Histophilus Somni
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''Histophilus somni'' is a non-motile,
gram-negative Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...
, rod or
coccobacillus Bacterial cellular morphologies are the shapes that are characteristic of various types of bacteria and often key to their identification. Their direct examination under a light microscope enables the classification of these bacteria (and archae ...
shaped,
facultative anaerobe A facultative anaerobic organism is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent. Some examples of facultatively anaerobic bacteria are ''Staphylococcus' ...
bacterial species belonging to the family
Pasteurellaceae The Pasteurellaceae comprise a large family of Gram-negative bacteria. Most members live as commensals on mucosal surfaces of birds and mammals, especially in the upper respiratory tract. Pasteurellaceae are typically rod-shaped, and are a notabl ...
. Prior to 2003, it was thought ''Haemophilus somnus'', ''Histophilus ovis'', and ''Histophilus agni'' were three different species, but now are all classified as ''Histophilus somni''. ''Histophilus somni'' is a
commensal bacteria Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fr ...
of mucous membranes of the upper respiratory tract and reproductive tract with a global prevalence and is found in cattle and other small ruminants. ''Histophilus somni'' is also a known causative agent that is a part of the Bovine Respiratory Disease (BRD) complex, which typically involves multiple pathogens residing together in
biofilm A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...
environments. ''Histophilus somni'' may also cause Histophilosus symptoms and clinical presentation will depend on the tissue affected. When disease does occur, it can be difficult to catch in time and is often diagnosed post mortem. This means that treatment often involves metaphylactic mass treatment or no treatment at all. This organism is more fastidious than others and requires knowledge for sample collection, storage and culture.
Genomic Genomics is an interdisciplinary field of molecular biology focusing on the structure, function, evolution, mapping, and editing of genomes. A genome is an organism's complete set of DNA, including all of its genes as well as its hierarchical, ...
studies related to this bacteria have enabled scientists to pinpoint
antibiotic resistance Antimicrobial resistance (AMR or AR) occurs when microbes evolve mechanisms that protect them from antimicrobials, which are drugs used to treat infections. This resistance affects all classes of microbes, including bacteria (antibiotic resis ...
genes.   


Description

''Histophilus somni'' is a member of the
Pasteurellaceae The Pasteurellaceae comprise a large family of Gram-negative bacteria. Most members live as commensals on mucosal surfaces of birds and mammals, especially in the upper respiratory tract. Pasteurellaceae are typically rod-shaped, and are a notabl ...
family and was first isolated from cattle in 1956. ''Histophilus somni'' is a gram-negative, rod or
coccobacillus Bacterial cellular morphologies are the shapes that are characteristic of various types of bacteria and often key to their identification. Their direct examination under a light microscope enables the classification of these bacteria (and archae ...
shaped bacteria that does not express pili or
flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
making it non-motile. Many species of the Pasteurellaceae family are encapsulated; however, based on electron microscopy and ruthenium red staining results ''H. somni'' is not encapsulated. Prior to 2003 ''Histophilus somni'' was considered to be three different species ''Haemophilus somnus, Histophilus ovis,'' and ''Histophilus agni''. ''Histophilus somni'' is a normal part of mucosal
microbiota Microbiota are the range of microorganisms that may be commensal, mutualistic, or pathogenic found in and on all multicellular organisms, including plants. Microbiota include bacteria, archaea, protists, fungi, and viruses, and have been found ...
in the upper respiratory and genital tract of
bovine Bovines (subfamily Bovinae) comprise a diverse group of 10 genera of medium to large-sized ungulates, including Bos, cattle, bison, African buffalo, Bubalus, water buffalos, and the four-horned and spiral-horned antelopes. The members of this gro ...
and
ovine Sheep (: sheep) or domestic sheep (''Ovis aries'') are a domesticated, ruminant mammal typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus ''Ovis'', in everyday usage it almost always refers to dom ...
animals, yet, under the right conditions is one of multiple bacteria responsible for causing bovine respiratory disease. Under normal conditions BRD complex bacteria symbiotically reside together in
biofilm A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...
environments. The biofilm protects ''H. somni'' from harmful substances while allowing co-existence with the host by down regulating
virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its abili ...
factor productions. Although ''H. Somni'' is known to survive the harsh intracellular environments of host
phagocytic Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...
cells, the bacteria is considered fragile in the external environment and does not survive well outside of the host.


