About 12 weeks ago green fluid started leaking into her feeding tube, signalling an issue with her bowel.
The green fluid, called aspirate, continued for weeks during which time Brennagh was fed very little so her bowel could rest.
"That is what has contributed to this rapid decline in Brennagh's weight loss," said McKay.
McKay decided to try giving Brennagh breast milk and adding a product called Juice Plus to the milk - a dietary supplement that contains concentrated fruit and vegetable juice extracts with added vitamins and nutrients.
A Givealittle page was set up for mothers to donate their breast milk to keep a steady supply for Brennagh.
After recent stories in the media, more than 50 people called in one day to offer their milk, McKay said.
"Honestly, like, all the mums from around the country have just jumped on board and offered to donate milk as far afield as Whangarei, and down south, Canterbury. We've had calls from overseas, I've had offers from Australia."
They are starting to see improvements in Brennagh's health - yesterday she was able to visit the beach.
McKay said it's been about six months since Brennagh has played outside, and while she wasn't able to play at the beach, she did manage to sit up and enjoy her surroundings.
McKay has set up Facebook pages for the mothers who have been helping her, and sent them pictures and videos of Brennagh at the beach.
"We're so appreciative, we just didn't want to sort of take this beautiful gift from them and then they never hear from us again. We almost feel now we've got a special connection with all these mums."
People around the country have been banding together to get the milk to McKay at her home in Rongotea, near Palmerston North. Husbands have even been recruited to drop off the milk for their wives.
"I'm so moved by it, I didn't expect this response," McKay said.
"We just wanted enough breast milk to keep my daughter alive until we could find a solution for her.
"We're so thankful to all these mummies for supporting us. It's just an incredible group of women that have come together."
McKay said some people questioned why she didn't just let Brennagh die.
"This whole journey wasn't about keeping someone here that doesn't belong here. I've got comments saying it's not right, you should let her go."
She understood, but said each time they went through a medical crisis it was up to Brennagh to pull through, and she "always springs back up".
People can head to the Givealittle page to offer milk or to make donations of money so mothers don't have to dig into their own pockets to arrange refrigerated shipping to get the milk to McKay.
Healthy Food Guide nutritionist Claire Turnbull said a huge amount of research goes into the types of feeds used when individuals need to be tube fed. This included looking at all the requirements for energy, carbohydrate, protein and fat as well as vitamins and minerals.
"The limitation with breast milk is that [it] is a primary source of nutrition for a child up to around six months. Beyond six months babies need additional nutrients which is why we need to start introducing supplementary food.
"There are some huge advantages of breast feeding and lots of fantastic things about breast milk but still, beyond that six-month mark it's not enough for even a baby."
Turnbull said the other issue with breast milk was that its nutritional value can vary from mother to mother depending on the needs of their baby. This means that sourcing breast milk from a range of mothers could make it hard to get a milk that's nutritionally consistent.
"There is a reason why specially formulated tube feeds exist. However I really do understand when people get desperate they will do anything. I completely get that."
