paratuberculosis

Preferred Scientific Name

• paratuberculosis

International Common Names

• English: Johne's disease; johne's disease, mycobacterium avium subsp paratuberculosis in cattle; johne's disease, mycobacterium avium subsp paratuberculosis in goats; johne's disease, mycobacterium avium subsp paratuberculosis in sheep

English acronym

• PTB

Johne's disease or paratuberculosis caused by Mycobacterium avium subspecies paratuberculosis, initially infects the ileum causing a granulomatous enteritis. It then gradually spreads to other parts of the intestine and regional lymph nodes. The disorder known as paratuberculosis or Johne's disease was first described in 1895 by Johne and Frothingham, who identified organisms in the granulomatous lesions in the intestines of affected cattle. These microorganisms stained acid-fast, indicating some type of mycobacterial organism. The bacteria were cultured from cattle in 1910, classified as a mycobacterium and named Mycobacterium chronicae pseudotuberculosae bovis Johne (Twort, 1910; Twort and Ingram, 1912). The organism was later named Mycobacterium paratuberculosis and the disease was referred to as Johne's disease or paratuberculosis (Bergey et al., 1923). In 1990, the organism was reclassified as M. avium subsp. paratuberculosis due to the close genetic relationship between M. paratuberculosis and M. avium (Thorel et al., 1990).

Paratuberculosis is widely distributed both nationally and internationally in domestic ruminants such as cattle, sheep and goats, as well as in wildlife such as deer, antelope and bison. The prevalence of the disease is difficult to ascertain because few comprehensive studies have been conducted. A review by Whittington et al. (2019) provides prevalence estimates for 32 countries. Very few countries have a low prevalence (<1%) in their dairy cattle herds (Norway, Sweden, Thailand) with most countries having >20%. In 2007, the National Animal Health Monitoring System conducted a survey of dairy farms in the USA using faecal environmental sampling and reported the apparent herd prevalence to be 70.4% and Bayesian estimates to be nearer 91% (Lombard et al., 2013). Corbett et al. (2018) used environmental sampling to estimate true herd prevalences ranging from 24-66% in dairy cattle across Canada. European countries have reported similar prevalence rates for paratuberculosis. True herd-level prevalence of M. avium subsp. paratuberculosis infected herds was estimated to be 28-43% of dairy herds in the UK, 20-71% of herds in The Netherlands and 85% of herds in Denmark (Geraghty et al., 2014). Approximately 19% of cattle (dairy and beef) herds were serologically positive for paratuberculosis in Austria in 2003 (Khol et al., 2007). Infection with M. avium subsp. paratuberculosis was reported to be endemic at high prevalence in sheep flocks and cattle and deer herds in New Zealand with 69% of farms reporting infection in at least one species (Verdugo et al., 2014). The accuracy of prevalence estimates from these studies is limited by the sensitivity of the diagnostic tests used, accurate recognition and reporting of the disease, and the number of animals sampled. It is estimated that annual losses in the USA from paratuberculosis in cattle herds may exceed US $220 million (Ott et al., 1999). This figure is extrapolated from estimated prevalence values together with computation of financial losses due to culling or death of clinically infected cows, and reduced reproductive efficiency, feed efficiency and decreased milk production in subclinically infected animals. The significance of subclinical infection on economic losses to the producer are detailed in a review (Johnson-Ifearulundu and Kaneene, 1997) with a 15-16% reduction in milk production accounting for the major portion of net monetary loss (Abbas et al., 1983; Benedictus et al., 1987). M. avium subsp. paratuberculosis-infected cows beyond the second lactation have demonstrated losses of 1300-2800 lb (590-1270 kg) of milk per lactation (Wilson et al., 1993).

Prevalence rates suggest that paratuberculosis can also be a significant economic factor in small ruminant populations. A survey in 2019 found that almost half of 11 countries with prevalence estimates of M. avium subsp. paratuberculosis in sheep based on objective criteria had >10% of flocks affected and almost one in five countries had >40% flock-level prevalence (Whittington et al., 2019). Studies have reported that 2-5% and 67% of sheep flocks in parts of Spain and Canada, respectively, were infected with M. avium subsp. paratuberculosis (Aduriz et al., 1994; Bauman et al., 2016). Paratuberculosis has had a major impact on the sheep industry in Australia and New Zealand (Sergeant, 2001; Verdugo et al., 2014). Similarly, reported herd-prevalence estimates for paratuberculosis in goats from 15 countries ranged from <1% to >40% (Whittington et al., 2019). Paratuberculosis has also been reported in farmed and free-ranging deer populations in Europe, New Zealand, and North America (Temple et al., 1979; Chiodini and Van Kruiningen, 1983; Power et al., 1993; Fawcett et al., 1995).