Culture and Biochemistry


Culture and Identification

''Histophilus'' is a fastidious bacterium and has some special requirements for culturing. A differentiating factor between ''Histophilus somni'' and the ''Haemophilus'' species is that ''Histophilus'' species are able to grow in the absence of Factor X (heat stable hemin) and Factor V (NAD) although their growth is increased in the presence of these factors. ''Histophilus'' grows best on
chocolate agar Chocolate agar (CHOC) or chocolate blood agar (CBA) is a nonselective, Growth medium#Enriched media, enriched growth medium used for isolation of pathogenic bacteria. It is a variant of the blood agar plate, containing red blood cells that have ...
at 37 °C with 5-10% and is unable to grow on
MacConkey agar MacConkey agar is a selective and differential culture medium for bacteria. It is designed to selectively isolate gram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentatio ...
. Cultures have been developed in hopes to make culturing ''Histophilus'' easier, but trials are usually unsuccessful. ''Histophilus'' can be grown in Columbia broth or
brain heart infusion broth Brain heart infusion (BHI) is a growth medium for growing microorganisms. It is a nutrient-rich medium, and can therefore be used to culture a variety of fastidious organisms. In particular, it has been used to cell culture, culture streptococci, ...
with the supplementation of thiamine monophosphate. Culturing in a broth before plate culturing will help isolate ''Histophilus'' from other commensal bacteria at the sample site. The colonies are quite small and usually only reach a size of 1-2mm. When grown on blood agar, ''Histophilus'' will have a clear areas around the colonies. The dew-drop shaped colonies grown on chocolate agar are tinged yellow which is a distinguishing factor of these colonies. Because ''Histophilus'' is fragile outside the host, care must be taken when collecting samples for laboratory diagnosis. Samples can try out very easily and should be cultured as soon as possible. The best storage method for samples to stay viable is for deep freezing below -60 degrees Celsius. The best way to identify ''H. somni'' is through
16S rRNA gene 16S ribosomal RNA (or 16 S rRNA) is the RNA component of the 30S subunit of a prokaryotic ribosome (SSU rRNA). It binds to the Shine-Dalgarno sequence and provides most of the SSU structure. The genes coding for it are referred to as 16S rRN ...
PCR amplification. Microagglutination assays have been developed for identification in cattle however many animals have positive cross-reactive antibodies without having had the infection. Methods have been developed for the differentiation between ovine and bovine strains of ''H. somni,'' including restriction enzyme analysis. Microscopy can be used for identification when bacteria are stained with fluorescent antibody stain.


Biochemistry

''Histophilus'' species can be difficult to identify with biochemical reactions as many of the tests used for identification do not have consistent results. Tests consistent throughout ''Histophilus'' colonies are oxidase and catalase tests, with oxidase having positive results and catalase being negative. ''Histophilus'' does not exhibit consistent
hemolysis Hemolysis or haemolysis (), also known by #Nomenclature, several other names, is the rupturing (lysis) of red blood cells (erythrocytes) and the release of their contents (cytoplasm) into surrounding fluid (e.g. blood plasma). Hemolysis may ...
. Their ability to reduce nitrogen allows them to be
facultative anaerobes A facultative anaerobic organism is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent. Some examples of facultatively anaerobic bacteria are ''Staphylococcus' ...
and their growth on culture will increase with an increased atmospheric . ''Histophilus'' has the ability to ferment glucose, has variable results with lactose and mannitol and are unable to ferment sucrose. Antimicrobial testing has demonstrated that ''H. somni'' is still susceptible to most antimicrobials, though some strains have shown resistance to tetracycline.


