Worm infection management in sheep

07/09/2022

Worms - The signs to look out for
Worm diseased sheep.
A typical sign of a worm problem is unthrifty sheep. An unthrifty sheep is one that is not eating properly, is losing condition, tends to lag behind the mob when moved and, in severe cases, is clearly weak.
A worm problem often (but not always) results in sheep scouring and becoming daggy. In severe cases, affected sheep may scour profusely. Other conditions can however produce these signs.
Young sheep are far more susceptible to worms than older sheep.
Sheep under stress (eg during the later stages of pregnancy, during lactation, during drought or winter feed shortages) are more susceptible to worms.
A high-risk time in Tasmania for worm infections in sheep is an especially long period of cold and wet weather - especially if this coincides with the later stages of pregnancy or lactation.
Other signs of worm infection you may see are anaemia or swelling under the jaw (commonly called "bottlejaw").
If you suspect a worm problem, it is worth doing a worm test to confirm it. Remember that "ill thrift" and scours, the major signs of a worm problem, can also occur with other diseases. So, acting purely on the clinical signs may result in a wrong (and costly) treatment.

07/09/2022

Worm management plan
It is important that everyone with sheep has a worm management plan - and this includes hobby farmers. An effective plan is simple and will save a lot of money, effort and heartache. The information on this webpage will help you prepare a worm management plan for your situation. Commercial sheep farmers should involve their vet in preparing the plan.

The essential elements of a worm management plan include
Worm testing. It is cheap and easy to do. Without regular testing, you won't know whether you have a problem, whether your worm management plan is working or whether you have an emerging drench resistance problem on your property.
Grazing strategy to create safe or low contaminant pastures for weaners and lambing ewes.
Maintaining good nutrition during periods of poor pasture growth.
Building worm resistance in the flock.
Biosecurity measures for new sheep arrivals on the property or any outbreak of worm disease in your flock (a sure sign that your worm management plan has failed).
Minimising the risk of drench resistance developing on your property.
It is strongly recommended that you routinely do a worm test before you drench your sheep.

07/09/2022

There are three broad types of internal parasite that can cause significant health issues in sheep - worms, flukes and protozoa.
Worms are thought to cost sheep owners more than any other disease.
Worm infestation is probably the most common cause of "ill thrift" in sheep.
Roundworms are the most common internal parasites of sheep, with the Small Brown Stomach Worm, the Black Scour Worm and Nematodirus being the more common worms in Tasmania. The Barber's Pole worm also occurs in Tasmania but it has to date only been the cause of localised problems, generally in the summer and early autumn, when the local climatic conditions have been warm and moist.
Drench resistance is an emerging problem in managing sheep worms.
Worm infection is a significant animal welfare issue and, if not treated, can cause death.
There is only one species of liver fluke known to infect sheep in Australia. It is, however, a problem in some areas of Tasmania.

07/09/2022

Gastrointestinal nematode (GIN) infections are common in domestic sheep and impact directly and indirectly on the health of infected animals as well as on the associated economic production. In this study, we aim at summarizing the current knowledge on the influence of GIN infections on sheep production by conducting a systematic review. A subsequent meta-analysis of relevant studies was performed to provide an estimate of the effect of GIN infections on weight gain, wool production and milk yield.

07/09/2022

Sheep represent an important source of income in many countries [8, 9] and although the effects of parasitism on production have been recognized there is still a need to quantify these losses. Anthelmintic resistance and climate change is likely to alter the geographical distribution of parasites and their impact on production animals, thus increasing the need for a clear understanding of the cost of parasitism in order to develop sustainable control strategies

07/09/2022

Gastro-intestinal parasitism is one of the most common infections in livestock. Clinical signs and sequelae are dependent on the parasite fauna present and the intensity of infection. In sheep, these can range from subclinical weight loss to lethal pathologies such as anaemia, diarrhoea and severe protein loss [1]. In addition, parasitism can have indirect consequences on metabolism such as mobilisation of proteins for an immune-response, reduced feed intake due to anorexia or increased susceptibility to other pathogens [2–4]. Since the 1960s the use of anthelmintics has become an important strategy to control nematode infections in livestock and increase their production performance [5]. For example, Sanchez et al. [6] reported the results of a meta-analysis which concluded that dairy cattle gained an estimated increase in milk production of 0.35 kg/day following treatment against gastro-intestinal nematodes.

