The genetics of adaptation of cattle to high altitude environment: The case of brisket disease in northwestern Ethiopia

High altitude or brisket disease of cattle is common at high altitude areas. It is characterized by right ventricular hypertrophy and edema of the chest and brisket, because of reduced blood oxygen saturation at high elevation. It is similar to altitude sickness in humans and frequently ends with the death of the affected animal unless transferred to lower altitude. The incidence and severity of the disease increase with altitude. Pulmonary artery pressure (PAP) is an indicator of proneness to the disease. High values (>50 mm HG) indicate high risk and low values (<35 mmHG) indicate resistance to the disease. Analyses of heritability of PAP, mostly performed on Angus cattle in the Rocky Mountains at altitudes of 2000-3800 m indicate a relatively high heritability, in the range of 0.30-0.50. This provides options for selection, which is performed routinely in the Rocky Mountains. The Semien Mountains are the highest mountain range of Ethiopia, peaking at the Ras Dashen (4620 m). Cattle are kept at altitudes of up to almost 4000 m. Along the western side of the mountain range, there is a rather continuous drop in altitude down to about 600 m, over a rather short distance (250 km). The cattle in the region are partly of different type (Zebu, Sanga) and partly a mixture (Zenga = Zebu x Sanga). The phenotypic differentiation in terms of body size along levels of altitude is strong. A study was conducted to assess the prevalence rate of high altitude disease and as well as to compare adaptive characteristics of indigenous cattle populations and their crosses with European types towards altitude, in particular, to high altitude disease. In January 2007, 218 animals situated within an altitude range of 1730 - 3500 m were tested for PAP by an experienced veterinarian from Colorado State University. Local breeds and crosses with Holstein Friesian and Jersey were measured. The results in Table I indicate that no sign of brisket disease is observed among the studied populations. All PAP scores (21- 47 mm Hg) fall within the range of low to moderate risks. Differences in means were not significant for any pair of populations. Some of the readings (values <28 mm HG) for the Semien cattle group measured at 3500 m are out of the range of readings of approx. 100,000 cattle that the veterinarian has taken in the Rocky mountains in the course of 20 years. Crosses of the local cattle with Holstein Friesian and Jersey were not more prone to brisket disease than local cattle measured at the same altitudes. In a study comparing PAP readings in yak, cattle and their crosses, the crosses had equally low PAP readings as the yaks. Yaks are known to be resistant to high altitude disease due to an adaptation of vascular system, indicated by thin-walled small pulmonary arteries. The authors speculated about an autosomal dominant gene transmitting genetic attenuation of the hypoxic vasoconstrictor response. We conclude that cattle breeds of North Western Ethiopia are genetically adapted to high altitude. To get an insight on the mechanism of adaptation an in depth histological study on the internal anatomy of cardiovascular and respiratory systems of these genotypes is currently being undertaken. (author)