This disease is on the list of diseases notifiable to the World Organisation for Animal Health (OIE). The distribution section contains data from OIE's WAHID database on disease occurrence. For further information on this disease from OIE, see www.oie.int.

Gross pathology of affected animals is primarily associated with the lower small intestine and regional lymph nodes. Paratuberculosis is characterized by a granulomatous inflammation involving, mainly the ileo-caecal valve, ileum, and associated lymph nodes (Chiodini et al., 1984). This inflammation makes the intestinal wall appear thickened and corrugated, associated lymph nodes become enlarged and oedematous. However, in advanced stages of clinical disease, infection may become systemic and affect many tissues including the mammary gland, kidneys, liver, and male and female reproductive organs (Doyle, 1954; Kopecky et al., 1967; Larsen and Kopecky, 1970; Larsen et al., 1981; Hines et al., 1987). Atherosclerosis and calcification of the aorta and heart have been observed in cattle with end-stage clinical disease (Buergelt et al., 1978).

Tissue changes are accompanied by increased leakage of plasma proteins across the intestinal wall and malabsorption of amino acids and other nutrients from the intestine, with little or no evidence of mucosal necrosis. A decrease of calcium total protein and albumin has been observed in serum from cattle and sheep with clinical paratuberculosis (Jones and Kay, 1996). Accumulations of lymphocytes, macrophages, epithelioid cells, and other inflammatory cells are found in the lamina propria. Microscopic lesions consist primarily of macrophage infiltrate; they are multifocal in the early stages of disease but become more diffuse as the disease progresses to more advanced stages. Giant cells are a characteristic of paratuberculosis and occur more frequently in advanced clinical disease.

In some clinical cases, inflammatory cells may be observed in the submucosa as a band of epithelioid cells and lymphocytes along the muscularis mucosa. In general, granulomas or diffuse cellular infiltrates containing numerous acid-fast bacilli are present in clinical cases. Villi become blunt and may fuse together. It should be noted that cases with severe lesions and few bacilli can be observed, mainly in sheep (Marin et al., 1992; Pérez et al., 1996; Clarke, 1997). Caseous necrosis has been observed in paratuberculous lesions most frequently in goats, and certain captive wild species such as deer (Stehman, 1996). In sheep and goats lymphangiectasia, is most often observed in clinical cases (Marin et al., 1992; Perez et al., 1996; Clarke, 1997). Moreover, in sheep and goats gross changes associated with a thickening of the ileum and jejunum are sometimes difficult to detect, and do not resemble those observed in cattle (Marin et al., 1992; Clarke, 1997).

The clinical signs that occur, following a prolonged incubation period of one to ten years, include gradual weight loss despite a normal appetite. Other than the loose consistency the faeces appear normal. As the disease progresses, the affected animals become increasingly lethargic and emaciated. Cachexia and ‘waterhose’ diarrhoea characterize the terminal stages of the disease in cattle, but diarrhoea is not a common feature in sheep and goats (Chiodini et al., 1984; Marin et al., 1992). The main clinical signs of the disease are progressive weight loss and a decrease in milk production; therefore, most farmers cull the animals before severe clinical signs are present and before the diagnosis of paratuberculosis is made. Consequently, the number of clinical cases of paratuberculosis may be underestimated (Chiodini et al., 1984; Marin et al., 1992).