06/09/2022

Controlling nematodes in dairy calves using targeteWith increasing concerns of anthelmintic resistance in cattle nematode populations worldwide, there is a need to explore alternative approaches to nematode control. One alternative approach is the use of targeted selective treatments (TST) where only individual animals are treated instead of the entire group. This study reports the findings of a TST approach in dairy calves conducted over their first grazing season (FGS) to control both gastrointestinal nematode and lungworm challenge. Ninety-six calves with an initial mean (s.d.) age and live weight of 130 (28.3) days and 120 (23.6)kg, respectively, were randomised by breed, age and live weight to one of two treatments; Control (n=24; ×2) and TST (n=24; ×2). Control calves were treated three times at pasture with ivermectin by subcutaneous injection. Individual calves in the TST group were treated at pasture with ivermectin when one of the following thresholds was met: (1) positive for lungworm larvae using the modified Baermann technique or (2) positive or negative for lungworm larvae using the modified Baermann technique with plasma pepsinogen concentration (PP) ≥ two international units of tyrosine/litre and faecal egg count (FEC) ≥ 200 strongyle eggs per gram of faeces. Calves were rotationally grazed from July 3rd 2012 (day 0) to November 2nd 2012 (day 122) when calves were housed. Calves were weighed and sampled (blood and faecal) every three weeks. There was an effect of treatment and time on both FEC [treatment (P=0.023), time (P

29/08/2022

Degree of the establishment of H. contortus and consequent sickness depend upon the number of infective larvae ingested, age (Saccareau et al. 2017), immunity level, and nutritional status of the host. Major pathogenic damage occurs due to the sucking of blood by the free-living parasites and eruption of ulcerative lesions in the abomasal mucosa leading to digestive syndrome and anaemic disorders (Besier et al. 2016b). The infected sheep may lose up to 30 μL of blood every day due to one parasite and even death in the pre-patent period (Emery et al. 2016). Loss of blood which is either ingested or let oozed out from the mucosal lesion to faeces leads to anaemia, which appears 10–12 days after getting infected (Roeber et al. 2013b), and a fall in packed cell volume (PCV) (Storey et al. 2017; Ferreira et al. 2019) detectable even at 4th day. The PCV value further drops by 3–6 weeks due to increased blood loss by the accelerated activity of parasites and bleeding from haemorrhagic gastritis lesions. A concurrent reduction in the concentration of haemoglobin and plasma protein is also observed (Swarnkar and Singh 2018). Extensive damage to abomasal mucosa affects the passage rate of ingesta, produces pain and inflammatory cytokines, and changes in gastric secretions as well as the level of gastrointestinal hormones in plasma which lead to prolonged loss of appetite (Angulo-Cubillán et al. 2007). Due to raised pH of the abomasum, rumen microbes do not get inactivated and lysed resulting in the non-availability of amino acids. Physical and chemical damage caused by the parasite induces the inflammatory response in the gastric tissues leading to a collection of numerous neutrophils, lymphocytes, and eosinophils which further aggravate the situation (Alam et al. 2020). Clinically haemonchosis can be sub-divided into hyper-acute, acute, and chronic forms. In a hyper-acute case, sudden death is the only sign. Acute form involves severe anaemia, lethargy, weakness, increased respiratory and heart rate, dark mushy faeces, loss of wool, pale to white conjunctiva, ascites, and sub-mandibular and cervical oedema. Quite recently fatal outbreaks of haemonchosis in lambs have been reported (Paul et al. 2020). Chronic disease is characterized by anorexia, loss of weight, agalactia, pallor of the conjunctiva, and mucosa (Besier et al. 2016b; Iliev et al. 2017).

