Hemorrhagic Bowel Syndrome, HBS, is characterized by sudden drop in milk production, abdominal pain due to obstructed bowel, and anemia. Death comes within 48 hours of bowel obstruction by the blood clot plug. Many potential causes of HBS have been investigated and discarded. Moldy feed was observed on several dairies which experienced cow losses due to HBS. Several studies have demonstrated potential for Aspergillus species to infect the ruminant gut at various sites and to cause enteric hemorrhage. An additional factor, in tandem with Aspergillus fumigatus, (possibly immunosuppression) predisposes dairy cattle to HBS. Proper storage and rotation of feed ingredients and ensiled forage inventories can greatly reduce exposure to Aspergillus fumigatus. Attention to detail when ensiling forages is paramount in the prevention of mold growth. Other recommendations include avoidance of feeding spoiled or moldy grains and other feedstuffs to pregnant or lactating animals. Summarized by Milkproduction.com staff.
In 1991, Anderson reported a new disease in Idaho dairy herds (1). The syndrome, which he termed “Point Source Hemorrhage”, was observed in five high-producing Holstein cows from one dairy. Symptoms included point-source sub-mucosal hematomas, each affecting 10-20 cm of the jejunum. One of the five cows exhibited a ruptured hematoma with exsanguination into the lumen of the jejunum. The origin of the hematoma was traced to the jejunal submucosa which dissected mucosa from underlying connective tissue. Despite the hemorrhage, clotting time was normal. No bacteriological assays proved definitive and no ulcerative processes, parasitism or vaculitis were apparent. Feeding and management practices on the dairy were described as “exemplary”.
Since 1991, awareness of “Point Source Hemorrhage” (now more commonly known as “Hemorrhagic Bowel Syndrome (HBS)”), in dairy cattle has grown. HBS incidence is increasing (3,4) and is responsible for 2% of the deaths of dairy animals in the US (5). Estimates of incidence may be somewhat inaccurate due to the following; a marked seasonality to the disease (more cases occur in cooler winter months), producer and veterinarian unfamiliarity with the disease, symptoms mimic other common ruminant digestive diseases and an unknown proportion of afflicted cattle are submitted for necropsy.
Signalment and etiology
HBS is characterized by sudden drop in milk production, abdominal pain due to obstructed bowel, and anemia (6). Death comes within 48 hours of bowel obstruction by the blood clot plug. Fatal factors are presumed to be the anemia combined with digesta stagnation in much of the severely dilated small intestine proximal to the plug (6).
What is the cause of HBS?
Despite finding C. perfringens in most HBS cows, Dennison et al (2) commented that “it is unclear whether proliferation of C. perfringens is part of the primary disease process in cows with HBS or occurs as a secondary response.” Evidence against C. perfringens playing the primary etiologic role includes the observations that C. perfringens is ubiquitous (7,8). Furthermore, immunization against Clostridium spp. does not appear to protect animals from HBS.
Many potential causes of HBS have been investigated and discarded. Agents which do not play an etiologic role include parasitism (1), BVD, coccidia, salmonella, coagulopathies, intestinal foreign bodies, physical obstructions and deformities including volvulus and intussuception (2). Furthermore, analyses of diets, ages of cow, levels of milk production and a full spectrum of blood chemistry and biochemical assays failed to reveal a consistent clinical correlate to HBS (2).
An alternative etiology
Moldy feed was observed on several dairies which experienced cow losses due to HBS. In “human literature”, Aspergillus fumigatus is described as pathogenic, causing intestinal bleeding and invasive aspergillosis in immunocompromised patients. We hypothesized that immunocompromised dairy cows might develop HBS when exposed to A. fumigatus –laden feed.
Pathogenicity of Aspergillus fumigatus
Aspergillus is a large fungal genus containing > 100 species. Of these, A. fumigatus and flavus are the most pathogenic (9,10). Pathogenicity of Aspergillus is attributed to three virulence factors: 1) production of iron (Fe)-sequestering siderophores, 2) secretion of complement- and phagocytic- inhibitory lipids and 3) secretion of proteases (9,10).
Specifically, A. fumigatus is able to meet its iron requirement, and thereby maintain growth, by the actions of its proteolytic enzymes. These liberate the host’s iron stores from transferrin and lactoferrin and allow iron transfer to triacetylfusarimine and ferriciocin (10).
In addition, invasiveness is facilitated by secretion of polar and neutral lipids, phenolic compounds and heterocyclic toxins (including aflatoxins and other toxins). Some of these inhibit phagocytosis while others suppress the immune response of the host by inhibiting complement factors C3a and C5a (10). Finally, pathogenic species of Aspergillus, like an invasive tumor, secrete proteases which facilitate hyphal penetration from a colonization site into the underlying parenchymal tissue (10).