Transmission of M. avium subsp. paratuberculosis in cattle herds varies considerably, depending on the management practices followed. In some herds, the disease may persist for years before clinical signs appear. The most common route of exposure of calves to M. avium subsp. paratuberculosis is by ingestion of contaminated faeces on the surface of the dam’s mammary gland during suckling (Chiodini and Davis, 1992; Thoen and Haagsma, 1996). Young calves, seem to be more susceptible to the infection than older or adult cows (Hagan, 1938; Payne and Rankin, 1961; Larsen et al., 1975). Faeces contaminate pastures so feed and water are sources of infection. M. avium subsp. paratuberculosis can resist destruction for several months in the natural environment, making prevention and control difficult (Jorgensen, 1977). Moreover, colostrum, placenta and uterus as well as the foetus may be infected adding to the difficulty in the control of paratuberculosis (McQueen and Russell, 1979; Seitz et al., 1989; Rohde and Shulaw, 1990; Sweeney et al., 1992a,b; Streeter et al., 1995). The primary route of infection is through ingestion of faecal material, milk or colostrum containing M. avium subsp. paratuberculosis microorganisms (Chiodini et al., 1984) which, once ingested, survive and replicate within macrophages of the small intestine and in regional lymph nodes. The organisms have been isolated from semen and reproductive organs of bulls, but the importance of these findings in transmission is unclear. The incubation period, which is the interval between infection and the observation of clinical disease (diarrhoea and weight loss), usually varies from 1 to 5 years or more. Cattle become infected with M. avium subsp. paratuberculosis usually as calves but often do not develop clinical signs until 2-5 years old.

It is most important to emphasize that clinical disease may be observed in less than 30% of the M. avium subsp. paratuberculosis-infected cattle in a herd. Some animals remain subclinically infected throughout their lifetime and never show clinical signs of disease. Clinical disease is most often observed in cattle 3 to 6 years old. While in sheep and goats, most of the cases are present in animals 2 to 4 years of age or mainly during the first and second lactation (Clarke, 1997). Stress is an important factor that may contribute to the onset of clinical disease; the stress may be related to parturition and/or increased milk production. In beef bulls, stress may be associated with the breeding season.

When clinical signs are evident, the animal usually sheds M. avium subsp. paratuberculosis in the faeces; however, some animals shed intermittently and micro-organisms may not be found on the examination of a single faecal sample. M. avium subsp. paratuberculosis has been isolated from the milk of cows with clinical paratuberculosis and from milk samples and supra-mammary lymph node specimens in cows with subclinical infection (Taylor et al., 1981; Sweeney et al., 1992a; Streeter et al., 1995).

After an incubation period of several years, extensive granulomatous inflammation occurs in the terminal small intestine, leading to malabsorption of nutrients and protein-losing enteropathy. During this period the animal may suffer from chronic watery diarrhoea, rapid weight loss, diffuse oedema and rough hair coat. The chronic diarrhoea fails to respond to antibiotic treatment and animals continue to lose weight, despite adequate food intake. Clinical disease may persist for 6 months or more; diarrhoea may be intermittent and cattle sometimes seem to recover for a few weeks. In terminal stages, diarrhoea results in emaciation and death. However, in small ruminants diarrhoea can be absent in a high percentage of clinical cases (Stamp and Watt, 1954; Clarke, 1997).

A decrease in total serum protein, albumin, triglycerides and cholesterol concomitant with increased creatine kinase and aldolase may occur in the latter stages of disease due to muscle damage (Patterson et al., 1965, 1967, 1968). Some animals will progress rapidly to an end-stage of recumbency and death, whereas others may go into remission for a period of time, although generally these animals will succumb to clinical signs at a later date. Clinical signs of paratuberculosis in sheep, goats and deer are limited to weight loss, unthriftiness and slight softening of their stools. Poor condition of wool is associated with end-stage disease in sheep (Cranwell, 1993). Intestinal thickening is variable in these species with some enlargement of lymph nodes.

Pathogenesis

M. avium subsp. paratuberculosis penetrates the intestinal epithelial layers primarily through the follicle-associated epithelium or M cells (Momotani et al., 1988) but also via regional enterocytes (Sigurðardóttir et al., 2005; Ponnusamy et al., 2013) and possibly dendritic cells. Phagocytes engulf the organisms and are usually unable to degrade them and they remain viable and protected from humoral factors. The first small and limited granulomatous lesion is detected in the ileo-caecal and jejunal Peyers patches, and can persist as latent infection for long periods of time, as has been observed in experimental infections in sheep and goats as well as in natural infections (Pérez et al., 1996). If the infection progresses, lesions spread to mucosa affecting different parts of the small intestine and associated lymph nodes (Juste et al., 1994; Pérez et al., 1996).

