Within-farm transmission of bovine paratuberculosis: recent developments

Abstract

Mycobacterium avium subspecies paratuberculosis is the causative agent of paratuberculosis in cattle which causes a chronic infection of the small intestine. Since the transmission is only partly understood current control programs have been able to only decrease prevalence but not to eradicate disease from a herd. Unknown and therefore uncontrolled routes of transmission were suggested and infective bioaerosols were hypothesized as a potential candidate. This review gives an overview concerning disease transmission and focuses on consequences of bioaerosols on the within-herd transmission of paratuberculosis.

Keywords:

• bovine
• cattle
• paratuberculosis
• Mycobacterium avium subspecies paratuberculosis
• bioaerosols
• transmission
• Johne's disease

1. Introduction

Paratuberculosis (Johne's disease) is a chronic infection of the small intestine in cattle caused by Mycobacterium avium subsp. paratuberculosis (MAP), a slow growing, acid-fast bacterium (Lombard 2011). The common understanding is that infection is predominantly transmitted from infectious adult cattle to susceptible young stock by the fecal-oral route, whereas susceptibility to infection is highest in newborn calves and decreases with age (Windsor and Whittington 2010; Lombard 2011). Slow course of disease in combination with limited options for detection of early infection imply that, once introduced on a farm, MAP can spread unnoticed until the first clinical case is identified. MAP infections on dairy farms occur around the world. Herd-prevalence in Europe has been estimated to be over 50% (Nielsen and Toft 2009). Similar figures have been estimated for Canadian (Sorensen et al. 2003) and US dairy herd prevalence levels (Johnson-Ifearulundu and Kaneene 1998). This review gives an overview concerning disease transmission and focuses on consequences of bioaerosol transmission on the epidemiology of paratuberculosis.

Economic losses due to lower milk production and a decrease in slaughter value as well as the ongoing discussion about the association between paratuberculosis in cattle and Crohn's disease in humans led to the development of control programs. According to simulation models, the main focus of control programs should be to prevent fecal-oral transmission by improving calf management, along with the removal of shedding animals once they have been identified (Groenendaal and Galligan 2003; Groenendaal et al. 2003; Marce et al. 2011). Recommended preventive measures are the improvement of hygienic measures associated with calving, separation of calf and dam as soon as possible after birth, and separation of young stock from adult cattle (Cetinkaya et al. 1997; Johnson-Ifearulundu and Kaneene 1998; Muskens et al. 2003). MAP control programs based on improving calf management and hygiene and removing test positive cows from the herd have been successful in decreasing prevalence but not to eradicate MAP (Benedictus et al. 2008; Wells et al. 2008; Ferrouillet et al. 2009; Collins et al. 2010). These findings indicate that not all MAP transmission routes are controlled by commonly implemented measures (Schukken et al. 2009).

2. Transmission routes

The most likely important route of MAP transmission is the fecal-oral route between infectious cows and susceptible young calves via ingestion of contaminated milk, water, other food products, or uptake from the environment (Lombard 2011). In addition, MAP has been identified in udder tissue, mammary lymph nodes, and milk/colostrum samples in clinically as well as in subclinically infected cows making MAP transmission likely through infected milk/colostrum (Taylor et al. 1981; Sweeney et al. 1992a; Streeter et al. 1995). However, recent studies investigating the risk of fecal shedding of calves born to fecal culture positive dams or fed with MAP containing colostrum were not able to confirm these proposed risk factors (Pithua et al. 2010, 2011). Intra-uterine transmission of MAP has been described in cows in subclinical and clinical stages of the infection (McQueen and Russell 1979; Sweeney et al. 1992b). A recent meta-analysis indicated that intra-uterine MAP transmission is a significant risk in dairy herds with a high prevalence (40%) and may be responsible for maintaining the infection in these herds even in situations where good hygiene measures have been implemented (Whittington and Windsor 2009). Despite difficulties in detecting fecal shedding of MAP in calves using current diagnostic tests, MAP transmission may also occur between calves (van Roermund et al. 2007). Model studies suggest, however, that calf-to-calf transmission does not constitute a major transmission route leading to increasing animal level prevalence (Marce et al. 2011; Weber and Groenendaal 2012). MAP transmission via the respiratory tract in cattle was postulated as an additional route of infection (Corner et al. 2004). It was hypothesized that MAP-containing aerosols may be generated. In sheep, the intra-tracheal application of MAP caused intestinal lesions which were comparable to lesions which occurred after oral inoculation (Kluge et al. 1968). In an infection study we recently showed that nasal as well as transtracheal trickle inoculation of calves with low dose of MAP leads to intestinal infection in these calves (Eisenberg et al. 2011).