Aspergillus in the ruminant gut
Several studies have demonstrated potential for Aspergillus species to infect the ruminant gut at various sites and to cause enteric hemorrhage. Jensen et al (13) surmised that the ruminant gut provided two portals for fungal invasion: intestinal Peyer’s patches and the pre-gastric digestive compartments and proposed that A.fumigatus was the primary invader.
In more recent studies (14,15) Jensen et al evaluated predisposing factors for mycotic infections in ruminants. The most common mycoses included aspergillosis, candidosis and zygomycoses. Principle etiologic agents were A. fumigatus, Candida albicans and Absidia corymbifera, respectively. Mucor pusillus and Rhizopus spp. were also identified as common etiologic agents in zygomycoses (14). Portals for infection were identified in the respiratory and GI tracts. GI mycosis was identified with the omasum representing the main organ for infection.
In one study (15), 32 of 694 cattle submitted for necropsy had gastrointestinal mycoses with an elevated incidence in cooler months (i.e., 75% incidence was in October through March, a time during which stored feed is fed to cattle). Aspergillus and zygomycetes were detected in gut wall vasculature with thromboses and vasculitis. Hematogenous spread of fungi to the liver, lung and kidney was detected. A. fumigatus was detected in 10 of 21 cows, A. corymbifera was detected in 8 of 23 cows and Candida was detected in 1 cow. Of interest, animals were never infected with more than one fungal species. Predisposing factors for mycotic infections included: 1) feeding of moldy feed, 2) immunocompromizing diseases, 3) acidosis, 4) antimicrobial therapy, 5) reflux of abomasal contents, 6) metabolic disturbances, 7) post-partum stress, 8) viral erosive diseases such as IBR, 9) anti- inflammatory treatment, and 10) abortion.
Aspergillosis in immunocompromised humans
A. fumigatus is ubiquitous. Yet it rarely causes serious disease in healthy individuals. Immunoincompetence is the primary predisposing factor in Aspergillus infection (invasive aspergillosis; 16-19) in humans. Patients with AIDS, cancer and those receiving organ transplants are particularly susceptible to invasive aspergillosis (17-19). For example, invasive aspergillosis occurs in 2.6 – 10.3% of all bone marrow transplant patients and has a mortality rate of 56 to 88.1% (16).
Immunosuppression in dairy cows
Mallard et al (20) have reported that immunosuppression is common in dairy cows and accounted for the high incidence of disease. Changes in both immune function and nonspecific host defense mechanisms have been reported in dairy cows at onset of lactation (21-25). Stressors in lactation include 1) a high energy diet (and potential acid reflux), 2) ketosis, 3) milk fever, 4) lameness, 5) regular handling, 6) post-partum stress, 7) inconsistent or poor feeding practices, (26), 8) social isolation when sick animals are placed in a “hospital pen” (27) and 9) artificial insemination (28). A stressed, immunocompromised dairy animal is susceptible to mycotic infection. The provision of A. fumigatus-infected feed to this animal may be a “trigger” which elicits HBS.
Feed-borne Aspergillus fumigatus infects the GI tract, tissues and blood of ruminants
A. fumigatus genomic analysis was conducted in eight HBS cows, one abomasal hemorrhage (AH) cow and one AH gazelle. A. fumigatus was detected in 3 of 3 submitted feed samples, gut contents (7 of 7 cases), gut wall (5 of 5 cases), and mesenteric lymph node (3 of 5 cases). Invasive aspergillosis was indicated by detection of A. fumigatus DNA in blood (6 of 6 cases) and in liver (1 of 1 case). One case of HBS was associated with a late-term abortion. Cotyledons from that case were sampled and also found to harbor A. fumigatus.
DNA from A. fumigatus was also detected in two cases of abomasal hemorrhage (AH), one in a dairy cow and another in a Dama gazelle.
Seventeen non-HBS, negative control cows have also been tested and, of these, 14 have been negative for A. fumigatus. The remaining cows (n=3) contained very low levels of A. fumigatus DNA; near the detection limit of our assay (i.e., < 0.02 X 106 A. fumigatus genomic units/ml of blood). These levels were 1/20th to 1/50,000th of the levels detected in HBS cows. Two of the negative controls (which tested negative for A. fumigatus) were from cows which had died from unknown causes at two different dairies. Both had exhibited rumen stasis and sudden death but did not have HBS.