Genetics

In 1995 ''
Haemophilus influenzae ''Haemophilus influenzae'' (formerly called Pfeiffer's bacillus or ''Bacillus influenzae'') is a Gram-negative, Motility, non-motile, Coccobacillus, coccobacillary, facultative anaerobic organism, facultatively anaerobic, Capnophile, capnophili ...
'' was genetically sequenced, a close relative to ''Histophilus somni'' and the first free living organism to have its complete
genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
sequenced. Due to the economic importance from production losses the genomes of both ''Histophilus somni'' pneumonia strain 2336 (2,263,857
base pair A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...
s with 1,980 protein coding
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
s) and preputial strain 129Pt (2,007,700 base pairs with 1,792 protein coding genes) have since been sequenced. The genome studies of ''Histophilus somni'' strains have identified specific markers encoding
tetracycline Tetracycline, sold under various brand names, is an antibiotic in the tetracyclines family of medications, used to treat a number of infections, including acne, cholera, brucellosis, plague, malaria, and syphilis. It is available in oral an ...
resistance, and
virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its abili ...
factors, while allowing a better understanding of the role of
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
in the evolution of these strains
Plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria and ...
-borne
antimicrobial resistance Antimicrobial resistance (AMR or AR) occurs when microbes evolve mechanisms that protect them from antimicrobials, which are drugs used to treat infections. This resistance affects all classes of microbes, including bacteria (antibiotic resista ...
is an important topic in modern microbiology and occurs commonly in members of the ''Pasteurellaceae'' family. ''Histophilus somni'' with tetracycline resistant
plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria and ...
s have been cultured from nasal swabs of calves. Using plasmid profiling as a way to identify isolated ''Histophilus somni'' from field samples gives veterinarians the ability to practice
antimicrobial stewardship Antimicrobial stewardship (AMS) refers to coordinated efforts to promote the optimal use of antimicrobial agents, including drug choice, dosing, route, and duration of administration. AMS has been an organized effort of specialists in infectious d ...
and reduce the impact of antimicrobial resistance.


Pathogenesis and Virulence

''Histophilus somni'' can be characterized as an opportunistic pathogen and successful disease can be established because of poor environmental factors and the bacteria's own
virulence factor Virulence factors (preferably known as pathogenicity factors or effectors in botany) are cellular structures, molecules and regulatory systems that enable microbial pathogens (bacteria, viruses, fungi, and protozoa) to achieve the following: * c ...
s. ''H. somni'' has numerous virulence factors including surface proteins, binding to and induction of apoptosis in host endothelial cells, immunoglobulin binding proteins, phase variation, endotoxin, biofilm formation, free radical inhibition, and survival of phagocytosis that allow the bacteria to colonize host tissues and evade the host's immune system. Depending on the strain of ''H. somni'', not all of the listed virulence factors may be present. Due to the role of ''H. somni'' as a commensal bacterium in the respiratory tract, an important part of establishing disease lies in the relationship between respiratory epithelial cells and the bacterium. It has been suggested that pathogenesis begins when the bacteria invades and crosses the pulmonary alveolar membrane or that it can cause both bovine turbinate (BT) and bovine alveolar type 2 (BAT2) cells to retract allowing passage of the bacteria into the bloodstream. In order successfully cross into the blood, in addition to causing respiratory endothelium cells to retract, ''H. somni'' disrupts the basement membrane via digestion by increasing production of
metalloproteinase A metalloproteinase, or metalloprotease, is any protease enzyme whose catalytic mechanism involves a metal. An example is ADAM12 which plays a significant role in the fusion of muscle cells during embryo development, in a process known as myoge ...
s from BAT2 cells. After entry into the bloodstream, the bacteria can colonize other tissues around the body such as the heart and is involved in
biofilm A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...
production on cardiac endothelium in bovine myocarditis. ''H. somni'' surface proteins have been studied in association to virulence and pathogenesis. In serum resistant virulent strains, outer membrane proteins such as a sialic acid-modified
lipooligosaccharide Lipopolysaccharide (LPS), now more commonly known as endotoxin, is a collective term for components of the outermost membrane of the cell envelope of gram-negative bacteria, such as ''Escherichia coli, E. coli'' and ''Salmonella'' with a common ...
(LOS) and immunoglobulin-binding protein-A (IbpA) were found to be important . ''H. somni'' LOS provides critical protection to the bacterium against host defences by undergoing phase variation both structurally and antigenically and acts as an endotoxin producing apoptotic activity in bovine epithelial cells, a classical sign of histophilosis. Upper respiratory tract colonization is possible through decoration of LOS with phase-variable phosphorycholine (ChoP) that binds to host cell platelet-activating factor receptors allowing ''H. somni'' to colonize host tissues while evading the immune response. ''H. somni'' immunoglobulin-binding proteins have two repeat domains (DR1 and DR2) that have cytotoxic Fic motifs as well as an ability to bind to bovine IgG2. IbpA DR2 can be taken up by BAT2 cells via
pinocytosis In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small molecules dissolved in extracellular fluid are brought into the cell through an invagination of the cell me ...
and decreasing its cytotoxicity. DR1 and DR2 Fic motifs aid in cell infection by transmigration by causing retraction of respiratory cells. ''H. somni'' has the ability to produce a branching, mannose-galactose biofilm made primarily of polysaccharide. As previously mentioned, the bacteria is capable of producing biofilm in the heart of affected animals with clinical bovine
myocarditis Myocarditis is inflammation of the cardiac muscle. Myocarditis can progress to inflammatory cardiomyopathy when there is associated ventricular remodeling and cardiac dysfunction due to chronic inflammation. Symptoms can include shortness of bre ...
. Evidence suggests that biofilm formation allows ''H. somni'' to remain protected and persist within the host. Another way that ''H. somni'' can evade the host immune system is by preventing intracellular killing. Although the mechanism is unknown, the bacteria is able to reduce the amount of reactive oxygen intermediates (ROIs) and therefore the oxidative burst from bovine polymorphonuclear cells (PMNs) reducing intracellular killing. In calves infected with ''H. somni'' it has been shown that phagocytosis of the bacteria is reduced in comparison to naive calves.