3. Environment and MAP infection

MAP is an obligate intracellular pathogen of ruminants indicating that it needs a host to be able to replicate (Thorel et al. 1990). However, a survival time of MAP outside the host in spiked slurry, water, and urine samples up to 250 days depending on the temperature was reported (Larsen et al. 1956; Jorgensen 1977). Survival of MAP was even longer in naturally contaminated soil and grass, up to 55 weeks after removing the ruminants in dry fully shaded locations (Whittington et al. 2004). Repeatedly collected environmental samples on sheep farms showed that 5 months after destocking, MAP could hardly be detected anymore (Whittington et al. 2003). These findings indicate that the environment does stay contaminated and probably infectious for considerable time after infected animals have been removed.

MAP exposure from environmental reservoirs has been accepted as a direct route of disease transmission (Fecteau et al. 2010; Lombard 2011) and has been proposed as a useful parameter in MAP monitoring programs for herd classification (Raizman et al. 2004; Berghaus et al. 2006; Lombard et al. 2006). Milking parlor exits, common alleyways, lagoons, and manure storage areas were identified as locations which best predicted MAP infection status in disease monitoring. 70% of the infected herds could be identified using environmental samples compared with individual milk or serum ELISA or pooled fecal samples (Lombard et al. 2006). Advantages of environmental sampling are the lower costs due to the requirement of fewer samples, as well as the avoidance of additional animal handling (Berghaus et al. 2006; Lombard et al. 2006; Raizman et al. 2007). The number of MAP positive environmental samples detected on a farm was found to be positively correlated with the prevalence of MAP positive animals (Raizman et al. 2004; Berghaus et al. 2006). Despite the well-documented presence of MAP in manure and the long environmental survival times, manure is still used as an organic fertilizer on grassland in many countries. After application on grassland, MAP tends to stay on the upper layers of soil and on grass – indicating that grassland can be seen as an infection hazard after fertilization with infected cow manure (Salgado et al. 2011). Transmission of MAP following grazing on pasture fertilized with MAP positive manure was described in sheep (Muskens et al. 2001).

The environment may not only be a source of infection when calves are in contact with fecal-contaminated areas, but it might also become infective in a more indirect way. Animal movement in barns leads to dust production which causes environmental contamination without the necessity of infective animals being present in a particular calf area. Recently it was shown that dust in infected herds can contain viable MAP and spreads throughout the whole barn (Eisenberg, Nielen et al. 2010; Eisenberg, Koets et al. 2010). Transmission via infective dust might therefore be a previously unrecognized route of within-farm transmission (Eisenberg, Koets et al. 2010).

4. Bioaerosols

4.1. Uptake of bioaerosols

Aerosols are defined as solid or liquid particles suspended in air, e.g. dust, smoke, and fog. Bioaerosols are aerosols containing particles of biological origin (Stark 1999). Particle sizes may range between 0.5 and 100 µm (Pearson and Sharples 1995). Particles > 10 µm are visible to the naked eye. Calves can take up dust particles by ingestion, inhalation, or by a combination of both routes. Particles of all sizes can settle in the environment, and due to explorative calf behavior, dust will be ingested by licking and suckling. Particles between 5 and 20 µm can be inhaled but will be removed by the mucociliary epithelium in the upper respiratory tract, presented to the nasal associated tissue and subsequently swallowed (CEN 1993; Pearson and Sharples 1995; Lugton 1999). The ‘fecal-oral route’ is then re-established, with pathogens entering either through the tonsils when swallowed or through the digestive tract (Payne et al. 1960; Payne and Rankin 1961; Neill et al. 1988; Lombard 2011). In humans particles lesser than 5 µm are referred to as ‘respirable’, which means that the particles will reach the lungs (alveoli) after inspiration. Similar patterns of respiratory deposition have been described for calves and other animals (Hatch 1961; Davies and Webster 1987). After inhalation of ‘respirable dust’, the lung can act as the portal of entry through the uptake of the infectious agent by macrophages (Pritchard 1988) or bacteria can be removed by alveolar macrophages and the mucociliary system (Green and Kass 1964). Once the epithelial barrier is passed, bacteria incorporated in macrophages can invade the local lymph node and from there the whole body via the reticuloendothelial system.