04/05/2022

Haemonchosis is one of the most significant parasitic diseases of livestock worldwide, affecting hundreds of millions of ruminants such as sheep and goats, causing billion dollar losses to the livestock industry annually.137 The causative agent, Haemonchus contortus (barber's pole worm), feeds on blood in the stomach (abomasum) and the host suffers gastritis, anemia and associated complications, leading to morbidity and mortality in severely affected animals. Picria fel-terrae has been used to treat helminthiasis in Indonesia activity but exact active components remain unknown. Consequently, eight extracts from the plants Picria fel-terrae Lour., Linariantha bicolor, Lansium domesticum and Tetracera akara were tested for nematocidal activity against seven strains of the free-living nematode Caenorhabditis elegans (one wild-type and six strains with GFP-tagged stress response pathways), and the stress responses caused by these extracts was determined.138 These plants are widely distributed throughout Asia and have been used by indigenous Malaysian healers to treat worm infections and gastrointestinal disorders in humans.138–140 Five of the eight plant extracts possessed significant nematocidal activity against both larval and adult stages of C. elegans. The most effective extracts were from P. fel-terrae and triggered stress response pathways that were distinct from commercially available anthelmintics (doramectin and levamisole) and so the mechanism of action may be novel.

01/07/2021

01/07/2021

Making the Case for Selective Anthelmintic Treatment
In small ruminants, worm burdens are unevenly distributed among animals; approximately 20% to 30% of the animals harbour 80% of the parasite population.3,11 A similar disaggregate distribution of trichostrongyles was noted in camelid herds.12 The majority of camelids in most herds have low parasite burdens, and they will not gain much benefit from anthelmintic treatment. If left untreated, however, these minimally infected animals provide a major source of refugia. Discriminating between camelids that need anthelmintic treatment and those that do not takes a combination of hands-on husbandry and periodic evaluation of quantitative f***l egg counts. This approach is more labor intensive than traditional calendar-based “whole herd” treatment programs, but the benefits of adopting a selective treatment program cannot be over-emphasized.3 Selective use of anthelmintics will prolong the efficacy of currently available anthelmintics, which should be regarded as precious and limited resources. Even though several new anthelmintics are on the horizon, they will be significantly more expensive than those that are currently available. It is highly likely that resistance will continue to evolve more quickly than new anthelmintics can be developed to replace those that are failing. As a result, sensible use of anthelmintics and management practices aimed at breaking the parasitic life cycle will remain essential aspects of current and future parasite control strategies.

Accurate identification of animals that require anthelmintic treatment to maintain health and productivity is the cornerstone of a selective treatment program. Body condition score (BCS) is a useful parameter to monitor camelid herds, as it is a good barometer of overall health. Generally, healthy-appearing, active animals that have optimal to high body condition scores are the least likely to be harboring health-threatening worm burdens. The BCS subjectively grades the amount of muscle and subcutaneous body fat over bony protuberances and categorizes them on a 1-to-9 scale, 1 for emaciation and 9 for obesity. A body condition score of 5 out of 9 is considered ideal, although some healthy individuals naturally maintain condition at a slightly higher or lower score. Camelids should be palpated from the withers to the loin and in the brisket area because fiber may conceal the actual body condition (Figure 6-1). It is important to notice any changes in the BCSs among individuals and the overall herd, especially if the scores are declining. Parasitism and inadequate nutrition are common reasons for decline in BCSs in multiple animals in a herd.

Since anemia is a prominent feature of haemonchosis, pallor of the conjunctiva and mucous membranes is an excellent indicator of the severity of H. contortus infections. Francois Malan noticed the association between anemia and eyelid pallor in parasitized sheep in South Africa.13 He subsequently developed a laminated card that depicted five illustrations of ocular membrane colors. The membrane colors are categorized as follows:

1 = deep red (not anemic)

2 = red-pink (not anemic)

3 = pink (mildly anemic)

4 = white-pink (anemic)

5 = white (severely anemic)

The ocular colors correlate with mean packed cell volumes (PCVs) in parasitized sheep. Studies conducted in the United States on sheep and goats with haemonchosis have demonstrated that the FAMACHA© eye score, PCV, and f***l egg counts are all highly correlated.14,15 Since introduction of the FAMACHA system in 2004, over 15,000 FAMACHA cards have been distributed in the United States, and numerous workshops have been conducted to teach small ruminant producers how to implement the concepts in their herds and flocks (Figure 6-2).16

