How did Islam spread to Nkhotakota and the Shire Highlands?

Islam spread into the Malawi region from the east coast of Africa during the 19th century.12

• Nkhotakota: The religion was first introduced to this area in the 1860s by the Jumbe, who were ruling Swahili-speaking slave traders.12
• Shire Highlands: Islam reached the Yao people of this region during the 1870s and 1880s, brought by traders who were returning from the coast.12