During early infection, a cell-mediated immunity (CMI) is predominant, but, as the lesions and disease progresses a humoral response appears due to the presence of large numbers of bacilli released by dying macrophages. The first response is a cell-mediated type Th1 with cytokines such as IL-2 and IFN-γ, as reported in other mycobacterial infections. The progression of the disease is associated with the T-gamma delta cell which inhibits CD4 and helper lymphocytes (Chiodini and Davis, 1992). Th2-like cytokines (IL-4 and IL-10) are predominant in clinical cases with multibacillary and non-lymphocytic lesions (Wards et al., 1995; Little et al., 1996; Burrells et al., 1998; Navarro et al., 1998).

In natural cases a similar situation has been reported with cellular immune response associated with latent-focal subclinical lesions and humoral immunity with high antibody response in diffuse-severe and multibacillary cases (Bendixen, 1978; Clarke, 1997; Pérez et al., 1997; Pérez et al., 1999). However, in some clinical cases with severe disease animals can show a strong CMI response (skin hypersensitivity and low antibody response) with predominant presence of lymphocytes in the lesion and few bacilli (Clarke, 1997; Pérez et al., 1997; Pérez et al., 1999). This is more common in sheep. Therefore, in M. avium subsp. paratuberculosis. infections it has been proposed that an immunopathological spectrum develops, which depends on the host response (Pérez et al., 1999). A clear relationship with pathology, number of bacilli, and the cellular or humoral immune mediated response has been established in sheep (Clarke, 1997; Pérez et al., 1997; Pérez et al., 1999).

Paratuberculosis is observed in cattle and small ruminants worldwide (Chiodini et al., 1984). Prevalence of the disease, on the basis of mycobacteriologic examination of tissue specimens collected from adult cattle at slaughter or on histopathological and serological studies (i.e., ELISA), varies considerably. A review by Whittington et al. (2019) provides prevalence estimates for 32 countries. Very few countries have a low prevalence (<1%) in their dairy cattle herds (Norway, Sweden, Thailand) with most countries having >20%. In 2007, the National Animal Health Monitoring System conducted a survey of dairy farms in the USA using faecal environmental sampling and reported the apparent herd prevalence to be 70.4% and Bayesian estimates to be nearer 91% (Lombard et al., 2013). Corbett et al. (2018) used environmental sampling to estimate true herd prevalences ranging from 24-66% in dairy cattle across Canada. These data may, however, markedly underestimate the true incidence of infection in animal populations. In Europe, true herd-level prevalences of 85% of dairy herds in Denmark and 28-43% of herds in the UK were estimated (Geraghty et al., 2014). A survey in 2019 found that almost half of 11 countries with prevalence estimates of M. avium subsp. paratuberculosis in sheep based on objective criteria had >10% of flocks affected and almost one in five countries had >40% flock-level prevalence (Whittington et al., 2019). Similarly, reported herd-prevalence estimates for paratuberculosis in goats from 15 countries ranged from <1% to >40%.

Cattle usually become infected with M. avium subsp. paratuberculosis as calves, but often do not develop clinical signs until 2-5 years of age (Larsen et al., 1975). Calves may become infected in utero or as neonates through ingestion of faecal material, milk or colostrum containing M. avium subsp. paratuberculosis microorganisms. Once ingested, it is believed that the M cells of the Peyer's patches serve as the primary portals of entry for M. avium subsp. paratuberculosis into the lymphatic system, as they do for other enteric intracellular pathogens such as Salmonella (Momotani et al., 1988). Live M. avium subsp. paratuberculosis will then traverse the M cell by transcytosis and be expelled on the basolateral side of the cell to be scavenged by the macrophages or dendritic cells. M. avium subsp. paratuberculosis may survive and replicate within the macrophages of the intestine and regional lymph nodes of the host animal, leading to a protracted subclinical phase of infection. Cattle shed minimal amounts of M. avium subsp. paratuberculosis in their faeces in the subclinical phase of infection, yet, over time, this shedding can lead to significant contamination of the environment and an insidious spread of infection throughout the herd. After an incubation period of several years, the immune system of the host animal may become compromised and the animal is no longer able to contain the infection. At this point, extensive granulomatous inflammation occurs in the terminal small intestine due to an influx of macrophages. Thickening of the intestinal epithelium occurs, resulting in malabsorption of nutrients and protein-losing enteropathy. The terminal stage of disease is characterized by chronic diarrhoea, rapid weight loss, diffuse oedema and inappetence.

