Pathogenesis and Stages of MAP Infection in Cattle

MAP infection has been divided into four stag­es, depending on the severity of clinical signs, the potential for shedding organisms into the

Table 11.1. The ‘iceberg effect’ with different stages of Mycobacterium avium subsp.

environment and the ease with which the dis­ease may be detected using current laboratory methods. As noted in Chapter 1 and the preface, an alternative, more condensed classification has been proposed in 2017, but the classical staging is presented here. It has been proposed that, for every cow with advanced JD that was born in the herd (on that farm), it is likely that 15-25 others are infected (Whitlock, 1992). Only 25-30% of these infected animals will be detectable by even the most sensitive molecular testing techniques (Whitlock, 2009). The clini­cal animal is the ‘tip of the iceberg'. As an ex­ample, consider a herd with 100 adult cattle and 100 replacement stock. Two cows born on the farm several years earlier develop clinical signs, with weight loss and diarrhoea. It is likely that 30-50 other cattle are infected but less than 30% of these will be detectable by faecal culture and/or polymerase chain reaction (PCR) or anti­body detection methodology. An outline of these stages, and their relative importance within an infected herd, is presented in Table 11.1.

11.3.1 Entry of the organism

Early studies suggested that 10³ bacilli were in­fectious (Gilmour et al., 1965) and that, with a concentration of 10⁶-10⁸ MAP colony-forming units (CFU) per gram of faeces, only a few milli­grams of manure ingested by a young calf would be infectious (Jorgensen, 1982; Whittington et al., 2000). Another review suggested 50­1000 CFU as infective for young calves (Chiodini, 1996). Experimental studies have shown that a

1.5 ? 10⁶ CFU/dose given orally at 21 and 22 days of age reliably induced infection in mul­tiple tissues, yet at a low level (Sweeney et al., 2006). Higher doses at younger ages resulted in greater tissue infection. Following oral inges­tion, most evidence points to the ileum, more specifically, the M cells, as the main portal of entry (Momotani et al., 1988). Following entry through the M cells, MAP organisms are re­leased on the submucosal side of the intestinal epithelium where they are captured by mac­rophages. The time required for intestinal trans­location from the mucosa to adjacent lymph nodes may be as short as 1 h (Wu et al., 2007). Moreover, studies in tissue cultures have dem­onstrated that MAP may affect the formation of tight junctions in the intestinal mucosa pro­viding a mechanism for increased permeability (Bannantine and Bermudez, 2013).

From this point on, whether an animal remains infected with MAP depends on its im­mune response. If the macrophages are success­ful at killing the phagocytosed MAP, the host has a chance of fighting and clearing MAP. But be­cause MAP organisms have the ability to survive within macrophages, most likely by preventing maturation and acidification of the phagocytic vacuole (Hostetter et al., 2003), the animal may remain infected and start the long incubation period that is typical of MAP infections.

11.3.2 Stage I: ‘Silent’ infection

Once infection occurs, the organism prolifer­ates slowly in the jejunal and ileal mucosa, and spreads to the regional lymph nodes (Clarke, 1997).

Stage I-infected cattle (typically replace­ment stock) show no outward clinical signs of infection, and there is no appreciable effect on growth or production. Cattle in this phase of infection have no detectable serum antibodies to MAP and may shed MAP in their faeces but at a level below levels of detection using current methods, including culture and PCR, hence the name ‘eclipse phase'. At post-mortem examina­tion, the organisms in the tissues may not be visible on microscopic examination but may be detectable by culture of multiple intestinal tis­sues, suggesting that widespread dissemination occurs early in the disease development (Sweeney et al., 2006; Stabel et al., 2009).

11.3.3 Stage II: The infection progresses

Cattle enter stage II disease with higher concentra­tions of MAP in their intestinal tissues. Although these animals still do not manifest weight loss or diarrhoea, they have histological changes consist­ent with the intestinal granulomatous inflamma­tory response characteristic of paratuberculosis. Cattle in stage II may have an altered immune re­sponse with increased interferon gamma response by sensitized T cells to specific mitogens and/or increased antibody response to MAP (Bassey and Collins, 1997). Animals in stage II may shed MAP in their manure, contaminating the environment and serving as sources of infection to other ani­mals on the farm.

The rate of disease progression through stage II is highly variable and is most likely influenced by a wide range of factors that may include: age at initial exposure to MAP, the dose of MAP at initial exposure, the frequency of re-exposure over time, genetic factors of both the host and the organism, environmental factors, nutritional factors, pro­duction effects and a variety of stressors. In addi­tion to the progression of the intestinal infection, dissemination of MAP organisms to other organs such as the uterus and mammary glands may now occur (Sweeney et al., 1992a, b; Streeter et al., 1995).

11.3.4 Stage III: Clinical disease begins

Animals at this stage have gradual weight loss and loose manure despite a normal appetite and vital signs. Milk production and reproductive ef­ficiency are affected. Nearly all animals at stage III are positive for MAP organisms on faecal cul­ture or faecal PCR and have increased antibodies

Fig. 11.1. Ziehl-Neelsen stain of ileum from a cow with advanced paratuberculosis. Copious fuchsin­staining organisms throughout the tissue indicate a multibacillary MAP infection. (Reproduced with the kind permission of Michael Collins.)

detectable by enzyme-linked immunosorbent assay (ELISA) test or agar gel immunodiffusion (AGID) test (Sweeney, 2011).

At this stage, the MAP population within the intestinal mucosal cells is very high (Fig. 11.1). The normal absorptive capacity of the bowel is ab­rogated, resulting in weight loss associated with a protein-losing enteropathy (Sweeney, 2011). The proliferation of reactive lymphocytes, epithelioid macrophages and giant cells results in infused blunted villi with decreased absorptive capac­ity. A thickened intestinal mucosa and associated lymphadenopathy along a substantial portion of the small intestinal tract is characteristic of the disease progression (Fig. 11.2). The infection becomes disseminated with MAP detectable in several extra-intestinal sites. These cows are at higher risk of transmitting the organism in utero and have a higher frequency of MAP isolated from the milk. Cattle in stage III shed high concentra­tions of MAP in their faeces, contaminating the environment.

11.3.5 Stage IV: Advanced clinical disease

Animals in stage IV of the disease are weak, emaciated and usually have chronic, profuse diarrhoea (Fig. 11.3). Intermandibular oedema or bottle jaw is characteristic of this phase of the disease. Animals can progress quickly from stage II to stage IV, sometimes within a few weeks, but a more gradual progression is more typical. Once the diarrhoea is profuse and hypoproteine­mia occurs, the animal's condition deteriorates rapidly, often in a matter of days. Most animals are sent to slaughter for salvage at this point. Otherwise, death occurs as a result of dehydra­tion and cachexia.

11.4