The FAMACHA system was recently evaluated on 17 llama and alpaca farms that had a high prevalence of H. contortus. PCV was highly correlated to FAMACHA eye scores, f***l egg counts, and BCSs.12 On the basis of these findings, the FAMACHA system has been deemed a useful tool for the management of H. contortus in llamas and alpacas in the area studied. It is most accurate when the prevalence of anemia associated with haemonchosis is high. The FAMACHA system is, however, not intended as a management tool for other types of internal parasites. Ocular examinations need to be conducted in bright natural light, using the FAMACHA card as a color reference. Alpacas and docile llamas may be restrained manually, but the more fractious camelids need to be restrained in a chute for safety purposes. The normal tendency of most handlers is to place a hand on top of the animal's forehead during ocular examination, but camelids often object to being touched in this area. Camelids are accepting of the scoring process if the examiner stands to the side of the animal and slowly slides one hand up the side of the animal's face towards its eye. The index finger is used to gently but firmly retropulse the globe through the upper eyelid, and the thumb of the same hand is used to evert the lower eyelid. The FAMACHA card is held beside the eye with the opposite hand. The examiner should expose as much conjunctiva as possible to accurately assess color. The color of the lower eyelid conjunctiva is compared with the FAMACHA card over a 1- to 3-second period and assigned a score. Both lower eyelids are scored, and if variation is noted between the two eyelid scores, the more anemic (higher) score is assigned. Some camelids have heavily pigmented inner eyelids, but usually, a nonpigmented area can be scored. Animals with haemonchosis with scores in categories 4 or 5 should be treated with an effective anthelmintic. Animals with scores in categories 1 or 2 are not anemic and therefore do not need treatment. Animals that receive a FAMACHA score of 3 should be handled according to prevailing circumstances. For instance, young animals and animals in herds or flocks where the majority of the animals have anemia, poor body conditions, or both should be treated, particularly during periods of high H. contortus transmission (warm, moist conditions). The main benefit of the FAMACHA system is that far fewer animals will be treated than when nonselective methods are used, so refugia are maintained. FAMACHA examinations should be conducted every 1 to 2 weeks during periods of high H. contortus transmission and less frequently during times of the year when transmission is typically low. Although haemonchosis is a major cause of chronic blood loss in camelids, other causes of anemia should be considered when pale ocular membranes are observed (Figure 6-3).17 Other etiologies include blood loss from causes other than endoparasitism, ingestion of wilted red maple leaves, chronic disease, naturally occurring iron deficiency, and symptomatic Candidatus Mycoplasma haemolamae (CMhl) infections. Although most CMhl infections are not clinical, red blood cell (RBC) destruction may occur if the animal becomes severely stressed or immunocompromised.

01/07/2021

Anthelmintics
In veterinary medicine, before modern anthelmintics became available, Areca (Areca catechu), extract of Male fern (Dryopteris fillix-mas), kamala, pomegranate (granatum), and santonin (from Artemisia spp) were used in different animal species with varying effects. Modern anthelmintics probably pose less of a risk for adverse effects; however, some animal owners prefer to refrain from using drugs and will wish to consider herbal options.
Other issues for consideration include comparative efficacy, safety, risk of zoonoses, resistance, and owner compliance (many protocols involve long-term use). For example, cucurbitine contained in crushed pumpkin seeds is only 55% efficacious against Taenia saginata (Pawlowski, 1970), and arecoline hydrobromide, derived from Areca catechu (the betel nut palm), is very effective against all kinds of tapeworms; however, emesis and diarrhea are common adverse events. A common procedure was to follow administration of these plant compounds with a purgative, causing diarrhea to expel the affected worms. Because of the difficulty involved in killing intestinal worms without harming the patient, skill was always required in administering these anthelmintics (Mills, 1989).

