Bighorn ram with domestic sheep in the Dome Valley, southwest of Kofa NWR
Disease in bighorn sheep is most prevalent when animals are stressed and during severe drought. Multiple diseases may also combine to increase mortality. Bighorns seem particularly susceptible to respiratory problems like bacterial pneumonia. Pasturella, for example, can be carried by healthy domestic sheep and goats, but is deadly when transmitted to wild sheep. Scabies is another common disease easily transmitted to bighorns; it was responsible for a significant decline on San Andres NWR in 1978. Disease transmission from burros or horses to bighorn sheep has not been substantiated; however, isolated cases of transmission from cattle to bighorn sheep have been documented. Since the late 1800s, diseases transmitted by domestic sheep and goats have caused large, recurrent population-level declines in bighorn sheep throughout the western US. These declines have been well documented, and subsequent regulations restricting contact between domesticated and wild sheep have been enacted. It is imperative to keep any domestic sheep or goats well away from bighorn sheep range.
Chronic sinusitis is prevalent in bighorn sheep throughout Arizona. In severe cases, necrosis of the frontal bone and thinning of the braincase creates holes and abscessing in the brain, which is fatal. The leading theory for cause of this condition is bacterial infection secondary to necrotic bot fly larvae (Oestrus ovis), which are deposited in the nostrils of bighorn sheep. Evidence of chronic sinusitis has been common in the Kofa bighorn sheep herd, though it appears to be less prevalent now than during the 1980s and 1990s.
No population-level outbreaks of disease have been documented in the Kofa herd, although disease is occasionally documented. Some of the 17 radio-collared sheep mortalities discussed above showed signs of chronic sinusitis, although sinusitis could not definitively be called the cause of mortality. A lamb captured during the 2005 transplant had contagious ecthyma. A ram with chronic sinusitis was discovered in October 2006, and a hunter reported a ram coughing in December 2006. In fall 2006, 6 bighorn sheep transplanted from the Kofa in 2005 and 2002 died of pneumonia on San Andres NWR, though this was attributed to unusual weather conditions at San Andres as opposed to any predisposition of the Kofa sheep. Blood samples have been drawn from all captured sheep over the years and tested for evidence of exposure to bovine respiratory syncytial virus (BRSV) bluetongue virus (BTV), parainfluenza III virus (PI3), contagious ecthyma virus (CE), Chlamydia spp. (CHLAM), and malignant catarrhal fever (MCF). In 2002 and 2005 captures, several animals were serologically positive for BRSV, CHLAM, or MCF but no clinical symptoms were noted. In these cases positive results most likely reflect past exposure or asymptomatic infection. Serum and feces are freezer banked for all animals should future analysis be required.