Disease

The general term for diseases caused by ''Histophilus somni'' is called Histophilosis; Disease mainly affects cattle but can affect other small ruminants, sheep, bighorn sheep, and bison.
Castration Castration is any action, surgery, surgical, chemical substance, chemical, or otherwise, by which a male loses use of the testicles: the male gonad. Surgical castration is bilateral orchiectomy (excision of both testicles), while chemical cas ...
and
weaning Weaning is the process of gradually introducing an infant human or other mammal to what will be its adult diet while withdrawing the supply of its mother's milk. In the United Kingdom, UK, weaning primarily refers to the introduction of solid ...
predispose young animals to disease caused by ''Histophilus somni''. Other stressors such as overcrowding, poor weather, or shipping are other predisposing factors for all cattle. Viral infections are another predisposing factor for all cattle. With predisposing factors in mind, there are still many unknowns in regards to how and where ''Histophilus somni'' is first able to establish infection. Histophilosis may be present as a component of the Bovine Respiratory Disease Complex which has a higher incidence in feedlot cattle. Histophilosis may also present as septucemia, thrombotic meningoencephalitis-myelitis,
pneumonia Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs known as Pulmonary alveolus, alveoli. Symptoms typically include some combination of Cough#Classification, productive or dry cough, ches ...
, pleuritis,
pericarditis Pericarditis () is inflammation of the pericardium, the fibrous sac surrounding the heart. Symptoms typically include sudden onset of sharp chest pain, which may also be felt in the shoulders, neck, or back. The pain is typically less severe whe ...
,
necrotizing Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. The term "necrosis" came about in the mid-19th century and is commonly attributed to German pathologist Rudolf Virchow, who is ...
myocarditis Myocarditis is inflammation of the cardiac muscle. Myocarditis can progress to inflammatory cardiomyopathy when there is associated ventricular remodeling and cardiac dysfunction due to chronic inflammation. Symptoms can include shortness of bre ...
, and
arthritis Arthritis is a general medical term used to describe a disorder that affects joints. Symptoms generally include joint pain and stiffness. Other symptoms may include redness, warmth, Joint effusion, swelling, and decreased range of motion of ...
. Clinical signs of Histophilosis may include
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...
signs such as depression, behavioral changes, and
ataxia Ataxia (from Greek α- negative prefix+ -τάξις rder= "lack of order") is a neurological sign consisting of lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes, and abnormalities in e ...
, respiratory signs,
abortion Abortion is the early termination of a pregnancy by removal or expulsion of an embryo or fetus. Abortions that occur without intervention are known as miscarriages or "spontaneous abortions", and occur in roughly 30–40% of all pregnan ...
,
conjunctivitis Conjunctivitis, also known as pink eye or Madras eye, is inflammation of the conjunctiva, the thin, clear layer that covers the white surface of the eye and the inner eyelid. It makes the eye appear pink or reddish. Pain, burning, scratchiness ...
, fever, poor body condition,
anorexia Anorexia nervosa (AN), often referred to simply as anorexia, is an eating disorder characterized by Calorie restriction, food restriction, body image disturbance, fear of gaining weight, and an overpowering desire to be thin. Individuals wit ...
, and exercise intolerance but in general clinical signs will depend on which body tissues are affected. Histophilosis may present acutely with sudden death, but may also present only upon post mortem examination with lesions that may be found in multiple locations in the body including the pleura, lung tissue, myocardium, joints, retina, and reproductive tissues. Damage to these areas are caused by
thrombus A thrombus ( thrombi) is a solid or semisolid aggregate from constituents of the blood (platelets, fibrin, red blood cells, white blood cells) within the circulatory system during life. A blood clot is the final product of the blood coagulatio ...
formation and thromboemboli in a septucemic animal followed by subsequent
infarction Infarction is tissue death (necrosis) due to Ischemia, inadequate blood supply to the affected area. It may be caused by Thrombosis, artery blockages, rupture, mechanical compression, or vasoconstriction. The resulting lesion is referred to as a ...
and
necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. The term "necrosis" came about in the mid-19th century and is commonly attributed to German pathologist Rudolf Virchow, who i ...
of tissue.


