Spitting is an early warning sign that your chickens may have worms, as the condition worsens they tend to make gurgling sounds because the worms are working their way through their system. The most likely culprit is the gape worm, a large round red worm. In some cases a chicken will spit from mishandling when picking them up you should be careful not to put pressure on their gullet because this will make them spit up, especially after drinking water. If you are sure that you have not mishandled your bird and he or she keeps spitting up or you notice spitting or gurgling sounds, then it is most likely time to get started deworming your chickens for gape worms. Gapeworms attach themselves to the lining of your chickens trachea which will cause the gurgling sound, spitting, and if left untreated, death.
"The gapeworm (Syngamus trachea) is a round red worm that attach to the trachea (windpipe) of birds and causes the disease referred to as "gapes". The term describes the open-mouth breathing characteristic of gapeworm-infected birds. Heavily infected birds usually emit a grunting sound because of the difficulty in breathing and many die from suffocation. The worms can easily block the trachea, so they are particularly harmful to young birds." Poultry Parasitic diseases 
Diseases of Poultry | Mississippi State University Extension Service. (2018). Extension.msstate.edu. Retrieved 21 December 2014, from http://extension.msstate.edu/agriculture/livestock/poultry/diseases-poultry​

Symptoms caused by gapeworms are:

• Spitting
• Gurgling sounds
• Pale wattles and comb
• Red worms in feces (can grow up to 1/4" in length)

In order to stop the gapeworm infestation you will need to deworm your chickens and continue to deworm them every 30 days.

The most common prevention and treatment for Gapeworm is Wazine 17%

• Wormer for use in drinking water for the removal of large roundworms from turkeys and chickens.
• Also removes large roundworms and nodular worms from swine.
• Worming every 30 days is necessary to break the large roundworm life cycle

How chickens contract gapeworms
When your chickens eat earthworms or worm eggs which are infected with the gapeworm larvae the female worm lays her eggs in your chickens trachea. Your chickens will cough up the eggs, swallow them, and pass them through their feces. 
Most likely gapeworm hosts

• Earth worms
• Snails
• Slugs

Appearance of gapeworms
This male and female red worm are joined together to make a Y or fork shape with the female being the larger of the two worms.
How to get rid of gapeworms

• Break the life cycle of the worms by deworming your chickens and other livestock and pets every 30 days. 
• Alternate your pens by using them every other year
• Till the soil in your pens at the end of each growing season
• Treat the soil to get rid of earthworms, snails, and slugs

"Gapeworms are best prevented by administering a wormer at fifteen to thirty day intervals or including a drug at low levels continuously beginning fifteen days after birds are placed in the infected pens. One drug that is effective for eliminating gapeworms is fenbendazole, however, its use is not presently approved for use in birds by the Food and Drug Administration" 
Reference: MSU cares.com http://msucares.com/poultry/diseases/disparas.htm

How to stop the gurgling sound & first aid for accidental poisoning in small animals
"Activated charcoal gel is made from hardwood charcoal and attapulgite clay is most often used for emergency first aid for treatment of poisoning in small animals from insecticides, organic chemicals, intestinal toxins and diarrhea. It is a black, tar-like substance that has extreme drying properties and will make your chicken thirsty. However, I have used it on our chickens when they were gurgling. I put a small amount on my finger and placed it on the bottom of the inside of the beak or in the throat. Then I put Vick's vapor rub on the comb, wattles, legs, under the wings, and on the middle of the back and within hours the gurgling stopped in most cases, if caught early enough within the first day or two. I also dewormed them and have had no fatalities. The charcoal gel also prevents the possibility of overdose or poisoning from too much worm medicine".-Mother Hen

Gapeworm is found worldwide, particularly in Asia and Africa. It is quite common in birds reared on soil in traditional farming: up to 25% of birds in a farm may be infected. See pictures and more details about gapeworms here: http://parasitipedia.net/index.php?option=com_content&view=article&id=2653&Itemid=2942