One of the future directions of herbal anthelmintics involves ethnoveterinary investigation into suitable plants for helminth control in production animals in tropical countries. For example, Spondias mombin has been studied in vivo for evaluation of the therapeutic efficacy of water and alcohol extracts administered to sheep naturally infected with gastrointestinal nematodes. The mean percentage of f***l egg reduction on day 12 in sheep drenched with 500 mg/kg S. mombin extract was up to 100% against Haemonchus species, Trichostrongylus species, Oesophagostomum species, Strongyloides species, and Trichuris species, varying with concentration (Ademola, 2005). In Pakistan, plants identified through ethnoveterinary research were screened for their in vitro anthelmintic activity. In vitro results showed that ginger killed all test worms (Haemonchus contortus) within 2 hours postexposure. Most worms exposed to control (normal saline) remained alive until 4 hours postexposure; then, 50% died within 6 hours postexposure. It was concluded that all studied plants had some anthelmintic activity (Zafar, 2001). An experiment was carried out to investigate the anthelmintic activity of papaya latex (Carica papaya) against natural infection of Ascaris suum in pigs. Pigs given 4 or 8 g of papaya latex per kilogram had worm count reductions of 80.1% and 100%, respectively. Some of the pigs receiving the highest dose of latex showed mild diarrhea on the day following treatment. Otherwise, no clinical or pathologic changes were observed in treated animals (Satrija, 1994). Other herbs that have been traditionally used for their anthelmintic activity in humans and animals are discussed in the following paragraphs.
WORMWOOD (ARTEMISIA SPP):
These herbs, including Artemisia absinthium, include bioactive compounds with some anthelmintic activity. The powdered shoots of Artemisia herba-alba were investigated in experimental hemonchosis in Nubian goats. Treatment with 2, 10, or 30 g of Artemisia shoots prevented caprine hemonchosis, suppressing egg production and the development of abomasal lesions (Idris, 1982).
GARLIC (ALLIUM SATIVUM):
This herb has been used in the treatment of patients with roundworm (Ascaris strongyloides) and hookworm (Ancylostoma caninum and Necator americanus). Allicin appears to be the anthelmintic constituent; diallyl disulphide was not effective (Kempski, 1967, Soh 1960). Allicin is formed through the action of allinase on alliin, which occurs on crushing fresh garlic. Minced garlic has been reported to be successful in reducing parasitism by Capillaria species in carp (Peoa, 1988), but it was unsuccessful as an anthelmintic in the treatment of 12 donkeys, when compared with control and fenbendazole treatment groups (Abells, 1999) (note that whole bulbs were used).
ELECAMPANE (INULA HELENIUM):
Alantolactone has been used as an anthelmintic in the treatment of patients with roundworm, threadworm, hookworm, and whipworm infection (Reynolds, 1982).
FUMITORY (FUMARIA PARVIFLORA):
As an ethanol extract, this agent caused a marked reduction in f***l egg count (100%) and 78.2% and 88.8% reduction in adult Haemonchus contortus and Trichostrongylus colubriformis, respectively, on day 13 posttreatment in lambs; it was as effective as the reference compound pyrantel tartrate (Hordegen, 2003).
BLACK WALNUT (JUGLANS NIGRA):
A decoction has been used to remove worms from people (Felter, 1898); the oil of black walnut (Juglans nigra) is often effectual in expelling worms and has even been known to cause ejection of the tapeworm (Cook, 1869). However, toxicity has been reported in dogs ingesting moldy walnuts, and horses exposed to walnut shavings.
GOLDENSEAL (HYDRASTIS CANADENSIS):
The effect of hydrastine on the protoscolices of the tapeworm (Echinococcus granulosus) has been studied in vitro and in vivo. Hydrastine at 0.3% concentration produced 70% mortality of the larvae in both experiments (Chen, 1991).
OTHER HERBS:
Others herbs used historically include the following:
Tansy (Tanacetum vulgare)
Rue (Ruta graveolens)
Thuja (Thuja occidentalis)

