Histophilosis : Introduction
(Thrombotic meningoencephalitis) |  |
| Histophilosis is a common disease in North American, primarily Canadian, cattle feedlots. It also is seen sporadically in individual beef and dairy cattle worldwide.
Histophilus
somni
(formerly
Haemophilus
somnus
) can cause an acute, often fatal, septicemic disease that can involve the respiratory, cardiovascular, musculoskeletal, or nervous systems, either singly or together. The reproductive system is often affected but usually without clinical signs and without other systemic involvement. |
| Etiology and Transmission: |
|
H
somni
is a gram-negative, nonmotile, nonsporeforming, nonencapsulated, pleomorphic coccobacillus that requires an enriched medium and a microaerophilic atmosphere for culture. Hemolysis on blood agar occurs within 48 hr due to an exotoxin produced by most disease-causing isolates. Pathogenic and nonpathogenic strains have been differentiated. The virulence of the organism may vary by region and age group. |
|
H
somni
is considered a commensal of bovine mucous membranes. Pathogenic and nonpathogenic strains of
H
somni
are found in the sheath and prepuce of males, the vagina of females, and in the nasal passages of both sexes. Nasal and urogenital secretions are believed to be sources of the organism. The organism may colonize the respiratory tract, presumably after inhalation, and gain access to the bloodstream via that route. Colonization of the male and female reproductive tracts may involve venereal spread. |
|  |
| Epidemiology: |
| All feedlot cattle are at risk of histophilosis for the duration of the feeding period. However, recently weaned calves are at higher risk of infection and death from histophilosis than are previously weaned older calves, yearlings, or mature animals. The risk of infection with
H
somni
is highest early in the feeding period with high-risk calves in the feedlot establishing peak titers to
H
somni
at ~21-23 days on feed. Although calves are generally exposed to
H
somni
earlier in the feeding period, the average days on feed at death of calves that die of histophilosis is 30-60 days. Sudden death due to peracute septicemia, especially, may occur throughout the feeding period. Sporadic reproductive disease manifestations, including sporadic abortion and mastitis, are seen in individual beef and dairy cattle. |
| |
| Pathogenesis: |
| Septicemia is likely required for most forms of histophilosis. Strains of
H
somni
that cause disease adhere to the endothelium of vessels, resulting in contraction, exposure of collagen, platelet adhesion, and thrombus formation. The primary lesion likely involves a thrombus, rather than a thromboembolism as once thought. Strains may adhere to endothelium in vessels of the pleura, myocardium, pericardium, synovium, or a variety of other tissues (eg, brain, larynx). Interruption of the blood supply in those areas results in destruction of tissue and
the development of clinical signs associated with the organ system involved. The susceptibility of individual animals and variations in the preference of strains of the organism for vessels in different tissues may be important in the development of the different forms of the disease, but the mechanisms involved are incompletely understood. Reproductive problems may not necessarily be preceded by bacteremia, but the pathogenesis in those situations is poorly defined. |
| |
| Clinical Findings: |
| Sudden death is often the first indication of
H
somni
infection in a feedlot animal. A profound depression is often the most noticeable clinical sign of histophilosis. Fever is also a common finding; however, animals diagnosed with undifferentiated fever may be suffering from
Mannheimia
pneumonia
, histophilosis, or both. Other findings are determined by the system(s) involved and may include rapid respiration, stiffness, muscle weakness, ataxia, paralysis, and opisthotonos. Specifically, animals affected with the fibrinous pleuritic form of the disease may exhibit extreme dyspnea; animals with myocarditis may exhibit sudden collapse and death on exertion (eg, being moved through a handling facility); and animals with the nervous form are predominantly depressed
with occasional signs of hyperesthesia (eg, convulsions) prior to death. Animals found dead and confirmed with
H
somni
infection often have a history of having been treated for undifferentiated fever or depression in the previous 14 days. |
Lesions:
|
Feedlot cattle that die of peracute or acute disease may exhibit postmortem lesions including fibrinous pleuritis, a myocardial infarct or abscess in the papillary muscle of the left ventricle, fibrinous pericarditis, fibrinopurulent bronchopneumonia, polyarthritis, and a fibrinous or abscessed laryngitis. Less common gross postmortem lesions include polyserositis, visible fibrin in the stifle joint fluid, and a fibrinopurulent meningitis
with cloudy CSF. Lesions of the reproductive tract may include suppurative vaginitis, cervicitis, and endometritis. |
| |
| Diagnosis: |
| A definitive diagnosis is based on sampling and examination of affected tissues collected during a postmortem or clinical examination. Isolation of the organism from CSF, brain, blood, urine, joint fluid, or other sterile, internal organs or fluids confirms the diagnosis. Because
H
somni
is a commensal of the mucous membranes of cattle, the bacterium should be isolated in predominant or pure culture from the respiratory or urogenital tract to be considered a significant etiologic agent. Characteristic histologic lesions include severe vasculitis, vascular thrombosis, infarction, necrosis, and heavy infiltrations of neutrophils in all tissues where localization of the bacteria occurs. Treatment often interferes with recovery of the organism; as a
result, the diagnosis may need to be confirmed with immunohistochemical staining techniques. |
| |
| Treatment and Prevention: |
| A major hindrance to successful antimicrobial treatment of individual histophilosis cases is the difficulty in identifying affected animals early in the course of disease due to its often rapidly fatal nature. Antimicrobial treatment is most effective in the early stages of disease. Prophylactic administration of injectable oxytetracycline on arrival at the feedlot (or within a few days of arrival) has not significantly affected mortality due to histophilosis. However, a
commercially available, oral formulation of chlortetracycline and sulfamethazine fed for the first 56 days of the feeding period has significantly reduced overall mortality due to histophilosis.
H
somni
is susceptible in vitro to a wide range of antimicrobials including florfenicol, tilmicosin, tetracyclines, trimethoprim-sulfadoxine, fluoroquinolones and ceftiofur. Florfenicol (20 mg/kg, IM, repeated in 48 hr, or 40 mg/kg, SC, once) may be the antimicrobial of choice if
H
somni
is a major cause of mortality in feedlot calves. |
| Bacterins containing different strains of the organism have been used to immunize cattle against
H
somni
. The humoral response generated by different commercial vaccines has been described, but the ability of current vaccines to protect cattle from
H
somni
-associated morbidity and mortality is not completely understood. |
| |
;)