Local feeds were tested for the presence of A. fumigatus. Feed samples have included mill run, ground corn, grass and corn silages and dried grass hay. Many were infected with A. fumigatus, although infection is not always visible (A. fumigatus on moist feed is dark blue- green). Hence, exposure to A. fumigatus may be common. An additional factor, in tandem with A. fumigatus, (possibly immunosuppression) predisposes dairy cattle to HBS.
Genotype analysis of five clostridial toxins indicated no correlation between HBS and toxin genes. In HBS and AH cases, genes encoding toxins A and E were detected in only 3 of 9 analyses. Toxin gene β and enterotoxin were not detected in any samples. The A and E toxin genes were detected in blood (1 of 3 analyses), jejunal or abomasal clot (2 of 4 analyses) and GI wall (1 of 5 analyses). In negative control cows, the ß toxin gene was detected in blood of 3 of 17 animals.
A Role for A. fumigatus in HBS
A clinical study showed HBS to be associated with high levels of A.fumigatus in the gut, gut wall and blood. In human studies, A. fumigatus is recognized for its invasive properties and pathogenicity in immunocompromised patients. We propose it has similar potential in ruminants. Alternatively, clostridial toxin genes were not correlated with HBS.
Limitations of this study include the small data set (i.e., eight HBS cows, two AH animals, 17 controls) and lack of GI tissues from negative control cows. A larger data set and completion of controlled studies with dairy animals are needed to definitively ascribe HBS to A. fumigatus.
A further limitation is that >100,000 fungal species are known. There are >100 Aspergillus species. Only a very small proportion of their ITS domains have been published. Hence, our detection of A. fumigatus could include unrecognized (non-sequenced) fungal species. Another issue with HBS is that many cows consume infected feed, yet only a small proportion develops HBS. We do not know what the other predisposing factor(s) may be. However, we propose that stress-induced immunoincompetance may play a role in the etiology of HBS. Finally, we developed and tested a product which inhibits fungal growth in vitro and in vivo. Application of this product on dairies shows potential for prevention of HBS and, possibly, other mycotic infections.
Limiting Exposure from Improperly Ensiled Forages
Proper storage and rotation of feed ingredient and ensiled forage inventories can greatly reduce exposure to Aspergillus fumigatus. Attention to detail when ensiling forages is paramount in the prevention of mold growth. Meeting silage moisture goals will help control mold growth. Moisture targets are 68 to 72% for corn silages and 64 to 68% for legume haylages going into bunker or drive over silos. Silage pits should be continuously and rapidly filled, and immediately packed with a heavy wheeled tractor for best results. Avoid interruptions in the filling process, if possible, to prevent layers of spoilage from forming. Properly fermented corn silages should reach a pH of 4.0 or less and legume forages will come in below pH 4.5. Inoculants can lower silage pH more rapidly.
After ensiling, molds will grow rapidly as lactic acid flashes off from exposed surface layers. Therefore, horizontal silos (bunker, pit, roll-over piles, etc.) should be immediately covered with quality plastic and the plastic weighted down to ensure that the cover will remains firmly in place. Horizontal silos should be designed to allow filling from the back, sloping away from the intended active feeding area to prevent rain runoff from draining into exposed feed. The spoiled layer at the top of the silo should be discarded, whenever possible, providing it doesn’t pose a safety risk to employees. In a 2003 OSU survey of Pacific Northwest feedstuffs, extremely high concentrations of Aspergillus fumigatus spores (>1.25 million per gram) were isolated from the spoiled layer from the top of corn silage bunkers. High levels of A.fumigatus spores were also isolated from separated manure solids used to anchor silo covers (>3 million spores per gram), making this a questionable practice, especially in cases where silos are sloped towards the active feeding face. Compromising hygiene at the feed bunks and water troughs often leads to explosive growth of molds including Aspergillus. Regular cleaning of feed bunks and water troughs should be part of every dairy’s HAACP program.
Other recommendations include avoidance of feeding spoiled or molded grains and other feedstuffs to pregnant or lactating animals. Feeding excessively high starch rations can lead to ruminal acidosis and severely damage the gut mucosa, predisposing animals to colonization by invasive molds and other pathogens.
We are grateful to Dr. Bruce Anderson DVM, PhD (University of Idaho), Dr. Mark Rasmussen DVM, PhD (National Animal Disease Center, Iowa State University), Dr. Rob Bidfell DVM and Jerry Heidel DVM, PhD (OSUCVM) for their advice and for samples from HBS cows. We thank Dr. Glenn Cantor DVM, PhD (Pharmacia, Kalamazoo, MI) for his advice and insights. We are also grateful to Dr. Francoise Symoens PhD (Scientific Institute for Public Health, Brussels, Belgium) for provision of fungal genomic DNA standards.
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