes in cattle and sheep. At the label dose of 10mg/kg, this product controls both adult F. hepatica and F. magna in sheep but only adult F. hepatica in cattle. However, studies have demonstrated that using 1.5 times label dose, or 15 mg/kg, will control on average, 94% of adult F. magna populations in cattle. Keep in mind, using this drug at a dose that is not in accordance with the label is considered extra-label drug use and is only permissible when prescribed by a veterinarian.
Timing of administration is an important consideration when using albendazole. The label cautions against administering it to pregnant animals during early gestation so this will need to be factored into the timing. In addition, the drug is most effective against adult flukes, not immature stages, and treatment should occur when the majority of flukes in the animal are mature and risk of reinfestation with metacercaria is at a minimum.
In northern states, this often described as 12 to 15 weeks after a heavy killing frost. The 12 to 15 weeks accounts for the amount of time from ingestion of metacercaria to when they develop into adults and the cold temperatures cause snails to burrow into soil to hibernate thus interrupting the fluke life cycle. In Missouri, the ideal treatment time would likely be mid to late winter but will depend on the year. Timing of treatment is not as critical for F. magna as it is with F. hepatica, since its impact is usually minor and cattle do not perpetuate F. magna populations.
Although F. magna is often thought of as having a minor impact on cattle, there are exceptions. Each operation should evaluate their risk for this parasite and determine if control and treatment strategies are necessary. Keep in mind, such strategies are seldom 100% effective so liver damage can still occur leading to liver condemnations at slaughter and increased risk for bacillary hemoglobinuria or infectious necrotic hepatitis. Because of potential losses associated with these clostridial diseases, it is highly recommended that Clostridium haemolyticum and Clostridium novyi be included in the vaccine program in areas where fluke populations exist.