Transplacental infection of foetuses with M. avium subsp. paratuberculosis occurs most frequently in foetuses from infected cows in the clinical stage of disease. A study examining infection of foetuses from cows with clinical signs of paratuberculosis found that 26.4% of foetuses had tissues that were culture-positive (Seitz et al., 1989). A lower level of foetal infection was noted in a more recent study evaluating foetal infection in cows that were faecal-culture positive for M. avium subsp. paratuberculosis but asymptomatic for clinical signs (Sweeney et al., 1992a). Foetal tissues were positive for M. avium subsp. paratuberculosis in only 8.6% of infected cows and these cows were shedding high numbers of the bacterium in their faeces. Results from these studies suggest that foetal infection is highly correlated with the level of faecal shedding of M. avium subsp. paratuberculosis and most commonly associated with cows demonstrating clinical signs of disease.

Faecal contamination of the environment poses the most likely threat for calfhood infection. In the clinical stage of disease, cows may shed up to 10⁸ organisms per gram of faeces (Chiodini et al., 1984). Contamination of the maternity pen is a primary source for calves to gain contact with the bacterium. However, contamination of water sources, feed troughs and pasture may also contribute to transmission of infection. Early studies established that M. avium subsp. paratuberculosis is capable of surviving in water, soil and manure for periods up to 1 year. A recent study, however, indicates that the organism does not survive in high numbers during composting and liquid manure storage methods (Grewal et al., 2006). M. avium subsp. paratuberculosis is also shed in the colostrum and milk of infected dams, albeit in low numbers. Although shedding of the bacterium in colostrum has not been adequately quantified it is highly correlated with heavy shedding in the faeces of infected dams (Streeter et al., 1995). Shedding of M. avium subsp. paratuberculosis in the milk of infected dams is also associated with the degree of faecal shedding by the dam, and M. avium subsp. paratuberculosis has been found in viable numbers at low concentrations (5-8 cfu/50 ml of milk) (Sweeney et al., 1992b).

Viable M. avium subsp. paratuberculosis organisms have also been isolated from the reproductive organs of infected females and the reproductive organs and semen of infected bulls, which may contribute to disease transmission (Philpott, 1993). Although it is unclear whether embryo transfer contributes to the incidence of disease, viable M. avium subsp. paratuberculosis organisms have been cultured from uterine washings of clinically infected dams (Rohde and Shulaw, 1990). Experimental evidence also indicates that M. avium subsp. paratuberculosis may attach to bovine ova, suggesting a potential source of uterine infection of the recipient cow (Rohde et al., 1990).

Economic losses caused by paratuberculosis are attributable to decreased value of breeding stock, lower milk production, reduced salvage or slaughter value of animals with advanced clinical disease, increased cull rates of high-producing animals, decreased feed conversion rates, and reduced fertility (Merkal et al., 1987; Thoen and Moore, 1989). The significance of subclinical paratuberculosis on economic losses to the producer are detailed in a review by Johnson-Ifearulundu and Kaneene; a reduction in milk production of 15-16% accounts for the major portion of net monetary loss (Johnson-Ifearulundu and Kaneene, 1997). Cows that are infected with M. avium subsp. paratuberculosis beyond second lactation have demonstrated losses of 1300-2800 lb (590-1270 kg) of milk per lactation (Wilson et al., 1993).

Although the estimated losses from paratuberculosis vary, in commercial dairy herds in which M. avium subsp. paratuberculosis infection persists, losses are considerable. Decreases in milk production have been estimated between 4% to 18% in subclinical infected animals and about 20% or higher in clinical cows (Buergelt and Duncan, 1978; Goodger et al., 1996; Spangler et al., 1992). A survey of dairy herds in the USA suggested an annual loss of more than US $200 per cow for positive herds with at least 10% of their cull cows showing clinical signs of paratuberculosis (Ott et al., 1999). Most of this cost could be attributed to a significant reduction in milk production per cow (1500 lb; 680 kg). The national average cost across all herds in the USA was US $22 per cow with an economic loss of US $220 million per year to the dairy industry. Other studies have reported costs per cow ranging from US $145 to US $1094 per cow with paratuberculosis. Among Canadian herds with a within-herd seroprevalence of 12.7%, the average loss was approximately CDN$50 per cow per year with 46% of the cost attributed to additional culling (Tiwari et al., 2008). A recent survey of the profitability of dairy farms in Victoria, Australia, reported that respondents who declared participation in paratuberculosis control programs made an additional net profit of $43.80 per cow per year compared with non-participants and respondents also using a vaccine gained an additional $35.84 per cow per year compared to non-users (Burden and Hall, 2021).

