Signs of Fall 6: Mange on the Range!

Created by ElSenorDeLaNoche. Wikimedia Commons

(Click on the following link to listen to an audio version of this blog …. Mange on the Range

The San Guillermo Biosphere Reserve is a vast (2,420,000 acres) expanse located on the dry eastern slopes and broad steppes of the Andes Mountains in northwest Argentina. Mountain peaks, long valleys and dry grass and shrublands all at elevations of 6900 to 21,000 feet generate the rugged landscape of the reserve. The center of the reserve is a 410,000 acre national park (San Guillermo National Park). The reserve was established in 1970’s and park was dedicated in 1999. Both areas were set aside especially to provide protected habitat for the preservation of wild camelids (including vicunas (Vicugna vicugna) and guanacos (Lama guanaco). The vicunas were especially in need of a broad protection plan since their total numbers in the Andes had in the 1960’s dropped to 6000 individuals as a consequence of uncontrolled hunting and wool gathering. The impact of a string of Andean reserves like San Guillermo, though, has help to bring the total numbers of vicuna up to 400,000 individuals.

Photo by R. Mario. Wikimedia Commons

The park and the reserve are fairly simple, low productivity ecosystems that sit in the rain shadow of the Andes and, so, are greatly limited by scant water supplies. There are no permanent waterways in the park and only limited wetlands. The herds of vicuna are the dominant grazing animals and cougars (Puma concolor) are their primary predators. Andean condors (Vultur gryphus) are the primary scavengers of the vicuna carcasses. This very straight forward predator/prey/scavenger system has attracted a great deal of research interest, and scientists have put radio collars on both the cougars and the condors in the park so their locations and activities can be closely monitored.

A team of researchers from universities in Argentina and the United States were in the San Guillermo National Park in early 2020 studying the vegetation of the park, the vicuna herds, the cougars and the condors. One of their areas of interest was the impact of vicuna/cougar interactions on the nutrient deposition and distribution in the park.

Vicunas. Photo by F. Mahe. Wikimedia Commons

The park vicunas, though, had, since 2015, been afflicted with sarcoptic mange. The mites that caused this mange outbreak had probably come from domesticated lama herds (lamas are another native, South American camelid species) being raised on farms near the boundaries of the reserve.

Mange is an extremely contagious skin disease seen in hundreds of species of both wild and domesticated mammals. There is also a form of mange that affects birds. An animal with mange often has severe, scabby skin lesions, extensive hair (or feather) loss and widespread secondary skin infections involving bacteria and a diverse array of fungi.  Mange is caused by several species of ectoparasitic mites that burrow into the outermost layer (stratum corneum) of the epidermis of the skin.

The most common type of mange in mammals is “sarcoptic mange” (or “scabies”). It is caused by the mite Sarcoptes scalsiei. There are specific subspecies/varieties of S. scalsiei that affect different hosts and, although almost any mammal can pick up any of these sarcoptic mites, cross-species infestations are usually short-lived and limited. Apparently, the similarlity of lamas and vicunas allowed for an effective sharing of their sarcoptic mites.

Sarcoptes mite. Photomcrograph by Kalumet. Wikimedia Commons

In an appropriate host, the mated female mite digs its s-shaped burrow into the host’s stratum corneum and lays its eggs. A female mite will live on average for 30 days and is able to produce three eggs per day through this life span. The eggs hatch into larvae in three days and immediately begin feeding on the host’s lymphatic and interstitial fluids and on its epithelial cells. Within three or four days the larvae develop into nymphs which continue to feed primarily on tissue fluids. The nymphs move about in their stratum corneum burrows and also may emerge onto the surface of the skin. The feces produced by the larvae and nymphs triggers inflammation and, frequently, allergic reactions in the host’s skin. These reactions are typically accompanied by intense itching. The host animal tears away at its own skin causing lesions and opening the skin to serious secondary infections.

The nymphs develop into adult in five to seven days (so the life cycle of this mite is less than two weeks!). These newly emerged adults wander about on the skin surface and mate. The newly mated females then burrow into the stratum corneum to lay their eggs and refresh the reproductive cycle.

Mites on the skin surface (either larvae, nymphs or adults) may be directly transferred to another host by skin to skin contact. These surface dwelling life stages may also drop off into the surrounding environment and, depending on environmental conditions, persist for several weeks off of a host. Animals that share bedding or nesting materials often also share the dispersing life stages of these mites.

Sarcoptic mites, then, can explode in a population of a suitable host species and can cause persisting and debilitating cases of sarcoptic mange!

Since mange began to infest the vicuna the sizes of their herds and their abundance on the grasslands of the park have greatly decreased. Examination of the radio-collar telemetric data from the park’s cougars indicated that they were not taking more vicunas than they had been before the mange outbreak, so the researchers concluded that the mange itself was fatally debilitating the vicunas.

Photo by Malcom. Wikimedia Commons

So, in San Guillermo, the vicuna decreased in numbers because of mange. The cougars in the park, though, continued to take the same number of vicuna as they had before, which, in the time of mange, meant that they were killing a larger and larger percentage of the park’s vicuna. This caused the vicuna numbers to decline even more precipitously, and led, eventually, to a decline in total vicuna kills by cougars and then to a decline in cougar numbers in the park. The decline in cougar mediated vicuna kills then lead to a decrease in the time of visitation of the Andean condors who sought the cougar killed vicuna carcasses.

So, mange in the vicuna herds caused significant declines in all of the other consumer components of the park!

Photo by C. Villani. Wikimedia Commons

Further, detailed examination of the grasslands in the park revealed that those areas that had been preferred by the vicuna (open grasslands in which approaching cougars could easily be spotted) began to produce very lush, tall grasses that were loaded with fully developed seed heads (an indication of lack of grazing). These grasslands were robustly utilizing the nutrients previously deposited by the vicuna (vicuna produce large quantities of feces!). Whether these lush grasslands are sustainable without the continued nutrient delivery by the vicuna herds is not yet possible to say.

Grasslands in the mountain valleys, though, (areas avoided by vicuna because of the danger of hiding cougars) showed no changes in grass biomass or seed production as a consequence of the dwindling numbers of vicuna.

There were, then, massive ecological changes in the ecosystems of San Guillermo all because of the invasion of tiny mite! As Barry Commoner put it in his first of four “informal laws” of ecology, “Everything is connected to everything else!”