The chief effects of altitude on the atmosphere are to make it lighter, drier, and colder, and to decrease the percentages of carbonic acid gas and dust in it. Dryness and diminution of carbonic acid and dust are conditions which promote the health of horses. The benefit to be obtained from cold produced by altitude, is dependent on the latitude, other things being equal. Thus in India, Sumatra, and Belgium for instance, the Spiti ponies, Deli ponies, and Ardenne mountain horses are respectively far stronger for their size than their relatives in the plains. A difference in altitude (say, of 7,000 ft.) which would be highly beneficial to equine life in hot countries, would, by decrease of temperature, be equally prejudicial in cold ones. We may therefore take for granted that cold limits the favourable influence of altitude on the atmosphere. In Tibet, at a latitude about the same as that of Syria, the hill ponies, which are very sturdy animals, live and thrive at an altitude up to at least 14,000 ft. Altitude by rarefying the air, within healthy limits, strengthens the lungs by increasing their work, and in this way often produces a good effect in the early stages of human consumption.
The beneficial influence of altitude in checking the development of many disease germs is apparently due, as a rule, to cold and diminution of humidity in the atmosphere; damp heat being generally favourable to their growth. We find that Horse Sickness in South Africa is practically unknown in places which are at least 6,000 ft. high, unless it has been introduced by affected stock. At similar altitudes in India, bursatee (a disease characterised by ulcers which somewhat resemble the "Delhi boils" of man) becomes spontaneously cured.
The observations I have made, chiefly in the Himalayas, on the distressing effects of high altitudes, lead me to think that they do not manifest themselves more severely on horses than on men. On leaving the plains in India and going to a height of, say, 10,000 ft., one experiences as a rule no new sensation, except a slight exhilaration of spirits. Ponies of all breeds, when taken from the plains and run on race-courses at Simla, Mussooree and Kujear (near Dalhousie), at altitudes of from 5,000 to 7,000 ft. above the level of the sea, show all their accustomed spirit and endurance. At an altitude of over, say, 15,000 ft. in the Himalayas or Tibet, human beings, especially new-comers, become more or less affected with giddiness, breathlessness, faintness, depression, headache, and other more or less painful symptoms of a deficiency in the supply of air to the lungs, which symptoms are naturally aggravated by muscular exercise. Horses in similar circumstances also exhibit exhaustion and distress, though probably, in such cases, they are handicapped by the severe toil they are as a rule enduring.
Fleming (Veterinarian, May, June, July and August, 1868) has drawn attention to the fact that horses are said to suffer from various respiratory diseases caused by crossing mountain ranges in Mexico and Peru. In fact it would appear from these reports, that horses are peculiarly susceptible to disease brought on by high altitudes, which is a supposition that cannot be accepted without further proof. My own experience leads me to infer that going over even the highest passes of the Himalayas does not predispose horses to disease. I have bought several and examined many Yarkund ponies which came to Kangra Valley via Chanchengmo, and have never seen more healthy animals.
Residence in places of high altitudes, confers a large degree of immunity against the distressing symptoms brought on by the breathing of highly rarefied air.