Reduced production of meat, wool and milk has been reported for sheep with paratuberculosis, together with increased mortality rates (Aduriz et al., 1994; Chaitaweesub et al., 1999). An average loss of $29 per sheep was reported for flocks with paratuberculosis in Australia, but some highly infected flocks demonstrated losses of $64 per sheep.

It is not clear whether M. avium subsp. paratuberculosis is a human pathogen and what potential danger it may present to consumers exposed to dairy or meat products from infected animals. Several species of mycobacteria, including M. fortuitum, M. avium-intracellulare, M. cheloni and M. kansasaii, have been found in intestinal biopsy tissue from Crohn's disease patients (Chiodini, 1989). M. avium subsp. paratuberculosis has also been identified by primary isolation from intestinal tissue and by PCR analysis of DNA specific for this organism. Because the clinical symptoms of Crohn's disease closely mimic those found in animals with paratuberculosis in the late stages of disease, it has been proposed by a number of investigators that M. avium subsp. paratuberculosis may be a causative agent of this disorder in humans (Sanderson et al., 1992; Mishina et al., 1996). An up to date review of the evidence for and against the potential relationship between M. avium subsp. paratuberculosis and Crohn’s disease has been written by Duffy and Behr (2020). A recent study employing three different culture techniques, a phage assay and four antibody assays performed in separate laboratories on parallel blood samples, revealed that viable M. avium subsp. paratuberculosis bacteraemia was widespread in a significant number of humans including Crohn’s and ulcerative colitis patients, those with other autoimmune disorders and asymptomatic healthy individuals (Kuenstner et al., 2020).

The potential relationship between Crohn's disease and M. avium subsp. paratuberculosis has become an issue for the dairy industry since the publication of a report in 1994 by a group in the UK that suggested that viable M. avium subsp. paratuberculosis organisms were present in pasteurized milk purchased from retail markets (Millar et al., 1996). Studies evaluating the optimal time and temperature for heat inactivation of M. avium subsp. paratuberculosis have followed (Grant et al., 1996; Hope et al., 1996; Stabel et al., 1997; Sung and Collins, 1998; Keswani and Frank, 1998). Viable M. avium subsp. paratuberculosis has been detected in retail pasteurized milk in the USA, Brazil and Argentina (Ellingson et al, 2005; Carvalho et al., 2012; Paolicchi et al., 2012) and also in infant formula (Botsaris et al., 2016) and calf milk replacer (Grant et al., 2017).

Further concerns regarding exposure of humans to M. avium subsp. paratuberculosis have been raised by observations of viable M. avium subsp. paratuberculosis in cheese (Spahr and Schafroth, 2001; Williams and Withers, 2010); meat (Mutharia et al., 2010; Whittington et al., 2010), dust (Eisenberg et al., 2010) and water (reviewed in Gill et al., 2011).

Despite the enduring controversy on the causal etiological role of M. avium subsp. paratuberculosis in the development of Crohn’s disease in humans (BANR, 2003b) the concerns have been instrumental in the establishment of national control programmes in multiple countries including, Australia, Denmark, the Netherlands and the United States.

Antimicrobial therapy for treatment of animals infected with M. avium subsp. paratuberculosis is not recommended unless the animal has a high genetic value. There is no cure for paratuberculosis, so drug therapy is used only to prolong the life of an infected animal and reduce clinical signs of disease. Standard antituberculosis drugs such as clofazimine, isoniazid, rifabutin, rifampin, and streptomycin have been tested both in vitro and in vivo for effects on M. avium subsp. paratuberculosis (St. Jean, 1996). Although many in vivo studies have demonstrated an improvement in clinical signs of paratuberculosis after treatment, the organism may be still be shed in the faeces. Combination therapy with two or more of the antimycobacterial agents has proven to be more effective than one compound (Rankin, 1955; Slocombe, 1982; Das et al., 1992). Treatment times are protracted and treatment may be necessary for the remainder of the animal's life. This is a significant disadvantage because the cost of therapy can range from US $1.00 to US $219.00 per animal per day, depending upon the compound used (St. Jean, 1996).