01/07/2021

Haemonchus contortus
H. contortus is one of the most fecund strongyle nematodes; individual females are capable of producing thousands of eggs per day, which can lead to rapid larval pasture contamination and associated outbreaks of haemonchosis (Levine, 1968). In sheep, the pre-patent period of H. contortus is 18–21 days; adult worms are short-lived, surviving in their hosts for only a few months. The main pathogenic effects are caused by the L4s and adults, which both feed on blood, causing severe anaemia which usually becomes apparent after two weeks of infection (Baker et al., 1959). Acute disease is usually intensity-dependent and is associated with signs of haemorrhagic anaemia, dark-coloured faeces, oedema, weakness, reduced production of wool and muscle mass, or sometimes sudden death. In cases of chronic disease, decreased food intake, weight loss and anaemia are most commonly observed (Kassai, 1999). Unlike many other gastrointestinal parasites, H. contortus is not a primary cause of diarrhoea, and its effects on a flock are often not readily detected by routine observation

28/06/2021

Faulty Digestion, Absorption, or Metabolism
Faulty digestion and absorption are commonly manifested by diarrhea, and diseases that have this effect are dealt with under the heading of malabsorption syndromes (see Chapter 7). In grazing ruminants, the principal causes are the nematode worms Ostertagia, Teladorsagia, Nematodirus, Trichostrongylus, Chabertia, Cooperia, and Oesophagostomum and the flukes Fasciola and Paramphistomum. In cattle the additional causes are tuberculosis, coccidiosis, sarcosporidiosis, and enzootic calcinosis. In sheep and goats there are Johne's disease, viral pneumonia without clinical pulmonary involvement, and hemonchosis. In horses there are strongylosis, habronemiasis, and heavy infestations with botfly larvae. In pigs there are stephanuriasis, hyostrongylosis (including the thin sow syndrome), infestation with Macracanthorhynchus hirudinaceus, and ascariasis. Gastrointestinal neoplasia must also be considered as a possible cause.

•
Chronic villous atrophy occurs most severely with intestinal parasitism or as a result of a viral infection.

•
Abnormal physical function of the alimentary tract, as in vagus indigestion of cattle and grass sickness in horses, is usually manifested by poor food intake and grossly abnormal f***s.

•
Inadequate utilization of absorbed nutrients is a characteristic of chronic liver disease. It is usually distinguishable by a low serum albumin concentration (although this is an uncommon manifestation of liver diseases in horses), by liver function tests, and by measurement of activity in serum of liver-derived enzymes. A clinical syndrome including edema, jaundice, photosensitization, and weight loss is a common accompaniment.
Neoplasia in any organ. The metabolism of the body as a whole is often unbalanced by the presence of a neoplasm so that the animal wastes even though its food intake seems adequate.
Chronic infection, including specific diseases such as tuberculosis, sarcocystosis, East Coast fever, trypanosomiasis (nagana), maedi-visna, caprine arthritis–encephalitis, enzootic pneumonia of swine, metastatic strangles in horses, and nonspecific infections such as atrophic rhinitis of pigs, abscess, empyema, and chronic peritonitis reduce metabolic activity generally as well as reducing appetite. Both effects are the result of the toxemia caused by tissue breakdown and of toxins produced by the organisms present. Less well understood are the means by which systemic infections, e.g., equine infectious anemia, scrapie in sheep, and other slow viruses, produce a state of weight loss progressing to emaciation.

