Shipping-Fever in Horses
Transport in general has been identified by a number of authors as an important risk factor for the development of respiratory disease (Austin et al. 1995; Mair and Lane 1989; Raphel and Beech 1982). The risk clearly increases with journey duration, especially in excess of 10 h. A severe manifestation of the effects of transport on the respiratory tract may be the development of “shipping–fever”. Affected horses most usually exhibit pyrexia, depression and reduced appetite with or without specific respiratory signs including cough, nasal discharge and increased respiratory rate. The reported causative agents implicated in shipping fever include Streptococcus equi, Streptococcus suis, Streptococcus zooepidemicus and Pasteurella caballi (Foreman et al. 1992; Hayakawa et al. 1993; Oikawa et al. 1994; Oikawa et al. 1995). However, it is generally believed that the most common agent involved is Streptococcus zooepidemicus.
The risk of shipping–fever increases with duration of travel. Road transport in excess of 10 h appears to markedly increase the risk of shipping fever (Austin et al. 1995; Oikawa and Kusunose 1995; Raphel and Beech 1982), although the author has experienced horses developing shipping–fever following journeys of as little as 6 h by road. In a study carried out in Japan over a 5-year period from 1989 to 1994, the incidence of shipping fever following road transport of 25-28h was 11.9% (Hirano 1994). Another study in Japan more recently over a 4-year period indicated that for all horse movements by road the incidence of shipping fever was 1.4% (Oikawa and Jones 2000).
Whilst most studies assess the development of shipping fever in the transport or immediate post-transport period, a study by Foreman et al. (Foreman et al. 1992) demonstrated that the incidence of shipping fever was higher when they were monitored for 14 days following transport. In a total of 130 horses less than 36 months of age, transported 515km, 70% developed pneumonia within 14 days of arrival.
The incidence of horses completing transport and showing clinical signs of respiratory disease, which does not progress to shipping fever, can also be high. When respiratory disease was defined on the basis of rectal temperature in excess of 38.6°C, cough, nasal discharge and lethargy, the percentage of horses showing such signs following 36h of road transport was 45% (Oikawa and Kusunose 1995). Similarly, the prevalence of pyrexic horses was found to increase markedly beyond 20-24h of road transport (Oikawa and Kusunose 1995).
Hungerford et al. (Hungerford 1992) examined risk factors for respiratory disease associated with transport in a cohort of 60 horses. There was no difference in risk between sex, age and breed groups, but fat horses (condition score 8 or 9) were at increased risk of disease (relative risk 1.9, P<0.03). Animals in the bottom back section of the trailer were also found to be at increased risk. Other risk factors for the development of shipping fever may include air quality, number and density of animals in the transport compartment and the presence of respiratory disease in individual horses prior to the onset of transport.