Diagnosis and Treatment

Diagnosis of ''Histophilus somni'' infection is difficult to do because the range of disease can be broad and vague. Clinicians can try to diagnose in herds using a microagglutination test but this also proves difficult because clinically healthy herds might have high
antibody titer Titer (American English) or titre (British English) is a way of expressing concentration. Titer testing employs serial dilution to obtain approximate quantitative information from an analytical procedure that inherently only evaluates as positi ...
s, there are many cross reactive antibodies, there are conflicting effects due to herd vaccination and there is asymptomatic colonization at mucosal sites. Histophilosis is often diagnosed on post mortem examination of cattle and gross lesion include pinpoint bloody lesions called
petechia A petechia (; : petechiae) is a small red or purple spot ( 1 cm in diameter) and purpura (3 to 10 mm in diameter). The term is typically used in the plural (petechiae), since a single petechia is seldom noticed or significant. Causes Physical t ...
, larger areas of hemorrhage called
ecchymoses A bruise, also known as a contusion, is a type of hematoma of tissue, the most common cause being capillaries damaged by trauma, causing localized bleeding that extravasates into the surrounding interstitial tissues. Most bruises occur c ...
and
necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. The term "necrosis" came about in the mid-19th century and is commonly attributed to German pathologist Rudolf Virchow, who i ...
in the brain,
vasculitis Vasculitis is a group of disorders that destroy blood vessels by inflammation. Both artery, arteries and veins are affected. Lymphangitis (inflammation of lymphatic vessels) is sometimes considered a type of vasculitis. Vasculitis is primarily c ...
caused by endothelial damage and more. ''H. somni'' can be a part of the bovine respiratory disease (BRD) and causes
pneumonia Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs known as Pulmonary alveolus, alveoli. Symptoms typically include some combination of Cough#Classification, productive or dry cough, ches ...
and has been detected in up to 40% of lungs with pneumonia. When ''H. somni'' is detected in pneumonic lungs, it presents as fibrinosuppurative bronchopneumonia and/or severe, diffuse fibrinous pleuritis. Diagnosis can be made by testing blood,
cerebrospinal fluid Cerebrospinal fluid (CSF) is a clear, colorless Extracellular fluid#Transcellular fluid, transcellular body fluid found within the meninges, meningeal tissue that surrounds the vertebrate brain and spinal cord, and in the ventricular system, ven ...
s, joint or pleural fluids for bacterial DNA via PCR or bacterial culture techniques. Although it is difficult to diagnose ''H. somni'' in herds, it is important to attempt it because bovine r disease is a production limiting disease and is reportable for domestic and international trade. Typically there are three approaches to dealing with ''Histophilus somni'' in a herd of cattle; mass
antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy ...
treatment, ''H. somni''
vaccination Vaccination is the administration of a vaccine to help the immune system develop immunity from a disease. Vaccines contain a microorganism or virus in a weakened, live or killed state, or proteins or toxins from the organism. In stimulating ...
or vaccination for other pathogens that are known to be a part of the BRD. These are not treatment per se, more methods of control or metaphylactic treatment, as we are treating many animals before there is an actual diagnosis. The goal is to reduce the onset of BRD or other clinical presentations of ''H. somni'' infections.   Like other bacterial infections, antibiotic susceptibility assays should be performed and it has been reported that ''Hisotphilus somni'' is usually susceptible to ceftiofur,
