"Heavy infestations" means a large number or amount of something undesirable, such as pests or harmful organisms, that have invaded or taken over a place. Full definition
The cat flea has even been known to kill animals as large as dairy calves through heavy infestation. (vin.com)
The animals were all extremely dirty and suffering from heavy infestations of internal and external parasites. (animalrescuecorps.org)
Newly infected dogs show no outward signs of infestation, while dogs with heavy infestations present with coughing, fatigue, inappetence, weight loss, anemia, fainting and heart failure. (dogcare.dailypuppy.com)