4.2. Aerosol transmission of infectious disease

Several conditions must be fulfilled before airborne disease transmission can occur. Bioaerosols must be generated; they have to be transported to and be inhaled by susceptible individuals (Stark 1999). Survival of bacteria in bioaerosols depends on relative humidity, temperature and radiation, and these factors differ between types of bacteria (Stark 1999). Aerosol transmission has been reported for different bacterial diseases targeting different organ systems (Kaufmann et al. 1980; Hardman et al. 1991; Kristensen et al. 2004).

For some bacteria, a dogma shift took place after considering bioaerosols responsible for infection. Transmission of Mycobacterium tuberculosis via aerosols is well established today, but at the beginning of the twentieth century transmission via the alimentary tract was believed to be the most important transmission route (Pritchard 1988). Several research groups had to prove independently that significantly smaller amounts of bacteria could cause tuberculosis via inhalation compared to ingestion (Findel 1907; Reichenbach 1908; Pritchard 1988).

A similar development took place for the understanding of the transmission of Mycobacterium leprae. It was believed to be transmitted by close contact with people having open skin lesions until around 1970, even though large numbers of acid-fast bacteria had already been detected in nasal discharge and aerosols produced by coughing and sneezing of patients at the end of the nineteenth century (Schaeffer 1898). Studies conducted in 1974 showed M. leprae isolated from the nose of patients could infect mice, and comparative epidemiological research with M. tuberculosis showed a similar spread of both infections in humans, indicating a similar route of transmission (Rees and Meade 1974; Rees and McDougall 1977). Today, bioaerosols are accepted as the main transmission route in M. leprae infection.

The phenomenon of suspension in the air after being shed in feces and causing disease has already been described for other bacteria in farm animals (Wathes et al. 1989; Hardman et al. 1991; Wilson et al. 2002). In animal housings, it has been shown that bacteria become airborne together with skin cells, hair, feed, bedding, and excretion particles in the form of dust and move through the barn (Collins and Algers 1986). Aerosol infection has been shown to be important in transmission of bacterial diseases of the respiratory tract, e.g., for Mycobacteria spp. in humans and Actinobacillus pleuropneumoniae in piglets (Falkinham 2003; Kristensen et al. 2004). It has also been proven for diseases of the digestive tract, e.g., Salmonella typhimurium infection in calves and Escherichia coli infection in piglets (Wathes et al. 1988, 1989; Hardman et al. 1991). As described before, MAP shed in the environment can survive there for some time. Bioaerosol formation of dried feces can occur due to animal movement, and thus calves will be exposed to infectious dust (Eisenberg, Koets et al. 2010). Nasally inoculated calves showed infected tonsils and retropharyngeal lymph nodes whereas transtracheally inoculated calves showed infected tracheobroncheal lymph nodes in a recent infection experiment (Eisenberg et al. 2011). These findings indicate that MAP uptake also occurred via the respiratory tract although secondary uptake via the intestinal tract might have occurred as well.

5. Conclusion

The well-established transmission route of MAP is the oral uptake of the bacteria by susceptible calves. However, under experimental conditions nasal and tracheal infection was successful in calves and sheep. Follow-up studies of control programs confirmed that MAP prevalence in dairy herds could be reduced but MAP could not be eradicated. Bioaerosols containing viable MAP have been identified and are not controlled for in current control programs. Therefore, MAP-contaminated bioaerosols represent a plausible infection route via ingestion after inhalation and/or ingestion by licking and suckling. Per respiration only a small amount of dust can be taken up. However, due to continuous exposure it might amount to an infective dose. Earlier suspicions about the presence of an unknown and therefore uncontrolled transmission route have thus been confirmed. Uptake of bioaerosols with viable MAP via the respiratory tract is a possible route of within-herd transmission of paratuberculosis on dairy farms.

In commercial dairy farms, viable MAP could not be detected in young stock housings when calves were in buildings spatially separated from the dairy herd (Eisenberg, Koets et al. 2010). Current control programs aimed at reducing fecal-oral transmission should therefore consider immediate separation of calves after birth and fully separated housing and rearing of young stock in addition to existing measures.