Predicting parasite dynamics in mixed-use trans-Himalayan pastures to underpin management of cross-transmission between livestock and bharal

The complexities of multi-use landscapes require sophisticated approaches
to addressing disease transmission risks. We explored gastro-intestinal
nematode (GINs) infections in the North India Trans-Himalayas through a
socio-ecological lens, integrating parasite transmission modelling with
field surveys and local knowledge, and evaluated the likely effectiveness
of potential interventions. Bharal (blue sheep; Pseudois nayaur), a native
wild herbivore, and livestock share pasture year-round and livestock
commonly show signs of GINs infection. While both wild and domestic
ungulates had GINs infections, egg counts indicated significantly higher
parasite burdens in bharal than livestock. However, due to higher
livestock densities, they contributed more to the total count of eggs and
infective larvae on pasture. Herders also reported health issues in their
sheep and goats consistent with parasite infections. Model simulations
suggested that pasture infectivity in this system is governed by
historical pasture use and gradually accumulated larval development during
the summer, with no distinct short-term flashpoints for transmission. The
most effective intervention was consequently predicted to be early-season
parasite suppression in livestock using temperature in spring as a cue. A
one-month pause in egg output from livestock could lead to a reduction in
total annual availability of infective larvae on pasture of 76%,
potentially benefitting the health of both livestock and bharal. Modelling
suggested that climate change over the past 33 years has led to no overall
change in GINs transmission potential, but an increase in the relative
influence of temperature over precipitation in driving pasture
infectivity. Our study provides a transferable multi-pronged approach to
investigating disease transmission, in order to support herders’
livelihoods and conserve wild ungulates.