penicillin Penicillins (P, PCN or PEN) are a group of beta-lactam antibiotic, β-lactam antibiotics originally obtained from ''Penicillium'' Mold (fungus), moulds, principally ''Penicillium chrysogenum, P. chrysogenum'' and ''Penicillium rubens, P. ru ...
, enrofloxacin, florfenicol, and
tetracycline Tetracycline, sold under various brand names, is an antibiotic in the tetracyclines family of medications, used to treat a number of infections, including acne, cholera, brucellosis, plague, malaria, and syphilis. It is available in oral an ...
. This is troublesome because we know that it is difficult to diagnose before a post mortem is completed and there is no point in treating a deceased animal. Therefore, producers often treat metaphylactically, or on suspicion of infection. Tilmicosin can be added to the feed and has been shown to reduce the levels of ''H. somni'' in herds. It should only be added to the feed for 14 days, should be used sparingly as resistance is likely to occur otherwise, producers should be aware of withdrawal times. Problems with antibiotic treatment is emerging resistance to tetracyclines, enrofloxacin, and florfenicol.
Biofilm A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...
s that are formed by ''H. somni'' function to protect the bacteria from host immune defenses, they also provide a barrier that impedes antibiotic function. A new line of research is treating susceptible populations with bacterial isolates from
subclinical Asymptomatic (or clinically silent) is an adjective categorising the medical conditions (i.e., injuries or diseases) that patients carry but without experiencing their symptoms, despite an explicit diagnosis (e.g., a positive medical test). P ...
carriers of ''Histophilus somni'' to act like a
probiotic Probiotics are live microorganisms that provide health benefits when consumed, generally by improving or restoring the microbiota in the gut. Probiotics are considered generally safe to consume, but may cause bacteria– host interactions ...
bacteria for the respiratory tract. A nasal inoculation of a nonpathogenic strain of ''H. somni'' could allow for the
respiratory tract The respiratory tract is the subdivision of the respiratory system involved with the process of conducting air to the alveoli for the purposes of gas exchange in mammals. The respiratory tract is lined with respiratory epithelium as respirato ...
mucosa A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It ...
to be colonized with the
commensal bacteria Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fr ...
. There are many licensed vaccines against ''H. somni'', however licensed does not mean effective. There is a push to develop new vaccines that are multi-pathogenic and DIVA compatible. ''H. somni'' vaccines are usually killed cells or specific outer membrane proteins but have not been proven to be effective at protecting cattle against disease. There are many constraints to vaccine use on top of ineffectiveness, including the timeliness of administration, adverse reactions from individuals in the herd, and research into the
epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and Risk factor (epidemiology), determinants of health and disease conditions in a defined population, and application of this knowledge to prevent dise ...
of the disease. In order to treat this infection, one must know which organ system it is affecting and some systems, such as the neurological presentation does not allow for timely treatment because by the time the disease is detected, it is too late for treatment.


References


External links


Overview of Histophilosis
— The Merck Veterinary Manual
Type strain of ''Histophilus somni'' at Bac''Dive'' - the Bacterial Diversity Metadatabase
{{Taxonbar, from=Q1620746 Bacterial diseases Pasteurellales Bovine diseases