28/06/2021

Diagnostic Confirmation
In fluke-endemic areas, fasciolosis must be considered as a possible factor in any outbreak of chronic ill health in sheep, either as the main cause or as a contributory factor along with other debilitating disease processes. To support a diagnosis, grazing history and the seasonality of fasciolosis in that locality should be taken into account. There should be fluke eggs in the f***s and characteristic hepatic lesions at necropsy. As these may be ubiquitous findings in endemic areas, a judgment is necessary to determine whether the severity of the lesions is sufficient to incriminate the fluke as the sole or major contributing etiologic factor. ELISAs are available for use with blood or milk and are particularly useful for the diagnosis of infection in cattle on an individual or herd basis.8,9 A rise in antibody can be detected by 2 weeks after infection and keeps rising until week 6. A commercially available coproantigen ELISA (BIOK 201, Bio-X Diagnostics, Belgium) has been developed for use in cattle7,9-12 that has the ability to indicate the intensity of fluke infestations in cattle.9,10 Experimentally, PCR methods for species-specific diagnosis of Fasciola species targeting the nuclear and/or mitochondrial DNA of the parasite have been developed.13-19
Acute disease can only be confirmed at necropsy.
Differential Diagnosis
Acute fasciolosis
Hemonchosis
Infectious necrotic hepatitis
Eperythrozoonosis
Anthrax
Enterotoxemia
Chronic fasciolosis
Nutritional deficiencies of copper or cobalt
Other internal parasitisms, including parasitic gastroenteritis (particularly hemonchosis) in sheep and ostertagiosis in cattle
Johne's disease
Treatment
Treatment and Control
Treatment
Cattle
Triclabendazole (12 mg/kg, PO) (R-1)
Combination of clorsulon (2 mg/kg SC) and nitroxynil (10.2 mg/kg SC) (R-1)2
Albendazole (10 mg/kg, PO) (R-2)
Clorsulon (13.2 mg/kg SC) (R-2)
Nitroxynil (10 mg/kg SC) (R-2)
Oxyclozanide (10 mg/kg, PO) (R-2)
Sheep
Triclabendazole (10 mg/kg, PO) (R-1)
Albendazole (7.5 mg/kg, PO) (R-2)
Clorsulon (13.2 mg/kg SC) (R-2)
Nitroxynil (10 mg/kg SC) (R-2)
Oxyclozanide (10 mg/kg, PO) (R-2)
Closantel (10 mg/kg, PO) (R-2)
PO, orally; SC, subcutaneously.
Not all compounds are equally effective against all stages of development of F. hepatica in the body. Oral triclabendazole comes closest to this ideal. For treatment of acute fasciolosis, it is essential to choose a product highly effective against the juveniles that damage the liver parenchyma. For chronic disease, a compound active against the adult fluke is required. Product safety is an important consideration, as hepatic detoxicating mechanisms are already impaired. Flukicides can be used therapeutically for treating disease or prophylactically to prevent outbreaks. Some bind to plasma proteins (e.g., closantel) or erythrocytes (clorsulon), extending their period of protection. All flukicides either have milk-withholding periods or are prohibited from use in animals providing milk for human consumption, so the best time to treat dairy cattle is at the drying off stage. Many products combine a flukicide with a nematocide, but these should only be used when there is simultaneous risk from the two types of parasite.
Triclabendazole is an orally compound specifically for use against F. hepatica in sheep (10 mg/kg) and cattle (12 mg/kg). Higher doses are required for the control of F. gigantica in buffalo. It is highly effective against all stages of fluke from 2 days old in sheep and 2 weeks in cattle, and is the drug of choice in outbreaks of acute fluke disease. An 8- to 10-week dosing interval is recommended for use in control programs. Fluke populations resistant to triclabendazole have developed following intensive control regimens in Australia, UK, Europe, and South America.11,21-27 Combining compounds has been shown to increase efficacy against immature stages of flukes. An example of a combination is SC clorsulon (2 mg/kg) and nitroxynil (10.2 mg/kg), which increases efficiency up to 99%.20 Oral triclabendazole has been used with success in horses and donkeys (12 mg/kg) but is not licensed for this purpose.
Albendazole is a broad-spectrum compound also active against nematodes and cestodes. It is effective against adult F. hepatica at an oral dose rate of 7.5 mg/kg in sheep and 10 mg/kg in cattle. It is ovicidal and will kill any F. hepatica eggs present in bile ducts or the alimentary tract at the time of treatment. Netobimin (20 mg/kg , PO) is metabolized to albendazole in the body and has similar activity against F. hepatica.
Closantel will kill the majority of flukes older than 4 weeks in sheep at an oral dose rate of 10 mg/kg PO and will delay fluke egg output by animals grazing contaminated pasture for up to 12 weeks. It also has a residual effect against Haemonchus contortus.
Clorsulon is supplied in combination with ivermectin for combined fluke and roundworm control in cattle. At the recommended dose rate of 2 mg/kg by subcutaneous injection, clorsulon is effective against adult and 12- to 14-week-old immature flukes, but activity against 8-week-old F. hepatica is variable.
