THE MERCK VETERINARY MANUAL
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Enzootic Pneumonia of Calves and Shipping Fever Pneumonia

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Enzootic pneumonia and shipping fever pneumonia share many similarities in their respective etiologies and pathogeneses and in general measures for control and prevention.

Enzootic Pneumonia of Calves (Dairy calf Pneumonia or summer Pneumonia of beef calves)

Enzootic pneumonia of calves refers to infectious respiratory disease in calves. The term “viral pneumonia of calves” is sometimes used but is not preferred based on the current understanding of etiology and pathogenesis. Enzootic pneumonia is primarily a problem in calves <6 mo old with peak occurrence from 2–10 wk, but it may be seen in calves up to 1 yr of age. It is more common in dairy than in beef calves and is a common problem in veal calves. It is also more common in housed dairy calves than in those raised outside in hutches. Peak incidence of disease may coincide with decline of passively acquired immunity. Morbidity rates may approach 100%; case fatality rates vary but can reach 20%.

The etiology is similar to that for BRD complex in general (see Bovine Respiratory Disease Complex). The pathogenesis involves environmental and management stressors and possibly an initial respiratory viral infection followed by a secondary bacterial infection of the lower respiratory tract. Stress results from environmental and management factors, including inadequate ventilation, mixing by adding calves to an established group, crowding, and nutritional factors such as poor-quality milk replacers. Partial or complete failure of passive transfer of maternal antibodies is an important host factor related to development of disease. Any of several viruses may be involved, and a variety of bacteria may be isolated from affected calves. Mycoplasmal and bacterial agents, including Pasteurella multocida, Mannheimia haemolytica, and Mycoplasma bovis, represent the most frequently isolated pathogenic organisms. The individual viral and bacterial etiologies, clinical signs, lesions, and treatment are discussed under Viral Respiratory Tract Infections in Cattle and Bacterial Pneumonia in Cattle.

When calves of varying ages are placed in communal pens, control of enzootic pneumonia is difficult. The severity of the pneumonia may be decreased by improved husbandry, proper housing, adequate ventilation, and good nursing care. Prevention begins with vaccinating the cows against specific respiratory viruses and bacteria 3–4 wk prepartum to improve the quality of colostral antibodies. Calves should receive good-quality colostrum at 8%–10% of body wt in the first 6 hr after birth. Newborn dairy calves should be housed individually in hutches or stalls and fed whole milk or a high-quality milk replacer with a fiber content of <0.25% until 8–12 wk old. The use of calf hutches in dairy herds is the preferred standard for calf housing and has been shown to significantly improve calf respiratory health. However, delivering milk to a large number of hutches in cold weather presents a significant challenge. Single calf housing in naturally ventilated calf barns is the next best alternative. Calves should be vaccinated against respiratory viruses 3–4 wk before the first grouping, although in some situations, the presence of passive immunity may interfere with an active immune response. Calves should be of similar age when assembled into groups, and a group should be limited to ≤10. As calves mature, groups can become larger as the size of the herd, facilities, and available labor dictate. An “all in/all out” management style should be practiced when establishing and terminating a group. Newborn beef calves and their dams should be moved from concentrated calving areas as soon as the calf is nursing well and strong enough to travel. The use of screening systems such as the Calf Respiratory Scoring Chart (University of Wisconsin) may serve as an objective tool to evaluate calves for treatment.

Shipping Fever Pneumonia

Shipping fever pneumonia, or undifferentiated fever, is a respiratory disease of cattle of multifactorial etiology with Mannheimia haemolytica and, less commonly, Pasteurella multocida or Histophilus somni (see Histophilosis) being the important bacterials agents involved. Shipping fever pneumonia is associated with the assembly into feedlots of large groups of calves from diverse geographic, nutritional, and genetic backgrounds. Morbidity in feeder calves often peaks within 7–10 days after assembly in a feedlot. Morbidity can approach 35%–50%, and case fatality is 5%–10%; however, the level of morbidity and mortality strongly depends on the array of risk factors present in the cattle being fed.

The pathogenesis of shipping fever pneumonia involves stress factors, with or without viral infection, interacting to suppress host defense mechanisms, which allows the proliferation of commensal bacteria in the upper respiratory tract. Subsequently, these bacteria colonize the lower respiratory tract and cause a bronchopneumonia with a cranioventral distribution in the lung. Multiple stress factors are believed to contribute to suppression of host defense mechanisms. Weaning is a significant stressor, and the incidence of this disease is highest in recently weaned calves. Transportation over long distances serves as a stressor; it may be associated with exhaustion, starvation, dehydration, chilling and overheating depending on weather conditions, and exposure to vehicle exhaust fumes. Additional stressors include passage through auction markets; commingling, processing, and surgical procedures on arrival at the feedlot; dusty environmental conditions; and nutritional stress associated with a change to high-energy rations in the feedlot. The individual viral and bacterial etiologies, clinical signs, lesions, and treatment are discussed under Viral Respiratory Tract Infections in Cattle and Bacterial Pneumonia in Cattle.

Prevention of shipping fever pneumonia should focus on reducing the stressors that contribute to development of the disease. Cattle should be assembled rapidly into groups, and new animals should not be introduced to established groups. Mixing of cattle from different sources should be avoided if possible; however, in the North American beef industry, this risk factor is almost unavoidable for large intensive feedlots. Transport time should be minimized, and rest periods, with access to feed and water, should be provided during prolonged transport. Calves should ideally be weaned 2–3 wk before shipment, and surgical procedures should be performed in advance of transport; however, the availability of these “preconditioned” calves is quite limited. Cattle should be processed within 48 hr after arrival at the feedlot. Adaptation to high-energy rations should be gradual, because acidosis, indigestion, and anorexia may inhibit the immune response. Vitamin and mineral deficiencies should be corrected. Dust control measures should be used.

Metaphylaxis with long-acting antibiotics such as oxytetracycline, tilmicosin, florfenicol, gamithromycin, tildipirosin, or tulathromycin has been widely adopted as a control measure given “on arrival” to cattle at high risk of developing shipping fever pneumonia. Metaphylaxis on arrival has been shown to significantly reduce morbidity and improve rate of gain and, in some cases, reduce mortality. Mass medication in feed or water is of limited value because sick animals do not eat or drink enough to achieve inhibitory blood levels of the antibiotic, and many of these oral antibiotics are poorly absorbed in ruminants.

On arrival, processing usually involves administration of modified-live vaccines for viral antigens and for bacterial components of shipping fever pneumonia. Because most cases of pneumonia occur during the first 2 wk after arrival, these on-arrival vaccines may not have adequate time to stimulate immunity. When possible, vaccinations for the viral and bacterial components of shipping fever pneumonia should be given 2–3 wk before transport or earlier and can be repeated on entry to the feedlot.

Last full review/revision March 2015 by John Campbell, DVM, DVSc

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