Nitroxynil is given subcutaneously at 10 mg/kg and has good efficiency against the adult fluke, but the dose has to be increased by up to 50% to obtain adequate control of acute disease. In sheep, spillage stains the fleece yellow. It cannot be given orally because the rumen microflora reduce the compound to an inactive metabolite.
Oxyclozanide used in cattle (10 mg/kg, PO) has a shorter milk-withholding period than most other flukicides. It has a significant effect against adult fluke but is inactive against immature forms. It may cause transient softening of f***s. This compound has been combined with levamisole to provide activity against fluke and gastrointestinal nematodes.
Control
Preventive measures are required in endemic areas because fasciolosis can cause death without warning or significant production losses. An integrated strategic approach is more cost beneficial than reliance on routine dosing and is less likely to induce anthelmintic resistance, but it requires detailed knowledge of the local epidemiologic cycle. In some countries in which risk varies from year to year, predictions of likely disease levels are issued based on analysis of meteorologic data and field observations. This enables control measures to be intensified when necessary. Computer models have been devised to assist this process.3
Segregation of stock from sources of infection is the ideal method of control but not always feasible in practice. Identification and mapping of snail habitats may enable grazing plans to be devised that avoid danger areas at times of high risk. Where habitats are restricted in size and clearly defined, it may be possible to exclude stock by fencing.
Stock on heavily contaminated land may be protected from acute fasciolosis by taking advantage of the interval between the ingestion of metacercariae and the onset of disease. Treatment during this period with a product effective against young flukes will eliminate the migrating parasites before they cause serious liver damage. A further dose may be necessary depending on the duration of metacercarial intake and residual activity of the chosen product. Some metacercariae will continue to be ingested after the main danger period has passed, so treatment with a product active against adult F. hepatica will be needed some weeks later to ensure against possible losses from chronic fasciolosis. Additional strategic doses may be required in regions where the winter infection of the snail is of significance. The precise timing of each of these doses depends on the local epidemiologic pattern.
Reduction of pasture contamination with metacercariae will reduce future risk. This can be done by preventing the snails from becoming infected with F. hepatica or by diminishing the size of the snail population. To achieve the first objective, adult flukes should be eliminated from the bile ducts of all grazing stock in spring and early summer. This prevents egg excretion and minimizes the numbers of snail-seeking miracidia at this crucial stage in the epidemiologic cycle. There may, however, be wildlife sources of F. hepatica eggs that cannot be controlled in this way. Snail numbers can be reduced by restricting the size of their habitat. This can be done, where feasible, by draining boggy areas and by making sure that ditches, land drains, water troughs, and so forth, are well maintained.
With stall-fed buffaloes in the tropics advantage can be taken of the fact that the metacercariae of F. gigantica concentrate on the lower part of forage plants, for example, rice straw. This can be cut off and used for other purposes, and the upper, uninfected, part can be fed to the farm stock.
Chemical snail control was widely practiced before reliable animal treatments became available. Lymnaeid snails have an enormous reproductive capacity and can quickly recolonize wet land. Therefore application has to be very thorough to have a significant season-long effect, and there must be no possibility of invasion from neighboring land. Chemicals can be applied in spring for maximum impact on the snail population before breeding starts, or later in the season when snails are plentiful, but before cercariae start to emerge. Efficacy is reduced if luxuriant plant growth hinders pe*******on to soil level. Inorganic compounds such as copper sulfate or sodium pentachlorophenate are effective but may be potentially hazardous to humans, stock, and the environment. Safer and more selective low-volume molluscicides such as n-trityl morpholine have been developed but are not commercially available.
Vaccines for F. hepatica are under development. One of these that uses recombinant fluke cathepsin L proteinases has produced up to 79% protection against infection in cattle and sheep.28 Successful vaccination strategies elicit T-helper-1 (Th1) rather than Th2 immune responses induced by natural infection.